When designing the foundations of buildings and structures , many factors must be taken into account. Particular attention should be paid to the composition and structure of the soil. Some of its species are capable of sagging with increasing humidity in tension under its own mass or from an external load. Hence the name of such soils is “subsidence ”. Let us further consider their features.
Kinds
This category includes :
- Loess soils (suspensions and loesses).
- Clays and loams.
- Some types of coating suspensions and loams.
- Bulk industrial waste. These, in particular, include ash, grate dust.
- Dusty clay soils with high structural strength.
Specificity
At the initial stage of the organization of construction, it is necessary to conduct a study of the soil composition of the site to identify possible deformations. Their occurrence is determined by the features of the process of soil formation. The layers are not densely packed. In loess soil, such a state can persist throughout its entire existence.
An increase in load and humidity causes, as a rule, additional compaction in the lower layers. However, since the deformation will depend on the strength of the external influence, insufficient compaction of the thickness with respect to external pressure exceeding the stress from its own mass will remain.
The possibility of fixing soft soils is determined in laboratory tests by the ratio of the decrease in strength during wetting to the indicator of the current pressure.
The properties
In addition to under compaction, subsidence soils are characterized by low natural humidity, dusty composition, and high structural strength.
Soil saturation with water in the southern regions, as a rule, is 0.04-0.12. In areas of Siberia, the middle band indicator is in the range of 0.12-0.20. The degree of humidity in the first case is 0.1-0.3, in the second 0.3-0.6.
Structural strength
It is mainly determined by cementitious adhesion. The more moisture enters the ground, the lower the strength.
The research results showed that thin water films have a proppant effect on the layers. They act as a lubricant, facilitate the sliding of subsiding soil particles. Films provide a more dense laying of layers under external influence.
The adhesion of moisture-saturated subsidence soil is determined by the influence of molecular attraction. This value depends on the degree of density and composition of the earth.
Process characteristic
Drawdown is a complex physicochemical process. It manifests itself in the form of compaction of the soil due to displacement and more dense (compact) laying of particles and aggregates. Due to this, the overall porosity of the layers is reduced to a state corresponding to the level of effective pressure.
An increase in density leads to some change in individual characteristics. Subsequently, under pressure, the seal continues, respectively, and the strength continues to increase.
Conditions
For a drawdown to occur:
- The load from the foundation or its own mass, which when wet will overcome the forces of adhesion of the particles.
- Adequate humidity level. It contributes to a decrease in strength.
These factors must work together.
Humidity determines the duration of deformation of subsidence soils . As a rule, it occurs within a relatively short time. This is due to the location of the land mainly in a low-moisture state.
Deformation in a water-saturated state lasts longer, since water is filtered through the thickness of the soil.
Methods for determining soil density
Relative subsidence is determined by samples of undisturbed structure. For this, a compression device is used - a densitometer for soil . In the study, the following methods are used:
- One curve with the analysis of one sample and its soaking at the final stage of the current load. Using this method, it is possible to determine the compressibility of the soil at a given or natural humidity, as well as the relative tendency to deformation at a certain pressure.
- Two curves with the test of 2 samples with an equal degree of density. One is investigated at natural humidity, the second - in a saturated state. This method allows you to determine the compressibility at full and natural moisture, the relative tendency to deformation when the load changes from zero to final.
- Combined. This method is a modified combination of the previous two. The test is carried out on one sample. It is first examined in its natural state to a pressure of 0.1 MPa. Using the combined method allows you to analyze the same properties as the method of 2 curves.
Important points
When testing in soil densitometers when using any of the above options, it is necessary to take into account that the results of the studies differ significantly. In this regard, some indicators, even when testing a single sample, may differ by 1.5-3, and in some cases by 5 times.
Such significant fluctuations are associated with a small sample size, material heterogeneity due to carbonate and other inclusions, or the presence of large pores. Unavoidable errors in the study are also important for the results.
Influence factors
In the course of numerous studies, it was found that the indicator of the tendency of soil to subsidence depends mainly on:
- Pressure.
- Degrees of soil density with natural moisture.
- The composition of subsidence soil .
- Humidity increase level.
The dependence on the load is reflected in the curve, according to which, with an increase in the indicator, the value of the relative propensity to change first reaches its maximum value. With a subsequent increase in pressure, it begins to approach zero.
As a rule, for loesslike loams, loesses, loams, the pressure is 0.2-0.5 MPa, and for loesslike clays it is 0.4-0.6 MPa.
