Modern problems of science and education. Structural Strength Lecture Notes on Soil Mechanics

The magnitude of the structural strength of soils is a very important characteristic of soils. Its value can be determined from the compression curve of the undisturbed structure, testing soils (until structural strength is reached) with very small load steps (approximately 0.002-0.010 MPa), then a sharp break in the compression curve will correspond to the structural strength of soil compression. The pressure value corresponding to the point of intersection of the curve with the pressure axis is equal to the value of the structural compressive strength .

Drawing a) relative compression of water-saturated soil depending on pressure p, b) relative compression of clay soil with partial decompaction depending on pressure.

The law of soil compaction: the change in soil porosity is directly proportional to the change in pressure.

13. Compression dependence during volumetric compression

Porosity coefficient changes e soil under compressive compression in the general case will depend not only on the magnitude of the vertical normal stresses But also from horizontal and

Let us determine the sum of the principal stresses in the case of compression of the soil layer without the possibility of its lateral expansion, highlighting the elementary paralepiped, which, under the conditions of this problem, will experience only more normal (principal) stresses

Since horizontal deformations (soil expansion to the sides) are impossible, the horizontal relative deformations will be equal to zero, i.e. , whence it follows that . In addition, from the equilibrium condition we have

It is known that the relative deformation of an elastic body in accordance with Hooke's law is found from the expression

Where is the modulus of elasticity of the material, is the coefficient of lateral expansion of the soil (Poisson's ratio). Substituting into this expression , , , we obtain

Where is the coefficient of lateral soil pressure at rest, i.e. in the absence of horizontal movements

Most clay soils have structural strength, and the water in the pores of these soils contains gas in dissolved form. These soils can be considered as a two-phase body consisting of a skeleton and compressive water in the pores. If the external pressure is less than the structural strength of the soil P page . , then the process of soil compaction does not occur, but there will be only small elastic deformations. The greater the structural strength of the soil, the less applied load will be transferred to the pore water. This is also facilitated by the compressibility of pore water with gas.

At the initial moment of time, part of the external pressure will be transferred to the pore water, taking into account the strength of the soil skeleton and the compressibility of water P w o - initial pore pressure in water-saturated soil under load R. In this case, the coefficient of the initial pore pressure

In this case, the initial stress in the soil skeleton:

pz 0 = P – P w O. (5.58)

Relative instantaneous deformation of the soil skeleton

 0 = m v (P – P w O). (5.59)

Relative deformation of the soil due to the compressibility of water when the pores are completely filled with water

 w = m w P w O n , (5.60)

Where m w is the coefficient of volumetric compressibility of water in pores; n- soil porosity.

If we accept that in the initial period at stresses P z the volume of solid particles remains unchanged, then the relative deformation of the soil skeleton will be equal to the relative deformation of the pore water:

 0 =  w = . (5.61)

Equating the right sides of (5.59) and (5.60), we obtain

. (5.62)

Substituting P w o into equation (5.57), we find the coefficient of the initial pore pressure

. (5.63)

The coefficient of volumetric compressibility of water in pores can be found by the approximate formula

, (5.64)

Where J w– coefficient of water saturation of the soil; P a - atmospheric pressure 0.1 MPa.

The diagram of vertical pressures in the soil layer from the load with compressible pore water and the structural strength of the soil is shown in Fig.5.14.

In view of the foregoing, formula (5.49) for determining the settlement in time of a soil layer under a continuous uniformly distributed load, taking into account the structural strength and compressibility of the gas-containing liquid, can be written as follows:

. (5.65)

Fig.5.14. Diagrams of vertical pressures in the soil layer under continuous load, taking into account structural strength

Meaning N determined by the formula (5.46). At the same time, the consolidation ratio

.

Similar changes can be made to formulas (5.52), (5.53) to determine the settlement over time, taking into account the structural strength and compressibility of the gas-containing liquid for cases 1 and 2.

5.5. Influence of the initial head gradient

Clay soils contain strongly and loosely bound water and partially free water. Filtration, and hence the compaction of the soil layer, begins only when the gradient is greater than the initial i 0 .

Consider the final settlement of a soil layer with a thickness h(Fig.5.15), which has an initial gradient i 0 and loaded with a uniformly distributed load. Water filtration is two-way (up and down).

