Classification of sandy and clay soils. Types of soils and their characteristics Characteristics of the state of silty clay soils

Dust-clay soils, depending on the amount of water they contain, can have a consistency (dough density) from solid to fluid. To determine the consistency, the characteristic moisture content of silty clay soils is found, which are called the rolling boundary and the yield boundary.

The rolling boundary is the moisture content of the soil, at which it loses its ability to roll into a cord with a diameter of 2..3 mm.

The yield point is the soil moisture at which the standard cone is immersed in the sample to a depth of 10 mm.

Rice. 1.4. Determining the border of soil rolling

The plasticity number of the soil is the difference between the yield boundary and the rolling boundary:

(1.18)

The consistency of silt-clay soil is estimated by the fluidity index:

(1.19)

Table 1.5. Condition of clays and loams

For sandy loam, due to the low accuracy of determining the values ​​and, only three states are distinguished: solid, plastic, and fluid.

Table 1.6. Sandy loam condition

In the group of silt-clay soils, loess soils and silts are distinguished - they have specific unfavorable properties.

Loess soils contain more than 50% of silt particles with the presence of salts, mainly calcium carbonate, have a predominantly macroporous structure and belong to the category of structurally unstable subsidence soils. Subsidence is a rapidly developing settlement caused by a sharp change in the structure of the soil. Significant precipitation in violation of the structure of subsiding soils is due to the fact that under natural conditions they are undercompacted. In the process of their formation, there is no complete compaction due to the action of its own weight due to the formation of new structural bonds. Such soils become macroporous and, under certain external influences (soaking, vibration), which destroy the bonds that have arisen, they can be additionally compacted, which causes significant precipitation. The possibility of manifestation of subsidence properties of soils is preliminarily assessed by the degree of their moisture content and the subsidence index, which is determined by the formula:

where: e - coefficient of porosity of natural soil; - coefficient of porosity corresponding to moisture content at the yield point (1.16).

Clay soils are one of the most common types of rocks. The composition of clay soils includes very fine clay particles, the size of which is less than 0.01 mm, and sand particles. Clay particles are in the form of plates or flakes. Clay soils have a large number of pores. The ratio of pore volume to soil volume is called porosity and can range from 0.5 to 1.1. Porosity characterizes the degree of soil compaction. Clay soil very well absorbs and retains water, which, when frozen, turns into ice and increases in volume, increasing the volume of the entire soil. This phenomenon is called heaving. The more clay particles are contained in soils, the more they are prone to heaving.

Clay soils have the property of cohesion, which is expressed in the ability of the soil to maintain its shape due to the presence of clay particles. Depending on the content of clay particles, soils are classified into clay, loam and sandy loam.

The ability of soil to deform under the action of external loads without rupture and retain its shape after the load is stopped is called plasticity.

The plasticity number Ip is the moisture difference corresponding to two states of the soil: at the yield boundary WL and at the rolling boundary Wp, WL and Wp are determined according to GOST 5180.

Table 1. Classification of clay soils according to the content of clay particles.

The plasticity number of clay soils determines their building properties: density, moisture, compressive strength. With decreasing humidity, the density increases and the compressive strength increases. With increasing humidity, the density decreases and the compressive strength also decreases.

Sandy loam.

Sandy loam contains no more than 10% of clay particles, the rest of this soil is sand particles. Sandy loam practically does not differ from sand. Sandy loam is of two types: heavy and light. Heavy sandy loam contains from 6 to 10% of clay particles, in light sandy loam the content of clay particles is from 3 to 6%. Lumps of sandy loam in a dry state easily crumble and crumble on impact. Sandy loam almost does not roll into a tourniquet. A ball rolled from moistened soil crumbles under light pressure.

Due to the high sand content, sandy loam has a relatively low porosity of 0.5 to 0.7 (porosity is the ratio of pore volume to soil volume), so it can contain less moisture and therefore be less prone to heaving. The lower the porosity of dry sandy loam, the greater its bearing capacity: with a porosity of 0.5 it is 3 kg / cm 2, with a porosity of 0.7 - 2.5 kg / cm 2. The bearing capacity of sandy loam does not depend on moisture, so this soil can be considered non-rocky.

Loam.

The soil, in which the content of clay particles reaches 30% by weight, is called loam. In loam, as in sandy loam, the content of sand particles is greater than clay particles. Loam has more cohesion than sandy loam and can be preserved in large pieces without breaking up into small ones. Loams are heavy (20% -30% clay particles) and light (10% - 20% clay particles).

