Mechanical properties are those that have a decisive influence on the deformation and strength of soil under load.
Deformation of soils under load is accompanied by complex processes: compression of solid particles, compression of water and air located in the pores of the soil, destruction of bonds between particles and their mutual displacement, changes in the thickness of water films and squeezing out free water from the pores of the soil.
These processes lead to deformations, which can be divided into elastic, i.e., disappearing after the load is removed, and residual.
The load on the soil foundation can be increased until there is a sharp increase in foundation deformations associated with the development of shifts in it. The higher the soil's shear resistance, the greater the load it can bear.
Soil shear resistance. The strength of soils at the base depends mainly on the resistance to shear of particles relative to each other due to the presence of friction and adhesion forces between them. The resistance to mutual displacement of two particles or two groups of particles can be illustrated schematically using the example of the displacement of two bodies.
Compressibility of soils and their compression tests. Modulus of soil deformation. The characteristic properties of soils to change their volume under the influence of load due to elastic compression or compression with mutual movement of particles without breaking the continuity are called their deformative properties. The main characteristics of the deformative properties of soils are the modulus of total deformation and the coefficient of lateral expansion.

Deformation properties characterize the behavior of soil under loads that do not exceed critical loads and, therefore, do not lead to destruction. These properties can be expressed by two pairs of indicators: either the deformation modulus and Poisson's ratio, or the shear and volumetric compression moduli.

Strength properties characterize the behavior of soil under loads equal to or exceeding critical ones, and are determined only when the soil is destroyed. Shear and rupture are the two main mechanisms of strength loss in a body. The shift occurs under the influence of tangential forces; When shearing, one part of the body moves relative to another. The rupture of a body occurs under the influence of normal tensile forces and is morphologically expressed in the form of cracks and separation of one part of the body from another. The main indicator of the strength of soils is their shear resistance; tensile strength is determined much less frequently. In the practice of geotechnical surveys, the resistance of soils to uniaxial compression is often determined.

End of work -

This topic belongs to the section:

Structure, tasks of geology, its role in the construction industry

In construction practice, any rocks and soils are called soils, soil is a mineral or organomineral dispersed phase... and rocks that are located in the upper part of the lithosphere and are... analyzed to select optimal design solutions for the placement of structures and production methods..

If you need additional material on this topic, or you did not find what you were looking for, we recommend using the search in our database of works:

What will we do with the received material:

If this material was useful to you, you can save it to your page on social networks:

Tweet

All topics in this section:

History of the development of geology. Main stages of development
As a science, historical geology began to take shape at the turn of the 18th-19th centuries, when W. Smith in England, and J. Cuvier and A. Brongniard in France came to the same conclusions about the successive change of layers and

What are the tasks of engineering geology in construction
In the process of engineering-geological research, information is collected about the physical-geographical situation, climate, vegetation, wildlife, experience in the construction and operation of structures, economics

Methods used in engineering geology
Using geophysical methods, a number of important engineering and geological problems can be solved. When conducting geotechnical research they often use: electric

Basic technological sequence of structure design
Engineering-geological surveys are needed to determine the features of the geological structure of the construction site. Survey work includes drilling wells, taking rock samples

What hypotheses about the origin of the Earth do you know?
Kant-Laplace hypothesisThey believed that the progenitor of the Solar system was a hot gas-dust nebula, slowly rotating around a dense core at the center. Under in

Describe the structure of the globe and its outer and inner shells
The structure of the globe was the result of complex processes occurring both in the interior of the Earth and on its surface. The earth has the shape of a geoid (Greek ge - earth, eidos - view), i.e. a ball, several

What does paleontology study?
Paleontology (from ancient Greek παλαιοντολογία) is the science of fossil remains of plants and animals, attempting to reconstruct

What does geotectonics study?
Geotectonics is a branch of geology, the science of the structure, movements and deformations of the lithosphere, its development in connection with the development of the Earth as a whole. Geotectonics is the theoretical core of all geology[

The main features of the relief of the earth's surface
The most characteristic feature of the face of the Earth is the antipodal, i.e. opposing, arrangement of oceanic and continental spaces. The antipodes of the continents on one side of the globe are the oceans on the opposite side.

Main tectonic structures
Tectonic structures - These are large areas of the earth's crust bounded by deep faults. The structure and movements of the earth's crust are studied by the geological science of tectonics.

As you already know, croup
Tectonic movements of the earth's crust

Tectonic disturbances are movements of matter in the earth's crust under the influence of processes occurring in the deeper interior of the Earth. These movements cause tectonic disturbances, i.e. changes
How are the formation elements determined?

Elements of occurrence of geological boundaries (strata, bedding surfaces and unconformities, tectonic) are not always possible to measure in outcrops. They can be identified: by visible slopes in the naked
Folds and their elements

Among the folds, elementary types of folds are distinguished - anticlinal and synclinal, neutral, as well as antiforms and synforms.
Anticlinal folds or anticlines are called izg

Fold elements
The following elements are distinguished in a fold: lock or arch, wings, axial surface, center line or fold axis, fold hinge, ridge and keel, ridge and keel surface, inflection line and

Types of discontinuous and continuous faults (dislocations)
The platform is a relatively stable block of continental crust. Platforms are vast, sedentary areas of the earth's crust - the most stable blocks that create its solid frame. Straw

List the main properties of minerals
For a long time, the main characteristics of minerals were the external shape of their crystals and other secretions, as well as physical properties (color, luster, cleavage, hardness, density, etc.)

List the processes of mineral formation
PROCESSES OF MINERAL FORMATION - physico-chemical. processes occurring in the earth's crust and causing the formation, change and destruction of minerals. The classification of P. m. is based, on the one hand, on the source of

The most important rock-forming minerals
Among the wide variety of natural minerals, only a small part of them participates in the formation of rocks. These minerals, called rock-forming minerals, include quartz, feldspars,

What is the Mohs scale used for?
To measure the hardness of minerals, attempts have been made to apply all sorts of methods based on the resistance of stones to scratching, abrasion, drilling, surface deformation... But all these attempts do not

Engineering - geological features of igneous and metamorphic rocks
Engineering-geological features of metamorphic rocks The physical and mechanical properties of metamorphic rocks are in many ways close to igneous ones, which means

What forms of intrusive bodies do you know?
Theoretically, intrusive bodies come in all sizes and shapes, but they can usually be classified into one of a variety of specific sizes and shapes.

Dykes - pla
What types of metamorphism do you know?

Metamorphism is a complex physical and chemical phenomenon caused by the complex effects of temperature, pressure and chemically active substances. It flows without being
What factors determine metamorphism

Metamorphism is the transformation of rocks under the influence of endogenous processes that cause changes in physicochemical conditions in the earth's crust. Any rock can undergo transformation - o
What metamorphic rocks do you know?

Metamorphic rocks are the result of the transformation of rocks of different genesis, leading to a change in the primary structure, texture and mineral composition in accordance with the new physical and chemical
Breeds of biochemical origin. Depending on the composition, siliceous (tripoli, opoka, some jaspers), carbonate (limestones, dolomites, marls) and phosphate rocks are distinguished.

Physical properties of soils. Indicators of physical properties of soils. Methods for their determination
physical properties of soils: density, moisture, strength, cohesion, lumpiness, loosening, angle of repose and erosion.

Density p is the ratio of soil mass, ink
Soil density, main indicators

Density p is the ratio of the mass of soil, including the mass of water in its pores, to the volume occupied by this soil. Density of sandy and clayey soils - 1.5...2 t/m3; semi-rocky, not loosened
Basic properties of clayey rocks

The special properties of clayey rocks are largely determined by the crystal chemical properties of clay minerals and their high dispersity (that is, extremely small particle size). Most
Determination of soil shear resistance. Coulomb formula. Devices. Building graphs. Shift passport

The shear resistance of soils is their most important strength indicator. It is necessary for calculating the stability and strength of foundations, assessing the stability of slopes, calculating soil pressure on soil
Which rock is the strongest?

Deep rocks (igneous) are characterized by high density, frost resistance and low water absorption. The main types of deep rocks are granites, syenites, gabbros, labradorites
Physico-chemical properties of soils, their significance in construction practice. Thixotropy

Physical properties: First of all, physical properties include: specific and volumetric mass, as well as porosity (porosity) of soils. The ratio of the solid phase of dry soil to the weight of an equal volume of water n
Soil as a multiphase system. The nature of the structure of bonds in the soil

Dispersed soils are a multiphase system. They consist of two or more substances distributed one within the other. An example of such a system is a clay suspension consisting
Rock mass as an object of engineering-geological research

Based on engineering-geological data of the rock mass, optimal design solutions for field development are selected, and therefore the costs of engineering-geological work are justified
When interacting with engineering structures

Fracture assessment, control measures
The degree of rock fracturing, together with other tectonic disturbances, characterizes the structure of the rock mass, its spatial heterogeneity and anisotropy of properties. It influences the

Criteria for assessing the degree of fracturing
The criteria for quantitative assessment of the degree of fracturing are indicators that take into account the size and density of cracks.