In the presence of an initial gradient from an external load R at all points along the depth of the layer in the pore water there is a pressure equal to P/ w ( w - specific gravity water). On the excess pressure diagram, the initial gradient will be represented by the tangent of the angle I:

R
is.5.15. The scheme of soil compaction in the presence of an initial pressure gradient: a - the compaction zone does not reach the depth; b - the compaction zone extends to the entire depth, but the compaction is incomplete

tg I = i 0 . (5.66)

Only in those areas where the pressure gradient will be greater than the initial (
), water filtration will begin and soil compaction will occur. Figure 5.15 shows two cases. If at z < 0,5h gradient is less than initial i 0 , then water will not be able to filter from the middle of the layer, because there is a "dead zone". According to Fig. 5.15, a we find

, (5.67)

Here z max< 0,5h. In this case, the sediment is

S 1 = 2m v zP/ 2 or S 1 = m v zP. (5.68)

Substituting value z max in (5.68), we get

. (5.69)

For the case shown in Fig. 5.15, b, the draft is determined by the formula

. (5.70)

When you need to take into account many factors. Particular attention should be paid to the composition and some of its types are capable of sagging when the humidity is increased in tension under its own weight or from an external load. Hence the name of these soils - "subsidence". Consider further their features.

Kinds

The category under consideration includes:

• Loess soils (suspes and loesses).
• Clays and loams.
• Separate types of cover slurries and loams.
• Bulk industrial waste. These include, in particular, ash, grate dust.
• Dusty clay soils with high structural strength.

Specificity

On initial stage construction organization it is necessary to conduct a study of the soil composition of the site to identify probable deformations. Their occurrence due to the peculiarities of the process of soil formation. The layers are in an insufficiently compacted state. In loess soil, such a state can persist throughout the entire time of its existence.

An increase in load and humidity usually causes additional compaction in the lower layers. However, since the deformation will depend on the strength of the external influence, the insufficient compaction of the stratum relative to the external pressure exceeding the stress from its own mass will remain.

The possibility of fixing weak soils is determined in laboratory tests by the ratio of the decrease in strength when wetted to the indicator of the effective pressure.

Properties

In addition to undercompaction, subsiding soils are characterized by low natural moisture content, dusty composition, and high structural strength.

Soil saturation with water in the southern regions, as a rule, is 0.04-0.12. In the regions of Siberia, middle lane the indicator is in the range of 0.12-0.20. The degree of humidity in the first case is 0.1-0.3, in the second - 0.3-0.6.

Structural strength

It is mainly due to cementation adhesion. The more moisture enters the ground, the lower the strength.

The research results showed that thin water films have a wedging effect on the formations. They act as a lubricant, making it easier for particles of subsiding soil to slide. Films provide more dense laying of layers under external influence.

Moisture saturated grip subsidence soil determined by the influence of the force of molecular attraction. This value depends on the degree of density and composition of the earth.

Process characteristic

Drawdown is a complex physical and chemical process. It manifests itself in the form of soil compaction due to the movement and denser (compact) packing of particles and aggregates. Due to this, the total porosity of the layers is reduced to a state corresponding to the level of acting pressure.

An increase in density leads to some change in individual characteristics. Subsequently, under the influence of pressure, the compaction continues, respectively, the strength continues to increase.

Conditions

For a drawdown to occur, you need:

• The load from the foundation or its own mass, which, when wet, will overcome the cohesive forces of the particles.
• Sufficient level of humidity. It contributes to the reduction of strength.

These factors must work together.

Humidity determines the duration of the deformation subsiding soils. As a rule, it occurs within a relatively short time. This is due to the fact that the land is predominantly in a low-humidity state.

Deformation in a water-saturated state lasts longer, since water is filtered through the soil.

Methods for determining soil density

Relative subsidence is determined from samples of undisturbed structure. For this, a compression device is used - soil density meter. The following methods are used in the study:

• One curve with the analysis of one sample and its soaking at the final stage of the acting load. With this method it is possible to determine the compressibility of the soil at a given or natural moisture, as well as the relative tendency to deform under a certain pressure.
• Two curves with the test of 2 samples with the same degree of density. One is studied at natural humidity, the second - in a saturated state. This method allows you to determine the compressibility under full and natural moisture, the relative tendency to deformation when the load changes from zero to ultimate.
• Combined. This method is a modified combination of the previous two. The test is carried out on one sample. It is first examined in its natural state to a pressure of 0.1 MPa. Using the combined method allows you to analyze the same properties as the 2-curve method.

Important Points

During testing in soil density meters when using any of the above options, it is necessary to take into account that the results of studies are characterized by significant variability. In this regard, some indicators, even when testing one sample, may differ by 1.5-3, and in some cases by 5 times.

Such significant fluctuations are associated with small size samples, heterogeneity of the material due to carbonate and other inclusions, or the presence of large pores. The inevitable errors in the study are also important for the results.

Influencing factors

In the course of numerous studies, it has been established that the indicator of the soil's tendency to subsidence depends mainly on:

• Pressure.
• Degrees of soil density under natural moisture.
• Composition subsidence soil.
• Humidity level.

Dependence on the load is reflected in the curve, according to which, with an increase in the indicator, the value of the relative propensity to change first also reaches its maximum value. With a subsequent increase in pressure, it begins to approach zero.