Pieces of soil in a dry state are less hard than clay. On impact, they break into small pieces. When wet, they have little plasticity. When grinding, sand particles are felt, lumps are crushed more easily, there are larger grains of sand against the background of finer sand. A tourniquet rolled out of damp soil turns out to be short. A ball rolled from moistened soil, when pressed, forms a cake with cracks along the edges.

The porosity of loam is higher than sandy loam and ranges from 0.5 to 1. Loam can contain more water and, therefore, is more prone to heaving than sandy loam.

Loams are characterized by sufficiently high strength, although they are prone to slight subsidence and cracking. The bearing capacity of loam is 3 kg / cm 2, in moistened - 2.5 kg / cm 2. Loams in a dry state are non-rocky soils. When moistened, clay particles absorb water, which turns into ice in winter, increasing in volume, which leads to heaving of the soil.

Clay.

Clay contains more than 30% clay particles. Clay has a lot of cohesion. Clay in a dry state is hard, in a wet state it is plastic, viscous, sticks to the fingers. When rubbing with fingers, sand particles are not felt, it is very difficult to crush lumps. If a piece of raw clay is cut with a knife, then the cut has a smooth surface on which grains of sand are not visible. When squeezing a ball rolled from raw clay, a cake is obtained, the edges of which do not have cracks.

The porosity of clay can reach 1.1; it is more prone to frost heaving than all other soils. Clay in a dry state has a bearing capacity of 6 kg / cm 2. Clay saturated with water in winter can increase in volume by 15%, losing a bearing capacity of up to 3 kg / cm 2. When saturated with water, clay can change from a solid to a fluid state.

Table 2 shows the methods by which you can visually determine the type and characteristics of clay soils.

Table 2. Determination of the mechanical composition of clay soils.

Classification of clay soils.

Most clay soils in natural conditions, depending on the water content in them, can be in a different state. The building standard (GOST 25100-95 Classification of soils) defines the classification of clay soils depending on their density and moisture content. The state of clay soils is characterized by the fluidity index IL - the ratio of the difference in moisture content corresponding to two soil states: natural W and at the rolling boundary Wp to the plasticity number Ip. Table 3 shows the classification of clay soils in terms of fluidity.

Table 3. Classification of clay soils in terms of fluidity.

According to the particle size distribution and plasticity number Ip, clay groups are subdivided according to table 4.

Table 4. Classification of clay soils according to particle size distribution and plasticity number

According to the presence of solid inclusions, clay soils are subdivided according to table 5.

Table 5. The content of solid particles in clay soils .

Clay soils should include:

The soil is peaty;

subsidence soils;

Swelling (heaving) soils.

Peaty soil - sand and clayey soil containing in its composition in a dry sample from 10 to 50% (by weight) of peat.

According to the relative content of organic matter Ir, clay soils and sands are subdivided according to table 6.

Table 6. Classification of clay soils according to the content of organic substances

Swelling soil is a soil that, when soaked with water or another liquid, increases in volume and has a relative swelling strain (under conditions of free swelling) greater than 0.04.

Subsiding soil is a soil that, under the action of an external load and its own weight or only from its own weight, when soaked with water or another liquid, undergoes vertical deformation (settlement) and has a relative deformation of subsidence e sl ³ 0.01.

Depending on the subsidence and its own weight during soaking, subsiding soils are divided into two types:

• type 1 - when the subsidence of the soil from its own weight does not exceed 5 cm;
• type 2 - when the subsidence of the soil from its own weight is more than 5 cm.

According to the relative deformation of subsidence e sl, clay soils are subdivided according to table 7.

Table 7. Relative deformation of subsidence of clay soils.

Heaving soil is a dispersed soil, which, upon transition from a thawed to a frozen state, increases in volume due to the formation of ice crystals and has a relative deformation of frost heaving e fn ³ 0.01. These soils are not suitable for construction and must be removed and replaced with soil with good bearing capacity.

According to the relative deformation of swelling without load e sw, clay soils are subdivided according to table 8.

Table 8. Relative deformation of swelling of clay soils.

]: rocky (soils with rigid bonds) and non-rocky (soils without rigid bonds).

GOST 25100-95 Soils. Classification

In the class of rocky soils, igneous, metamorphic and sedimentary rocks are distinguished, which are subdivided according to strength, softening and solubility in accordance with Table. 1.4. Rocky soils, the strength of which in a water-saturated state is less than 5 MPa (semi-rocky), include clay shales, sandstones with clay cement, siltstones, mudstones, marls, and chalks. With water saturation, the strength of these soils can decrease by 2-3 times. In addition, in the class of rocky soils, artificial soils are also distinguished - fissured rocky and non-rocky soils fixed in their natural occurrence.

