About Determination of Moisture Changes during Presoaking an Excavation Pit in Loess Strata

Abstract

Introduction. Due to intensified construction in the areas of loess subsiding soils, the issue of forecasting the evolvement of underflooding processes is becoming a relevant objective, since these processes can cause the emergency seepage, uneven groundwater surge, change of the soil stress-strain state  and, as a result, impossibility to operate the buildings or structures. Loess soils are attributed with the distinct anisotropic permeability. Subsidence and water infiltration take place in condition of incomplete water saturation. Emergence of the advanced computer technologies makes it possible to improve the mathematical modeling and develop the mathematical models by means of numerical computation, which reliably reflects the intra-soil processes. The article strives to improve a mathematical model of the moisture transfer problem in nonhomogeneous loess soils with anisotropic permeability, taking into account their structural features. Materials and Methods. The research includes:mechanical and mathematical modeling the infiltration and moisture transfer processes in loess subsiding soils, in which the percolation rate is determined according to Darcy's law; suction pressure, water permeability are the given functions of saturation;analysis of the physical and mechanical and structural properties of loess soils;study of the formulations and solutions of initial value and boundary value problems during presoaking the loess strata; – numerical experiments on the forecast of excavation pit presoaking; – comparing the results with the experimental data.Results. It has been acknowledged that the existing calculation methodologies do not always reliably reflect the process of infiltration and moisture transfer in loess soils. An equation determining the moisture transfer was formulated. A mathematical model of the moisture transfer problem in nonhomogeneous media with anisotropic permeability was proposed, which took into account the structural properties of loess soils. The initial value and boundary value problems were solved by the iterative methods with linearization of the solution over sufficiently small time spans. For practical implementation of the theoretical solution, a flow chart of the program algorithm was developed, which included the calculation of the physical and mechanical properties of finite elements and the computational domain dimensions, as well as partitioning the computational domain into the units and triangular elements, determination of the permeability and diffusion coefficients, formation of the parameters of an equation according to the Krank-Nicholson scheme, solution of the system of equations by the compact elimination method and constructing  a vector of moisture. An algorithm for solving the axisymmetric problem of moisture transfer in condition of incomplete water saturation has been developed, which is characterised by stability of direct integration of the moisture transfer equation. The results of the numerical experiments and field tests on presoaking a circular-shaped excavation pit have been compared. The results of the numerical experiments have been presented in the curves of the volume moisture values in different periods of time. It has been found that the results of the solution are well cohered with the experimental data. Discussion and Conclusion. The results of the theoretical research of the problem of moisture transfer in unsaturated loess soils have justified the formulation and the finite element solution of the problem of moisture transfer in unsaturated media, without taking into account the stress-strain state. Based on the experimental data on test excavation pit presoaking, the calculation methodology was verified, and showed the coherence of calculated and experimental results. The proposed methodology is recommended for calculation of the second group of limit states — the deformations.