As you know, under the influence of pressure, the soil is deformed. The nature and magnitude of the deformation depend on the nature of the soil, the method of loading, and the boundary conditions of soil deformation. The deformation properties of soils are determined by the following main natural factors: 1) structure and texture; 2) composition and concentration of the pore solution; 3) chemical and mineralogical composition of the soil skeleton; 4) ambient temperature. The influence of certain natural factors on the deformability of soils depends mainly on the structure of the soil, i.e. on dispersion, density and location of particles in space and relationships between particles. Depending on the method of loading the soil, deformations are distinguished under static (stepped), shock and dynamic methods of applying pressure. Most often, the deformation properties of the foundation soils of structures are determined under static loading. In special cases, the deformation properties of soils are determined under the action of an impact load (tamping, explosion, etc.), during vibration, and also under the influence of hydrostatic, mainly negative (capillary) pressure, which occurs during dewatering in dispersed soils.
The deformation properties of dispersed soils are determined by their compressibility under load, due to the displacement of particles relative to each other and, accordingly, a decrease in pore volume due to deformation of rock, water, and gas particles. When determining the compressibility of soils, indicators are distinguished that characterize the dependence of the final deformation on the load and the change in soil deformation over time at a constant load. The first characteristic of the indicators includes the compaction coefficient, the compression coefficient, the modulus of settlement, the second - the consolidation coefficient.
The deformation properties of soils are determined both in laboratory conditions on samples with broken or unbroken structural bonds, and in the field. Until now, laboratory tests have been the main method for studying the properties of soils, since they make it possible to relatively easily transfer various pressures on the soil, study the behavior of the soil in a wide range of changes in the physical state and environmental conditions, and simulate complex cases of soil operation in the foundation or body of structures. Field test methods make it possible to more correctly reflect the influence of soil textural features on its deformability.
To study the compressibility of soils in the field, a pressuremeter is used - a device based on compression and measurement of the deformation of the soil located in the walls of an open hole, and determining the compressibility modulus.
20. Back to main features strength properties of soils include: soil shear resistance along the soil and along freezing surfaces; resistance to compression, stretching; cohesion and angle of internal friction, equivalent cohesion.
There are simple and complex stress states in the soil.
A simple stress state corresponds to the manifestation of one of the types of stresses: compression, tension, shear. The stress state in the soil mass corresponds to a complex stress state, when all types of simple stress states appear simultaneously with a different combination.
They allow predicting the settlements of structures, determining the stability of rocks at their base, and when constructing foundations, the bearing capacity of soils can be used to the maximum. Indicators expressing the resistance of rocks to shear make it possible to design the laying of slopes of dams, embankments, dams, sides of quarries with a minimum amount of excavation, determine the stability of slopes and landslides, determine the rational section and the stability of various structures, incl. concrete dams. Compressibility rock is called its ability to reduce the volume under the influence of the load. When rock is compressed by a vertical load under conditions of free lateral expansion under uniaxial compression, relative deformation (e) is the ratio of the magnitude of the absolute decrease in the loaded sample (Δh) to its initial height (h 0) e \u003d Δh / h 0 The relationship between stress (δ) and the value relative strain (e) at loads less than the limit of propor-ity is determined by the expression: δ=Ee (E is the modulus of elasticity)..
Shear resistance. Strength properties of rocks determined by a number of indicators belonging to the category of direct calculated indicators. The strength of rocks is characterized by the ability to resist shear forces (shear resistance). Shear is the process of deformation and destruction of the rock due to the displacement of one part of it relative to another. Shear along a given site is caused by shear stress to it. Shear resistance depends on the amount of vertical load applied to the specimen. The strength of rocks is estimated mainly according to Mohr's theory, according to which the destruction of a body occurs at a certain limiting ratio of normal and shear stresses.
The determination of strength and deformation characteristics is carried out both in laboratory and in the field, under simple and complex stress states. The main types of tests are: uniaxial compression; gap; shift; torsion; compression; axisymmetric triaxial compression by vertical and radial load; axisymmetric triaxial compression with torsion; axisymmetric compression of a hollow cylinder with torsion; triaxial compression with independent setting of all three principal directions; dynamometric test in relaxation-creep mode.
21. Reol. holy soils. In the engineering-geological assessment of rocks, these properties are of great importance. However, the role of each of them is not the same, which depends on the composition of the rocks.1) Water resistance. Determination of water resistance is most important when evaluating clayey rocks, which, under the influence of water, lose their cohesion and change their consistency or soak and disintegrate. The rate and nature of soaking characterize water resistance. Some varieties of clay rocks swell strongly when moistened, and their volume increases by 25-30%. The change in the properties of clayey rocks occurs not only when moistened. Drying of wet clayey rocks is sometimes accompanied by their cracking, change in solidity, decrease in volume (shrinkage). Water, acting on rocks, can also dissolve, leach out water-soluble parts and thereby change their properties. 2) moisture capacity. The water capacity of a rock is its ability to contain and retain a certain amount of water. In accordance with this, rocks are distinguished: moisture-intensive (clays, loams), medium-moisture-intensive (skpes, sands m / s, s / s, dusty) and non-moisture-intensive (sands s / s, s / s, gravel, etc.). With regard to non-moisture-intensive rocks, we should talk about their water capacity. In moisture-intensive rocks, total, capillary and molecular moisture capacity are distinguished. Full moisture capacity complete saturation of the rock with water, i.e. filling all her pores. Comparing the natural moisture of the rock with the moisture corresponding to the total moisture capacity, the degree of its water saturation is judged. The capillary moisture capacity corresponds not to the complete saturation of the rock with water, but to one when only the capillary pores are filled with water. Molecular water capacity refers to the ability of rocks to hold a certain amount of physically bound water. The maximum amount of physically bound water that a rock can hold on the surface of its particles is called the maximum molecular water capacity. From sandy rocks saturated with water, not all water can flow freely, but only that part that is subject to gravity. The ability of sandy and other clastic rocks saturated with water to give it away by free runoff characterizes their water loss. This ability is possessed by non-moisture-intensive breeds. The water loss of rocks is approximately equal to the difference between their total water capacity (W p) and the maximum molecular capacity: W dep \u003d W p -W m . 3) Capillarity. With a significant increase in the humidity of sandy and especially clayey rocks, their building qualities decrease. Humidification of water may be due to infiltration of water from the surface of the earth or its entry from below from any aquifer under the influence of the pressure of capillary forces. Capillary forces form a capillary zone above the groundwater level, within which there is increased wetting or saturation of rocks. With intensive evaporation of capillary water, soil salinization and the formation of solonchaks occur. It is known that the maximum height of capillary rise in t/s and m/s sands can reach 1.5-2.0 m, in clayey rocks 3-4 m. In coarse-grained rocks it is small and has no practical significance. 4) Water permeability. Water permeability is one of the main water properties of rocks; the ability to pass water through itself under the action of pressure. Data characterizing the permeability of loose clastic and clayey rocks is widely used in practice to determine inflows into construction pits, underground workings, drainage methods, etc. The permeability of sands, pebbles, and other loose deposits depends on their porosity and off-duty ratio. Clay rocks at low pressures are very weakly permeable, tk. their pore size is small. The movement of water and other liquids through porous media (rocks) is called filtration. Therefore, the permeability of sandy and clayey rocks is their filtration capacity. A measure of the water conductivity of rocks is the filtration coefficient. In engineering-geological practice, they mainly use the velocity expression of the filtration coefficient, based on the equation v=K f I (k) . If I=1, then v=K f m/day, cm/day.