As a rule, for the pressure is 0.2-0.5 MPa, and for loess-like clays - 0.4-0.6 MPa.

The dependence is caused by the fact that in the process of loading the subsiding soil with natural saturation at a certain level, the destruction of the structure begins. In this case, a sharp compression is noted without a change in water saturation. Deformation in the course of increasing pressure will continue until the layer reaches its extremely dense state.

Dependence on the composition of the soil

It is expressed in the fact that with an increase in the plasticity number, the tendency to deformation decreases. Simply put, a greater degree of structure variability is characteristic of slurry, a smaller one - for clay. Naturally, in order to fulfill this rule, other conditions must be equal.

Initial pressure

At designing foundations for buildings and structures the load of structures on the ground is calculated. In this case, the initial (minimum) pressure is determined, at which deformation begins at full saturation with water. It disrupts the natural structural strength of the soil. This leads to the fact that the normal compaction process is disrupted. These changes, in turn, are accompanied by restructuring and intense compaction.

Given the above, it seems that at the design stage when organizing construction, the value of the initial pressure should be taken close to zero. However, in practice this is not the case. The specified parameter should be used such that the thickness is calculated according to general rules non-drawdown.

Purpose of the indicator

Initial pressure is used in the development of projects foundations on subsiding soils for determining:

• Estimated load at which there will be no change.
• The size of the zone within which compaction will occur from the mass of the foundation.
• The required depth of soil deformation or the thickness of the soil cushion, which completely excludes deformation.
• The depth from which changes from the mass of the soil begin.

Initial humidity

It is called the indicator at which soils in a stressed state begin to sag. A component of 0.01 is taken as a normal value when determining the initial humidity.

The method for determining the parameter is based on compression laboratory tests. 4-6 samples are needed for the study. The method of two curves is used.

One sample is tested at natural humidity with loading up to the maximum pressure in separate stages. With it, the soil is soaked until the subsidence stabilizes.

The second sample is first saturated with water, and then, with continuous soaking, is loaded to the limiting pressure in the same steps.

Humidification of the remaining samples is carried out to indicators that divide the moisture limit from initial to full water saturation into relatively equal intervals. Then they are examined in compression devices.

The increase is achieved by pouring the calculated volume of water into the samples with further holding for 1-3 days until the saturation level stabilizes.

Deformation characteristics

They are the coefficients of compressibility and its variability, modulus of deformation, relative compression.

The deformation modulus is used to calculate the probable indicators of foundation settlement and their unevenness. It is usually defined in field conditions. For this, soil samples are tested with static loads. The value of the modulus of deformation is affected by humidity, density level, structural cohesion and soil strength.

With an increase in soil mass, this indicator increases, with greater saturation with water, it decreases.

Coefficient of variability of compressibility

It is defined as the ratio of the compressibility under steady or natural moisture to the characteristics of the soil in a water-saturated state.

A comparison of the coefficients obtained in field and laboratory studies shows that the difference between them is insignificant. It is in the range of 0.65-2 times. Therefore, for practical application, it is sufficient to determine the indicators in the laboratory.

The coefficient of variability depends mainly on pressure, humidity, and the level of its increase. With an increase in pressure, the indicator increases, with an increase in natural humidity, it decreases. When fully saturated with water, the coefficient approaches 1.

Strength characteristics

They are the angle of internal friction and specific cohesion. They depend on structural strength, water saturation level and (to a lesser extent) density. With an increase in humidity, the adhesion decreases by 2-10 times, and the angle - by 1.05-1.2. With an increase in structural strength, adhesion is enhanced.

Types of subsidence soils

There are 2 in total:

1. Settling occurs predominantly within the deformable zone of the base under the action of the foundation load or other external factor. At the same time, the deformation from its weight is almost absent or is no more than 5 cm.
2. The subsidence of the soil from its mass is possible. It occurs predominantly in bottom layer thickness and exceeds 5 cm. Under the action of an external load, a subsidence may also occur in the upper part within the boundaries of the deformable zone.

The type of subsidence is used in assessing construction conditions, developing anti-subsidence measures, designing foundations, foundations, and the building itself.

Additional Information

Settling can occur at any stage of the construction or operation of a structure. It can manifest itself after an increase in the initial subsidence moisture.

During emergency soaking, the soil sags within the boundaries of the deformable zone quite quickly - within 1-5 cm/day. After the cessation of moisture supply, after a few days, the drawdown stabilizes.

If the initial soaking took place within the boundaries of a part of the deformation zone, with each subsequent water saturation, subsidence will occur until the entire zone is completely wetted. Accordingly, it will increase with increasing load on the soil.

With intensive and continuous soaking, soil subsidence depends on the downward movement of the moistening layer and the formation of a water-saturated zone. In this case, subsidence will begin as soon as the moistening front reaches the depth at which the soil sags from its own weight.