TABLE 1.4. CLASSIFICATION OF ROCK SOILS

These soils are subdivided according to the method of fixing (cementing, silicification, bitumization, resinization, firing, etc.) and according to the uniaxial compressive strength after fixing, just like rocky soils (see Table 1.4).

Non-rocky soils are divided into coarse-clastic, sandy, silty-argillaceous, biogenic and soils.

Coarse-clastic soils include non-consolidated soils in which the mass of fragments larger than 2 mm is 50% or more. Sandy soils are soils containing less than 50% of particles larger than 2 mm and not possessing the property of plasticity (plasticity number I p < 1 %).

TABLE 1.5. CLASSIFICATION OF LARGE-CLASTIC AND SANDY SOILS ACCORDING TO GRANULOMETRIC COMPOSITION

Coarse-clastic and sandy soils are classified according to their granulometric composition (Table 1.5) and the degree of moisture (Table 1.6).

TABLE 1.6. DIVISION OF LARGE CLASSIC AND SANDY SOILS BY THE DEGREE OF HUMIDITY S r

The properties of coarse-grained soil with a sand aggregate content of more than 40% and a silt-clay aggregate of more than 30% are determined by the properties of the aggregate and can be established by testing the aggregate. With a lower aggregate content, the properties of coarse soil are determined by testing the soil as a whole. When determining the properties of sand filler, the following characteristics are taken into account - humidity, density, porosity coefficient, and dusty-clay filler - additionally the plasticity number and consistency.

The main indicator of sandy soils, which determines their strength and deformation properties, is the bulk density. According to the density of addition, sands are subdivided according to the porosity coefficient e, soil resistivity during static sounding q with and conditional soil resistance during dynamic sounding q d(Table 1.7).

With a relative content of organic matter of 0.03< I from≤ 0.1 sandy soils are called soils with an admixture of organic matter. According to the degree of salinity, coarse-grained and sandy soils are divided into non-saline and saline. Coarse clastic soils are saline if the total content of easily and medium soluble salts (% of the mass of absolutely dry soil) is equal to or more than:

• - 2% - when the content of sand filler is less than 40% or dusty-clay filler is less than 30%;
• - 0.5% - with a sand aggregate content of 40% or more;
• - 5% - with a content of silt-clay filler of 30% or more.

Sandy soils are classified as saline if the total content of these salts is 0.5% or more.

Dusty clay soils are subdivided according to the number of plasticity Ip(Table 1.8) and according to the consistency, characterized by the fluidity index I L(Table 1.9).

TABLE 1.7. DIVISION OF SANDY SOILS BY BODY DENSITY

TABLE 1.8. DIVISION OF SILTY-CLAY SOILS BY THE NUMBER OF PLASTICITY

Among the silty-clay soils, it is necessary to distinguish loess soils and silts. Loess soils are macroporous soils containing calcium carbonates and capable of sagging under load when soaked in water, easy to soak and erode. Silt is a water-saturated modern sediment of water bodies, formed as a result of microbiological processes, having a moisture content exceeding the moisture content at the yield line, and a porosity coefficient, the values ​​of which are given in Table. 1.10.

TABLE 1.9. DIVISION OF SILTY-CLAY SOILS ACCORDING TO THE FLOW INDICATOR

TABLE 1.10. SILT DIVISION BY POROSITY COEFFICIENT

Silty clay soils (sandy loam, loam and clay) are called soils with an admixture of organic substances with a relative content of these substances of 0.05< I from≤ 0.1. According to the degree of salinity, sandy loam, loam and clay are divided into uninhabited and saline. Saline soils include soils in which the total content of easily and moderately soluble salts is 5% or more.

Among silty clay soils, it is necessary to single out soils that exhibit specific unfavorable properties during soaking: subsidence and swelling. Subsiding soils include soils that, under the action of an external load or their own weight, when soaked with water, give a sediment (subsidence), and at the same time, the relative subsidence εsl≥ 0.01. Swelling soils include soils that, when soaked with water or chemical solutions, increase in volume, and at the same time, relative swelling without load ε sw ≥ 0,04.