In clayey rocks, the effective porosity is always much less than the total porosity and is often equal to zero, because the pore space is largely occupied by physically bound water.
22. Relaxation. When loaded with a constant force F, deformations occur,
developing over time. To stop the development of these deformations, it is necessary to reduce the force according to a certain law F (t). The decrease in time of the stress necessary to maintain a constant deformation is called relaxation (relaxation) of stresses. From the standpoint of statistical physics, relaxation can be considered as the process of establishing a statistical equilibrium in a physical system, when microscopic quantities characterizing the state of the system (stresses) asymptotically approach their equilibrium values. The characteristic of the phenomenon of stress relaxation is relaxation time, equal to the time during which the stress decreases by a factor of e, which characterizes the duration of the “sedentary life” of molecules, i.e., determines the mobility of the material. The relaxation time is different for different bodies. For rocky soils, the relaxation time varies by hundreds and thousands of years, FOR glass - ABOUT a hundred years, and for water - 10-11 s. For example, the rocks that form the earth's crust have a relaxation time measured in millennia, air 10-10, water 10-11, ice hundreds of seconds. If the duration of the action of forces on the soil is less than the relaxation period, then mainly elastic deformations will develop.
Thus, within 100-1000 seconds, the ice behaves like an elastic body (for example, it breaks brittle upon impact under heavy load conditions). When the load decreases, the ice flows like a viscous liquid. Similar behavior - brittle fracture under rapid load application and viscous flow under prolonged load - is clearly manifested in frozen soils.
If the time of action of the force on the soil exceeds the relaxation time, then irreversible creep and flow deformations occur in the soil. In other words, depending on the ratio of the force action time to the relaxation time, the body will behave as a solid or as a liquid. The relaxation period is the "main constant that combines the properties of solid and liquid bodies. The value of the relaxation time can be determined from the ratio of viscosity r | to the modulus of elasticity (shear): For solid bodies, which include dispersed and rocky soils, the presence of a limiting shear stress Xk , called the yield point and coinciding with the elastic limit.
23-24. Basic physical and chemical properties of soils . These properties include properties that appear as a result of physical and chemical interaction between soil components. These include the corrosion properties of soils, diffusion, osmotic, adsorption, as well as stickiness, plasticity, swelling, soaking, shrinkage and other properties of rocks. Corrosive properties: Corrosion is the process of destruction of materials due to their chemical, electro-chemical or bio-chemical interactions with the environment. Underground corrosion is expressed in the destruction of building metal materials, structures and pipelines when they interact with soils. The main causes of underground corrosion are: 1) the impact of ground moisture on a metal structure; 2) the phenomenon of electrolysis. These phenomena occur around the pipeline, as well as in areas where tram and railway traffic is used. Such destruction occurs in soils, as a result of the action of stray electric currents on water - a saline solution in the pores of the soil, which, as a result of such interaction, will become an aggressive CISO4 electrolyte; 3) the actions of microorganisms in the soil, causing biocorrosion. In general, soil corrosion depends on many factors. The main ones include the chemical composition of soils and, first of all, the composition and amount of dissolved salts, as well as soil moisture content, gas content in them, soil structure, their electrical conductivity and the presence of bacteria. Diffusion (from Latin Diffusion - spreading, spreading, dispersion), the movement of particles of the medium, leading to the transfer of matter and the alignment of concentrations or to the establishment of an equilibrium distribution of the concentrations of particles of a given type in the medium. Osmosis (from the Greek Osmos - push, pressure), one-way transfer of a solvent through a semi-permeable partition (membrane) that separates the solution from a pure solvent or a solution of lower concentration. Diffusion and osmosis lead to the redistribution of substance ions and water molecules and are most materially manifested in clay soils. Osmosis in clays can cause swelling or shrinkage deformations. For example, if saline clay soil is placed in fresh water, then osmotic absorption of water will occur and, as a result, the soil will swell. In practice, such swelling can occur in various channels laid in saline soils after they have been flooded with fresh water. If the reverse ratio of concentrations takes place, that is, the solution in the soils is fresher than in the channel, then osmotic suction of water from the soils will occur as a result of their shrinkage. Soil adsorption is called their ability to absorb certain particles or elements of a substance from passing solutions. There are several types of adsorption: mechanical (particle retention due to pore configuration); physical (due to molecules interacting between particles from solution and surface pores); chemical (due to chemical interactions); biological (due to the action of plants and various microorganisms). Separate types of adsorption can manifest themselves together (physico-chemical adsorption).
25. Shrinkage soil . Soil shrinkage is a decrease in its volume as a result of the removal of water during drying or under the influence of physicochemical processes (osmosis, etc.). As a result of shrinkage, the soil becomes denser and, after drying, even hard. The compaction of clay soil during shrinkage increases its resistance to deformation, but the presence of cracks, which usually accompany shrinkage, increases water permeability and reduces the stability of the surface layer of soil in slopes. In a dry and hot climate, shrinkage cracks break up an array of clay soil to a depth of 7-8 m or more. Shrinkage manifests itself to the maximum extent in clays; it is less characteristic of other connected rocks.
stickiness soil manifests itself at humidity greater than Wm; it reaches its greatest value in clay soils. The stickiness of clays increases with an increase in external pressure and a decrease in humidity; in most cases, its maximum value is reached at the maximum molecular moisture capacity. The stickiness of the soil depends on the categories of water contained in the soil, the characteristics of its chemical-mineral part, the area of contact of the soil with the object, etc. The stickiness of clay soils at a certain ratio of their characteristics with external factors can reach 0.02-0.05 MPa. Therefore, the stickiness of the soil is one of the factors that determine the working conditions of buckets, road and tillage machines. Soil sticking to the surface of earth-moving and transport machines and mechanisms causes a decrease in their productivity when performing overburden work in quarries, when excavating pits, etc.
Water resistance- this is the ability of soils to maintain mechanical strength and stability when interacting with water. The interaction of rocks with water can be static and dynamic: the impact of calm water causes the phenomena of swelling and soaking, the hydrodynamic impact - the process of erosion.
Soakability- this is the ability of clay rocks, when absorbing water, to lose cohesion and turn into a loose mass with partial or complete loss of bearing capacity. The intensity of the soaking process depends on the nature of the structural bonds, the composition and condition of the soils. The rate and intensity of erosion depend both on the nature of the water impact and on the reaction of the rock to this impact - erosion. A sharp change in water resistance (for example, as a result of weathering) can lead to a significant decrease in the bearing capacity of the foundation soils of structures and to the occurrence of collapse and landslide phenomena in the sides of construction pits and deep pits.
