Sedimentary soils: types and characteristics. Method for determination of soil density

When design of foundations for buildings and structures there are many factors to consider. Particular attention should be given to the composition and structure of the soil. Some of its types are capable of sagging when the humidity increases in tension under its own weight or from an external load. Hence the name of such soils - "subsidence"Let us further consider their features.

Kinds

This category includes:

• Loess soils (suspsi and loess).
• Clays and loam.
• Separate types of cover supraspices and loam.
• Bulk production waste. They include, in particular, ashes, grate dust.
• Dusty clay soils with high structural strength.

Specificity

At the initial stage organization of construction It is necessary to conduct a study of the soil composition of the site to identify possible deformations. Their emergence is determined by the features of the processsoil formation. The layers are in an insufficiently compacted state. In loess soil such a state can persist for the duration of its existence.

The increase in load and humidity causes, asrule, additional compaction in the lower layers. However, since the deformation will depend on the strength of the external action, the insufficient compactness of the strata relative to the external pressure exceeding the stresses of its own mass will be preserved.

The possibility of fixing weak soils is determined in laboratory tests by the ratio of the reduction in strength at moistening to the value of the actual pressure.

Properties

In addition to lack of compactness, subsidence soils are characterized by low natural humidity, silty composition, high structural strength.

Saturating the soil with water in the southern regions, likethe rule is 0.04-0.12. In the regions of Siberia, the average band is in the range of 0.12-0.20. The degree of humidity in the first case is 0.1-0.3, in the second - 0.3-0.6.

Structural strength

It is mainly caused by cementation clutch. The more moisture enters the ground, the lower the strength.

The results of the studies showed that thinWater films have a wedging effect on the seams. They act as a lubricant, facilitate the sliding of subsidence ground particles. Films provide a more dense laying of layers under external influence.

Strength of saturated with moisture subsidence ground is determined by the influence of the force of molecular attraction. This value depends on the degree of density and composition of the earth.

Characteristics of the process

Drawdown is a complex physico-chemicalprocess. It manifests itself in the form of soil compaction due to displacement and more compact (compact) packing of particles and aggregates. Due to this, the total porosity of the layers is reduced to a state corresponding to the level of the acting pressure.

An increase in density leads to a certain change in individual characteristics. Subsequently, under the influence of pressure, the seal continues, accordingly, the strength also continues to increase.

Conditions

To create a drawdown, you need:

• Load from the foundation or own mass, which, when wet, will overcome the binding forces of the particles.
• Sufficient level of humidity. It helps to reduce strength.

These factors should work together.

Humidity determines the duration of deformation subsidence ground. As a rule, it occurs in a relatively short time. This is due to the presence of land mainly in a low-salinity condition.

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

Methods for determination of soil density

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

• One curve with the analysis of one sample and itssoaking the final load. With the help of this method, it is possible to determine the compressibility of soil at a given or natural moisture content, as well as the relative propensity to deformation at a certain pressure.
• Two curves with a test of 2 samples with equaldegree of density. One is investigated with natural humidity, the second - in a saturated state. This method makes it possible to determine the compressibility at full and natural moistening, the relative propensity to deformation when the load varies from zero to the final one.
• Combined. This method is a modified combination of the two previous ones. The test is carried out on the same sample. It is first examined in the natural state up to a pressure of 0.1 MPa. Using the combined method allows you to analyze the same properties as the method of the 2 curves.

Important points

During the tests in densitometers for soil when using any of the aboveit is necessary to take into account that the results of the studies differ significantly in their variability. In this regard, some indicators, even when testing one sample may differ in 1.5-3, and in some cases, in 5 times.

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

Factors of influence

In the course of numerous studies it has been established that the index of the propensity of the soil to subsidence depends mainly on:

• Pressure.
• Degrees of soil density with natural hydration.
• Composition subsidence ground.
• Level of humidity increase.

The load dependence is reflected in the curve, bywhich, with an increase in the indicator, the relative propensity to change first also reaches its maximum value. With a subsequent increase in pressure, it begins to approach the zero mark.

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

The dependence is caused by the fact that in the process of loadingsubsidence soil with natural saturation at a certain level begins the destruction of the structure. There is a sharp contraction without changing the water saturation. The deformation along the pressure increase will continue until the layer reaches its extremely dense state.

Dependence on soil composition

It is expressed in the fact that with an increase in the numberplasticity, the index of relative propensity to deformation is reduced. Simply put, a greater degree of structural variability is characteristic of suppositories, a smaller degree for clay. Naturally, to fulfill this rule, other conditions must be equal.

Initial pressure

When design of foundations for buildings and structures calculation of structural loads onpriming. In this case, the initial (minimum) pressure is determined, at which the deformation begins with full saturation with water. It violates the natural structural strength of the soil. This leads to the fact that the process of normal compaction is violated. These changes, in turn, are accompanied by a restructuring of the structure and intensive compaction.

In view of the foregoing, it appears thatDuring the design phase, the initial pressure should be taken close to zero in the organization of construction. However, in practice this is not so. The specified parameter should be used in such a way that the thickness is considered by the general non-shrinking rules.

Assignment of the indicator

Initial pressure is used in the development of projects foundations on subsidence ground for determining:

• The design load at which there will be no change.
• The size of the zone within which there will be a seal from the mass of the foundation.
• The required depth of soil deformation or the thickness of the soil cushion, completely eliminating deformation.
• Depth, from which changes start from the mass of the ground.

Initial humidity

It is called an indicator, in which the ground in a stressed state begins to sink. A relative value of 0.01 is taken as the normal value in determining the initial humidity.

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

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

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

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

The increase is achieved by pouring into the samples the estimated volume of water with further exposure for 1-3 days to stabilize the saturation level.

Deformation characteristics

The compressibility and its variability factors, the deformation modulus, and relative compression act as them.

The strain module is used to calculateprobable parameters of sedimentation of the basement and their unevenness. As a rule, it is determined in the field. To do this, soil samples are subjected to static loads. The value of the strain modulus is affected by humidity, density level, structural connectivity and soil strength.

With an increase in the mass of the soil, this index rises, with a greater saturation with water decreases.

Coefficient of variability of compressibility

It is defined as the ratio of the ability to contract with steady or natural moisture and soil characteristics in the water-saturated state.

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

The coefficient of variability depends mainly onfrom pressure, humidity, the level of its increase. With increasing pressure, the indicator increases, with increasing natural humidity - it decreases. When fully saturated with water, the coefficient approaches 1.

Strength characteristics

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

Types of subsidence soils

There are 2:

1. Drawdown occurs predominantly withindeformable base zone under the influence of the foundation load or other external factor. At the same time, the deformation from its weight is almost absent or not more than 5 cm.
2. It is possible to subsidence the soil from its mass. It occurs predominantly in the lower layer of the thickness and exceeds 5 cm. Under the influence of an external load, subsidence can occur also in the upper part within the boundaries of the deformed zone.

Type of drawdown is used in assessing the conditions of construction, the development of anti-planting activities, the design of the grounds, the foundation, the building itself.

Additional Information

Drawdown can occur at any stage of erection or operation of the structure. It can appear after increasing the initial subsidence humidity.

With emergency soaking, the soil sags within the boundaries of the deformed zone quite quickly - within 1-5 cm / day. After cessation of moisture intake after a few days, the subsidence stabilizes.

If the initial soaking took place within the boundariesparts of the deformation zone, with each subsequent water saturation, subsidence will occur until the whole zone is completely moistened. Accordingly, it will increase with increasing load on the soil.

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