In a special group in non-rocky soils, soils are distinguished that are characterized by a significant content of organic matter: biogenic (lake, marsh, alluvial-marsh). The composition of these soils includes peaty soils, peat and sapropels. Peaty soils include sandy and silty clay soils containing 10–50% (by weight) of organic matter in their composition. When the content of organic matter is 50% or more, the soil is called peat. Sapropels (Table 1.11) are freshwater silts containing more than 10% organic matter and having a porosity coefficient, as a rule, more than 3, and a flow index more than 1.

TABLE 1.11. DIVISION OF SAPROELS BY RELATIVE CONTENT OF ORGANIC MATTER

Soils are natural formations that make up the surface layer of the earth's crust and are fertile. Soils are subdivided according to their granulometric composition in the same way as coarse and sandy soils, and according to the number of plasticity, like silty clay soils.

Non-rocky artificial soils include soils compacted in their natural occurrence by various methods (tamping, rolling, vibration compaction, explosions, drainage, etc.), bulk and alluvial. These soils are subdivided according to the composition and characteristics of the state in the same way as natural non-rock soils.

Rocky and non-rocky soils that have a negative temperature and contain ice in their composition are classified as frozen soils, and if they have been in a frozen state for 3 years or more, then they are permafrost.

Soil moisture is determined by drying a soil sample at a temperature of 105°C to constant weight. The ratio of the difference in the masses of the sample before and after drying to the mass of absolutely dry soil gives the value of moisture, expressed as a percentage or fractions of a unit. The proportion of filling the pores of the soil with water - the degree of humidity S r calculated by the formula (see table. 1.3). The moisture content of sandy soils (with the exception of dusty ones) varies within small limits and practically does not affect the strength and deformation properties of these soils.

The plasticity characteristics of silty clay soils are the moisture content at the yield boundaries wL and rolling wp, determined in the laboratory, as well as the number of plasticity Ip and flow rate I L calculated by formulas (see Table 1.3). Characteristics wL, wp and I p are indirect indicators of the composition (granulometric and mineralogical) of silty clay soils. High values ​​of these characteristics are characteristic of soils with a high content of clay particles, as well as soils, the mineralogical composition of which includes montmorillonite.

1.3. SOIL CLASSIFICATION

The soils of the foundations of buildings and structures are divided into two classes: rocky (soils with rigid bonds) and non-rocky (soils without rigid bonds).

Non-rocky soils are divided into coarse-clastic, sandy, silty-argillaceous, biogenic and soils.

Coarse-clastic soils include non-consolidated soils in which the mass of fragments larger than 2 mm is 50% or more. Sandy - these are soils containing less than 50% of particles larger than 2 mm and not possessing the property of plasticity (plasticity number I p < 1 %).TABLE 1.5. CLASSIFICATION OF LARGE-CLASTIC AND SANDY SOILS ACCORDING TO GRANULOMETRIC COMPOSITION

Coarse-clastic and sandy soils are classified according to their granulometric composition (Table 1.5) and the degree of moisture (Table 1.6).

TABLE 1.6. DIVISION OF LARGE CLASSIC AND SANDY SOILS BY THE DEGREE OF HUMIDITY S r

The properties of coarse-grained soil with a sand aggregate content of more than 40% and a silt-clay aggregate of more than 30% are determined by the properties of the aggregate and can be established by testing the aggregate. With a lower aggregate content, the properties of coarse soil are determined by testing the soil as a whole. When determining the properties of a sand aggregate, the following characteristics are taken into account - humidity, density, porosity coefficient, and dusty-clay aggregate - additionally the plasticity number and consistency.

The main indicator of sandy soils, which determines their strength and deformation properties, is the bulk density. According to the density of addition, sands are subdivided according to the porosity coefficient e, soil resistivity during static sounding q with and conditional soil resistance during dynamic sounding q d(Table 1.7).

With a relative content of organic matter of 0.03< I from≤ 0.1 sandy soils are called soils with an admixture of organic matter. According to the degree of salinity, coarse-grained and sandy soils are divided into non-saline and saline. Coarse clastic soils are saline if the total content of easily and medium soluble salts (% of the mass of absolutely dry soil) is equal to or more than:

− 2% - when the content of sand filler is less than 40% or dusty clay filler is less than 30%

− 0.5% - with a sand aggregate content of 40% or more;

− 5% - with a content of silt-clay filler of 30% or more.

Sandy soils are classified as saline if the total content of these salts is 0.5% or more.