Blurring most often estimated by the coefficient of resistance of rocks to erosion.
Plasticity Soils are called the ability of soils to change their shape (deform) without discontinuity as a result of external influence and retain the new shape obtained during deformation after the external influence stops. The plastic properties of soils are closely related to moisture and vary depending on the quantity and quality of water in the soil. The transition of clay rock from one form of consistency to another takes place at certain moisture values, which are called characteristic moisture or limits. In engineering-geological practice, the upper and lower limits of plasticity are most widely used. The limits of plasticity and the plasticity number are widely used in the classification of clay soils, the determination of the calculated soil resistances and the approximate assessment of the stability of soils in pits, excavations, etc.
swelling soil is called the increase in its volume when interacting with water. Swelling of soils is often observed during the excavation of pits and excavations and leads to deformation of the lining, roadbed, foundations, etc. To determine the swelling, several methods have been proposed that can be combined into five groups based on the assessment of swelling: 1) by the heat of swelling; 2) by swelling pressure; 3) according to the volume of sediment deposited in the liquid; 4) by the amount (volume or weight) of water that caused the swelling; 5) according to the increase in soil volume during swelling.
The method of studying swelling by increasing the volume of soil in the process of saturating it with water (in the form in which it was developed by A. M. Vasiliev) has received the greatest distribution in the practice of engineering and geological work.
26. The movement of water and other liquids through porous media (rocks) is called filtration. Therefore, the permeability of sandy and clayey rocks is their filtration capacity. A measure of the water conductivity of rocks is the filtration coefficient. In engineering-geological practice, they mainly use the velocity expression of the filtration coefficient, based on the equation v=K f I (k) . If I=1, then v=K f m/day, cm/day. The speed of water movement through porous media (rocks) is directly proportional to the hydraulic gradient, i.e. the ratio of the effective pressure to the length of the filtration path. This is the most important law of water permeability of sandy and clayey rocks - the law of laminar filtration.
The speed of water movement is also determined by the equation: v \u003d Q / F (Q is the amount of water filtering through the rock, m 3; F is the cross-sectional area, m 2, through which water is filtered). Since the movement of water occurs only through the pores, the actual filtration rate (based on the smaller area of the actual section of the rock) is greater. Actual filtration coefficient: K fd =K f / n (n - porosity). The actual filter coefficient is sometimes called the filtration rate coefficient. In sandy rocks, K fd is always greater than the filtration coefficient, which is determined directly in laboratory conditions. In clayey rocks, the effective porosity is always much less than the total porosity and is often equal to zero, because the pore space is largely occupied by physically bound water. In construction, the filtration properties of the soil (its permeability) are associated with: 1. Engineering tasks (bank filtration as a result of dam construction). 2. With questions of temporary lowering of the groundwater level (U.G.V.) for drainage of pits. A laboratory device for determining the filtration properties of soils is a vessel with a porous bottom (see diagram), in which sand is placed. Water is poured from above and its consumption is measured (filtration through a sand sample) at various time intervals. If a hydraulic gradient less than the initial value is created in clay soil, there is no filtration in the soil and such soil is an aquiclude. Phil-nye characteristics of soils are used for: 1. Calculation of drainage. 2. Determination of the flow rate of the source of underground water supply. 3.Calculate the settlement of structures (foundations) in time. 4.Artificial reduction of U.G.V. 5. Calculation of sheet piling when digging pits, trenches.
Let us note a number of features characteristic of permafrost soils after their thawing:
The maximum values of water permeability are noted in the zones of tectonic crushing, and there is no attenuation with depth, which is explained by the high content of ice caused by the swelling of the dispersed aggregate. After the ice melts, powerful filtration passages are formed.
The water permeability of permafrost soils after their thawing is usually variable in time, since it is under the influence of two confrontation factors. On the one hand, the voids that have just formed in the swollen massif after the ice melts tend to close under the influence of the weight of the overlying soils or loads from structures, as a result of which the water permeability should decrease. On the other hand, finely dispersed filler, which, after ice melting, does not have a structure that ensures its filter strength, can be washed away by a filamentous flow. This entails an increase in the water supply of rocks. The filtering capacity of permafrost is estimated from the results of experimental work on pre-thawed areas or by indirect methods. Cosv-m methods for assessing the permafrost water permafrost include: calculation; comparison of dependences of water permeability indicators on fracturing for thawed and frozen soils; air testing of wells; geophysical. All of these methods are evaluative in nature.
Mechanical properties of soils Strength and deformation properties GOST 12248-96 METHODS FOR LABORATORY DETERMINATION OF STRENGTH AND DEFORMABILITY CHARACTERISTICS
Definition The mechanical or deformation and strength properties of the soil characterize its behavior under the influence of an external load.
Compressibility - the ability of soils to reduce volume under pressure. In dispersed clay soils, compressibility occurs mainly due to the squeezing of water and gases from the porous space. The compressibility of sands occurs as a result of changes in the structure of the skeleton of the rearrangement of particles. In rocky soils - due to the elastic deformation of the skeleton
Compressibility characteristics Compressibility characteristics or deformation properties include: u Deformation modulus u Poisson's ratio u Compressibility ratio u Consolidation ratios u Reconsolidation ratio
Stresses are internal forces (pressure) that arise in the body as a reaction to the effects of an external load.
Total and Effective Stresses Stresses arising in water-saturated soils are determined by two factors - the forces arising at the contacts between mineral particles (in the soil skeleton) and the pressure created by the water squeezed out of the pores. Effective stress (GOST 12248-96) is the stress acting in the soil skeleton, defined as the difference between the total stress in the soil sample and the pressure in the pore fluid. Apparent, imaginary, neutral and other voltage - stress created by the pressure of squeezed water Total stress - effective + apparent stress
Total and effective stresses Considering the soil as a two-phase system consisting of a skeleton - mineral particles and pore water, we introduce the concepts: u Pz - effective pressure, pressure in the soil skeleton (compacts and strengthens the soil). u Рw – neutral pressure, pressure in pore water (creates pressure in water, causing its filtration). At any time in a fully water-saturated soil mass, the relation takes place: P = Pz + Pw, where P is the total pressure. The effective voltage is determined, in this case, as: Pz \u003d P - Pw (according to Alekseev S.I., 2007)
Pw is the pressure created by water squeezed out of the pore space of the soil during deformation. This pressure causes stresses, referred to as "minimum". u Over time, imaginary stresses gradually relax (relax). In sandy soils, the relaxation process proceeds quickly (sometimes instantaneously), in clay soils, it is much slower. u The reason for this difference is the difference in the speed and nature of water filtration under load. u
Soil consolidation under compression In the general case of applying an external load to a water-saturated soil, compression initially occurs due to elastic deformations of the pore water and the soil skeleton. Then the process of filtration consolidation begins, due to the squeezing of water from the pores of the soil. u Upon completion of the filtration process, the process of secondary consolidation of the soil begins, determined by the slow displacement of particles relative to each other under conditions of slight squeezing of water from the pores of the soil. Primary consolidation is filtration consolidation, secondary consolidation is due to creep. u
Theory of filtration consolidation The main position of the theory of filtration consolidation is the compaction of dispersed water-saturated soil due to the squeezing of water from it during compression of the porous space. What stresses cause soil consolidation? Only effective, that is, transmitted to the soil skeleton. Neutral pressure does not affect the compression of the soil.
