Static analysis of the ultimate equilibrium of the collapse arch during the excavation of underground workings in rocky and semi-horizontal soils

The article is devoted to the improvement of methods for calculating mountain pressure in rocky and semi-horizontal soils. The need to develop the theory of calculating mountain pressure follows from the critical analysis of the existing normative methodology for determining this value presented here. Two major problems of the normative methodology are noted: the lack of methods for determining the strength coefficient, which directly determines the amount of mountain pressure, and in some cases higher values of the strength coefficient in dispersed soils compared to rocky ones. The latter circumstance contradicts the main idea of the theory of arch formation by M. M. Protodyakonov, on which the normative methodology is based.

Two new methods of mountain pressure calculation are proposed – for rocky and semi-horizontal soils. These methods are also based on the theory of vaulting and contain a static analysis of the force interaction of the collapse vault and the surrounding soil mass in the limiting state. At the same time, a search is performed for such a height of the roof of the building that maximizes the amount of mountain pressure. A distinctive feature of the proposed methods is the use of a soil strength passport to assess the ultimate effort. This made it possible to directly link the height of the collapse arch and the soil strength parameters determined in standard tests, while avoiding very conditional empirical dependencies.

As a result, it was found that with the specified parameters of the soil strength, a collapse column is either formed above the excavation, depending on its width (regardless of the depth of the foundation), or a collapse arch is formed, or the collapse of the soil is impossible. In this regard, it is proposed to introduce the concepts of two critical spans of underground workings. The first critical span separates the case of a completely stable state of the soil above the mine from the situation when a collapse arch is formed above the mine. When the width of the mine exceeds the value of the second critical span, the destruction of the soil occurs according to the scheme of the collapse column.

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