During the construction and operation of buildings, the soil is deformed, which can lead to shrinkage of the newly constructed structure. This phenomenon is very unpleasant, because it threatens the appearance of cracks or roll of the building. So, it will endanger its further operation. But you can calculate the coefficient of shrinkage by planning the limits of deformation of the base of the structure. For this, stamp tests of soils are carried out.
The essence and features of the method
Determination of the deformation properties of soil is how it behaves under high static loads. Knowing them is necessary in order to predict further compaction or subsidence. This is a whole team of specialists.
The stamp test method gives very accurate results, therefore it is one of the main ones for any engineering and geological types of work. Especially if you plan to build an underground structure or skyscraper. The accuracy of these tests is regulated by GOST.
The essence of the method is the phased load of hard dies installed in the soil. With the help of special devices, loads on them are monitored and shrinkage indicators are determined. Depending on the shrinkage of the stamp itself, the deformation properties of the soil are also calculated.
Stamping Testing
Unlike the sounding method, stamp tests of soils require more complex equipment and are labor intensive. The necessary instruments and tools, as a rule, are large-sized and have considerable weight. In addition, before starting work, it is necessary to prepare the soils for testing. The time factor is also important: to study the nature of soil shrinkage, it will take a long time. Add complex calculation formulas that require specific data, such as the Poisson's ratio for soils.
That is why this method is used not at the beginning of geological engineering studies, but at the stage of completion of all work, when a place for construction is selected and the dimensions of the structure are calculated. It is important to first find out the maximum level of load on the soil and calculate the geological features of this building site. And also take care of calculating the type of foundation and the depth of its laying. All this regulation is set out in the relevant paragraphs of GOST.
Die dimensions and their effect on deformation
As early as 1930, it was revealed that the size of the stamp used in many respects affects the degree of deformation of a particular soil breed. As a result of the research of the Press with square stamps on clay and sand, interesting information was established. Shrinkage did not change under the condition of a standard stamp size: with a side from 18 to 30 cm. If its dimensions deviated strongly from these values, up or down, the shrinkage parameter increased.
As a result of this, Soviet geologists calculated the standard sizes of dies, which were used for testing for many years. These are flat, round and hard stamps with an area of 2.5, 5 and 10 thousand square centimeters. If it is necessary to use a small stamp with an area of only a thousand cm 2 , its area is increased to minimum values using special rings. However, small dies are used only in special industries, mainly in wells with drilling rigs.
Recommended Dimensions
Here are the stamps recommended to use, depending on the density and type of soil:
- Clay, sand, loose and coarse-grained soils with an average degree of density - stamps with an area of 5 thousand cm 2 and a diameter of up to 80 cm. The IL coefficient in such soils is more than 0.25.
- Clay and sandy soils use stamps with an area of half that and a diameter of up to 57 cm. Their IL coefficient is less than 0.25.
Deformability characteristics
The required soil criteria can be determined using a special tool - a pressiometer. It is used in field stamp tests. This equipment allows you to identify many characteristics of soils based on their type.
For sandy, clay, organic soil or coarse soil, the deformation modulus is determined. Clay soils with a high probability of subsidence are checked for the initial subsidence pressure coefficient and relative deformation.
Formula Using Poisson's Ratio
The calculation of soil resistance according to the results of stamp tests is determined using a special formula. It uses a special coefficient, for each breed it is different. It is called the Poisson's ratio and is the following values for different rocks:
- Rocky soil - 0.15.
- Half-length - 0.25.
- Coarse - 0.27.
- Sand - 0.3.
- Loam - 0.35.
- Clay - 0.42.
The Poisson's ratio for soils is involved in the formula for calculating the deformation modulus. But, regardless of the type and accuracy of the equipment used, the value of this module can only be calculated on a straight line. Indeed, the behavior of the soil, the compressibility indicators of which must be identified, is very inconsistent. Therefore, it falls under the theory of linearly deformable bodies.
However, the deformation modulus itself is a constant, therefore, knowing it, as well as the composition and properties of the soil itself, it will not be difficult to calculate the level of shrinkage of the structure.
In addition to the deformation module, stamp tests of soils allow us to determine the subsidence of the soil during wetting and under a certain load, to identify the degree of critical values and to predict the nature of deformation loads during operation.
Soil shrinkage stages
In the process of shrinkage of the soil under the pressure of the foundation, which is modeled with the help of dies, several stages are distinguished.
- Seal The soil is compressed, which leads to a decrease in the characteristics of porosity.
- Shift. The soil is in extreme equilibrium, but with numerous shifts along the edges of the foundation.
- Destruction, complete or partial. The side walls of the soil begin to collapse, which leads to subsequent deformations of the structure. The soil section together with the stamp is displaced, and along its edges there is an outburst of soil rocks. If the load on the soil at this stage increases, even slightly, the level of deformation is growing rapidly.
Other researchers distinguish only two stages of shrinkage, without any clear separation. However, modern technologies have gone far ahead, today even the smallest shifts in the soil structure can be recorded without difficulty .
Recording and processing of received data
Any engineering and geological surveys for construction are supplemented by a journal in which the necessary data are entered. Here it is the amount of shrinkage of the dies under the influence of loads. Data processing, depending on the type of equipment used, can be carried out manually or using special devices.
When the test work is completed, the data is processed documented. Based on their results, charts are drawn up on which you can trace the interdependence of pressure and shrinkage of the stamp.
Nuances of work and precautions
There are certain restrictions on the size of the plants themselves and the diameter of the wells, regulated by GOST. In addition, it is important to monitor the degree of verticality of the well. In the process of work, its walls are fixed using casing pipes.
Before installing the stamp, the face is thoroughly cleaned using special equipment. The bottom after that becomes smoother. If it is not possible to achieve a perfectly flat surface, a sand cushion 2-5 cm thick is placed on it, depending on the type of soil.
To stamp as tightly as possible in contact with the ground, it is rotated a couple of times around the axis. After installation, carefully check the degree of horizontalness, and then carry out the installation of the rest of the equipment.
Measurement of the degree of shrinkage is carried out with an accuracy of a tenth of a millimeter. During the first hour after the start of stamp tests of soils, measurements are made every 15 minutes for deflections. In the second hour - every half hour, and then once an hour, until the degree of shrinkage is stabilized.
The achievement of critical pressure is signaled by the appearance of cracks around the stamp or bulging of the soil. On this test should be stopped.
Thanks to modern technologies, engineering and geological surveys for construction are carried out on any type of soil, as well as on sites of any complexity. It is permissible to investigate flooded territories and even landslide hazardous areas. With careful adherence to the technology and following the requirements of GOST, the test will be successful.
It is also possible to carry out stamp tests of soils on railway tracks. And if drilling is difficult or is carried out in cramped conditions, specialists use special tools and a motor drill.