There are three types of indicators: linear
Types of cracks

Cracks are flat discontinuities in a continuous medium if their magnitude exceeds the interatomic distances in the crystal lattice by an order of magnitude or more. There are three orders of cracks:
Fracture Characteristics

The correct choice of development system and drilling and blasting parameters depends on the degree of fracturing. In the old days, fracturing was assessed using the acoustic method, hitting the rock with a hammer and listening
Theories of the origin of groundwater

1. Infiltration theory. Basic provisions: groundwater comes from atmospheric precipitation, which penetrates into the ground through the smallest channels of rocks, where it accumulates, which happens
Underground and surface runoff

Surface runoff is the process of water moving across the earth's surface under the influence of gravity. Surface runoff is divided into slope and channel runoff. Slope runoff is formed from
Physical properties of groundwater

According to GOST, the physical properties of groundwater also include density, viscosity, electrical conductivity, radioactivity, etc. Density of water - mass of water, nah
Main chemical components of groundwater

ion-salt composition. Groundwater does not occur in chemically pure form. More than 60 elements of Mendeleev's periodic table were discovered in it. The main components (ions) that determine chemically
Aggressiveness and hardness of groundwater

Most often, water tests are performed on samples where the total dissolved solids account for only a small fraction of one percent of the total weight of the water sample. Therefore, water mineralization
Kurlov's formula

Kurlov, 1921, is a pseudo-formula that clearly depicts the basic properties of chemicals. comp. water. Anions are written in the numerator of the fraction, cations present in an amount of more than 5% equivalent are written in the denominator. (based on
Unloading

Kurlov, 1921, is a pseudo-formula that clearly depicts the basic properties of chemicals. comp. water. Anions are written in the numerator of the fraction, cations present in an amount of more than 5% equivalent are written in the denominator. (based on
Groundwater is the free water of the first permanently existing aquifer from the surface, which lies in the zone of complete saturation. The recharge area of ​​groundwater, as a rule, coincides

Maps of hydroisohypsum and hydroisobates. Their analysis
Hydroisohypsum map - a map that displays the position of the groundwater table in the form of hydroisohypses. HYDROISOBATHS - lines connecting on the plan (map) groundwater mirror points located

Unloading Elements of artesian pools. Hydroisopiesis maps
Artesian is called pressurized groundwater located in permeable (porous, fractured, karst) layers, covered and underlain by waterproof rocks. These waters are everywhere

Name the water and physical properties of rocks
The water properties of rocks are understood as those that appear in them when interacting with water: water permeability, moisture capacity, water loss, natural humidity, swelling, wetting,

Lifting, water loss, water absorption, water saturation
One of the main properties of a rock that determines its relationship to water is porosity and porosity. Porosity refers to the presence of small voids in rocks - capillary pores; porosity - n

Porosity, density, humidity
Physical properties characterize the physical state of rocks, i.e. qualitative certainty, manifested in their density, humidity, porosity, fracturing and weathering under conditions

Name the types of water in rocks
1)Water in the form of steam. This type of water is present in the air that fills cracks and voids between rock particles.

2) Water in the form of ice. Ice in soils and rocks
Movement. Darcy's formula. How do laminar and turbulent differ?

groundwater movement?
The speed of movement (filtration) of groundwater is characterized by Darcy’s law “The amount of water Q passing through any section F per unit time

Methods for determining the filtration coefficient (CF)
1) filtration devices in laboratories. The filtration coefficient k is determined in the laboratory using a special installation in which a sample of the soil being tested is placed.

Galleries, etc.). Describe how water intakes differ in the nature of opening
Pressure is the amount of liquid pressure expressed by the height of the liquid column above the selected reference level; measured in linear units.

PRESSURE GRADIENT
Main types

[edit] Reservoir drainage Reservoir drainage system is laid at the base of the protected structure directly on the aquifer. At the same time, it is hydraulically connected
The concept of depression funnel and radius of influence

When pumping water from wells, due to friction of water against soil particles, a funnel-shaped decrease in the water level occurs.
A depression funnel is formed, in plan having a shape close to a circle

Factors determining the development of geological and engineering-geological processes and phenomena
Exogenous (from the Greek éxo - outside, outside) are geological processes that are caused by energy sources external to the Earth: solar radiation and gravitational field.

Endogenous engineering-geological processes and phenomena. general characteristics
Endogenous (internal) processes are those geological processes whose origin is associated with the deep interior of the Earth. The substance of the globe develops in all its forms

What is an earthquake called, hypocenter, epicenter?
Earthquakes are tremors and vibrations of the Earth's surface caused by natural causes (mainly tectonic processes), or (sometimes) artificial

Seismic waves and their measurement
The sliding of rocks along a fault is initially prevented by friction. As a result, the energy causing movement accumulates in the form of elastic stresses in the rocks. When the voltage reaches critical

Types of seismic waves
Seismic waves are divided into compression waves and shear waves.

§ Compression waves, or longitudinal seismic waves, cause vibrations of the rock particles through which they pass, vd
Man-made earthquakes

Recently, information has emerged that earthquakes can be caused by human activity. For example, in areas of flooding during the construction of large reservoirs, tectonic influences intensify.
Magnitude scale

The magnitude scale distinguishes earthquakes by magnitude, which is the relative energy characteristic of the earthquake. There are several magnitudes and, accordingly, magnitude
Weathering is the process of destruction and change of rocks in the conditions of the earth's surface under the influence of mechanical and chemical influences of the atmosphere, ground and surface waters and organisms. P

What is evolution, deluvium, proluvium, colluvium, alluvium. Their engineering geological features
Eluvium (eluvial deposits) (Latin eluo - “I wash away”) - loose geological deposits and soils formed as a result of weathering of surface rocks in place of

River valleys. River erosion. Basis of erosion
Valley (river) is a negative, linearly elongated form of relief with a uniform fall. It is usually formed as a result of the erosive activity of flowing water. Rechnaya in

Linear erosion
In contrast to surface erosion, linear erosion occurs in small areas of the surface and leads to the dismemberment of the earth's surface and the formation of various erosion forms (ravines, ravines, beams

Mudflow processes, their division
According to the mechanism of movement, mudflows can be divided into two types. Type 1 - cohesive (“mud” and “mud-stone”) flows with a predominance of viscous flow. Type 2 - disconnected (“water-rock”) flows

Karst development
Negative relief forms are most characteristic of karst. Based on their origin, they are divided into forms formed by dissolution (surface and underground), erosive and mixed. By morph

The concept of suffusion, quicksand, cause of occurrence, control measures
Suffosion (from Latin suffosio - digging) is the removal of small mineral particles of rock by water filtering through it. The process is close to karst, but differs from it in that it

True quicksand
Often, quicksand properties are exhibited by silty sands and sandy loams, saturated with water, containing large quantities of very small particles (clayey and colloidal), which begin to play a lubricating role.

False quicksand
False quicksand is fine porous sand saturated with water. Since the formation is located at depth, the water in the pores of the quicksand is under pressure greater than atmospheric pressure. When opened, the formation is exposed, and water

Engineering-geological assessment of permafrost
The distribution of frozen strata is subject to latitudinal and altitudinal zoning. Based on average annual temperatures, the nature of distribution and thickness of permafrost, five zones are distinguished. Continuously

Stress state of rocks
The stressed state of the earth's crust characterizes not only the surface layers themselves, which can be observed directly, but also the deeper parts of the earth's crust, and the stress value is composition

What are the criteria for assessing the engineering and geological conditions of the area?
Engineering-geological surveys.

1 collection and processing of materials from previously completed work; 2 field work (drilling and testing wells, field soil research); 3 hydrogeological
Requirements for constructing maps. Reading geological sections and maps

A geological map showing the horizontal occurrence of rocks has its own characteristics:  the youngest rocks occupy the highest areas of the terrain (mountain tops),

Construction and analysis of hydroisohypsum maps
Determination of underground flow rate

The calculation is made using a map of hydroisohypsum, constructed according to data from measuring levels in wells, in places where springs emerge a) H1 = h1 and H2 = h2 b)
Practice of constructing a hydroisohypsum map based on borehole data

Groundwater recharge due to surface water occurs everywhere (the level of surface and groundwater fluctuates depending on the time of year). As a result, between superficial
Construction and analysis of engineering geological sections. Construction practice

Engineering geological sections (profiles) are a widely used form of graphical processing and summarization of information that characterizes engineering geological and hydrogeological conditions
Construction of the geological column of a well drilled within the geological map

To construct a geological column of a well, descriptions of boreholes drilled within the geological map are used. To build a geological column, for example well No. 6,
Stages of engineering and geological surveys for construction

Engineering surveys are an important part of construction design. As a result of a set of measures, the necessary data is received about the natural conditions of the area where construction is planned.
Modern methods of research and processing of engineering-geological information

To obtain, accumulate, store and process engineering geological information, various methods are used, which are usefully divided into methods: obtaining information - M11
Methods of engineering-geological sampling and sampling sequence

Engineering-geological testing is a method that includes methods for establishing the volume and parameters of data, methods for selecting soil samples and their conservation. This method, together with other methods (

ORGANIZATION STANDARD
Deformation
and strength characteristics

Jurassic clay soils of Moscow

STO 36554501-020-2010

Moscow

Preface

1 DEVELOPED AND INTRODUCED by the Laboratory of Electrical Engineering Technologies (head of the laboratory - Candidate of Technical Sciences Kh.A. Dzhantimirov) Research Institute-OSP named after. N.M. Gersevanov - Institute of OJSC "National Research Center "Construction" led by. scientific associate, cand. tech. Sciences O.I. Ignatova

3 APPROVED AND ENTERED INTO EFFECT by order of the General Director of OJSC “Scientific Research Center “Construction” dated February 10, 2010 No. 27

4 INTRODUCED FOR THE FIRST TIME

Introduction

In connection with the intensive development in recent years of construction in Moscow of high-rise and high-rise buildings with a deep underground part and underground structures, a need has arisen to assess the construction properties of soils located at great depths. These soils include soils of the Jurassic, Cretaceous and Carboniferous periods.