Pavlovsky's equation is the basis of the theory of filtration consolidation u This equation for the one-dimensional case has the form u where q is the unit flow rate of filtered water (velocity), m/s; n - soil porosity; z coordinate (filtering occurs along the z axis), m; t - time, s.
The equation for a one-dimensional problem is as follows: For a three-dimensional problem, it has the form u where c. V - consolidation ratio; - Proportion pressure
The consolidation coefficient Cv has the dimension m2/s. It indicates the speed of the consolidation process - the greater the consolidation ratio, the faster it goes.
Filtration in sands and clays Filtration occurs due to the difference in pressure or due to the presence of a filtration gradient.
Initial Gradient In clayey soils, there is no free water, the flow of which obeys the force of gravity. Water in clayey soils is contained in very small, often closed pores and cannot be filtered by itself. In order for filtration to begin in clay soil, it is necessary to apply some additional pressure to it, creating a certain gradient, which is called the initial gradient. The initial filtration gradient (i 0) is the value of the filtration gradient in clay soils, at which almost perceptible filtration begins
Darcy's law: Vpot= Kf * i, Vpot - flow rate i- pressure gradient Kf - filtration factor 0 for i
Creep (according to GOST) u Creep is the development of soil deformations over time at a constant stress. u The stage of undamped (non-steady) creep is the process of soil deformation at a constant or increasing rate at a constant stress
Deformations of the foundation of St. Isaac's Cathedral (according to Dashko and others) - a consequence of creep http: //georec. people. en/mag/2002n 5/7/7. htm Reliable, slightly compressible soil Weak, highly compressible soil (creeping soil) Reliable, slightly compressible soil
Theory of elasticity. Hooke's law. Elastic deformation of compression and/or tension is directly proportional to stress: ε = Рх/Е, where ε – relative strain Рх – stress (pressure), MPa Е-Young's modulus, MPa
The physical meaning of Young's modulus Young's modulus (E, MPa) reflects the proportion between relative linear deformation and stress. It is determined by the composition and properties of the material (in our case, soils) and varies depending on the composition and properties of the latter. Does not depend on the magnitude of the compressive stress.
Elastic deformation Elastic deformation is a relative change in the size and shape of a body under the influence of an external load. After the load is removed, the shape and dimensions are restored.
Elastic deformation According to the direction of deformation, they are divided into longitudinal (relative to the direction of the applied load) and transverse. Relative longitudinal deformation: x \u003d (h 1 -h 2) / h 1 Relative transverse deformation: y \u003d (S 2 -S 1) / S 1
Poisson's ratio () Poisson's ratio - the ratio of the relative linear deformations of the body in the direction transverse to the action of the load to the relative linear deformation in the longitudinal direction: = ε y / ε x
Compressibility coefficient () and volumetric deformation modulus (K) of elastic bodies u For the case of uniform compression of a solid body, Hooke's law takes the form: where p=(px+py+pz)/3. The value of p is called the average normal stress.
The compressibility coefficient (m 0) and the volumetric deformation modulus (K) of elastic bodies u Based on the previous one, we can find an expression for the compressibility coefficient or its reciprocal - the volumetric deformation modulus K of an elastic medium: Does not depend on the magnitude of the compressive stress.
Compression tests u 5. 4. 1. 1 A soil compression test is carried out to determine the following deformability characteristics: compressibility factor mo, deformation modulus E, consolidation set. . . u 5. 4. 1. 2 These characteristics are determined by the results of tests of soil samples in compression devices (odometers) ..., excluding the possibility of lateral expansion of the soil sample when it is loaded with a vertical load.
Deformations When compressed in a compression device, there is a decrease in volume and (primarily) a decrease in the volume of the pore space (and, consequently, the porosity kta). This makes it possible to express the volumetric deformation in terms of changes in the values of the porosity factor e.
Deformation of Soils The soil is not a perfectly elastic body. In clay soils, along with elastic soils, plastic deformations also appear, which violates the linear nature of the relationship between stress and strain.
The compression curve is a hyperbolic graph of the dependence of loads and the coefficient of porosity e The coefficient of porosity (volume-strain function) e 0 i load step e 1 e 2 i + 1 load step Rectilinear segment P, MPa Ps P 1 P 2 vertical pressure e 0 - initial natural the value of the porosity factor, Рs, is the minimum pressure at which noticeable deformation begins
Transverse deformation coefficient β-coefficient taking into account the absence of transverse expansion of the soil in the compression device β=1 - (2 2/(1 -)) The coefficient (Poisson's ratio) is determined from the data of triaxial tests. If these data are not available, its values are assumed to be: - For sands and sandy loams: 0. 30 -0. 35 - For hard loams and clays: 0.2 -0. 3 - For semi-solid loams and clays: 0. 30 -0. 38 - For tight-flowing loams and clays: 0. 38 -0. 45
Deformation modulus (E, MPa) - coefficient of proportionality of the linear relationship between increments of pressure on the sample and its volumetric deformation. By its nature, it is similar to the volumetric deformation modulus (K) in Hooke's law, but depends on the magnitude of the compressive stress. When determining E, the volumetric deformation V approximately corresponds to changes in the porosity coefficient e at the corresponding deformation steps: V e
Relative compressibility at the i-th step The coefficient of relative compressibility (relative vertical deformation) at the i-th load step is defined as the ratio of the height by which the sample has changed from a given load to the initial height of the compressible sample: εi = Δhi/h
Calculation of the porosity coefficient at the i-th stage of the load K-t of porosity at the i-th stage of the load is calculated as: e 0 - initial (initial) k-t of porosity ei- k-t of porosity at the i-th stage of loading i-th load stage
Calculation of the deformation modulus In accordance with GOST 12248-96, the total deformation modulus E is calculated by the formulas: Еi-(i+1)= ((Рi - Pi+1)/(ei - ei+1))*β Or Еi-(i +1)= ((1+eo)/mo)*β eo- coefficient of porosity of natural soil e- value of porosity at I and i+1 load steps mo- compressibility β - lateral expansion
Loads and compressibility Loads or specific pressure from many types of structures (block five-story buildings, earth embankments about 10 m high, etc.) are in the range from 200 to 300 KPa. Based on this, soils in terms of compressibility in the pressure range of 200300 KPa can be classified into: u mo mo >1/10 MPa - medium compressible u mo >1/10 MPa - slightly compressible
Consolidation ratio u. Filtration coefficient V and secondary with consolidation - indicators characterizing the rate of soil deformation at constant pressure due to water filtration (c. V) and soil creep with
Consolidation factor Consolidation factors are used to estimate the rate of settlement development. Cv - cm 2 / min, hour, year C - cm 2 / min, hour, year These to-you are determined by the graphic-analytical method according to the compression curve (Appendix H, GOST 12248 -96) or by special tests in a compression device.
