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 when the humidity increases in tension under its own weight or from an external load. Hence the name of such soils - "subsidence". Consider further their features.
Views
The category under consideration includes:
- Loess soils (slurry and loess).
- Clays and loams.
- Separate types of cover slurries and loams.
- Bulk industrial waste. These include, in particular, ash, grate dust.
- Silty clay soils with high structural strength.
Specifics
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 occurrencedue to the peculiarities of the process of soil formation. The layers are in an insufficiently compacted state. In loess soil, such a state can persist throughout the entire time of its existence.
Increase in load and humidity usually causes additional compaction in the lower layers. However, since the deformation will depend on the force of the external influence, the insufficient compaction of the thickness relative to the external pressure exceeding the stress from its own mass will remain.
The ability to fix weak soils is determined in laboratory tests by the ratio of the reduction in strength when wetted to the acting pressure.
Properties
In addition to undercompaction, subsiding soils are characterized by low natural moisture content, dusty composition, and high structural strength.
Soil saturation with water in the southern regions, as a rule, is 0.04-0.12. In the regions of Siberia, the middle zone, the 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 due to cementation adhesion. The more moisture enters the ground, the lower the strength.
Research results have shown that thin water films have a wedging effect on formations. They act as a lubricant, making it easier for particles of subsiding soil to slide. Films provide more dense laying of layers under external influence.
Clutch saturatedthe moisture of the subsiding soil is determined by the influence of the force of molecular attraction. This value depends on the degree of density and composition of the earth.
Process characteristic
Drawdown is a complex physical and chemical process. It manifests itself in the form of soil compaction due to the movement and denser (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 acting pressure.
Increase in density leads to some change in individual characteristics. Subsequently, under the influence of pressure, the compaction continues, respectively, the strength continues to increase.
Conditions
For drawdowns to occur:
- The load from the foundation or its own mass, which, when wet, will overcome the cohesive forces of the particles.
- Sufficient level of humidity. It helps reduce strength.
These factors should work together.
Moisture determines the duration of deformation of subsiding soils. As a rule, it occurs within a relatively short time. This is due to the fact that the land is predominantly in a low-humidity state.
Deformation in a water-saturated state takes longer as water filters through the soil.
Methods for determining soil density
Relative subsidence is determined by samples of undisturbed structure. For this, a compression device is used -soil density meter. The following methods are used in the study:
- Single curve with analysis of one sample and its soaking at the final stage of the current load. With this method, it is possible to determine the compressibility of soil at a given or natural moisture, as well as the relative tendency to deform at a certain pressure.
- Two curves testing 2 samples with the same degree of density. One is investigated at natural humidity, the second - in a saturated state. This method allows you to determine the compressibility under 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 2 curves method.
Important points
During testing in soil density meters using any of the above options, it must be taken into account that the results of the studies vary greatly. In this regard, some indicators, even when testing one sample, may differ by 1, 5-3, and in some cases even 5 times.
Such significant fluctuations are associated with the small size of the samples, the heterogeneity of the material due to carbonate and other inclusions, or the presence of large pores. The inevitableresearch errors.
Influencing factors
In the course of numerous studies, it has been established that the indicator of the soil's tendency to subsidence depends mainly on:
- Pressure.
- Degrees of soil density with natural moisture.
- Composition of subsidence soil.
- Humidity increase level.
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 also reaches its maximum value. With the subsequent increase in pressure, it begins to approach zero.
As a rule, for loess-like sandy loams, loesses, loams, the pressure is 0.2-0.5 MPa, and for loess-like clays - 0.4-0.6 MPa.
Dependence is caused by the fact that in the process of loading subsidence soil with natural saturation at a certain level, destruction of the structure begins. In this case, a sharp compression is noted without a change in water saturation. The deformation in the course of increasing pressure 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 number of plasticity, the indicator of the relative tendency to deformation decreases. Simply put, a greater degree of structure variability is characteristic of slurry, a smaller one - for clay. Naturally, for this rule to be true, other conditions must be equal.
Initial pressure
When designing the foundations of buildings and structuresthe load of structures on the ground is calculated. In this case, the initial (minimum) pressure is determined, at which deformation begins at full saturation with water. It disrupts the natural structural strength of the soil. This leads to the fact that the normal compaction process is disrupted. These changes, in turn, are accompanied by structural restructuring and intense densification.
Considering the above, it seems that at the design stage when organizing construction, the initial pressure should be taken close to zero. However, in practice this is not the case. The specified parameter should be used such that the thickness is considered non-subsidence according to the general rules.
Indicator assignment
Initial pressure is used when designing foundations on subsiding soils to determine:
- Designed load at which there will be no changes.
- The size of the zone within which compaction will occur from the mass of the foundation.
- The required depth of soil deformation or the thickness of the soil cushion, completely excluding deformation.
- The depth from which changes from the mass of the soil begin.
Initial humidity
It is called the indicator at which soils in a stressed state begin to subside. When determining the initial humidity, a relative value of 0.01 is taken as a normal value.
The method for determining the parameter is based on compression laboratory tests. 4-6 samples are needed for the study. The two method is usedcurves.
One sample is tested at natural humidity with loading up 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, loaded to the ultimate pressure in the same steps.
Moisturizing of the remaining samples is carried out to indicators that divide the moisture limit from initial to full water saturation into 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 holding for 1-3 days until the saturation level stabilizes.
Deformation characteristics
They are the coefficients of compressibility and its variability, modulus of deformation, relative compression.
The modulus of deformation is used to calculate the probable indicators of the settlement of the foundation and their non-uniformity. As a rule, it is determined in the field. For this, soil samples are tested with static loads. The value of the modulus of deformation is affected by moisture content, density level, structural cohesion and soil strength.
With increasing soil mass, this indicator increases, with greater saturation with water it decreases.
Compressibility variability factor
It is defined as the ratio of the compressibility under steady or natural moisture to the characteristics of the soil in a water-saturated state.
Matchingcoefficients obtained in 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 application, it is enough to determine the indicators in the laboratory.
The coefficient of variability depends mainly on pressure, humidity, and the level of its increase. With an increase in pressure, the indicator increases, with an increase in natural humidity, it decreases. When fully saturated with water, the coefficient approaches 1.
Strength characteristics
They are the angle of internal friction and specific cohesion. They depend on structural strength, water saturation level and (to a lesser extent) density. With increasing humidity, the adhesion decreases by 2-10 times, and the angle decreases by 1.05-1.2. As the structural strength increases, the adhesion increases.
Types of subsidence soils
There are 2 in total:
- Sagging occurs mainly within the deformable zone of the base under the action of the foundation load or other external factor. At the same time, the deformation from its weight is almost absent or is no more than 5 cm.
- Possible subsidence of the soil from its mass. It occurs mainly in the lower layer of the thickness and exceeds 5 cm. Under the action of an external load, a subsidence may also occur in the upper part within the boundaries of the deformable zone.
The type of subsidence is used in assessing construction conditions, developing anti-subsidence measures, designing foundations,foundation, the building itself.
Additional information
Sag may occur at any stage of construction or operation of a structure. It can appear after an increase in the initial subsidence moisture.
During emergency soaking, the soil sags within the boundaries of the deformable zone rather quickly - within 1-5 cm/day. After the cessation of moisture supply, after a few days, the drawdown stabilizes.
If the initial 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, soil subsidence depends on the downward movement of the moistening layer and the formation of a water-saturated zone. In this case, subsidence will begin as soon as the moistening front reaches the depth at which the soil sags from its own weight.