Assessing the characteristics of these soils based on statistical generalization of accumulated archival geotechnical survey data is an urgent task.

To carry out the work, archival materials of laboratory and field tests of pre-Quaternary soils in Moscow were collected from reports on engineering-geological surveys of 40 organizations conducting survey work in the city, which were received by the institute for specific design objects.

This standard provides research results for Jurassic J 3 clay soils.

The results of studies of the relationship between the deformation modulus according to stamp tests and the resistivity of the soil under the probe cone for the Jurassic clays of Moscow are presented in the work, but they were based on little statistical material.

Based on the research conducted for Jurassic clayey soils, tables of standard and calculated values ​​of strength and deformation characteristics were compiled and transition coefficients from compression deformation moduli to stamp ones were established. For these soils, an equation was also obtained for estimating the deformation modulus based on the results of static sounding. The results of the research were published in the work.

These results are recommended to be used in the practice of geotechnical surveys, design and installation of foundations and foundations, which will increase the reliability of the deformation and strength characteristics used in foundation calculations.

ORGANIZATION STANDARD

DEFORMATION AND STRENGTH CHARACTERISTICS JURASSIC CLAY SOILS OF MOSCOW Deformation and strength characteristics of Jurassic clay soils in Moscow

Date of introduction 2010-02-25

1 area of ​​use

1.1 This standard applies to the determination of deformation and strength characteristics of Jurassic J 3 clay soils of Moscow. These soils were represented by the following sediments: J 3 ν - Volgian stage; J 3 ox- Oxfordian stage and J 3 cl- Callovian stage. In table the ranges of variation and average values ​​of the main physical characteristics of the soils of the indicated deposits are given.

1.2 The standard is intended to determine the standard and calculated values ​​of the deformation and strength characteristics of soils using tables and equations depending on their physical characteristics and static sounding data.

1.3 Tables and equations for determining the standard and calculated values ​​of the deformation and strength characteristics of soils are recommended to be used for preliminary calculations of the foundations and foundations of buildings and structures of the I level of responsibility and final calculations of the foundations and foundations of buildings and structures of the II and III levels of responsibility.

Index	Characteristic values	ρ , t/m 3	e	w L, %	Ip, %	I L	h, m
J 3 ν		1,72	0,48			0,25
		2,14	1,14			0,90
	Average	1,92	0,77			0,29
J 3 ox		1,62	0,82			0,26
		1,93	1,52			0,40
	Average	1,75	1,20			0,04
J 3 cl		1,74	0,60			0,36
		2,04	1,22			0,35
	Average	1,84	0,98			0,06

2 Normative references

Static soil probing was carried out with a type II probe in accordance with GOST 19912.

Compression tests of soils were carried out in accordance with GOST 12248 for soils with natural moisture. For the research, the results of tests with a final vertical load were used R≥ 0.5 MPa. The values ​​of compression deformation moduli were calculated in the load range of 0.2 - 0.5 MPa.

Values φ And With were determined based on the data of consolidated-drained tests for shearing soils with natural moisture in accordance with GOST 12248.

The physical characteristics of soils were determined in accordance with GOST 5180.

3.3 To compile tables of standard and calculated values ​​of deformation and strength characteristics of soils during statistical processing of materials, a correlation-regression analysis apparatus was used, which makes it possible to establish correlations and regression equations between mechanical characteristics E, φ And With on the one hand, and physical characteristics and static sounding data q with another. The closeness of the connection is characterized by the correlation coefficient R and mean square (standard) deviation S(application ).

The following physical characteristics were used in the correlation analysis: plasticity number I r as an indicator of the type or clay content of the soil; porosity coefficient e as an indicator of soil density in its natural occurrence and an indicator of fluidity I L as an indicator of soil condition by consistency.

3.4 Correlation studies have been carried out between standard values ​​of mechanical and physical characteristics and probing resistance q, defined as the arithmetic mean of partial values ​​for engineering geological elements (IGE) identified during surveys (GOST 20522).

To determine standard and calculated values E, φ And With According to tables and equations, it is necessary to use standard values ​​of physical characteristics and probing resistance q for IGE.

4 Determination of the deformation modulus by physical characteristics

4.1 Standard values ​​of the field modulus of deformation E should be taken according to equation () or table. , compiled on the basis of statistical processing of the results of testing soils with a stamp and a pressuremeter (Fig.).

Turnover rateI L	Standard values ​​of deformation modulus E, MPa, at porosity coefficient e, equal
	0,6 - 0,7	0,8 - 0,9	1,0 - 1,1	1,2 - 1,3	1,4 - 1,5
0,25 ≤ I L ≤ 0
0 < I L ≤ 0,25
0,25 < I L ≤ 0,5
0,5 < I L ≤ 0,75

Picture 1- Dependence of the deformation modulus according to stamp data ( E m) And
pressureometric ( En) tests ( n IGE = 75; n i= 280) from the coefficient
porosity e and turnover rate I L for Jurassic clay soils:
I L:1 - (-0,25); 2 - 0,0; 3 - 0,25; 4 - 0,5; 5 - 0,75

5 Determination of the deformation modulus from static sounding data

5.1 Standard values ​​of the field modulus of deformation E should be taken depending on the resistivity of the soil under the probe cone q according to equation (), obtained on the basis of statistical processing of the results of soil testing with a stamp, pressure meter and static probing (Fig. ).

Figure 2- Dependence of the deformation modulus E according to stamp data
and pressureometric tests on soil resistivity
under the probe cone q :
experimental points: 1 - For J 3 ox; 2 - For J 3 ν; 3 - addiction E = f(q)

6 Coefficients of transition from the compression modulus of deformation to the die modulus

6.1 Transition factors m k from the compression modulus of deformation to the die one should be taken or depending on the porosity coefficient e and turnover rate I L(table), or depending on the plasticity number I r and turnover rate I L(Table).

Turnover rateI L	Coefficient valuesm kat porosity coefficient e, equal
	0,6 - 0,8	0,9 - 1,1	1,2 - 1,5
0,25
0,25
0,75

Turnover rateI L	Coefficient valuesm kat plasticity numberIp equal
	≤ 7	8 - 17	18 - 30	31 - 50
0,25
0,25
0,75

Figure 3- Coefficient dependence m k on porosity coefficient e
and turnover rate I L for Jurassic clay soils
(n = 32; m k = 2,47 + 0,53e - 1,60I L; R = 0,79; S = 0,42):
I L:

Figure 4- Coefficient dependence m k from the plasticity number I r
and turnover rate I L for Jurassic clay soils
(n = 32; m k = 2,51 + 0,02I r - 1,24I L; R = 0,83; S = 0,38):
I L:1 - (-0,25); 2 - 0,0; 3 - 0,25; 4 - 0,5; 5 - 0,75

When using coefficients m k according to table and to adjust the compression deformation moduli, the latter must be calculated in the range of vertical pressures of 0.2 - 0.5 MPa, and the coefficient values β , taking into account the impossibility of lateral expansion of the soil in a compression device, is 0.4 for clays, 0.62 for loams and 0.72 for sandy loams.

7 Determination of strength characteristics based on physical characteristics

7.1 Standard values ​​of strength characteristics of Jurassic clay soils - angle of internal friction φ and specific adhesion With, obtained from the results of consolidated-drained (CD) shear tests of soils, should be determined depending on the plasticity number I r and turnover rate I L according to equations () and () or table. (Fig. and):

Turnover rateI L Characteristic designation Standard values φ ° and With, kPa, at plasticity numberI r,% equal ≤ 1 8 - 17 18 - 30 31 - 40 41 - 50 0,25 ≤ I L ≤ 0 φ ° With, kPa 0 < I L ≤ 0,25 φ ° With, kPa 0,25 < I L ≤ 0,5 φ ° With, kPa 0,5 < I L ≤ 0,75 φ ° With, kPa 7.2 Design values φ And With should be calculated based on standard values ​​(table), reducing them by the value of the confidence interval Δ, calculated using the method of app. 2 SRT with confidence probability α = 0.85 and α = 0.95 (SP 50-101). Confidence interval Δ for φ And With is: Δ φ = 1° Δ With= 7 kPa (at α = 0.85); Δ φ = 2° Δ With= 11 kPa (at α = 0.95). Figure 5- Dependence of the angle of internal friction φ ° from the plasticity number I r and turnover rate I L Appendix A J 3v- Upper Jurassic deposits of the Volgian stage J 3 ox- Upper Jurassic deposits of the Oxfordian stage J 3cl- Upper Jurassic sediments of the Callovian stage ρ - soil density e- soil porosity coefficient I r- soil plasticity number I L- soil fluidity indicator h- depth of soil sampling or testing with a stamp (pressiometer) E w - deformation modulus according to the results of stamping tests E n - deformation modulus according to the results of pressureometric tests q- soil resistivity under the probe cone during static probing KD - consolidated-drained soil section R- correlation coefficient S- standard deviation (standard deviation) Appendix B To study the relationships between mechanical at and physical x i characteristics, the apparatus of correlation and regression analysis was used. The calculations were carried out on a computer using a standard program, which provides for the construction of a linear dependence of the form by the least squares method To approximate a nonlinear relationship, a 2nd or 3rd degree polynomial or equation () is most often used. However, due to the fact that statistical estimates in correlation theory are developed only for linear dependencies, nonlinear dependencies must be converted into linear ones by replacing variables. m- average number of definitions φ And With in IGE; n- total number of standard values φ And With(total number of IGE); d 2 - functional characterizing the change in the width of the confidence interval along the dependence. It should be noted that the value d 2 /n at those values n, which occurred in the studied sample of experimental data, turned out to be negligibly small. Calculated values φ And With calculated with confidence probabilities α = 0.85 and α = 0.95, regulated