1

The work is devoted to the characterization of the initial state of dispersed soils - their structural strength. Knowing its variability makes it possible to determine the degree of soil compaction and, possibly, the features of the history of its formation in a given region. Evaluation and consideration of this indicator during testing of soils is of paramount importance in determining the characteristics of their physical and mechanical properties, as well as in further calculations of the settlement of foundations of structures, which is poorly reflected in regulatory documents and is little used in the practice of engineering and geological surveys. The paper briefly outlines the most common graphical methods for determining the index based on the results of compression tests, the results of laboratory studies of the structural strength of dispersed soils in the territory of the Tomsk region. Relationships between the structural strength of soils and the depth of their occurrence, the degree of their compaction are revealed. Brief recommendations on the use of the indicator are given.

Structural strength of soils

pre-sealing pressure

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structural strength pstr called strength, due to the presence of structural bonds and characterized by stress, to which the soil sample, when loaded with a vertical load, practically does not deform. Since compaction begins at stresses in the soil that exceed its structural strength and when testing soils, underestimation of this indicator leads to errors in determining the values ​​of other characteristics of mechanical properties. Importance of defining an indicator pstr has been celebrated for a long time, as N.A. Tsytovich - “... in addition to the usual indicators of the deformation and strength properties of weak clay soils, in order to assess the behavior of these soils under load and establish the correct prediction of the magnitude of the settlement of structures erected on them, it is necessary to determine the structural strength during surveys pstr". The phenomenon in surveying the degree of compaction of soils is important for predicting the settlement of the designed structure, since settlement on overcompacted soils can be four or more times less than on normally compacted soils. For values ​​of the overconsolidation coefficient OCR > 6, the lateral soil pressure coefficient at rest K about may exceed 2, which must be taken into account when calculating underground structures.

As noted in the paper: “Initially, conditions of normal compaction prevail during the process of sedimentation and formation and subsequent compaction of marine, lacustrine, alluvial, deltaic, eolian and fluvial deposits of sands, silts and clays. However, most soils on Earth have become slightly/moderately/severely overconsolidated as a result of various physical, environmental, climatic and thermal processes over many thousands to millions of years. These mechanisms of overconsolidation and/or visible prestressing include: surface erosion, weathering, sea level rise, sea level increase ground water, glaciation, freeze-thaw cycles, repeated wetting/evaporation, desiccation, mass loss, seismic loads, tidal cycles, and geochemical forcings.” The topic of determining the state of soil compaction is still very relevant and is found in publications from almost all continents. Factors and indicators that determine the over-compacted or under-compacted state of clay soils, the causes and influence on the physical and mechanical parameters of such strong cementation are considered in the works. The results of determining the indicator also have a wide range of applications in practice, starting from the calculation of the settlement of the foundations of structures; preservation of the natural structure of samples intended for laboratory testing; to very specific topics, predicting soil compaction in eucalyptus and coffee plantations by comparing their structural strength with the load from machinery.

Knowledge of indicator values pstr and their variability with depth characterize the features of the composition, bonds and structure of soils, the conditions of their formation, including the history of loading. In this regard, of particular scientific and practical interest are studies pstr V different regions, these studies are especially important in the territory of Western Siberia with a thick cover of sedimentary deposits. In the Tomsk region, detailed studies of the composition and properties of soils were carried out, as a result of which both the territory of the city of Tomsk and the surrounding areas were studied in sufficient detail from engineering-geological positions. At the same time, it should be noted that the soils were studied specifically for the construction of certain facilities in accordance with the current regulatory documents, which do not contain recommendations for further use. pstr and, accordingly, do not include it in the list of required soil characteristics to be determined. Therefore, the purpose of this work is to determine the structural strength of dispersed soils and its changes along the section in the most actively developed and developed areas of the Tomsk region.

The objectives of the study included a review and systematization of methods for obtaining pstr, laboratory determinations of soil composition and characteristics of the main physical and mechanical properties, the study of variability pstr with depth, comparison of structural strength with domestic pressure.

The work was carried out in the course of engineering and geological surveys for a number of large objects located in the central and northwestern regions of the Tomsk region, where the upper part of the section is represented by various stratigraphic and genetic complexes of Quaternary, Paleogene and Cretaceous rocks. The conditions of their occurrence, distribution, composition, state depend on age and genesis and create a rather heterogeneous picture; only dispersed soils were studied in terms of composition, in which clay varieties of semi-solid, hard and rigid-plastic consistency predominate. To solve the tasks set, wells and pits were tested at 40 points, more than 200 samples of dispersed soils were selected from a depth of up to 230 m. Soil tests were carried out in accordance with the methods given in the current regulatory documents. Were determined: particle size distribution, density (ρ) , density of solid particles ( ρs) , density of dry soil ( p d) , humidity ( w), moisture content of clay soils, at the border of rolling and fluidity ( w L And wp), indicators of deformation and strength properties; calculated state parameters such as porosity factor (e) porosity, total moisture capacity, for clay soils - plasticity number and flow index, soil compaction coefficient OCR(as ratio of pre-compression pressure ( p ") to domestic pressure at the sampling point) and other characteristics.