Household pressure Household (lithostatic or natural or mountain, etc.) pressure (Pb) is defined as: Pb \u003d * H H - depth, m - specific gravity (MN / m 3)
The specific gravity of the soil, taking into account the weighing effect of water (for water-saturated soils), is determined by the formula u = (s - w) / (1 + e), where: u s - the specific gravity of the soil particles is calculated: u s = s * g where: u s - density soil particles t/m 3 u g – free fall acceleration = 9.81 m/s2 u w – specific gravity of water = 0.01 MN/m 3 u e – porosity coefficient (dimensionless) u
Diagram of vertical stresses Soil massifs in their natural occurrence are in a stressed state due to pressure from soil layers. In conditions where there is no possibility of lateral bulging, the vertical stress increases with depth: bz \u003d ∑ gi * i * hi, i- number of layers, g acceleration of gravity, i- specific gravity of the i-th layer, hi- depth of the roof (foot) i- layer.
Definitions GOST 30416 -96 Stabilized state of the soil, characterized by the end of seal deformations under a certain load and the absence of excess pressure in the pore fluid. u An unstabilized state of the soil, characterized by incomplete deformation of the seal under a certain load and the presence of excess pressure in the pore fluid. u
Overconsolidated and Underconsolidated Soils Soils whose compressibility is lower than expected at a given ambient pressure are called overconsolidated soils. Overconsolidation is a consequence of the compression of soils in the depth of the stratum and their subsequent exit to the surface as a result of erosion of overlying sediments, the result of compression under the pressure of ancient glaciers, etc. They are characterized by low compressibility, sometimes they swell. In general, they are reliable bases.
Soils whose compressibility is greater than expected at a given ambient pressure are said to be undercompacted. They are formed as a result of very rapid accumulation (avalanche sedimentation) and other causes. Typical undercompacted soils are loesses, as well as marine and alluvial-marine silts, sapropels, and peat. They are characterized by the presence of excess pore pressure exceeding the hydrostatic pressure; high compressibility; instability under dynamic loading, in general, are very unreliable grounds.
Overconsolidation and underconsolidation I - range of loads not exceeding the domestic pressure II - interval of loads exceeding the domestic pressure e Ps - maximum domestic pressure that has occurred in the geological history (pre-compaction pressure) For overcompacted soils: Рs>Pb For undercompacted soils: Рs
Reconsolidation kit To assess soil compaction, the reconsolidation kit KPU is used. According to the KPU values, soils can be classified: u undercompacted KPU 4.
The set of re-sealing KPU is calculated as: KPU= Ps/Pb, where: u Ps - pre-seal pressure, MPa u Pb - current domestic pressure, MPa
Overconsolidation kit Underconsolidated soils tend to settle under their own weight. At the same time, they are characterized by low strength, high compressibility and instability under dynamic loads. In general, they are unreliable grounds. u Overconsolidated soils have high strength, low compressibility, and may swell. With KPU>6, the lateral soil pressure k-t can exceed 2, which must be taken into account when designing underground structures. In general, they are reliable bases. u
Strength properties The shear strength of soils is determined by cohesion (presence of structural bonds) and friction between particles. Structural connections - connections between structural elements (particles, aggregates, crystals, etc.) that make up soils
Characteristics of strength properties С- adhesion (specific adhesion), MPa φ - angle of internal friction, degrees τ - resistance of soil to shear, MPa R- resistance to uniaxial compression Su- resistance to undrained shear, MPa
Structural bonds according to the degree of strength Mechanical - friction between particles (in sands, coarse clastic and clay soils) Water-colloidal or coagulation (essentially - adhesion of particles) - due to electromagnetic (van der Waals - Van der Waals) forces of intermolecular attraction (clay dispersed soils) Cementation - arise due to the filling of the porous space with mineral mass, cementing particles (semi-rocky rocks) Crystallization - inside the crystals and between the crystals (rocky igneous and metamorphic rocks)
Strength and fracture The strength of soils is determined mainly by structural bonds between individual particles (crystals or grains) and/or aggregates of particles and crystalline intergrowths. The strength of the elementary crystals, particles, or mineral aggregates themselves is of secondary importance. Soil destruction occurs when, upon reaching certain limiting stresses, structural bonds are broken and an irreversible movement of particles relative to each other occurs.
The pressure P from the weight of the above-ground part of the structure and the self-weight of the foundation is dissipated in the soil mass. We decompose the resultant R into two components and, compress the soil particles to each other and practically cannot destroy them (soil particles - quartz, feldspar, etc.) destroy 2000 kgf / cm 2 200 MPa - such stresses practically do not occur under the foundation .
u So the destruction of the soil occurs from the action of shear stresses (). Under the action of these stresses, the soil particles are displaced relative to their contacts, the grains enter the pore space, the process of soil compaction occurs with the appearance of sliding surfaces in some areas.
Mohr-Coulomb theory According to this theory, the soil strength is determined by the ratio between normal and shear stresses: = σ * tgφ + С, where - - shear stress - σ- Normal stress - C - adhesion - φ - angle of internal friction
Physical and geometric meaning C and φ Geometric meaning (according to GOST 30416 -96): u The angle of internal friction is a parameter of direct dependence of soil resistance to shear from vertical pressure, defined as the angle of inclination of this straight line to the abscissa axis. u Specific soil cohesion - a parameter of direct dependence of soil resistance to shear on vertical pressure, defined as a segment cut off by this straight line on the y-axis. Physical meaning: u Specific adhesion - strength or strength of structural bonds u Angle of internal friction - friction forces between particles Two components of adhesion can be distinguished: 1 - strength of structural bonds (Cc) 2 - strength due to friction (ΣW) - mechanical bonds
The strength of clay soils τ In bonded clay soils containing sand particles with cementation or water-colloidal bonds, the strength is determined by both adhesion and the angle of internal friction φ τ = σ * tg φ + C С σ 0
The strength of clay soils τ In bonded clay soils that do not contain sand particles, with cementation or water-colloidal bonds, the strength is determined as cohesion τ \u003d C С σ 0
Strength of sandy soils τ In unbound sandy soils, the strength is mainly determined by the angle of internal friction, and the C values are relatively small τ = σ * tg φ φ σ
Determination of strength characteristics by the single-plane cut method u u 5. 1. 1. 1 Soil testing by the single-plane cut method is carried out to determine the following strength characteristics: soil shear resistance τ, internal friction angle φ, specific adhesion C, for sands (except for gravel and coarse), clayey and organic-mineral soils. 5. 1. 1. 2 These characteristics are determined by the results of testing soil samples in single-plane shear devices with a fixed shear plane by shifting one part of the sample relative to its other part with a tangential load while simultaneously loading the sample with a load normal to the shear plane
Shear device u The single-plane shear device consists of two rings (lower and upper). The lower ring is fixed in the shift box motionlessly. The top can move relative to the bottom.
