Stamp testing of soils. Engineering and geological surveys for construction

During the construction and operation of buildings, the ground is deformed, which can lead to shrinkage of the newly constructed structure. The phenomenon is very unpleasant, because it threatens the appearance of cracks or heeling of the building. So, it will endanger its further exploitation. But it is possible to calculate the shrinkage coefficient by planning the limits of deformation of the base of the structure. For this purpose, stamp tests of soils are carried out.

Essence and features of the method

The definition of the deformation properties of the soil is how it behaves under large static loads. Know them is necessary in order to predict further compaction or subsidence. This involved a whole team of specialists.

The die test method gives very accurate results, therefore it is one of the main for any engineering-geological activities. Especially if it is planned to build an underground structure or high-rise buildings. The accuracy of these tests is regulated by GOST.

The essence of the method consists in the step-by-step loading of rigid stamps installed in the soil. With the help of special devices, the loads on them are tracked and the shrinkage values are determined. Depending on the shrinkage of the die itself, deformation properties of the soil are also calculated.

Testing with anti-sounding punches

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 and have considerable weight. In addition, before starting work, it is necessary to prepare soils for testing. The time factor is also important: to study the nature of shrinkage of the soil, it will take a long time. Add here complex calculation formulas that require special data, for example, Poisson's ratio for soils.

That is why this method is not used at the beginning of engineering and geological research, but at the stage of completion of all works, when a site for construction is chosen and the dimensions of the structure are calculated. It is important to first know the maximum level of soil loading 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 these regulations are set out in the corresponding paragraphs of GOST.

Dimensions of the stamp 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 rock. As a result of research, the press with square stamps on clays and sands had interesting information. Shrinkage did not change under the condition of the standard size of the stamp: with a side from 18 to 30 cm. If its dimensions deviated significantly from these values, more or less, the shrinkage parameter increased.

As a result, Soviet geologists calculated the standard sizes of dies, which for many years were used for testing. These are flat, round and rigid 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 one thousand cm ² , its area is increased to the minimum values by means of special rings. However, small dies are used only in special sectors, mainly in wells with drilling rigs.

Recommended dimensions

Here what stamps are recommended to use, depending on the density and type of soils:

• Clay, sand, loose and coarse-ground 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 coefficient of IL for such soils is more than 0.25.
• Clay and sandy soils use stamps with an area of half the diameter and a diameter of up to 57 cm. The coefficient of IL is less than 0.25.

Deformability characteristics

Determine the required soil criteria can be done with a special tool - pressiometer. It is used in field stamp testing. This equipment allows you to identify many characteristics of soils based on their type.

For the sandy, clay, organic soil or coarse ground, the deformation module is determined. Clay soils with a high probability of subsidence are checked for initial settling pressure and relative deformation.

The formula using Poisson's ratio

Calculation of soil resistance by the results of die tests is determined using a special formula. It uses a special coefficient, for each breed it has its own. It is called the Poisson's ratio and makes the following values for different rocks:

• Rocky ground - 0,15.
• Semicall - 0.25.
• The overwrap is 0.27.
• Sand - 0,3.
• Loam - 0,35.
• Clay - 0.42.

The Poisson coefficient for soils participates in the formula for calculating the strain 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 segment. After all, the behavior of the soil, the compressibility parameters of which must be identified, is highly variable. Therefore, it falls under the theory of linearly deformable bodies.

However, the deformation modulus itself is a constant value, therefore, knowing it, and also the composition and properties of the soil itself, it is not difficult to calculate the shrinkage level of the structure.

In addition to the deformation module, stamp tests of soils allow determining the subsidence of the soil under moistening and under a certain load, to reveal the degree of critical values and to predict the nature of deformation loads during operation.

Stages of shrinkage of soil

In the process of shrinkage of soil under the pressure of the foundation, which is modeled by means of stamps, several stepwise stages are distinguished.

• Sealing. The ground is compressed, which leads to a decrease in the porosity characteristics.
• Shift. The ground is in utmost equilibrium, but with numerous shifts along the edges of the basement.
• Destruction, complete or partial. The lateral walls of the soil begin to break down, which leads to subsequent deformations of the structure. The section of the soil together with the stamp is displaced, and along the edges of it the excavation of soil rocks occurs. If the load on the soil at this stage increases, even slightly, the level of deformations is growing rapidly.

Other researchers distinguish only two stages of shrinkage, without any clear separation. However, modern technologies have gone far ahead, today it is easy to fix even the smallest shifts in the soil structure.

Fixing and processing of received data

Any engineering and geological survey for construction is supplemented with the maintenance of the journal, in which the necessary data are entered. Here, this is the shrinkage value of the stamps under the influence of loads. Data processing, depending on the type of equipment used, can be handled manually or with the help of special devices.

When the test work is completed, the data is processed documentarily. Based on their results, graphs are drawn up, on which the interdependence of pressure and shrinkage of the stamp can be traced.

Nuances of work and precautions

There are certain limitations in 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 course of work, its walls are fixed by means of casing pipes.

Before installing the stamp, the face is carefully cleaned using special equipment. The bottom after this becomes smoother. If a perfectly flat surface can not be reached, a sand pillow 2-5 cm thick is placed on it, depending on the type of soil.

To make the stamp as closely as possible contact with the ground, it is rotated a couple of times around the axis. After installation, carefully check the degree of horizontality, and then carry out the installation of the remaining equipment.

Measurement of the degree of shrinkage is carried out to within a tenth of a millimeter. During the first hour after the beginning of the stamp tests of the soils, the deflections are measured every 15 minutes. At the second hour - every half hour, and then once an hour, until the degree of shrinkage stabilizes.

The attainment of the critical pressure is signaled by the appearance of cracks around the stamp or ground protrusions. This test should be stopped.

Thanks to modern technologies, engineering and geological surveys for construction are carried out on any types of soil, as well as on sites of any complexity. It is permissible to investigate flooded areas and even landslide areas. With careful observance of the technology and following the requirements of the GOST the test will be successful.

It is also possible to conduct stamp tests of soils on railway tracks. And if drilling is difficult or carried out in cramped conditions, specialists use special tools and a motobur.