CONSTRUCTION AND GEOTECHNICS

During the construction of multi-storey and high-rise buildings, much attention is paid to the choice of the type of foundation and the features of its design. One of the most common types of foundations in the construction of buildings and structures in difficult soil conditions is pile foundations, which ensure the safe operation of the objects under con-struction. The introduction of an intermediate transfer layer between the pile field and the foundation slab is an effective solution under seismic and other dynamic loads. The studies presented in this article were carried out numerically and analytically (with the development of an analytical method of calculation). The results of the research were implemented in the construction of multi-storey and high-rise buildings in the South of Russia.

The results of physical modeling of interaction between cross-beam foundation and inclined ground (α = 15°, 30°) at formation of karst-rockfall processes are presented. The purpose of the research was to study the influence of the base slope angle, karst cavity diameter and its location on the parameters of the stress-strain state of the cross-beam foundation bounded by a rigid vertical wall in the lower part of the slope in case of sudden collapse of the karst cavity vault. The methodology of research planning, force and measuring equipment is described. According to the obtained results of the experimental studies, the influence of the main factors (the angle of inclination of the foundation, the size of the karst cavity diameter, the location of the karst cavity relative to the retaining structure and the foundation footing) on the changes in settlement, the nature of the distribution of normal contact stresses was analyzed. The parameters of change in the shape and radius of the sinkhole at the boundary of the foundation resting on the base were obtained empirically. The obtained results make it possible to evaluate chang-es in the VAT parameters of the cross-beam foundation operation after the cavity vault collapse and formation of a complex shape of the collapse prism, to determine the criteria of the most unfavorable location of the karst cavity in relation to the cross-beam foundation structures. The analytical formula allowing to determine the approximate value of the design diameter Dk of the cavity is obtained.

The problems of undrained behavior of weak clayey soils of St. Petersburg are considered. The importance of taking into account the mechanism of shear hardening in calculations of excavations in weak clayey soils has been demonstrated. The problem of using undrained shear resistance as a strength parameter in full stress evaluation is shown. The modification of Y.I. Soloviev's theory of instantaneous strength has been presented with account for the formation of excess pore pressures under deviatoric load-ing in the plane formulation. With its help the calculated value of resistance to undrained shear through effective strength parameters for use in numerical modeling complexes is obtained. Numerical realization of the nonlinear model of soil with shear hardening in the software package implementing the finite element method has been performed. This model utilizes the proposed methodology for estimating undrained strength. Comparison of the calculation results of the proposed model and the Mohr-Coulomb model has been performed.

Frost heaving of clay soils can contribute to the development of significant defor-mations and mobilization of tangential forces during freezing of soils in the seasonally thawed layer of permafrost soils or on seasonally freezing soils. The existing characteris-tics for assessing soil frost heaving differ significantly in the methods of their determina-tion according to international and Russian standards. The process of frost heaving is conditioned by the volume of moisture supplied to the freezing front and the rate of soil freezing. The purpose of the study is to identify the classification index determined by the intensity of moisture migration to the freezing front, under closed system conditions. The studied clay soil samples had water content of 21.3 % and 22.8 %, dry density of 1.80 g/cm3 and 1.68 g/cm3, porosity factor of 0.50 and 0.60, respectively. Soil tests were carried out in a freezer on an installation designed by the authors and including 3 cells covered with a layer of thermal insulation and equipped with electronic temperature and displacement sensors which provide automatic recording of readings. The experi-ments were carried out according to three schemes, in two of which waterproof mem-branes were placed in the samples at different depths. Based on the results of the exper-iments it was obtained characteristic curves of deformations of heaving and freezing, as well as moisture redistribution in the samples. According to the value of moisture redistri-bution, the reduced moisture migration index which varies from 0.006 to 0.088 day-1was calculated. The power dependence of the reduced migration index on the freezing rate was obtained. This dependence can be used for numerical modelling of the stress-strain state of freezing freezing soil.