NN, KN and KD (according to GOST 30416 -96) Consolidated-drained soil test to determine the strength and deformability characteristics with preliminary compaction of the sample (in the odometer) and squeezing water out of it during the entire test. Consolidated non-drained soil testing to determine the strength characteristics with pre-compaction of the sample and squeezing water out of it only during compaction. Unconsolidated-undrained soil test to determine the strength characteristics without preliminary compaction of the sample in the absence of water squeezing out of it during the entire test.
Shear resistance Soil shear resistance is a soil strength characteristic determined by the value of shear stress at which fracture (shear) occurs. u Soil resistance to shear (τ, MPa) is defined as shear load Q divided by shear area A of the sample at a given normal load F. u τ = Q/A, MPa
Why do we need at least three points? τ - soil resistance to shear, MPa The third point plays a corrective role
Shear test schemes non-consolidated-undrained test - for water-saturated clay and sandy soils - test without preliminary compaction and without water squeezing; u consolidated non-drained test - for non-stabilized clay soils - test with pre-compaction (in odometer) under pressure equivalent to domestic pressure + pressure from the structure and without water extraction; u Consolidated-drained test - for stabilized clay soils and sands - test with pre-consolidation and water extraction u
Uniaxial compression method 5. 2. 1. 1 Soil testing by the uniaxial compression method is carried out to determine the following strength characteristics: uniaxial compressive strength (R) for rocky semi-rocky soils; resistance to undrained shear for water-saturated clay soils (Su). 5. 2. 1. 2 The uniaxial compressive strength is determined as the ratio of the vertical load applied to the sample, at which the sample breaks, to the area of its initial cross section.
Triaxial compression (most advanced method) 5. 3. 1. 1 Soil testing by triaxial compression is carried out to determine the following characteristics of strength and deformability: angle of internal friction φ, specific cohesion C, undrained shear resistance Su, deformation modulus E and transverse deformation coefficient v for sands, clayey, organomineral and organic soils. 5. 3. 1. 2 These characteristics are determined by the results of testing soil samples in triaxial compression chambers, which enable lateral expansion of the soil sample under conditions of triaxial axisymmetric static loading ...
Features of the method During testing, a cylindrical soil sample is placed in a rubber shell. The pressure on the sample is created by the working piston (vertical load F) and all-round water pressure. Unlike compressive compression, shear and uniaxial compression, not only vertical and longitudinal (during shear) deformations are measured, volumetric deformation (by measuring the volume and pressure of water in the chamber)
Triaxial tests of soils with cyclic loads The purpose of this method is to evaluate the strength properties under dynamic loads (earthquakes, sea waves, vibration of a structure, etc.). In this method, a soil sample is subjected to alternating compressive and tensile loads. Cycles of compression and extension alternate with a period and frequency corresponding to the expected dynamic impact. Test methods are not standardized.
6. Strength and deformability of frozen soils Determined by the following methods: Tests with a ball punch u One-plane cut along the freezing surface u Uniaxial compression u All tests are carried out at a negative external temperature, which, ideally, should correspond to the natural temperature of the frozen soil
What to do if the deformation and strength properties of soils are not determined and only the values of physical properties are available? 1. 2. Strength and deformation properties are taken from materials obtained in adjacent areas. For preliminary calculations of foundations ... it is allowed to determine the normative and design values of the strength and deformation characteristics of soils according to their physical characteristics from Appendix 1 of SNi. P 2. 01-83. Foundations and foundations.
Normative values of specific adhesion cn, k. Pa (kgf / cm 2), internal friction angle n, deg. , silty-argillaceous non-loess soils of Quaternary deposits
Normative values of specific adhesion cn, k. Pa (kgf / cm 2), internal friction angle n, deg. and deformation modulus E, MPa (kgf / cm 2), sandy soils of Quaternary deposits
SP 22.13330.2011
Updated version of SNiP 2.02.04-88
Author NIIOSP named after N.M. Gersevanov
Chapter 5.3. P.:
- The main parameters of the mechanical properties of soils that determine the bearing capacity of foundations and their deformation are the strength and deformation characteristics of soils (angle of internal friction φ, specific adhesion c, ultimate strength for uniaxial compression of rocky soils Rc, deformation modulus E and coefficient of transverse deformation υ of soils). It is allowed to use other parameters characterizing the interaction of foundations with the base soil and established empirically (specific heaving forces during freezing, base stiffness factors, etc.).
NOTE Further, except where otherwise noted, the term "soil characteristics" means not only mechanical, but also physical characteristics of soils, as well as the parameters mentioned in this clause.
SP 50-101-2004 "Design and arrangement of foundations
and foundations of buildings and structures"
Author NIIOSP them. N.M. Gersevanova, State Unitary Enterprise Mosgiproniselstroy
clause 5.1.8
The physical and mechanical characteristics of soils include:
- - density of soil and its particles and humidity (GOST 5180 and GOST 30416);
- - coefficient of porosity;
- - particle size distribution for coarse soils and sands (GOST 12536);
- - humidity at the limits of plasticity and fluidity, plasticity number and fluidity index for clay soils (GOST 5180);
- - angle of internal friction, specific cohesion and soil deformation modulus (GOST 12248, GOST 20276, GOST 30416 and GOST 30672);
See Regulatory values of these characteristics - Appendix A SP 22.13330.2016
- - tensile strength under uniaxial compression, indicators of softening and solubility for rocky soils (GOST 12248).
The physical characteristics of soils include:
For specific soils, the design features of the foundations of which are set out in Section 6 of SP 22.13330.2011, and when designing the foundations of the underground parts of structures (see Section 9), the characteristics specified in these sections must be additionally determined.
Soils with specific unfavorable properties include:
- subsidence soils
Swelling soils
Saline soils
Organo-mineral and organic soils
Eluvial soils
Bulk soils
Alluvial soils
Heaving soils
Fixed soils
When determining the design soil resistance R bases of wooden houses belonging to the 3rd reduced responsibility class, according to tabular values R0(B.1-B.10 of Appendix B) it is not required to determine such physical and mechanical characteristics as:
Angle of internal friction, specific adhesion, modulus of deformation and coefficient of transverse deformation of soils (GOST 12248, GOST 20276, GOST 30416 and GOST 30672);
See an example of determining the properties of soils for replacing a foundation on the website page: "Example of calculating the foundation of a wooden house"
Definitions
Appendix A. p.:
- Porosity coefficient e determined by the formula (See A.6 GOST 25100-2011)
e = (ρ s - ρ d)/ρ d , (A.5)
- ρ s - density of particles (skeleton) of soil, mass per unit volume of solid (skeleton) particles of soil g/cm3;
ρ d is the density of dry soil, the ratio of the mass of soil minus the mass of water and ice in its pores to its initial volume, g/cm3, determined by the formula
- Density of dry soil (skeleton) ρ d determined by the formula (see A.16 GOST 25100.2011)
ρ d = ρ/(1+ w), (A.8)
- where ρ is the density of the soil, g / cm 3 (see GOST 5180);
w- natural soil moisture, %
- Flow index I L- the ratio of the moisture difference corresponding to two states of the soil: natural W and at the rolling boundary Wp, to the plasticity number Ip
A.18 GOST 25100-2011, Flow index I L d.u., is an indicator of the state (consistency) of clay soils; determined by the formulaI L = (w - w p)/I p , (A.9)
- where w is the natural moisture content of the soil, % (see GOST-5180-84);
w p - humidity at the border of rolling,% (see GOST 5180);
I p - plasticity number,%, (see A.31 GOST 25100-2011)
- Plasticity number I p(See A.31 GOST 25100-2011), %; determined by the formula
I p = w L - w p , (A.17)
- where w L is the moisture content at the yield point, % (see 4 GOST 5180);
w p - humidity at the border of rolling,% (see 5 GOST 5180)
Compressibility- the ability of the soil to decrease in volume under the action of an external force, characterized by the compressibility coefficient m0(the tangent of the slope of the compression curve), determined by the formula (See 5.4 GOST 12248-2010)
m 0 = (e i - e i+1)/ (p i+1 - p i) 5.32
- e i and e i+1 - porosity coefficients corresponding to pressures p i and p i+1 .