The main indicators of the mechanical properties of soils, which determine the bearing capacity of the foundations, as well as their deformation, are the angle of internal friction, specific adhesion WITH, deformation modulus E. To determine the mechanical properties of soils, you can use the tables in Appendix 1 of SNiP 2.02.01-83*. For sandy soils, standard adhesion values ​​are
(kPa), angle of internal friction (deg.) and deformation modulus E(MPa) (Table 1.2.1) is determined depending on the type of soil and porosity coefficient. For silt-clay soils of size
,(Table 1.2.2) and E(Table 1.2.3) are determined depending on the type of soil, fluidity index and porosity coefficient. The required standard value of the soil mechanical properties indicator is determined using, if necessary, linear interpolation based on the porosity coefficient. If the values e, soils exceed the limits provided in the table, the characteristics
,And E should be determined based on direct testing of these soils in field or laboratory conditions. It is allowed to take into account the characteristics as a safety margin
,And E according to the corresponding lower limits e, , if the soils have values e, less than these values.

Table 1.2.1. – Extract from Table 1, Appendix 1, SNiP 2.02.01-83*. Standard values ​​of specific adhesion With n j n, deg. and deformation modulus E, MPa (kgf/cm2), sandy soils of Quaternary deposits

Sandy soils		Characteristics of soils at porosity coefficient e, equal
Gravelly and large	c n
	j n
Medium size	c n
	j n
	c n
	j n
Dusty	c n
	j n

Table 1.2.2. – Extract from Table 2, Appendix 1, SNiP 2.02.01-83*. Standard values ​​of specific adhesion With n, kPa (kgf/cm 2), angle of internal friction j n, deg. silty-clayey non-loess soils of Quaternary deposits

		Designations of soil characteristics	Characteristics of soils at porosity coefficient e, equal
	0 £ I L£0.25	c n j n
	0,25 < I L£0.75	c n j n
Loams	0 < I L£0.25	c n j n
	0,25 < I L£0.5	c n j n
	0,5 < I L£0.75	c n j n
	0 < I L £0.25	c n j n
	0,25 < I L£0.5	c n j n
	0,5 < I L£0.75	c n j n

Table 1.2.3. Extract from Table 3, Appendix 1, SNiP 2.02.01-83*. Standard values ​​of the deformation modulus of silty-clayey non-loess

Origin and age of soils

Name of soils and limits of standard values ​​of their fluidity index

Soil deformation module E, MPa (kg/cm 2), with porosity coefficient e, equal

Quaternary deposits

Alluvial, Diluvial, Lacustrine-alluvial

0 £ I L£0.75

Loams

0 £ I L£0.75

0,25 < I L£0.5

0,5 < I L£0.75

0 £ I L£0.75

0,25 < I L£0.5

0,5 < I L£0.75

Fluvioglacial

0 £ I L£0.75

Loams

0 £ I L£0.75

0,25 < I L£0.5

0,5 < I L£0.75

Moraine

Loams

I L£0.5

Jurassic deposits of the Oxfordian stage

£0.25 I L £ 0

0 < I L£0.25

0,25 < I L£0.5

Soil strength is called their ability to resist destruction. In general, soil destruction can be caused by forces of different nature (mechanical, thermal, electrical, etc.), therefore, the corresponding types of soil strength are distinguished according to the nature of the destructive effects. For engineering-geological purposes, it is first of all important to know mechanical strength soils, i.e. their ability to resist destruction under the influence of mechanical stress. If the deformation characteristics of soils are determined at stresses that do not lead to destruction (i.e., subcritical), then the soil strength parameters correspond to critical failure stresses and are determined at extreme loads that cause either separation of the body into parts (for elastic soils) or an irreversible change in shape bodies as a result of deformation of plastic flow (for plastic soils).

The physical nature of soil strength is determined by the forces of interaction between their structural elements - crystals, grains, fragments, aggregates, particles, i.e., it depends on the type and characteristics of structural connections. The greater the interaction force between the structural elements of the soil, the higher its strength as a whole. Therefore, rocky soils, among which strong chemical (crystallization and cementation) structural bonds predominate, have greater strength than dispersed soils with weak physical and physicochemical structural bonds.

Since the tested soil sample can be subject to different stresses (normal, tangential, volumetric, or combinations thereof), then measures of its strength different types of critical stresses or their ratios can be selected, these are the measures that are strength parameters.

By now, more is known two dozen strength conditions developed to describe the behavior of clay and sandy soils. According to the classification proposed by W.-F. Chen, all stressed states of soils can be divided into one- and two-parameter models. One-parameter models include the Tresca, Mises, Lade, and Duncan strength conditions. Two-parameter models include the conditions proposed by Mohr-Coulomb, Drucker-Prager, R. Lade, M.V. Malyshev and others. After the publication of W.-F. Chen many years have passed (1984), and during this time strength conditions or soil models have been proposed, which can be called multi-parameter. The most complex of them include up to 6 independent parameters, determined from very complex and expensive experiments. Despite the variety of strength conditions, only a few of them are used in practice. This is primarily a condition for the strength of the Mohr-Coulomb, Cap-model and multi-surface models (Prevost, 1977, 1985; Dafalias, 1985). The last two groups of soil models are more complex and do not allow obtaining solutions in analytical form, therefore they are used in nonlinear mechanics and numerical problem solving.

When assessing the strength of soils, they most often use limit state theory, according to which certain parameters of critical (limiting) stress values ​​that a soil sample can withstand without destruction are determined. Strength limits are those limits, when exceeded, the soil is destroyed and it does not perceive the forces applied to it. The critical values ​​of the parameters correspond to different types of stressed states of the soil in which it may be located and which can be characterized by the values ​​of the principal stresses σ1, σ2 and σ3, and σ1, σ2 and σ3 The following conditions are most often considered as such (Fig. 8.27):

• plane shear ( σ1> 0, r > 0, fig. 8.27, A);
• uniaxial tension σ1 0, σ2= σ3= 0, fig. 8.27, b);
• uniaxial compression (when σ1 > 0, σ2 = σ3= 0, fig. 8.27, V)
• triaxial compression (σ2 = σ3 ≠ σ1> 0, fig. 8.27 (g, d, e).

Rice. 8.27. Experimental schemes: shear (a): uniaxial tension (b); for uniaxial compression (c): for triaxial compression: for determining the undrained strength of soils (d): drained strength of sandy (e) and clayey (f) soils

Strength characteristics of dispersed soils (angle of internal friction <р and specific adhesion c) can be obtained by testing soils using laboratory methods: shear or triaxial compression, tension, but the angle of repose, indentation of a stamp with a spherical or cone-shaped surface, and in the field - by shear testing of soil pillars in pits or pits. The parameters of strength properties and laboratory methods for their determination, regulated by current regulatory documents, are given in Table. 8.30.

For water-saturated clay soils with a fluidity index //,>0.5, organomineral and organic soils, for which preparing pillars for field tests or selecting samples for laboratory tests is difficult, strength characteristics (c„) for calculating foundations from these soils in an unstabilized state can be determined by the field method of rotational cutting in wells or massifs.

Values (rice sand and clay soils for structures of II and III levels of responsibility can be determined by field methods of translational and annular cutting in wells. At the same time, for structures of the 11th level of responsibility, the obtained values <р and c should be clarified based on their comparison with the results of parallel tests of the same soil by laboratory methods for shear or triaxial compression, and in field conditions - tests for shearing of soil pillars in pits or pits.

Values (R And With sand and clay soils can be determined static probing method. and sands (except for silty water-saturated ones) - dynamic sensing method. For structures of I and II levels of responsibility, the values ​​obtained by sounding (rice should be clarified on the basis of their comparison with the results of parallel tests of the same soil by laboratory methods for shear or triaxial compression, and in field conditions - tests for shearing soil pillars in pits or pits. In other cases, it is allowed to determine the values (rice only according to sounding data [114].

Rotary shear testing with an impeller should be carried out to assess maximum shear strength values with and organic-mineral and organic soils and clayey soils of soft plastic, fluid consistency in undrained conditions. The test methodology and interpretation of the results obtained should be carried out in accordance with GOST 20276-99 (or ASTM D2573, NEN 5106 when carrying out surveys jointly with foreign investors or according to their technical specifications).

Determination of the strength characteristics of soils in laboratory conditions should be carried out using the triaxial compression method (GOST 12248), and their results should be used to correct single-plane shear test data. Other types of stress states can be realized in direct and annular shear devices (Fig. 8.28, i), in installations with sample tilting (Fig. 8.28, b), using laboratory vane shear meters (Fig. 8.28, V) and when testing solid and hollow cylindrical samples for torsion (Fig. 8.28, d, d). Soil samples can be in the form of: a cube, a parallelepiped, a solid or hollow cylinder, a solid or hollow coil.