When choosing graphical methods for determining the indicator pstr, except methodCasagrande methods used abroad for determining the pre-compaction pressure were considered σ p ". It should be noted that in the terminology of a geological engineer, "pre-compaction pressure" ( Preconsolidation Stress) , begins to displace the familiar concept of "structural strength of the soil", although the methods for determining them are the same. By definition, the structural strength of the soil is the vertical stress in the soil sample, corresponding to the beginning of the transition from elastic compressive deformations to plastic ones, which corresponds to the term Yield Stress. In this sense, the characteristic determined in compression tests should not be taken as the maximum pressure within the "historical memory" of the sample. Burland believes that the term yield stress is more precise, and the term preconsolidation stress should be used for situations in which the magnitude of such pressure can be determined by geological methods. Similarly, the term Over Consolidation Ratio (OCR) should be used to describe a known history of stresses, otherwise the term Yield Stress Ratio (YSR) . In many cases Yield Stress is taken as the effective pre-compaction stress, although the latter is technically related to mechanical stress relief, while the former includes additional effects due to diagenesis, cohesion due to organic matter, the ratio of soil components and its structure, i.e. is the structural strength of the soil.

Thus, the first step towards identifying the features of soil formation should be a quantitative determination of the profile Yield Stress, which is a key parameter for separating normally compacted soils (with a predominantly plastic response) from overconsolidated soils (associated with a pseudo-elastic response) . and structural strength pstr, and pre-compaction pressure p" are determined in the same way, as noted, mainly by laboratory methods based on the results of compression tests (GOST 12248, ASTM D 2435 and ASTM D 4186). There are many interesting works investigating the state of the soil, pre-compaction pressure p" and methods for its determination in the field. Graphical processing of the results of compression tests is also very diverse, below is given short description most commonly used abroad methods for determining p ", which should be used to obtain pstr.

MethodCasagrande(1936) - the most old method to calculate structural strength and pre-compaction pressure. It is based on the assumption that the soil undergoes a change in strength from an elastic response to a load to a ductile response at a point close to the pre-compaction pressure. This method works well when there is a well-defined inflection point on the compression curve graph. of the form e - log σ"(Fig. 1a), through which a tangent and horizontal line is drawn from the porosity coefficient, then a bisector between them. The straight section of the end of the compression curve is extrapolated to the intersection with the bisector and a point is obtained , meaning when projected onto the axis log σ", corresponds to the overconsolidation pressure p"(or structural strength). The method remains the most commonly used compared to others.

Burmister Method(1951) - presents the dependence of the form ε-Log σ", Where ε - relative deformation. Meaning p" is determined at the intersection of the perpendicular coming from the axis Log σ" through the point of the hysteresis loop upon repeated loading of the sample, with a tangent to the end section of the compression curve (Fig. 1b).

Schemertmann method(1953), the compression curve of the form is also used here e - log σ"(Fig. 1c). Compression tests are carried out until a distinct straight section is obtained on the curve, then unloaded to domestic pressure and reloaded. On the graph, draw a line parallel to the midline of the decompression-recompression curve through the point of domestic pressure. Meaning p" determined by drawing a perpendicular from the axis log σ" through the point of unloading, to the intersection with a parallel line. From a point p" draw a line until it intersects with a point on a straight section of a compression curve having a porosity coefficient e\u003d 0.42. The resulting true compression curve is used to calculate the compression ratio or compaction ratio. This method is applicable to soft soils.

MethodAkai(1960), presents the dependence of the creep coefficient εs from σ" (Fig. 1d), is used, respectively, for soils prone to creep. The consolidation curve represents the dependence of the relative deformation on the logarithm of time and is divided into the section of seepage consolidation and creep consolidation. Akai noted that the creep factor increases proportionally σ" up to value p ", and after p" proportionately log σ".

Janbu method(1969) is based on the assumption that the pre-compaction pressure can be determined from a graph like ε - σ" . In the Janbu method for clays with high sensitivity and low OCR pre-compacting pressure can be determined by plotting the load-strain curve using a linear scale. Second way Janbu is a graph of the secant modulus of deformation E or E 50 from effective stresses σ" (Fig. 1 e). And one more option Christensen-Janbu method(1969), presents a dependence of the form r - σ", obtained from the consolidation curves , Where t- time , r= dR/dt, R= dt/dε.

