Influence of Change of Compacted Sand Structure on the Design Strength of Subsoil

The values of deformation modulus and strength properties of subsoil increases during compaction. The increase depends on the content, grading of the soil, mechanical influence on the soil and other properties. The influenced space with changed soil properties may be formed by tamping the pits for the foundation. It is very important by using physical and mechanical properties of natural soil to forecast the density of compacted soil. For this purpose we have to determine the maximum density value of dry soil and calculate the void ratio of the compacted soil. Change of sand soil density is low. Thus its mean value is commonly used in engineering calculations. The tip resistance CPT of sand can be calculated according to correlation between tip resistance CPT and void ratio (7), (8). When tip resistance CPT is obtained, it is possible to calculate the bearing capacity of the compacted subsoil and to design the foundation. The assumption is made, that the design bearing capacity of foundation installed in the tampered pits is equal to the stresses in the subsoil when the settlement of the foundation is equal to 1…3% of the foundation diameter. According to the tests, the correlation between tip resistance CPT and the bearing capacity of subsoil was made. It should be pointed out that the design bearing capacity of the foundations in the tampered pits are different depending on the shape of foundation. Design bearing capacity of the pyramidal foundations according to the tests results: R sn = 0,04nq c . Design bearing capacity of the cylindrical foundations according to the tests results: R sn = 0,16nq c . It is obvious that bearing capacity of the cylindrical shape foundation installed in tampered pits of sandy soil is bigger than the pyramidal one. The reason is that the main part of the bearing capacity of pyramidal shape foundation is realised by the foundation side bearing capacity. The foundation side bearing capacity of the cylindrical shape foundation is smaller. Design bearing capacity of the foundation in tampered pits may be calculated according to the equations (15), (16), (18). The value of the bearing capacity of the sandy subsoil may be increased up to five times by tampering the pits.

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