The dependence is caused by the fact that in the process of loading subsidence soil at natural saturation at a certain level, the destruction of the structure begins. At the same time, sharp compression is observed without changing water saturation. Deformation along the pressure amplification will continue until the layer reaches its extremely dense state.
Soil composition
It is expressed in the fact that with an increase in the number of plasticity, the index of relative tendency to deformation decreases. Simply put, a greater degree of variability of the structure is characteristic for suspensions, a smaller one for clay. Naturally, to fulfill this rule, other conditions must be equal.
Initial pressure
When designing the foundations of buildings and structures , the calculation of the load of structures on the ground is carried out. In this case, the initial (minimum) pressure at which deformation begins when completely saturated with water is determined. 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 intense compaction.
Given the above, it seems that at the design stage during the organization of construction, the initial pressure should be taken close to zero. However, in practice this is not so. The specified parameter should be used such that the thickness is considered to be non-subsiding according to the general rules.
The purpose of the indicator
The initial pressure is used in the development of draft foundations on subsidence soils to determine:
- Estimated load at which there will be no changes.
- The size of the zone within whose compaction will take place from the foundation mass.
- The required depth of soil deformation or the thickness of the soil cushion, completely eliminating deformation.
- Depths from which changes from soil mass begin.
Initial humidity
It is called the indicator at which soils in tension begin to sag. For a normal value in determining the initial humidity, a relative value of 0.01 is taken.
The method for determining the parameter is based on compression laboratory tests. For research, 4-6 samples are needed. The two-curve method is used.
One sample is tested at natural humidity with loading to maximum pressure in separate steps. With it, the soil is soaked until the subsidence stabilizes.
The second sample is first saturated with water, and then, with continuous soaking, it is loaded to the ultimate pressure in the same steps.
Humidification of the remaining samples is carried out to indicators that divide the moisture limit from initial to full water saturation at relatively equal intervals. Then they are examined in compression devices.
The increase is achieved by pouring the calculated volume of water into the samples with further aging for 1-3 days until the saturation level is stabilized.
Deformation characteristics
They include compressibility and variability coefficients, deformation modulus, and relative compression.
The deformation modulus is used to calculate the probable parameters of the foundation sediments and their unevenness. As a rule, it is determined in the field. For this, soil samples are tested with static loads. The value of the deformation modulus is affected by humidity, density, structural cohesion and soil strength.
With increasing soil mass, this indicator increases, with greater saturation with water decreases.
Compressibility variability coefficient
It is defined as the ratio of compressibility at steady or natural humidity and soil characteristics in a water-saturated state.
A comparison of the coefficients obtained from field and laboratory studies shows that the difference between them is insignificant. It is in the range of 0.65-2 times. Therefore, for practical use it is enough to determine the indicators in the laboratory.
The coefficient of variability depends mainly on pressure, humidity, its level of increase. With increasing pressure, the indicator increases, with an increase in natural humidity - decreases. When fully saturated with water, the coefficient approaches 1.
Strength characteristics
They are the angle of internal friction and specific adhesion. They depend on structural strength, water saturation, and (to a lesser extent) density. With increasing humidity, the adhesion decreases by 2-10 times, and the angle - by 1.05-1.2. With an increase in structural strength, adhesion increases.
Types of subsidence soils
There are 2 of them:
- The subsidence occurs mainly within the deformable zone of the base under the influence of the load of the foundation or other external factor. Moreover, the deformation from its weight is almost absent or not more than 5 cm.
- Possible subsidence of the soil from its mass. It occurs mainly in the lower layer of the stratum and exceeds 5 cm. Under the influence of an external load, subsidence may occur in the upper part within the boundaries of the deformable zone.
The type of drawdown is used in assessing construction conditions, developing anti-subsidence measures, designing foundations, foundations, and the building itself.
Additional Information
Drawdown may occur at any stage of the construction or operation of the structure. It can occur after increasing the initial subsidence humidity.
During emergency soaking, the soil sags within the boundaries of the deformable zone rather quickly - within 1-5 cm / day. After the termination of the flow of moisture after a few days, the drawdown stabilizes.
If the primary soaking took place within the boundaries of a part of the deformation zone, with each subsequent water saturation, subsidence will occur until the entire zone is completely moistened. Accordingly, it will increase with increasing load on the soil.
With intensive and continuous soaking, the subsidence of the soil depends on the downward movement of the moisture layer and the formation of a 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.