Dusty clay soils are subdivided according to the number of plasticity Ip(Table 1.8) and according to the consistency, characterized by the fluidity index I L(Table 1.9). TABLE 1.7. DIVISION OF SANDY SOILS BY BODY DENSITY

TABLE 1.8. DIVISION OF SILTY-CLAY SOILS BY THE NUMBER OF PLASTICITY

Among the silty-clay soils, it is necessary to distinguish loess soils and silts. Loess soils are macroporous soils containing calcium carbonates and capable, when soaked with water, to give a subsidence under load, easily soak and erode. Silt is a water-saturated modern sediment of reservoirs, formed as a result of microbiological processes, having a moisture content exceeding the moisture at the yield boundary, and a porosity coefficient, the values ​​​​of which are given in Table. 1.10.

TABLE 1.9. DIVISION OF SILTY-CLAY SOILS ACCORDING TO THE FLOW INDICATOR

TABLE 1.10. SILT DIVISION BY POROSITY COEFFICIENT

Silty clay soils (sandy loam, loam and clay) are called soils with an admixture of organic substances with a relative content of these substances of 0.05< I from≤ 0.1. According to the degree of salinity, sandy loam, loam and clay are divided into uninhabited and saline. Saline soils include soils in which the total content of easily and moderately soluble salts is 5% or more.

Among silty clay soils, it is necessary to single out soils that exhibit specific unfavorable properties during soaking: subsidence and swelling. Subsiding soils include soils that, under the action of an external load or their own weight, when soaked with water, give a sediment (subsidence), and at the same time, the relative subsidence εsl≥ 0.01. Swelling soils include soils that, when soaked with water or chemical solutions, increase in volume, and at the same time, relative swelling without load ε sw ≥ 0,04.

5. Sandy soils consist of particles of grains of quartz and other minerals with a particle size of 0.1 to 2 mm, containing no more than 3% clay and do not have the property of plasticity. Sands are divided according to the grain composition and the size of the prevailing fractions into gravel lines d>2 mm, large d>0.5 mm, medium size d>0.25 mm, small d>0.1 mm and dusty d=0.05 - 0.005 mm.

Soil particles with a particle size of d = 0.05 - 0.005 mm are called dusty . If there are from 15 to 50% of such particles in the sand, then they are classified as dusty . When there are more dusty particles in the soil than sandy particles, the soil is called dusty .

The larger and cleaner the sands, the greater the load the base layer from it can withstand. The compressibility of dense sand is low, but the rate of compaction under load is significant, so the settlement of structures on such foundations quickly stops. Sands do not have the property of plasticity.

gravelly, large and medium size sands are significantly compacted under load, slightly freeze.

The type of coarse-grained and sandy soils is determined by the granulometric composition, the variety - by the degree of moisture.

clayey - cohesive soils, consisting of particles with a particle size of less than 0.005 mm, which are mainly scaly in shape, with a small admixture of fine sand particles. Unlike sands, clays have thin capillaries and a large specific surface area of ​​contact between particles. Since the pores of clay soils are in most cases filled with water, when the clay freezes, it swells.

Clay soils are divided depending on the plasticity number into clay (with a content of clay particles over 30%), loams (10...30%) and sandy loam (Z...10%).

The bearing capacity of clay bases depends on humidity, which determines the consistency of clay soils. Dry clay can withstand a fairly large load.

The type of clay soil depends on the plasticity number, the variety depends on the fluidity index.

Classification of soils by particle size.

6. According to the size of the mineral particles of the soil, their mutual connection and mechanical strength, the soils are divided into five classes: rocky, semi-rocky, coarse-grained, sandy (non-cohesive) and clay (cohesive).

To rocky ground include cemented waterproof and practically incompressible rocks (granites, sandstones, limestones, etc.), which usually occur in the form of continuous or fractured massifs.

To semi-rocky soils include cemented rocks capable of compaction (marls, siltstones, mudstones, etc.) and non-water resistant (gypsum, gypsum-bearing conglomerates).

Coarse clastic soils consist of unconsolidated pieces of rock and semi-rock; usually contain more than 50% of rock fragments larger than 2 mm.

sandy soils consist of unconsolidated rock particles with a size of 0.05 ... 2 mm; are, as a rule, naturally destroyed and transformed to varying degrees of rocky soils; do not have plasticity.

Clay soils are also a product of natural destruction and transformation of primary rocks that make up rocky soils, but with a predominant particle size of less than 0.005 mm.

Classification of sandy soils according to the degree of moisture.

7. LARGE CLASSIC AND SANDY SOILS ARE SEPARATED BY THE DEGREE OF HUMIDITY.