Criteria For Damage To The Surrounding Rock Of Large Section Underground Cavern Group And Its Application

In this paper,we adopt the combination of geological investigation,numerical simulation and theoretical analysis to reveal the damage characteristics of the excavation of the underground chamber group,and investigate the effectiveness of the overload factor method,the response ratio method and the local strength reduction method in the calculation of the damage criterion of the underground chamber group.To investigate the effectiveness of overload factor method,response ratio method and local strength reduction method in the calculation of damage criterion of underground chamber group.The “danger zone” penetration damage criterion based on the local strength reduction method is proposed and applied to the analysis of an underground water-sealed gas reservoir cavern group.The main results are as follows.(1)The engineering geological survey,ground stress measurement and indoor test are carried out in the area of the reservoir site to obtain the engineering geological parameters and ground stress field in the reservoir area.The rock quality of the reservoir area is mainly Ⅲ+Ⅳ,and the rock quality of the surrounding rock of a few main caverns is Ⅳ.The main structural surfaces in the dense zone of joints are mainly at a slow dip angle,and the orientation is mainly concentrated in three directions: NE70°,NW60° and near east-west.Chlorite weak alteration is prevalent on the jointed surfaces.The rock alteration and fragmentation are obvious and the integrity is poor.The maximum horizontal principal stress is 10.42 MPa,the minimum horizontal principal stress is 3.46 MPa,the dominant principal stress direction is NW25°～40°,and the main cavern axis direction is designed to be NW30°.(2)The numerical analysis model of underground cavern group is established,and the shear strain energy calculation formula is embedded into 3DEC software through FISH language programming to analyze the weak location and damage mechanism of the cavern group.The energy release of the arch waist and bottom of the cavern group,the energy accumulation of the roof and foot of the cavern,the largest energy release rate of the internal arch waist,presenting the main tensile stress,and the stress-controlled damage is the main;the NW60° joint group,the F6 fault and the axial(NW30°)angle of the main cavern are all less than 45°,and the structural surface-controlled damage is the main.The overload factor method and theoretical equations of additive and unloadability response ratio are embedded into 3DEC software to solve the stability safety factor of the cavern group,and it is found that the equivalent plastic strain zone under the critical instability load has been penetrated,the deformation is large,the vertical load is too large while the horizontal structural stress remains unchanged,and the lateral pressure coefficient is changed,which is not consistent with the actual ground stress environment.(3)The area with Yield Approach Index(YAI)is less than 0.1 is defined as the“danger zone”,and the YAI criterion is embedded in the 3DEC program through FISH language programming and combined with the local strength reduction method.Then,we propose to use the penetration of the “danger zone” of the surrounding rock between caverns as the quantitative criterion for the overall damage of the cavern group,and the corresponding discount factor when the “danger zone” is penetrated is defined as the safety factor for the damage of the cavern group to quantitatively evaluate the overall stability of the cavern group.The results show that the order of safety coefficients for each damage criterion is as follows: equivalent plastic strain penetration > plastic zone penetration > sudden increase of displacement at critical point > penetration of“danger zone”.The errors of the safety coefficients of the first three criteria are 33.7%,13.9% and 2.6%,respectively;the calculation results of the safety coefficients based on the “danger zone” penetration are closer to those of the displacement penetration criteria.The damage zone based on “danger zone” penetration is consistent with the damage characteristics of the actual surrounding rock.