Study On The Mechanism And Effect Of Separation In Near-horizontal Layered Surrounding Rocks

The excavation of a cavern in a layered rock mass leads to the bending deformation of the strata in the direction of the air,and with the development of this deformation,it causes the opening of the rock layers to produce separation,which in turn accelerates the instability and damage process of the layered surrounding rocks.A near-horizontally layered surrounding rocks in the engineering area of a pumped storage power station was used as the object of study.Based on the principle of dynamic evolution of surrounding rocks and plate and shell theory,methods of geological field investigation,testing,theoretical analysis and numerical calculation are used.The mechanism of separation of horizontally layered surrounding rocks in large caverns is discussed,the conditions of separation are analysed,the progressive damage characteristics of separation are explained,and the spatial and temporal distribution of separation are studied.The main research results are as follows:(1)The deformation of horizontally layered surrounding rocks is mainly caused by the structural deformation of the strata,and the structural bending deformation of the strata is the basis for the deformation of separation,which is essentially a result of the non-synchronous and non-coordinated structural deformation of the adjacent strata.The generation and development of separation is mechanically related only to the horizontal stress and the self-weight of the rock stratum,but not to the vertical rock pressure.In addition,the separation is influenced by the physical and mechanical parameters of the surrounding rocks,the strata thickness of the surrounding rocks and the size of the cavern.Separation is beneficial when there are significant differences in mechanical properties or strata thickness between adjacent stratum.The area of influence of the separation is controlled by the location of the key layer,the closer the key layer is to the cavern,the smaller the area of influence of the separation,and when there is no key layer in the surrounding rocks,the strong impact of separation is half of the span of the cavern.(2)The generation and development of separation has an obvious time effect,with the amount and area of separation increasing with time.In addition,The separation has the typical progressive layer-by-layer deformation and damage characteristics.In terms of separation deformation,the stratum close to the cavern is the first to separate,and then develop layer by layer away from the cavern.In terms of separation damage,the strata collapse layer by layer in the form of "structural bending deformation — separate— tensional fracture due to bending — collapse" and the span of the collapse decreases layer by layer until it equilibrates and finally forms a trapezoidal arch.(3)The separation in the surrounding rocks at the bottom of the cavern has the same spatial distribution characteristics as the separation in the surrounding rocks at the top of the cavern.The separation in the surrounding rocks at the top and bottom of the cavern decreases as the depth of the surrounding rocks increases.The distribution of separation within the same layer is consistent with the deformation law of the top and bottom plate of the cavern,which is larger in the central part of the cavern and smaller in the surrounding area.For large cavern with a high sidewall,the stress difference between the top and bottom of the cavern results in greater values of separation in the bottom surrounding rocks,which are more strongly affected by horizontal stresses.This further confirming that the horizontal stress is an important mechanical factor controlling the generation and development of separation.