The experimental diagrams between stress and strain components for soft soils are non-linear. Nonlinear diagrams qualitatively differ for soils of undisturbed and disturbed structures. It is believed that the manifestations of nonlinear properties of soil are associated with micro-destruction of soil structure under compression and, therefore, with changes in its mechanical characteristics under strain. It follows that the modulus of elasticity, Poisson’s ratio, viscosity and other mechanical parameters are the variables in the process of soil strain. Based on this, from the experimental results given in scientific literature, the changes in the modulus of elasticity and plasticity of soil are determined depending on the values of compression strain. In the process of static and dynamic compression of soil it is almost impossible to determine the boundaries of elastic and plastic strains in soft soil. So, the modulus under soil compression is called the strain modulus. From published results of experiments on dynamic and static compression of soil the most informative ones have been selected. Processing the selected compression diagrams of soft soil, the secant moduli of strain for loess soil and clay have been determined. It is established that the moduli of strain of clay and loess soil under static and dynamic strain vary depending on the rate of strain, the state of the structure and the level of compressive load.
The reliability of underground trunk pipelines, in most cases, is estimated based on their strength. When determining the strength of underground pipelines under seismic effects, it is necessary to take into account the forces of pipe-soil longitudinal interaction (the friction force) in calculations. Currently, linear relationships are used to describe the longitudinal interaction forces, where the interaction force is determined depending on the value of the pipe displacement relative to soil. The dependences of the interaction forces on the seismic stress state of soil are not taken into account at all. The paper shows that the interaction forces on the contact surface of an underground pipeline with soil are complex in nature and depend on the wave parameters in soil and in the pipeline itself. Based on the numerical results of solving one-dimensional unsteady-state coupled wave problems for the interacting system “underground pipeline-soil”, the pattern of changes in interaction forces on the pipeline-soil contact surface are determined. It was found that an account for static and dynamic soil pressure normal to the contact surface leads to quantitatively different results than when only static pressure is considered. It is shown that when solving the reliability problems of underground trunk pipelines under seismic effects, first it is necessary to take into account the dynamic stress-strain state of soil in the vicinity of the pipeline.
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