Sellforce Method(1975) is a dependency of the form ε - σ" (Fig. 1f), is mainly used for the CRS method. The stress-strain axis is chosen at a fixed ratio on a linear scale, typically 10/1 for the ratio of stress (kPa) to strain (%). This conclusion was made after a series of field tests, where the pore pressure of pores and sediment were measured. This means that the Sallfors method for estimating overconsolidation pressure gives more realistic values ​​than estimates made in field trials.

Pacheco Silva Method(1970), seems to be very simple with respect to plotting, also of the form e - Log σ"(Fig. 1 g) , gives accurate results when testing soft soils. This method does not require subjective interpretation of the results and is also scale independent. Widely used in Brazil.

MethodButterfield(1979) is based on the analysis of the dependence of the sample volume on the effective stress of the form log(1+e) - log σ" or ln (1+e) - ln σ"(Fig. 1h). The method includes several different versions where the pre-compaction pressure is defined as the intersection point of two lines.

Tavenas method(1979), suggests a linear relationship between strain energy and effective stress for the recompression portion of the test in a graph like σ"ε - σ" (Fig. 1n, at the top of the graph). It is used directly on the basis of the compression curve without taking into account the reset part of the test. For more consolidated samples, the stress/strain plot consists of two parts: the first part of the curve rises more sharply than the second. The point of intersection of the two lines is defined as the pre-compaction pressure.

Oikawa Method(1987), represents the intersection of lines on the dependency graph log(1+e) from σ" -

Jose Method(1989), presents a dependence of the form log e - log σ" a very simple method for estimating the pre-compaction pressure, the method uses the intersection of two straight lines. It is a direct method and there are no errors in determining the location of the point of maximum curvature. MethodSridharanetal. (1989) is also a dependency graph log(1+e) - log σ" to determine structural strength of dense soils, so the tangent crosses the horizontal line corresponding to the initial porosity coefficient, which gives good results.

MethodBurland(1990) is a dependency graph porosity indexI v from stress σ" (Fig. 1 and). The porosity index is determined by the formula I v= (e-e* 100)/(e* 100 -e* 1000), or dl i weaker soils: I v= (e-e* 10)/(e* 10 -e* 100), Where e* 10, e* 100 and e* 1000 porosity coefficients at loads of 10, 100 and 1000 kPa (Fig. b) .

MethodJacobsen(1992), structural strength is assumed to be 2.5 σ to, Where σ to c is the point of maximum curvature on the Casagrande plot, respectively, also a dependence of the form e-log σ" (Fig. 1 l).

Onitsuka Method(1995), represents the intersection of lines on the dependency graph log(1+e) from σ" - effective stresses plotted on the scale on a logarithmic scale (decimal logarithms).

Van Zelst Method(1997), on a species dependency graph ε - log σ", the slope of the line (ab) is parallel to the slope of the discharge line ( cd). Point abscissa ( b) is the structural strength of the soil (Fig. 1m).

MethodBecker(1987), like the Tavenas method, determines the strain energy for each compression test load using the relationship W- σ", where. The strain energy (or, on the other hand, the work of the force) is numerically equal to half the product of the magnitude of the force factor and the displacement value corresponding to this force. The amount of stress corresponding to the total work is determined at the end of each voltage increment. The dependence on the graph has two straight sections, the overconsolidation pressure will be the point of intersection of these straight lines.

MethodStrain Energy-Log Stress(1997),Senol and Saglamer(2000 (Fig. 1n)), transformed by Becker and/or Tavenas methods, is a dependence of the form σ" ε - log σ", 1 and 3 sections are straight lines, the intersection point of which, when extended, will be the structural strength of the soil.

MethodNagaraj & Shrinivasa Murthy(1991, 1994), the authors propose a generalized relationship of the form log σ"ε - log σ"- to predict the magnitude of pre-consolidation pressure for over-compacted saturated non-consolidated soils. The method is based on the Tavenas method and compared to Senol method et al. (2000), this method gives a higher correlation coefficient in particular cases.

Chetia and Bora method(1998), primarily considers the history of soil loads, their characteristics and evaluation in terms of overconsolidation ratio (OCR), the main goal of the study is to establish an empirical relationship between OCR and the ratio e/e L .

MethodThogersen(2001), is the dependence of the consolidation ratio on effective stresses (Fig. 1o).

MethodwangandFrost, DissipatedStrainEnergymethod DSEM (2004) also refers to energy methods for calculating strain. Compared with Strain Energy method, DSEM uses the dissipated strain energy and the slope of the unload-reload compression cycle to minimize the effect of broken sample structure and eliminate the effect of elastic deformation. The dissipated strain energy, from the point of view of micromechanics, is directly related to the irreversibility of the consolidation process. Using the slope of the compression curve in the unload-reload section simulates elastic reload during the recompression stage and can minimize the impact of sample disruption. The method is less operator dependent than most existing ones.

