Investigation of physical and mechanical properties of clay soils around deep shafts

Underground mining of coal resources deep below the surface of the earth requires the excavation of deep shafts. The thickness of the Cenozoic in a minefield in Shandong, China, exceeds 700 m, which is one of the mines with the largest thickness of loose layers. The large thickness of the layer has made the excavation of the mineshaft and its subsequent maintenance difficult. In order to understand the state change of deep large thickness geotechnical loose layers after disturbance and the impact on engineering, a large number of clay samples of different depths from the surface to more than 700 m were obtained. The influence of shaft excavation on the physico-mechanical properties of deep clay was studied through systematic tests. The test results indicated that the physical properties of deep clay soil do not change significantly as the soil depth increases and that soil dispersion was large because of the disturbance caused to the clay sample during sampling. The deep clay structure is obvious, and microscopic cracks are generally developed. After loading–unloading compression and water expansion had occurred, the original clay structure displayed significant damage, and the cracks in the sample displayed an uneven expansion. The number and width of the cracks have significantly increased compared with those of the original sample. Thus, soil disturbance due to shaft excavation and soil expansion due to water immersion will lead to the destruction of the original structure of clay, and shaft excavation will have a significant impact on the structure of deep clay.