Table 8.30

Methods for determining the strength characteristics of unfrozen soils

End of table. 8.30

Rice. 8.28. Diagrams and photographs of devices:

a - annular shear: b - direct shear with distortion of the sample; c - laboratory version of the impeller and field tester impeller; d, e - testing schemes for solid and hollow cylindrical samples for torsion (81. 92]

Ring shear devices are used to determine the strength of soils under both small and large shear deformations (hundreds of percent). Most soils exhibit a decrease in strength with increasing shear strain after reaching the peak condition. This process can be recorded in a ring shear device, as well as using a direct cut device under kinematic loading of the sample. In the annular shear device (Fig. 8.29), in addition to the values ​​of the maximum and limit angle of internal friction, the parameter is measured residual strength (r g, used in calculating the stability of slopes, pit slopes, retaining walls and in modeling landslide processes or soil movement in a fault zone along an already formed sliding plane. The main advantage of circumferential shear tests is the shear deformation with a constant area of ​​the sample throughout the entire experiment, as well as the ability to test soils with shear deformation of more than 10...30%, which is not possible with direct shear or simple shear devices. In addition, under annular shear conditions, the particle orientation in the post-peak state does not change, which is characterized by almost zero adhesion and minimal friction.

When tested in a ring shear device, the soil is located in two rings (upper or lower), one of which rotates, and the other (upper or lower) lies motionless. The experiment is carried out at constant normal pressure, which is determined by the dependence:

Where R- load from the weight of cargo, stamp and rod; G 0 and g, are the inner and outer radii of the ring die, respectively.

Shear stress is calculated from the torque value M

Rice. 8.29. Shear devices that determine direct and residual stresses: a - experimental diagrams with ring devices; o - diagram of a ring device; c - photograph of an annular shear device (manufacturer Wykeham Farrance)

The ring shear method makes it possible to recreate in the laboratory conditions similar to natural ones and obtain very accurate values ​​of residual resistance, which depend not only on the value of the normal pressure in the shear plane, but also on the shear rate. Typically, when slopes shift, the speed of movement of soil masses from 5 cm/year to 50 cm/day is observed.

Simple shear devices with sample distortion (Fig. 8.28, b) allow you to simulate various conditions of shear loads. The results are used in calculating the stability of underwater slopes of continental shelves characterized by layered clayey soils; when predicting the behavior of soils under the foundations of offshore platforms or near the side surface of piles. The installation is designed to compact the drainage sample and then shift it. Shear deformation is caused by a horizontal displacement of the bottom of the sample relative to the top, the rings slide over each other and the diameter of the sample remains constant, so any changes in volume are the result of the vertical movement of the upper clamping device. During the shear phase of testing, the vertical height of the specimen is kept constant by a vertical actuator feedback-connected to a displacement sensor. Soil samples can be in the shape of a cylinder, rectangle or cube.

The advantage of this device is that if in conditions straight cut destruction of a soil sample occurs along a pre-fixed horizontal plane, then under conditions simple shift failure will occur along a series of horizontal (or vertical) shear planes along weakened areas of the soil with the least resistance. Unlike direct shear tests (where it is practically impossible to withstand undrained conditions), direct shear tests contain the sample in a rubber sheath, which allows drained and undrained tests to be carried out while maintaining soil volume, as well as pore pressure measurements. Tests under simple shear conditions make it possible to determine not only strength parameters, but also the shear modulus G.

Direct single-plane or circumferential shear tests are carried out mainly for soil stability conditions when obvious rupture planes arise or when strength characteristics are determined at the soil-foundation contact surface. The results of these tests agree well. Stresses under circumferential shear conditions are more uniform, and in this test it is easier to obtain large shear deformations and determine the residual strength of the soil than in a direct shear test. Preparing a sample for testing under direct shear conditions is less labor intensive compared to circumferential shear.

Comparison of the results of tests under simple shear conditions with the results of tests under triaxial compression or direct shear conditions indicates that under simple shear conditions the maximum strength is lower and the difference in residual strength values ​​is less significant. Given these differences, it is recommended to take peak shear strength values ​​with reduction factors of 0.77-0.85.

For field studies of the strength of soft soils (peat, silt, fluid and fluid-plastic clay soils), a vane shear meter is used. A similar mini-device is also used in laboratory conditions. The impeller consists of two identical rectangular mutually perpendicular plates mounted on a vertical axis (Fig. 8.28, V), to which the torque is applied and its limiting value is measured, used for calculation resistance to undrained shear with and.

In installations operating according to the schemes of torsion shear (Fig. 8.28, d) and torsion of a hollow cylinder (Fig. 8.28,<)), образцы фиксируются в основании, и вращение производится вокруг вертикальной оси в верхней части образца. Изначально для этих схем испытаний применялись стабилометры кручения, в 1957 г. W. Kirpatric предложил использовать полые цилиндры грунта, что позволило приводить во вращение верхний нагрузочный штамп, а также создавать давление внутри и с внешней стороны образца. За рубежом приборы для испытаний получили название НСА (Hollow Cylinder Apparatys). При испытании полых цилиндрических образцов (рис. 8.30, V) true triaxial compression is modeled with rotation of the directions of the principal stress axes (Fig. 8.30, A). As a result, a wide range of possible variants of a complex stress state is created in a soil sample, which is especially important for anisotropic soils: it is possible to change the vertical (

Rice. 8.30. Testing of hollow cylindrical samples: a - maximum and minimum stresses in foundation soils: b - NSA device (manufacturer Wykeham Farrance); in sample preparation devices; d - soil sample before installation in the triaxial compression chamber

As already noted, when testing soils, it is necessary to select conditions that most fully correspond to the actual operating conditions of the soil at the base of the future structure. The main external factors affecting the strength of soils include: type of stress state, test conditions (closed or open system, influence of pore pressure, etc.), loading rate, nature of sample loading (static or dynamic), etc.

Influence of the type of stress state under conditions of pure shear, uniaxial tension and compression, as well as triaxial compression(schemes of experiments are shown in Fig. 8.27) on soil strength soil strength passports can be analyzed using Mohr circles (Fig. 8.31). Soil strength certificate is a curve that envelops the Mohr stress limit circles in the coordinates of normal and tangential stresses. Mohr's limit circle corresponds to the maximum stress state achieved

for a given ratio of the largest and smallest principal normal stresses, and has a radius R=/2 with center coordinates ( / 2; 0). To construct a strength passport based on the determination of strength limits in volumetric compression, uniaxial compression and tension by a set of paired values o c v= ffmax and oh = <7 П ип (полученных при объемном сжатии не менее чем при трех различных значениях бокового давления <7з) в координатах строят полуокружности радиусами /2 с координатами центров / 2; 0) К семейству полуокружностей добавляют полуокружности радиусами (т р /2и<т с /2с координатами центров (-я р / 2; 0) и (я с / 2; 0), где <т р - предел прочности при одноосном растяжении; я с - предел прочности при одноосном сжатии.

Rice. 8.31. Strength data sheet based on determination of strength limits in volumetric compression, uniaxial compression and tension

From the diagrams (Fig. 8.31) it follows that the same soil, depending on the type of stress state, will have different values ​​of the limiting strength parameters, the lowest value is typical for conditions of simple uniaxial tension (rupture), the highest for conditions of volumetric compression.

The strength characteristics of soils depend on sample loading speed , parameters of the shear resistance of rocky and cohesive soils (angle of internal friction (R and cohesion c) are different for the same soil tested under fast or slow shear conditions. With decreasing loading speed (increasing test duration), the value of specific adhesion naturally decreases, and the angle of internal friction increases. In order to identify the type of stress state in which tangential stresses reach the ultimate strength, terms such as short-term and long-term stability.

Short term stability assumes the occurrence of a number of conditions in an array of weak water-saturated clay soils with low permeability, both during construction and during operation of the structure. These conditions include rapid rates of loading of the base, lack of drainage, and the occurrence of excess pore pressure. In this case, the strength of clay soils is estimated at conditions of undrained loading.

Long-term stability is assessed under the conditions of the possibility of drainage and partial (or complete) consolidation of the soil with dissipation of pore pressure and stabilization of deformations. These conditions arise instantly during construction on coarse and sandy soils; in clayey soils, stabilization of deformations continues for a longer time. When these conditions occur, the strength of the soil is estimated at drained loading conditions.

In some cases, it is necessary to determine both the short-term and long-term stability of the base. For example, during the construction of an embankment in water-saturated foundation soils, there will be virtually no drainage, and after its construction, the strength will change during the process of drainage and consolidation. In the first case, it is necessary to conduct unconsolidated-undrained tests, in the second - consolidated-drained or consolidated-undrained.

Test conditions affecting the strength of soils primarily include closed or open (undrained or drained) test circuits.

Drained strength parameters determined in direct shear and triaxial compression installations (consolidated-drained tests). When determining strength in an open system Water can be squeezed out of the soil when loaded. Due to this, the load that occurs when transferring to the ground (O) pore pressure (And) gradually dissipates and with slow loading can drop to zero. In soils that are not fully saturated with water, pore pressure is not taken into account. With drained loading The strength of soils depends to a large extent on whether the soil experiences compression or expansion under external loads. If the soil expands (for example, the area in front of a retaining wall) or contracts (behind the retaining wall), the strength of the soil will vary. The strength of soils during expansion is less than the strength during compression.