Method Einavandcarter(2007), is also a graph of the form e-logσ", A p" expressed by a more complex exponential dependence .

The case of soil transition to the stage of consolidation creep after overcoming p" described in the works, if the end of the action of the next load step coincides with the end of the primary consolidation and the porosity coefficient on the dependency graph e - log σ" falls sharply vertically, the curve enters the stage of secondary consolidation. When unloading, the curve returns to the end point of primary consolidation, creating an overconsolidation pressure effect. There are a number of works offering calculation methods for determining the indicator p".

a) b) V)

G) e) e)

g) h) And)

To) l) m)

m) O)

Methods:

A)Casagrande, b)Burmister, c) Schemertmann,G)Akai, e)Janbu, f) Sellfors, g) Pacheco Silva, h)Butterfield, and)Burland, To)Jacobsen, l)Van Zelst, m)Becker, n)Senol and Saglamer, O)Thø gersen

Rice. Fig. 1. Schemes of graphical processing of the results of compression tests, used in determining the structural strength of the soil, by various methods

In general, graphical methods for determining the reconsolidation pressure based on the results of compression tests can be divided into four main groups. First group solutions includes dependences of the porosity coefficient ( e)/density (ρ) / relative strain ( ε )/volume change ( 1+e) from effective stresses (σ" ). The graphs are corrected by taking the logarithm of one or two of the listed characteristics, which leads to a straightening of the sections of the compression curve, and the desired result ( p ") is obtained by crossing the extrapolated straightened sections. The group includes the methods of Casagrande, Burmister, Schemertmann, Janbu, Butterfield, Oikawa, Jose, Sridharan et al., Onitsuka, and others. Second group links consolidation rates with effective stresses, these are methods: Akai, Christensen-Janbu and Thøgersen. The simplest and most accurate are methods of the third group - energy methods strain analysis: Tavenas, Becker, Strain Energy-Log Stress, Nagaraj & Shrinivasa Murthy, Senol and Saglamer, Frost and Wang, and others. Becker and others estimate the linear relationship between the total strain energy W and effective voltage without unloading and reloading. In fact, all energy methods are displayed in space. W- σ" , as well as the Butterfield method is reproduced in the field log(1+e)-log σ". If the Casagrande method focuses the reconsolidation pressure mainly on the most curved section of the graph, then the energy methods are adapted to the middle of the compression curve slope up to p". Part of the recognition of the superiority of these methods is due to their relative novelty and the mention in the development and improvement of a new method of this actively developing group. Fourth group combines methods with a variety of non-standard approaches to the graphic processing of curves, these include the methods of Jacobsen, Sellfors, Pacheco Silva, Einav and Carter, etc. Based on the analysis given in sources 10, 19, 22-24, 30, 31, 43-46] we note that the most common are the graphical methods of Casagrande, Butterfield, Becker, Strain Energy-Log Stress, Sellfors and Pacheco Silva, in Russia, the Casagrande method is mainly used.

It should be noted that if, in order to determine YSR ( or OCR) one value is enough pstr or p" , then when selecting straight sections of the compression curve before and after pstr when obtaining deformation characteristics, it is desirable to obtain two key points: the minimum pstr/min and maximum pstr / max structural strength (Fig. 1a). Here it is possible to use breakpoints tangent to the start and end sections, or use the methods of Casagrande, Sellfors and Pacheco Silva. As guidelines for the study of compression parameters, it is also recommended to determine the corresponding minimum and maximum structural strength indicators physical properties soil: primarily the coefficients of porosity and moisture content.

In this work, the indicator pstrwas obtained according to the standard method set out in GOST 12248 at the ASIS NPO Geotek complex. For determining pstr the first and subsequent pressure stages were taken equal to 0.0025 MPa until the start of compression of the soil sample, which is taken as the relative vertical deformation of the soil sample e >0,005. Structural strength was determined by the initial section of the compression curve ei = f(lg σ" ), Where ei - coefficient of porosity under load i. The point of a clear break in the curve after the initial straight section corresponds to the structural compressive strength of the soil. Graphical processing of the results was also carried out using the classical methods of Casagrande and Becker. . The results of the determination of indicators according to GOST 12248 and the methods of Casagrande and Becker correlate well with each other (correlation coefficients r=0.97). Undoubtedly, knowing the values ​​in advance, you can get the most accurate results using both methods. In fact, the method Becker seemed somewhat more difficult when choosing a tangent at the beginning of the graph (Fig. 1m).

According to laboratory data, the values ​​​​change pstr from 0 to 188 kPa for loams, for clays up to 170, for sandy loams up to 177. The maximum values ​​are noted, of course, in samples taken from great depths. A dependence of the change in the indicator with depth was also revealed. h(r = 0,79):

pstr = 19,6 + 0,62· h.

