In this paper we consider the special multilevel algorithm for solving systems of linear algebraic equations obtained by the finite element discretization of the vector Helmholtz equation in the 3D domains with internal borders. The algorithm is based on the multilevel Schwarz method and the domain decomposition with an overlap. The results of calculations allow us to estimate the contribution of the each stage of the algorithm to the acceleration of the calculation procedures.

The article presents the results of the field research on distribution of normal stresses and relative deformations in the sand base along the vertical axis in the gap between the extended beam elements of strip foundation model with variable geometric shape of the foundation base. In the field research of Stress-Strain Behavior (SSB) of the subfoundation of strip foundation model based on beam elements, it is found that the height of the soil curved arch occurring between the extended elements of the foundation base, increases during loading and when approaching the ultimate loading the experimental value of the arch height tends to the theoretical value defined by the formula proposed by M.M. Protodyakonov.

Many authors noted in the field research on the models of rigid foundations that a compacted wedge-shaped core of soil is formed with the destruction of the bases, which is a natural extension of the foundation. In the research of the strip foundation model with variable geometric shape of the foundation base, a compaction soil area of spherical shape with a diameter close to the width of the foundation model is found on a sand base. A wedge-shaped core, along the edges of which there has been a shift in mass of the base when achieving the ultimate loading, has been noted in a compaction soil area. The solid part of the foundation base is the compacted wedge-shaped core base, the height of the wedge-shaped core being commensurate with the length of the elements forming the foundation. Identified geometric parameters of compacted core under the foundation base of strip foundation with geometrically variable shape can be used in determining the ultimate bearing capacity of soil of the base for such foundations.

The article presents the results of the field research on depth distribution of normal stresses in the mass of sand base in the transverse alignment of the foundation model with variable geometric shape of the foundation base. Diagrams of the distribution of normal stresses are constructed on the results of the experiments being held, and presence of the arching phenomenon in the gap between the extended elements of the foundation being confirmed.