Undrained parameters strength with and obtained from the results of non-consolidated undrained tests in direct shear and triaxial compression installations, which reflect the behavior of clayey soil with low permeability at any loading rate, even at a very slow one. The high speed of construction of the structure and the lack of drainage prevent the soil from consolidating and affect its strength. When determining the strength of water-saturated soils in a closed system the soil is isolated from the external environment, it cannot absorb or release water when loaded, its humidity remains constant. Pore ​​(or neutral) pressure arising when the sample is loaded (And) increases in proportion to the applied load (O) up to the moment of destruction of the sample or remains constant at a given constant voltage O.

Shear resistance s in water-saturated organic-mineral and organic soils can be identified with the value of specific cohesion With(according to the method (R= 0), which makes it possible to calculate the bearing capacity and stability of foundations and slopes according to existing design schemes using standard programs. Field studies of organomineral and organic soils using a four-blade impeller in some cases are the only possible way to determine their mechanical properties. Undrained strength is used as a classification indicator, for example in the UK BS standard. In table 8.31 shows the classification of soils according to undrained strength.

Presence or absence of normal pressure in soils is of considerable importance when studying their strength. In most cases, test results are processed using the Coulomb or Mohr-Coulomb strength condition. The Coulomb strength of soil depends on normal pressure, which can be expressed in terms of total and effective stresses. When determining strength parameters under full stress, pore pressure is not taken into account, assuming that under conditions of complete drainage it dissipates, therefore tests at the shear stage are carried out according to an open scheme, allowing drainage and loading of the sample in steps with holding until the shear deformation is completely stabilized. If the pore pressure is measured, which is only possible with complete water saturation of the samples and the absence of drainage, then when conducting experiments according to the scheme of non-consolidated-undrained or consolidated-undrained shear, it is possible to determine the strength parameters in effective stresses. The higher the pore pressure And, the smaller part of the external pressure is transferred to the soil skeleton. To take into account the influence of pore pressure, according to K. Terzaghi, effective pressure is introduced, then the Coulomb equation taking into account pore pressure takes the form:

Where O"- effective pressure; And- pore pressure; c" - specific adhesion (in terms of effective stresses).

Table 8.31

Shear strength of soils in undrained tests

Type of soil	Resistance to undrained shear c„. kPa
Extremely low strength
Very low strength	10 < с„ < 20
Low strength	20 < with and < 40
Medium strength	40 < with and < 75
High strength	75 < with and < 150
Very high strength	150 < с„ < 300
Extremely high strength	with and > 300

Thus, if burrow pressure is taken into account when calculating the stability of slopes or the bearing capacity of foundations, then the strength parameters are taken in effective stresses; if pore pressure is not taken into account, then in full.

Character of loading, also affecting the strength parameters of soils, manifests itself in different ways of transmitting external stresses to the soil. They can be static (under the action of constant or slowly changing loads) or dynamic (under the action of variable, cyclic, periodic, impulse loads, etc.). The features and patterns of destruction of the same soil under static or dynamic conditions are different, therefore, under dynamic influences, the strength of soils is studied using special methods.

Lecture outline:

1. The nature of soil strength.

2. Determination of soil strength:

– on uniaxial compression;

– in uniaxial tension;

– clutches and internal friction angle using simplified methods.

3. Determination of adhesion and angle of internal friction according to stabilometric tests.

4. Determination of adhesion and internal friction angle based on shear test data.

The strength properties of soils characterize the behavior of soil under loads equal to or exceeding critical ones, and are determined only when the soil is destroyed. The loss of strength of a material occurs, as a rule, through its rupture and (or) shear.

1. The nature of soil strength

Griffiths theory gives the internal mechanism and mathematical model of destruction based on physical parameters. This theory assumes that any material contains defects, and when a body is loaded, a stress concentration occurs around the defects, which causes the growth and propagation of cracks; this process ultimately leads to the formation of a main fracture crack, i.e., to macroscopic destruction of soils.

Figure 8.1 – Mechanism of strength formation according to Griffiths

Calculating the energy of crack formation is quite complex, so this theory has not been widely used in practice.

McClinton and Walsh proposed that during compression, Griffiths cracks close and frictional forces arise on their surface.

A mechanism for the destruction of materials is proposed, connecting theories Griffiths and Walsh– when the soil is loaded until it fails, the processes of formation of growth and grouping of rupture cracks (according to Griffiths), shearing and crushing of material in the zone of the main rupture (according to Walsh) occur in it. This entails changes in the structure and phase state of the soil in the zone of the main rupture, hence a change in its (material) properties.

Just like the Griffiths theory, this theory is not widely used due to the complexity of crack formation calculations.

Figure 8.2 – Mechanism of strength formation according to Griffiths and Walsh

In practice, the most widely used theory is the Coulomb–Mohr theory.

Coulomb's theory of greatest tangential stresses. According to this theory, the ultimate strength of a rock under a complex stress state should occur when the greatest shear stress (σ pr. ) will reach the value at which the ultimate strength of the sample occurs under simple compression (σ compression ) or stretching (σ r. ).

where σ compress.

σ pr.

τ ex. ≤ σ compress. (σ р.)

– uniaxial compressive strength;

– uniaxial tensile strength.

σ n.

Figure 8.3 – Mechanism of formation of Coulomb strength

The ultimate stress of the soil condition - the Coulomb strength criterion - is described by the following equation:

τ pr =σ tanϕ +c

where ϕ – angle of internal friction, degrees; с – adhesion, MPa; σ – normal stress, MPa;

τ ave. – shear stress, MPa.

The disadvantage of this theory is that in practice the ultimate shear stress is not always lower than the compressive strength. But in general, Coulomb's theory satisfies practice.

cos 2 α

It should be noted that the greatest tangential stresses are formed on the inclined area at an angle of about 45° to the surface of the horizontal section. Let's consider this statement using an example (Figure 8.4).

Rn. F′

α Р с

Figure 8.4 – Action of the normal (Рн.) and tangential (Рс.) component of the force P on an arbitrarily selected section

The figure shows that if a distributed load P acts on the surface of a horizontal section (α = 0) with area F, then normal stresses σ n. are equal:

σ n. = σ 1 = F P

The cross-sectional area at an angle α >0 is equal to:

F ′ = cos F α

The components of the force P, oriented normally (Рн.) and tangentially (Рс.) to this section are equal:

Rn. =P cos α, Pc. =P sin α

Then the normal (σ n.) and tangential (τ) stresses will be equal:

Pn.

Pcosαcosα

(1+ cos 2α)

τ =

PC.

P sinα cosα

sin 2α

Hence, at α = 0, σ n.

At α = 45° sin 2 α = 1,

small values ​​and are equal to:

reaches the maximum value, i.e. σ n. = σ s .

then the shear stresses take maximum

τ max. = σ 2 1

Thus, in the volume of rock, the sections in the most unfavorable state are those in relation to which the acting force is directed normally or at an angle close to 45°, i.e. sections in which the maximum normal and shear stresses act. That is why the greatest deformation of rocks during compression is observed in the direction of the force, and cleavage cracks appear along sections forming with the direction

acting force angle close to 45°, i.e. close to the angle θ max.

Mohr's theory is the theory of the ultimate stress state.

In a soil mass, any point is affected by three main and six tangential stresses (Figure 8.5), with σ 1 > σ 2 > σ 3.

σ 3 σ2

Figure 8.5 – Distribution of principal normal stresses at any point in the soil mass

According to Mohr's theory, two main normal stresses σ 1 and σ 3 determine the strength of soils, σ 2 does not affect the strength.

The strength condition according to Mohr’s theory will be written as follows:

σ 1 − [ σ [ σ сж р . . ] ] σ 3 ≤ [ σ compress . ]

where σ compress. – uniaxial compressive strength; σ r. – uniaxial tensile strength.

Graphic strength conditions can be reflected in the form of Mohr diagrams (Figure 8.6).

	(σ n.) min.= σ 3
		(σ n.) max.= σ 1

Figure 8.6 – Mohr diagram showing the stresses caused by the action of forces along three sections passing through the axes σ 1, σ 2, σ 3

The diagram shows that each point on the surface of the circle characterizes the normal (σ n.) and tangential stresses (τ) of a strictly defined area in the soil body, and these stresses can be calculated.

So, for example, in order to determine the stress σ n. and τ, acting on any area A-B, inclined at an angle α to the plane I-I of the main stresses, the values ​​of the main stresses σ 1 and σ 3 are plotted along the abscissa axis and a circle is constructed on their difference, as on a diameter ("stress circle ", or "Mohr's circle"), the center of which C lies at the midpoint of the distance between points A-D. At point C, setting aside the angle 2α, we obtain point B, the coordinates of which are OK and VC, respectively, equal to σ n. and τ.

From Figure 8.7 it follows:

	BC=DC=AC=			OD−OA		σ 1 − σ 3

Figure 8.7 – Determination of normal and tangential stresses acting at a given point of an arbitrary site,

using Mohr's diagram

From the right triangle VKS we have:

τ = BK = BC sin 2α = σ 1 − 2 σ 3 sin 2α

σ n. = OK = OA + AC + CK = σ 3		σ 1 − σ 3		σ 1 − σ 3	cos 2α

σ n. = σ 1 cos2 α + σ 3 sin 2 α

Thus, knowing the main normal stresses, it is possible for any site in the soil body to calculate the normal (σ n.) and tangential (τ) stresses acting on it.