- Based on the values of the horizontal shear and normal loads measured during the test, the shear and normal stresses τ and σ, MPa, are calculated using the formulas:
τ = 10Q / A; (5.3)
σ=10F/A; (5.4) - Specific adhesion c And angle of internal friction φ
soil are defined as linear dependence parameters
τ = σtan(φ) + c (5.5)
- τ and φ are determined by formulas (5.3) and (5.4) = Q/A, (5.1) are shear stresses and
= F/A, (5.2) - normal stresses
Q and F - tangential and normal force to the shear plane, respectively, kN
A - cut area, cm2
Source: GOST 12248-2010 soil density ρ - the ratio of the mass of soil, including the mass of water in its pores, to the volume occupied by this soil (g / cm 3 t / m 3)
dry soil density ρ d - the ratio of the mass of dry soil (excluding the mass of water in its pores) to the volume occupied by this soil (g / cm 3 t / m 3)
density of soil particles ρ s is the ratio of the mass of dry soil (excluding the mass of water in its pores) to the volume of the solid part of this soil (g / cm 3 t / m 3). Total moisture capacity Wo is the maximum possible content of all possible types of water in the soil when its pores are completely filled.
w sat = n.ρ w/pd
- where: n – porosity, f.u.,
ρ w is the density of water, g/cm3,
ρ d - density of dry soil.
Mechanical properties of soils- this is their ability to resist changes in volume and shape as a result of force and physical influences.
deformation- soil capacity strength– soil capacity
resist the development of deformations; resist destruction;
The mechanical properties are influenced by the nature of the structural bonds of the particles, the granulometric and mineral composition, and soil moisture. The main mechanical properties of soils are: compressibility; shear resistance; water permeability.
Compressibility.
The ability of soil to decrease in volume under the influence of compacting loads is called compressibility, settlement or deformation. According to the physical structure, the soil consists of individual particles of various sizes and mineral composition (soil skeleton) and pores filled with liquid (water) and gas (air). When compressive stresses occur, the change in volumes occurs due to a decrease in the volumes of pores located inside the soil filled with water. Thus, compressibility depends on many factors, the main of which are the physical composition, the type of structural bonds of particles, and the magnitude of the load.
According to the nature of shrinkage, elastic and plastic deformations are divided. Elastic deformations occur as a result of loads that do not exceed the structural strength of soils, i.e. not destroying the structural bonds between the particles and are characterized by the ability of the soil to return to its original state after the removal of loads. Plastic deformations destroy the soil skeleton, breaking bonds and moving particles relative to each other. At the same time, volumetric plastic deformations compact the soil due to a change in the volume of internal pores, and shear plastic deformations - due to a change in its original shape and up to destruction. When calculating soil compressibility, the main deformation characteristics are determined in laboratory conditions according to the relative compressibility coefficient, the lateral pressure coefficient and the transverse expansion coefficient.
Shear resistance
Ultimate shear resistance is the ability of the soil to resist the movement of parts of the soil relative to each other under the influence of shear and direct stresses. This indicator is characterized by the strength properties of soils and is used in calculations of the foundations of buildings and structures. The ability of a soil to bear loads without collapsing is called strength. In sandy and coarse-grained non-cohesive soils, resistance is achieved mainly due to the friction force of individual particles; such soils are called loose. Clay soils have a higher resistance to shear, tk. along with the friction force, shear is resisted by cohesion forces. In construction, this indicator is important in the calculation of foundation foundations and the manufacture of earthworks with slopes.
The shear resistance of clay soils t is determined by the Coulomb equation:
For sandy soils, due to the lack of cohesive forces, the shear resistance takes the form:
Water permeability
Water permeability is characterized by the ability of the soil to pass water through itself under the action of a pressure difference and is determined by the physical structure and composition of the soil. Ceteris paribus, with a physical structure with a lower content of pores, and with a predominance of clay particles in the composition, the water permeability will be less than that of porous and sandy soils, respectively. This indicator should not be underestimated, because in construction, it affects the stability of earthworks and determines the rate of compaction of foundation soils.
Deformation and strength properties of soils and their characteristics.
Compressibility soil characterizes their ability to deform without destruction under the influence of an external load. The deformation properties of soils are characterized by the total deformation modulus E , Poisson's ratio, compressibility and consolidation ratios, shear and bulk moduli. The compressibility of dispersed soils under load is due to the displacement of mineral particles relative to each other and, accordingly, a decrease in the pore volume.
Soil strength determined by their shear resistance , which can be described by the linear Coulomb dependence
τ = p tgφ + c,
Where τ – shear resistance, MPa; R – normal pressure, MPa; tan φ is the coefficient of internal friction; φ is the angle of internal friction, deg; c – clutch, MPa.
Quantities φ And c necessary for engineering calculations of strength and stability.
The strength of rocky soils is determined mainly by their structural bonds, i.e. adhesion, but to the greatest extent cracking.
The temporary resistance of rocky soil to uniaxial compression (ultimate compressive strength) is an important classification characteristic, according to which soil is classified as rocky (> 5 MPa) or non-rocky (< 5 МПа).
The chemical and mineral composition, structures and textures of soils, the content of organic matter are determined in geological laboratories equipped with the necessary equipment (X-ray electron microscope, etc.). The physical and mechanical properties of soils are studied in soil science laboratories and in the field at future construction sites. Particular attention is drawn to the reliability of the results obtained.
For each soil characteristic, several determinations are made and their statistical analysis is carried out. For any IGE, there must be at least three definitions.
Ground laboratory. Soil samples for laboratory research are taken by soil layers in pits and in boreholes at the facilities.
Soil samples are delivered to the laboratory in the form of monoliths or loose samples. Monoliths are soil samples with an undisturbed structure, which should have dimensions of 20 x 20 x 20 cm. In silty clay soils, it is necessary to maintain natural moisture due to a waterproof paraffin or wax shell on their surface. In loose soils (sand, gravel, etc.) .) samples are taken with a mass of at least 0.5 kg.
