4. Soils. Soil construction properties

The soil is a natural environment in which the underground part of buildings and structures is located. Grounds in construction are called rocks that occur in the upper layers of the earth's crust and are mainly loose and rocky rocks. The following main types of soil are distinguished: sand, sandy loam, loam, clay, loess soil, peat, gravel, vegetable soil, various rocky and compacted soils. The strength and stability of structures erected, production methods, labor intensity and cost of work depend on the construction properties of soils.

When choosing methods of excavation work, the following basic characteristics of soils should be taken into account: density, humidity, stickiness, looseness, adhesion, angle of repose, complexity (complexity) of development. Depending on these characteristics, soils in construction are considered from the point of view of:

■ suitability as bases of various buildings and structures and the size of the load on them;

■ the possibility of their use as permanent structures, that is, as a material for the construction of embankments and excavations;

■ the feasibility or possibility of applying a particular method of excavation.

Sandy soils are dry in bulk, do not have the property of plasticity. They are water permeable, at a certain speed of water flow they blur, the volume of sand changes with the change of humidity. The largest volume is sand in a wet state (the entire space between the particles is filled with water), the smallest volume is sand saturated with water (heavier sand settled on the bottom, water squeezed the air out of the pores and rose to the upper layers itself)
intermediate position is sand in a dry state (the free space between the particles is filled with air).

Clay soils - coherent and having the property of plasticity. Clay absorbs water strongly and at the same time swells greatly. When freezing, water increases in volume up to 9%, due to which clay soils are heavily puffed, when dried, soils, on the contrary, hardly give up moisture, decrease in volume and crack. When wet, the clay is plastic and almost waterproof, with increasing humidity, the adhesion of clay particles decreases, and the clay is easily eroded by running water.

Loam has the properties of clay, sandy loam - sand, but to a much lesser extent. Loesslike soils are especially highlighted in clay soils. In the dry state, the loess has considerable strength and hardness, but when in contact with water it easily absorbs it, it spreads, decreases greatly in volume, drastically loses its carrying capacity, becomes subsiding.

The granulometric composition of the soil. Depending on the average particle size, mm, constituting the soil, they are divided into:

clay - <0.005; dusty - 0,005 .. .0,05; sands is 0.03 ... 3; gravel-3 ... 40; pebble-40-200; stones, boulders -> 200

Sands, in turn, are subdivided into: fine - more than 50% of the volume is made up of particles with a size of 0.1 ... 0.25 mm; middle - the same, particles 0.25 ... 0.5; large - 0.5 ... 3 mm.

An important component of most soils is the presence of clay particles. Soils, depending on the content in their volume of clay particles are divided: sand - <3%; sandy loam-3-10%; loam - 10 ... 30%; sandy clay - 30 ... 60%; heavy clay -> 60%.

Humidity of the soil is characterized by the degree of saturation of the soil with water and is determined by the ratio of the mass of water in the soil to the mass of solid particles of soil. Depending on humidity, the soils are divided into low-moisture (up to 5%), wet (up to 30%), saturated with water (> 30%). Water in the pores of wet and water-saturated soils is called groundwater.

Soil filtration coefficient . The speed of groundwater movement depends on the porosity of the soil; it is different for different soils and rocks and therefore characterizes the water permeability of these soils. The speed of groundwater (m / day) is called the coefficient of soil filtration. The smaller the particle size of the soil, the smaller the pores between these particles, and hence the filtration rate of water between them and vice versa. Filtration factors for different soils, m / day: clay - 0; loam - <0.05; fine sand - 1 ... 5; gravel - 50 ... 150.

The density of the soil is the mass of 1 m3 of soil in its natural state, that is, in a dense body. The performance of construction machines depends on the density and strength of adhesion of soil particles to each other. The density of various types of soil varies significantly. So, the density of silt soils on average is 0.6 t / m3, sandy soils - 1.6 ... 1.7 t / m, rocky soil - 2.6 ... 3.3 t / m3.

The adhesion of the soil is characterized by the initial resistance to shear, it depends on the type of soil and its humidity. So, the adhesion force for sandy soils is 0.03 ... 0.05 MPa, for clayey -0.05 ... 0.3 MPa.

Looseness. During development, the soil is loosened and its volume increases in comparison with the initial one. For this reason, distinguish the amount of soil in the natural and loosened state. The increase in soil volume at loosening is very different for different soils and is called the initial loosening. Over time, this loosened soil under the influence of the load from the overlying layers, under the influence of precipitation or mechanical stress gradually compacted. However, the soil does not occupy the volume that it occupied before the development. The degree of loosening of the soil after its precipitation and compaction is called residual loosening. The magnitude of the initial and residual loosening expressed in% relative to the volume of soil in a dense state. Coefficients that take into account these increments in soil volume are called coefficients of initial and residual loosening (Table 2.1).

Table2.1

Loosening factors for different soils

To speed up the compaction of soils poured into the embankment, artificial compaction by rollers, tamping, vibration is used, and for sandy soils it is more convenient to shed it with water.

Stickiness - the ability of the soil at a certain moisture content to adhere to the surface of various objects. The great adhesion of the soil makes it difficult to unload the soil from the bucket of a car or body, the working conditions of the transport, etc. The stickiness is determined by the effort required to detach the stuck object from the ground (for clay, stickiness reaches 0.05 MPa).

Classification of soils on the difficulty of their development (specific cutting resistance ). The classification is given in ENRP 2-1-1 “Earthworks”. It takes into account the properties of various soils and the design features of the earth-moving and earth-moving machinery, which are used for the development of soils. For single-bucket excavators, soils are divided into 6 groups, for multi-bucket excavators and scrapers - into 2 groups, for bulldozers and graders - into 3 groups.

For the development of soil manually made 7 groups, namely: sand, sandy loam, loam, clay, loess - groups 1 ... 4; coarse soils - group 5; rocky soils - groups 6 and 7.

Soils 1 ... 4 groups are easily developed by manual and mechanized methods, the subsequent groups - soils require preliminary loosening, including by explosive means.

Slope steepness. According to safety conditions, digging of pits and trenches with vertical walls without their fastening is allowed only in soils of natural moisture to a depth not exceeding the following values: in bulk, sandy and gravelly soils - 1 m; in the sandy mountains - 1.25 m; in loams and clays - 1.5 m; in especially dense non-rocky soils - 2.0 m.

It is allowed to dig trenches with a depth of up to 3 m without fastenings in particularly dense non-rocky rocks, provided that they will be developed with the help of mechanisms and without lowering the workers into these trenches.

With a depth of more than this, the excavation and trenches are developed with slopes or with fastening walls.

The permissible slope steepness in soils of natural moisture from the conditions of safe work depends on the depth of the excavation or the height of the embankment and is taken from Table. 2.2.

Table 2.2

Permissible slope steepness

The slope slope depends on the angle of repose, in which the soil is in a state of ultimate equilibrium, the determining factors of which are the angle of internal friction of the soil, the internal cohesion forces and the pressure of the overlying soil layers.