To determine the strength of the soil, stress circles are constructed based on the particular values ​​of σ 1 and σ 3, which reflect the limiting equilibria at specific σ 1 and σ 3. These circles are called limit circles (Figure 8.8).

Figure 8.8 – Mohr diagram for the limiting state of a rock

On each of the limit stress circles (Figure 8.8), the ordinates of points B, B' and B'' are equal to the limiting tangential stresses at the moment immediately preceding the failure of the rock at the corresponding compressive normal stresses K, K ′, K ′′. If a tangent (envelope) is drawn to the limit circles of stress, then it forms an angle ϕ = θ max with the abscissa axis. , A

on the ordinate axis the segment C will be cut off. In accordance with the condition of limit equilibrium, points B, B ′ and B ′′ must be on this tangent, the equation of which has the form:

τ = σн. tg ϕ + C

The values ​​of ϕ and C in this equation are parameters of soil strength; C characterizes the presence and strength of structural bonds, i.e., the action of adhesion forces, or simply adhesion, in megapascals, and ϕ is the intensity of the increase in the shear (shearing) resistance of the rock with increasing normal load, i.e., its internal friction. The angle ϕ is conventionally called the angle of internal friction, and tan ϕ is the coefficient of internal friction.

From Figure 8.8 it is also clear that the direction AB determines the direction of the platform along which at a given point, at a limit state, rock spalling (shear) and its destruction can occur. This shearing (sliding) area forms an angle α with the direction of the area along which a large principal stress acts. Since the angle 2α = 90°ϕ, then α = 45°+ϕ /2, therefore, under the conditions of the limiting stress state, the “cleavage platform” will be

inclined at an angle of 45°+ϕ /2 to the direction of the area of ​​greatest principal stress. At each point of extremely stressed rock there can be two such sites. The mating areas are located at an angle of 45°±ϕ /2.

Thus, Mohr’s circles of limiting stresses and the envelope of Mohr’s circles, expressed by the Coulomb equation, are actually the theory of soil strength

comrade Kulona-Mora.

2. Determination of soil strength

In practice, the strength of soils is usually assessed by the following indicators: strength for uniaxial compression and tension, adhesion and angle of internal friction.

a) Strength of soils in uniaxial compression refers to the strength properties of soils. The strength of soils is often determined by crushing them under conditions of free lateral expansion. In this case, the destructive force acts only in one direction, therefore this test is called uniaxial compression, i.e. the condition of the limiting state of soils is met (Figure 8.9)

σ 1 > σ 2 = σ 3 = 0.

σ1

σ 2 =σ 3 =0

σ 2 =σ 3 =0

Figure 8.9 – Scheme of soil operating conditions under uniaxial compression

Calculation of compressive resistance is carried out based on the assumption of a uniform stressed state of the soil sample using the formula:

σ сж = Р F section

where P section is the crushing force;

F – cross-sectional area of ​​the sample, m2.

It should be noted that the compression test must be carried out with a ratio of sample height to diameter h/d ≥ 2. This is due to the fact that when the soil is loaded, compaction zones (a) in Figure 8.10 appear in it. Therefore, at h/d ≤ 2, these zones interact, which results in additional soil strength, i.e., we obtain overestimated values ​​of σcompressor. .

45° +ϕ /2

and α

Figure 8.10 – Compaction zones

Compressive strength can be expressed graphically using Mohr's circle

(Figure 8.11).

σ

σ 3=0 σ 1= σ compress.

Figure 8.11 – Compressive strength

Uniaxial compressive strength is, to a certain extent, a conditional characteristic of soil strength, depending on many factors. Nevertheless, the determination of σ co is widespread in engineering and geological practice, as it allows one to approximately estimate the bearing capacity of a foundation on rocky soils, determine the cohesion and angle of internal friction of the rock, and evaluate its strength as a building material.

b) Strength of soils in uniaxial tension

The tensile strength of rocks is one of the most important characteristics of rocks; it can be widely used both for comparative assessment of the strength properties of rocks and for calculating the angle of internal friction and the coefficient of adhesion. It, like uniaxial compression, models the work of the soil under the condition σ 1 > σ 2 = σ 3 = 0.

The uniaxial tensile strength of the rock (σ race, MPa) is calculated using the formula:

σ dis. = P F section .

where Psection – maximum value of tensile pressure; F is the cross-sectional area of ​​the sample.

Graphically, tensile strength is expressed through the Mohr stress circle in the following form (Figure 8.12).

σ r.

Figure 8.12 – Tensile strength

Experimental data on compressive and tensile strength. The table shows data on σ compress and σ dis.

Table 8.1 – Tensile strength σ р and uniaxial compression σ сж of some rocks

Rock		σ compress, kg/cm2	σ р , kg/cm2
Quartzites
Limestones
Sandstones
Shales
Rock salt

The table shows that the tensile strength is an order of magnitude less than the compressive strength. This is due to the fact that τ p evaluates only the strength of structural bonds, and in compressive strength, in addition to the strength of structural bonds, shear forces also participate.

c) Adhesion and angle of internal friction

Cohesion and the angle of internal friction of soils are the main indicators characterizing soil in various stressed states. There are quite a lot of ways to determine c and ϕ. Of these, the most widely used methods are:

– according to uniaxial compressive and tensile strength data;

– according to volumetric compression data (stabilometry);

– according to shear testing data.

Determination of adhesion and angle of internal friction of soils based on uniaxial compressive and tensile strength data

To determine c and ϕ, soils are tested for uniaxial compression and tension (Table 8.1). Construct soil strength certificates (envelopes of Mohr stress limit circles). Determine the angle of internal friction (ϕ) and adhesion (c).

σ r. σ compression

Figure 8.13 – Scheme for constructing a soil strength passport

The results obtained by this method are quite conditional, but they can be used as estimates.

Accelerated methods for determining the strength properties of soils:

1. The method for determining the shear strength of rock samples, developed by the author, is as follows. Initially, cylindrical samples are made from blocks of sandstone, gypsum, rock salt and other rocks being studied. Then the samples are sawed to form a crack, and the working surfaces of the crack are processed until irregularities with a height of 0.03–0.5 mm are formed. After that, the sample with a crack is loaded with stepwise increasing compressive forces, causing compressive stresses σ in the sample. In this case, σ should not exceed 0.6 of the average compressive strength of the sample material σcom. After that, multiple shifts of the parts of the sample separated by the crack are performed at each loading stage and the friction angle φ of the sample material is measured. Compressive stresses σ ≤ 0.6 σav do not cause microfractures and plastic deformations in the sample material, which allows the sample to be used for subsequent tests, and the height of the irregularities in the specified limits provides an accurate measurement of the true friction angles φ. If the height of the irregularities goes beyond the specified limits (0.03–0.5 mm) for the listed materials, then this leads to a sharp increase in the friction angle φ, i.e., measuring not the friction angle of the material, but the friction angle of rough surfaces, and to an increase measurement errors. After determining the friction angle φ of the material, the sample is loaded with compressive forces until it fails and the compressive strength σcompressor of the material of the test sample is determined.

Based on the data obtained, the parameter c is calculated:

c = σ compress / 2 tg (45° – φ 2)

And shear strength according to the formula

τ = σ tan φ + s .

WITH Using the proposed method, it is possible to calculate the shear resistance of rocks, especially rocky and semi-rocky rocks, using fairly easily determined indicators of compressive strength and rock friction angle.

2. Method for determining tensile strength by crushing cylindrical samples along a generatrix. A cylindrical sample with a height equal to the diameter is placed between the press plates so that the compressive forces are directed parallel to the side surfaces of the cylinder. End surfaces

The parts of the cylinder must be smooth and in close contact with the press plates. The calculation is carried out according to the formula

σ times = F P

where σtimes – tensile strength, MPa;

F – sample area along the split surface, m2.

The spread of the obtained rock tensile strength values ​​is, as a rule, much lower than when tested by any other method (the coefficient of variation for individual samples usually does not exceed 6–10%).

3. The coaxial punch method was developed at VNIMI to determine the tensile and compressive strength of rocks. It is based on the destruction of rock disks having a diameter of 30–120 mm and a height of 8–11 mm.

Determination of adhesion and angle of internal friction of soils based on uniaxial compressive strength data and friction

To determine C and ϕ, the soil is tested for uniaxial compression (σ compression), then the friction along the prepared shear surface (ϕ) is determined and a soil strength passport is constructed from these data (Figure 8.14).

	σ compression

Figure 8.14 – Scheme for constructing a passport of soil strength by σ compression. and ϕ

After that, C - clutch is determined. This method is evaluative.

3. Determination of adhesion and angle of internal friction according to stabilometric tests

Stabilometric tests mean the study of soils

V volumetric stress state according to the diagram (Figure 8.15):

σ 1 > σ 2 = σ 3

σ 2 =σ 3 >0		σ 2 =σ 3 >0

Figure 8.15 – Scheme of testing soils under triaxial compression conditions

It is known that at the base of the structure the soil is in a volumetric stress state. Therefore, obtaining strength characteristics under volumetric compression conditions most accurately simulates soil operating conditions.

Soil tests are carried out using devices called stabilometers. The design of the stabilometer is shown in Figure 8.16.

				Movable piston

Soil sample

Р2 = σ 2

The fitting through which oil pressure is supplied

Figure 8.16 – Stabilometer diagram

Methodology

The general test scheme is as follows:

– a sample in a waterproof shell is placed between two pistons in a chamber (stabilometer);

– the chamber is filled with liquid (for example, oil);

– set a fixed lateral pressure on the sample –σ 2 ;

– vertical pressure (σ 1) is transferred to the soil sample through the piston until the soil is completely destroyed;

– three to four cycles of such tests are carried out;

– carry out data processing.