Variability analysis OWITHR(Fig. 2) showed that soils below 20 m are normally compacted, i.e. structural strength does not exceed or slightly exceeds the internal pressure ( OCR ≤1 ). On the left bank of the river Ob in the intervals of 150-250 m, semi-rocky and rocky soils firmly cemented with siderite, goethite, chlorite, leptochlorite and cement, as well as dispersed soils with a high structural strength of more than 0.3 MPa, underlain and interbedded by less the effect of cementation on the structural strength of soils, which is confirmed by the systematization of similar actual materials in the work. The presence of more durable soils caused a large spread of values ​​in this interval, so their indicators were not included in the dependency graph OWITHR from the depth, as not typical for the whole area. For the upper part of the section, it should be noted that the scatter of the index values ​​is much wider - up to highly compacted (Fig. 2), since the soils of the aeration zone are often found in a semi-solid and solid three-phase state, and with an increase in their moisture content ( r\u003d -0.47), full moisture capacity ( r= -0.43) and degree of water saturation ( r= -0.32) the structural strength decreases. There are also, noted above, the option of transition to creep consolidation (and not only in the upper part of the section). Here, it should be noted that soils with structural strength are very diverse: some may be in an unsaturated two-phase state, others may have a very high coefficient of sensitivity to mechanical stress and a tendency to creep, others have significant cohesion due to cement, the fourth are simply quite strong. , fully water-saturated clay soils occurring at shallow depths.

The results of the studies made it possible for the first time to evaluate one of the most important indicators of the initial state of soils in the Tomsk region - its structural strength, which varies over a very wide range above the aeration zone, so it must be determined at each work site before testing to determine the physical and mechanical properties of the soil. Analysis of the obtained data showed that changes in the indicator OCR at a depth below 20-30 meters are less significant, soils are normally compacted, but their structural strength should also be taken into account when determining the mechanical characteristics of soils. The research results are recommended to be used in compression and shear tests, as well as to determine the disturbed state of samples with a natural structure.

Reviewers:

Savichev O.G., Doctor of Geological Sciences, Professor of the Department of Hydrogeology, Engineering Geology and Hydrogeoecology of the Institute of Natural Resources of the Tomsk Polytechnic University, Tomsk.

Popov V.K., Doctor of Geology and Mathematics, Professor of the Department of Hydrogeology, Engineering Geology and Hydrogeoecology of the Institute of Natural Resources of the Tomsk Polytechnic University, Tomsk.

Bibliographic link

Kramarenko V.V., Nikitenkov A.N., Molokov V.Yu. ON THE STRUCTURAL STRENGTH OF CLAY SOILS IN THE TERRITORY OF THE TOMSK REGION // Contemporary Issues science and education. - 2014. - No. 5.;
URL: http://science-education.ru/ru/article/view?id=14703 (date of access: 01.02.2020). We bring to your attention the journals published by the publishing house "Academy of Natural History"

Above, we considered the deformation of a soil that does not have structural strength, i.e., compacted under the action of even a small pressure. This phenomenon is usually characteristic of very weak soils.

In most cases, natural soils are compacted by the pressure of the overlying layers. As a result of compaction, the soil particles approached and water-colloidal bonds formed between them. In the process of long-term existence of soils under certain conditions, fragile crystallization bonds could additionally arise in them. In total, these bonds give the soil some strength, which is called structural strength soil pstr.

At a pressure lower than the structural strength ( p

), when it is perceived by water-colloidal and crystallization bonds, compaction practically does not develop. Only when p>p str soil compaction occurs. It is difficult to determine the exact value of the structural strength, since a partial violation of the soil structure occurs already during sampling, in addition, when the sample is compressed, the destruction of the structure occurs first at the individual most stressed points of particle contact even at low pressures. As the pressure increases, the destruction at the contact points increases rapidly, and the process passes into the stage of soil compaction in the entire volume of the sample (Fig. 3.4.a.).

Rice. 3.4. Compression curves of soil with structural strength in simple (a) and semi-logarithmic (b) coordinate systems.

The beginning of the primary soil compression is more clearly revealed when using a compression curve built in semi-logarithmic coordinates (Fig. 3.4.b). In this case, the primary compression curve will be straight SD. Continuation of this straight line up to the intersection with the horizontal (dashed) line EU" corresponding to the value of the initial porosity coefficient e o, allows you to find the value p o, which can be considered as the value of structural strength.

The structural strength of the soil can also be determined by the results of changes in the lateral pressure of the soil when testing it in a triaxial compression device (according to E.I. Medkov) or by the moment the pressure arises in the pore water.

The compression curve equation with a certain approximation can be represented, as shown by K. Terzaghi, in the form of a logarithmic dependence:

, (3.11)