In laboratory conditions, it is possible to determine all physical and mechanical characteristics, each according to its own GOST: natural moisture and soil density - GOST 5180-84, tensile strength - GOST 17245-79, granulometric (grain) composition - GOST 12536-79, etc. The laboratory determines the humidity, density of soil particles and some others.
Field work. The study of soils in the field gives an advantage over laboratory analysis, since it allows you to determine all the values of physical and mechanical characteristics in the natural occurrence of soils without destroying their structure and texture, while maintaining the moisture regime. At the same time, the work of soil massifs in the foundations of buildings and structures is simulated. Such studies of soils have been used more and more in recent years. At the same time, technical equipment is being improved, and computers are being used. Express methods allow you to quickly obtain soil properties. In order to predict the behavior of soil massifs for the period of operation of buildings and structures, it is advisable to reasonably combine laboratory and field studies.
Among the methods of deformation testing of soils for compressibility, one should consider the reference method field stamp tests (GOST 20278-85). The results of other test methods, both field (pressuremetry, dynamic and static leading) and laboratory (compression and stabilometric) must be compared with the results of stamp tests.
When determining the strength characteristics of soils, the most reliable results are obtained by field tests on the cut of soil pillars directly at the construction site (GOST 23741-79). Due to the high cost and labor intensity, these works are carried out only for structures of the I level (class) of responsibility. These include buildings and structures of great economic importance, social facilities and requiring increased reliability (main buildings of thermal power plants, nuclear power plants, television towers, industrial pipes above 200 m, buildings of theaters, circuses, markets, educational institutions, etc.).
For other cases of construction (II and III class of structures), sufficiently reliable indicators With And φ obtained as a result of laboratory tests of soils in flat-cut devices (GOST 12248-78) and triaxial compression (GOST 26518-85).
Strength characteristics can also be determined by the vane sounding method, the results of which, when designing critical structures, are compared with shear tests to ensure the reliability of the results.
Deformation testing of soils. The compressibility of soils is studied by stamp methods, pressuremeters, dynamic and static sounding.
Stamp method. IN non-rocky soils at the bottom of the pits or at the bottom of boreholes, stamps are installed to which static loads are transferred (GOST 20276-85). Stamp in the hole – it is a steel or reinforced concrete round slab with an area of 5000 cm 2. To create a given pressure under the stamp, jacks or platforms with a load are used (Fig. 49).
The draft of the stamps is measured by deflectometers. In the pit, at the mark of the sole of the stamp and outside it, soil samples are taken for parallel laboratory studies. The stamp is loaded in stages, depending on the type of soil and its condition, withstanding until the deformations stabilize. As a result of the tests, graphs of the dependence of the stamp settlement on pressure and on time are plotted by load steps. After that, the soil deformation modulus is calculated using the formula E , MPa.
Stamp in a boreholee. Soil testing is carried out in a well with a diameter of more than 320 mm and a depth of up to 20 m. A stamp with an area of 600 cm 2 is lowered to the bottom of the well. The load on the stamp is transmitted through the bar, on which the platform with the load is located. The deformation modulus is also determined by the formula.
Pressuremeter studies carried out in clay soils. The pressuremeter is a rubber cylindrical chamber lowered into the well to a predetermined depth and expanded by liquid or gas pressure. Under the created pressures, the radial displacements of the borehole walls are measured, which makes it possible to determine the deformation modulus and the strength characteristics of the soil.
Rice. 49. Determination of soil compressibility with stamps:
a, b - pits; c – borehole; 1 - stamps; 2 - jack;
3 - anchor piles; 4 – platform with cargo; 5 - rod
sounding(or penetration ) is used to study soil thicknesses up to a depth of 15–20 m. The density and strength of soils and their variability in a vertical section are determined by the resistance of penetration into the soil of a metal tip (probe). Probing refers to express methods for determining the mechanical properties of sandy, clay and organogenic soils that do not contain or have few impurities of crushed stone or pebbles. According to the method of immersion of the tip, probing is distinguished dynamic and static . With static probing, the cone is pressed into the ground smoothly, and with dynamic probing, it is driven with a hammer.
Static and dynamic sounding allow:
Divide the thickness of the soil into separate layers;
Determine the depth of rocky and coarse-grained soils;
Establish approximately the density of sands, the consistency of clay soils, determine the modulus of deformation;
Assess the quality of artificially compacted soils in embankments and alluvial formations;
Measure the thickness of organogenic soils in swamps.
On fig. 50 shows a penetration logging station.
Rice. 50. Penetration-logging station:
1 – probe-sensor; 2 - rod; 3 - mast; 4 – hydraulic cylinder; 5 - communication channel; 6 - hardware station; 7 - control panel
Strength testing of soils. The resistance of soils to shear is determined by the limiting values of stresses during destruction. Experiments are carried out in pits, leaving columnar pillars of undisturbed soil, to which compressive and shear forces are applied. For the correct determination of internal friction and specific adhesion, the experiment is carried out on at least three pillars with different compressive forces. The shift is also produced during the rotation of the impeller, which is a four-blade device. It is pressed into the ground and rotated, while measuring the torque, from which the shear resistance is calculated.
Experienced building work. During the construction of objects of the I level of responsibility (class), field studies of soils are of particular importance, therefore, they resort to experimental work.
Experienced piles. At the construction site, an inventory pile is immersed and the nature of its immersion and soil resistance are observed. By applying loads to the pile and measuring precipitation at each step, the bearing capacity of the soil is determined under conditions of natural moisture and during soaking. The test results are compared with calculated data based on laboratory soil tests.
Experienced Foundations. They arrange the foundations of the future building in full size and at the design depth. A load is applied to the foundation as from a future building and observations are made of the compression of the base soil. This is how the actual bearing capacity of the soil and the settlement of the future building are determined.
Experimental buildings. A quantitative assessment of the subsidence properties of loess is given according to the data of laboratory and field tests of soils. In real conditions, under erected buildings in full size, the loess base is saturated with water and observations are made of the nature of the development of the process, the values of subsidence are determined and the condition of the building structures is assessed. Similar experimental work is also carried out when assessing dynamic effects on building structures and foundations.
Processing the results of soil studies. The evaluation of the properties of soil massifs is carried out on the basis of physical and mechanical characteristics as a result of laboratory studies of individual soil samples and field work on the territory of the massif. The characteristics obtained in the laboratory and in the field correspond only to those places where samples were taken and field tests of soils were carried out. In this regard, disparate research results and standard indicators must be generalized, i.e., statistically processed in order to obtain average values and subsequent use in base calculations.
Stationary observations in engineering-geological and hydrogeological studies, they are carried out to assess the development of unfavorable geological processes (karst, landslides, etc.), the regime of groundwater and temperature. Measurements are carried out during the operation of buildings and structures, but they can also be started during the design periods. Duration of work - up to 1 year or more.