Study On The Strength Characteristics Of Layered Rock Masses And The Structural Effects Of Instability In Layered Surrounding Rocks

Layered rock is the most widespread rock masses.The anisotropic nature of layered rocks and the discontinuity caused by structural fractures within the rock formations can easily lead to instability of the surrounding rock in roadways,tunnels and chambers,posing a great challenge to the safety of underground construction.With the development of mining,tunnelling and hydropower projects,the stability of layered rock has become a hot issue in this field.The study takes layered rock masses and layered fractured rock masses as the research objects,and uses a combination of indoor tests,numerical simulations and theoretical analyses to study the anisotropic characteristics of layered rocks in uniaxial,triaxial and direct shear compression tests.Considering the structural effects arising from changes in the structural characteristics of layered rock masses,the expansion and evolution characteristics of equally-spaced fractures under the external loads are studied.Combined with the excavation disturbance tests of the tunneling project,the destabilization mechanism of the layered equally-spaced fractures confining rock is investigated.The main research results obtained are as follows:(1)Using 3D scanning and printing technology,layered specimens of different lithologies containing natural bedding plane were fabricated,which reveals the influence mechanism of lithology on the layered rocks mechanical properties,and establishes a strength model of layered rocks reflecting the characteristics of lithology.It has been shown that layered rock failure is controlled by both bedding plane dip and lithological composition.The peak strength of the layered rock masses generally shows an asymmetrical "U" curve that decreases and then increases at different confining pressures.It is predicted that the non-linear variation of the layered rock strength with increasing confining pressure will gradually change to a linear variation as it continues to increase.(2)Forward and reverse shear tests were carried out on layered rocks with different bedding plane dips to reveal the mechanical properties of the layered rocks under different shear directions.It has been shown that the shear strength,shear expansion properties and damage pattern of layered rock specimens with the same bedding plane dip are closely related to the shear direction.The strength of shear bearing capacity in direct shear of layered rock can be reflected by the shear stress concentration factor.(3)The ratio of fracture spacing to rock thickness(S/T)is used as a parameter of the layered rock structural characteristics,revealing the influence of the layer rock mass structural characteristics on the evolution of fracture extension,and clarifying the structural effect of the crack extension evolution in layered rock masses.The numerical calculation model of complex crack paths in layered rocks was established by CZM,and the validity of the numerical model was verified by using a modified wedge splitting test.The crack penetration or deflection behavior of the bedding plane is controlled by two strength thresholds of bedding plane.The strength thresholds of the bedding plane determine the crack penetration state.Fracture extension in layered rock was found to occur in three damage modes: deflection,direct penetration and coupling of the two.(4)Based on numerical simulation analysis,the instability mechanism of layered fractured rock masses in tunneling projects and structural effects were revealed by considering the S/T ratio(layer-thickness ratio)of layered fractured rock masses.The influence of parameters such as fissure spacing(S),layer thickness(T)and layer thickness ratio(S/T),as well as rock dip angle,on the damage characteristics of the tunnel surrounding rock was explored.The S/T is found to play a fundamental role in the damage and deformation of the rock structure.Under confining pressure,when the S/T is small,a ’triangular’ block-loose zone is formed at the top of the tunnel,and on the outside of the block-loose zone,stresses are applied in the form of stress arches across the tunnel and support the rock outside the block-loose zone.When the S/T exceeds a certain threshold,there is no free rock mass at the top of the tunnel and the damage mode of the surrounding rock changes from rock mass collapse to rock cracking at the top and bottom of the tunnel.