For example: we test sandstone soil.

Three fixed levels of lateral pressures are set: σ 3 = 5, σ 3 ′ = 10 and σ 3 ″ = 15 MPa. σ 1 , σ 1 ′ , σ 1 ″ are determined accordingly (Table 8.2).

Table 8.2

Test No.	σ 2, MPa	σ 1, MPa

Processing test results

Processing the results in the general case comes down to constructing Mohr circles and their limiting envelope.

To construct Mohr's circles, the maximum and minimum principal stresses σ 1 and σ 3 are plotted on the abscissa axis (Table 8.2) and circles are drawn from their difference, like a diameter (Figure 8.17). An envelope is built using three Mohr circles (see Figure 8.17). The determination of adhesion and the angle of internal friction of rocks located in given (modeled) conditions is carried out graphically or by calculation (see Figure 8.17).

τ, MPa

σ, MPa

Figure 8.17 – Envelope of Mohr stress limit circles according to test data

In practice, the envelope of Mohr's stress limit circles is called the soil strength passport.

In the event that the uniaxial compressive (σ compress.) and tensile (σ r.) strengths have also been determined for the soil under study, then a complete soil strength passport is constructed (Figure 8.18).

τ ,MPa

σр

σ2 "

σ1 "

σ ,MPa

Figure 8.18 – General case of the maximum stress envelope of Mohr circles:

1 – uniaxial tensionσ r. ;

2 – uniaxial compressionσ compression ;

3 – volumetric (triaxial) compression;

σ 1 > σ 2 = σ 3 ≠ 0;

ϕ – angle of internal friction, degrees;

WITH – clutch, kg/cm2.

It should be noted that with an increase in σ n. the angle of internal friction decreases. Therefore, when assessing c and ϕ, it is necessary to take into account the performance of the soil under specific conditions.

The given test schemes do not exhaust the entire variety of rock operating conditions, therefore, the triaxial compression devices are designed in such a way that they also make it possible to simulate various special cases of soil behavior encountered in practice. VNIMI has developed stabilometers that make it possible to create lateral and axial pressure, respectively, from 15–40 to 50–250 MPa and more. It is recommended to carry out soil tests in stabilometers when assessing and predicting the stability of the most critical engineering structures.

4. Determination of adhesion and internal friction angle based on shear test data

Shear is the process of destruction of soil due to the sliding of one part of it relative to another along a given surface, i.e. when using

When testing soils for shear, the conditions of a fixed failure surface are modeled (Figure 8.19).

Shear surface, σ n. emerging in the process

soil loading

σ τ

Fixed (shear) fracture surface

Figure 8.19 – Scheme of shear tests of soils:

A) in natural conditions; B) fixed shear surface (fracture)

The dependence τ = f (σ) is called the soil passport, sometimes it is called pre-

separate Mohr envelope (Figure 8.20).

τ, MPa

	0,05 0,1 0,15 0,20	σ, MPa

Figure 8.20 – Strength data sheet

In the pressure range 1÷ 20 MPa, the shear resistance of soils can be expressed by the Coulomb equation:

τ = σ tan ϕ + c

where c and φ are parameters of a given soil.

Shear resistance is also characterized by the value of the so-called shear angle ψ; tan ψ is called shift factor, numerically tg ψ = σ τ .

In laboratory conditions, the shear resistance of soils is determined by the methods of single-plane shear for dispersed soils and shear with compression for rocky soils.

Single plane cut

To determine the shear strength by the single-plane cut method, the Maslov-Lurie device is most often used in the modernization of the Hydroproject - GGP-30 (Figure 8.21) and VSV-25.

Fixed Movable ring ring

Figure 8.21 – Diagram of a single-plane soil cutting device (I – I" – specified cutting plane)

Using the GGP-30 device, the shear strength of a rock sample with a diameter of 71.4 mm and a height of 40.0 mm is determined. Maximum permissible vertical load 12 9.8 104 Pa ≈ 12 105 Pa ≈ 1.2 MPa.

Methodology

The test is carried out as follows (see Figure 8.21).

– soil preparation is carried out;

– a rock sample in a split ring is placed in a holder;

– a fixed vertical load is applied to the ground (σ );

– the shear stress is determined (τ );

– shear stressτ is determined for three different vertical loads σ 1 ;

– shear loadτ is increased in steps, the value of which is determined based on the selected soil testing scheme;

– processing of experimental data is reduced to constructing a passport of soil strength. tg valuesφ and c are calculated by processing the experimentally obtained values ​​of τ and σ using the least squares method.

Schemes for testing soils for shear differ in the conditions of preliminary soil preparation and shear rate.

Based on the nature of the preliminary preparation of clay soil for testing, there are three main test methods:

1. Shift of soil samples in a natural state without preliminary compaction (unconsolidated).

2. Shear of soil samples pre-compacted with different loads

And soil samples cut under compaction loads (consolidated);

3. Shear of soil samples pre-compacted with the same load, but cut under lower loads (consolidated).

Depending on the speed of the test, fast and slow shift are distinguished:

1. Rapid shear is carried out at such a speed that the density - the moisture content of the soil during the shear process - does not change (undrained shear).

2. Slow shear is carried out at such a speed when the density - the moisture content of the clay soil - manages to come into equilibrium with the acting load (drained shear).

The nature of the preliminary preparation and the test mode determine the value of the shear resistance parameters.

With rapid shear, the strength of clay soil will be determined only by adhesion, and the internal friction forces will be very small.

The results of unconsolidated undrained shear are usually used to calculate the stability of a soil mass during the construction stage (method

ϕ = 0).

At slow shift soils have the greatest shear resistance.

The results of consolidated-drained shear are used to calculate the stability of a clay soil mass at the stage of long-term operation.

For example: we test clay soil.

Three fixed normal stresses are specified: σ 1 = 0.1 MPa, σ 1 ′ = 0.15 MPa and σ 1 ′′ = 0.20 MPa. Shear stresses are then determined (Table 8.3).

Table 8.3

Figure 8.23 ​​– Sand strength data sheet

From Figure 8.23 ​​it can be seen that the adhesion is zero, then the Coulomb equation takes the following form:

τ = σ tan ϕ

And ϕ

C B

σ n.

Figure 8.24 – Scheme for selecting minimum normal stresses

It should be noted that when choosing the minimum normal stress (Figure 8.24) (σ) during shear tests, you need to take into account σ n. – the magnitude of the main normal stress at which soil destruction occurs. At

σ < σ н. моделируем разрушение грунта в точке В. Тогда полученные значения с 1 < С

and ϕ′ > ϕ, which is unacceptable, since the use of these data in engineering calculations leads to a decrease in the reliability of the stability of engineering structures.

Average values ​​of c and ϕ for dispersed soils. Table 8.4

	Indicators		Porosity coefficient, e
Gravelly sands
Medium sand
Dusty
Loams

s – kgf/cm2, ϕ – deg., clayey soils fluidity 0.25< I < 0,5.

Compression cut

To determine the shear strength by the compression shear method, inclined matrices are used (Figure 8.25). A special set of wedges that allow cutting at angles from 25 to 65° with an interval of 5°. The vertical load is transmitted by a press.

Soil sample

Figure 8.25 – Scheme for determining the volumetric strength of samples using the oblique cut method: α – sample cut angle:

a) testing samples of the correct soil shape; b) testing irregularly shaped soil samples

Methodology

The test is carried out as follows:

– samples of cylindrical and prismatic shapes are prepared; irregularly shaped samples can also be tested, which are filled with quick-hardening cement in special cages;

– a vertical load Q is applied to the ground (see Figure 8.25), creating

May by press, which is decomposed into normal (σ) and shear (τ);

– cutting angles are set (with wedges)α = 30°, α = 45° and α = 60° in inclined dies (see Figure 8.25);

– a vertical load (Q) is applied until the soil sample is completely destroyed, the load is fixed;

– carry out from 3 to 27 tests;

– the research results are processed, which boils down to

constructing a passport of soil strength (Figure 8.26) and determining c and ϕ.

Figure 8.26 – Soil strength passport

For example: we are testing mudstones.

1. Samples are prepared that have a cylindrical shape, size (mm): cylinder diameter 42±0.1; cylinder height 42 ± 2.5; taper and barrel shape ± 0.05.

2. α = 30°, α = 45° and α = 60° (Table 8.5) and normal stresses are calculated.

Table 8.5

		Tilt angle			Destructive	Normal
						stress, σ =
	tests	(α, deg.)	sample, cm		kgf/cm2

3. Data processing.

We set off an angle α = 30° from the ordinate axis and draw a straight line through the origin of the ordinate. On this straight line σ = 9.4 kgf/cm2 is deposited. We perform similar operations for α = 45° and α = 60°. Then we calculate c and ϕ (Figure 8.27).

with α =30°

45° 60°

Figure 8.27 – Claimstone strength data sheet

This method is very labor intensive. However, it is convenient for testing rocks from which it is impossible to produce samples of the correct geometric shape, as well as for determining the angle of internal friction and adhesion along weakened surfaces, cracks, layers of weak rocks, etc.

Thus, the nature of soil strength and methods for determining strength indicators σ r are considered. , σ compress. , s and ϕ .