A Comparative Study On Double-layer Support And Yielding Support For Tunneling In Squeezing Rocks

When constructing tunnels with squeezing surrounding rock,the surrounding rock is too weak to bear the stress itself and it will produce extreme large deformation,causing deformation of the supporting structure,cracking of the shotcrete,and distortion of the steel frame,and the support has to be dismantled and replaced,causing schedule delays and economic losses.In order to effectively control this type of large squeezing deformation,some special support methods,such as multiple support,yielding support,etc.,are gradually used in domestic and foreign projects.However,the design method for this kind of special support construction method is still immature,and the related surrounding rock-supporting mechanism needs further study.Based on the severely squeezed and large-deformed section of the Yuelongmen Tunnel of the Cheng-Lan Railway,the elastoplastic theory of the deep-buried circular tunnel considering the strength deterioration of the surrounding rock is firstly analyzed.Then use the ABAQUS finite element analysis software to carry out a three-dimensional numerical model of the three-step belt invert one-time excavation process,respectively study the mechanical mechanism of the double-layer support and the yielding support of the large deformation tunnel.And further,based on the theoretical analysis,numerical simulation and on-site monitoring data,the deformation and force characteristics of double-layer support and yielding support are analyzed and compared.The main findings are as follows:(1)Assuming that the cohesive force of surrounding rock decreases with the increase of plastic strain,through the analysis of elastoplastic theory,the calculation formula for the deformation of the surrounding rock of the deep-buried circular tunnel and the interaction with the support is deduced when considering the strength deterioration of the surrounding rock,and it is applied to the severely compressed and large deformation section of the Yuelongmen Tunnel.The theoretical calculation and analysis of the double-layer support and the yielding support are carried out respectively,and the interaction characteristic curve of surrounding rock and support is obtained.(2)The timing of the construction of the second layer of the double-layer support is the key element for the design of the double-layer support.Combined with the actual construction process on site,through three-dimensional numerical simulation,the deformation and force of the surrounding rock support under the first and second layer support and their combined action under the three construction timings are calculated and analyzed.The results show that it is more reasonable to construct the second layer of support at one time the hole diameter after the first layer of support is closed.(3)Aiming at the yielding block of a high-deformability material(hi DCon),through three-dimensional numerical simulation,the deformation and force characteristics of the yielding support in the three-step excavation support method are calculated and analyzed.The results show that after the support is closed,the support can still undergo a large allowable deformation(about 40-50% of the total deformation),and the support force is slowly increased without causing damage to the sprayed concrete part,reaching the overall stability of the yielding support.(4)The double-layer support can ensure that the second-layer support is in an elastic state by selecting the timing of the second-layer support,forming the overall stability of the support system,and the on-site operation is easy.The yielding support has less rigidity and greater deformation of the surrounding rock,and the design and construction of the yielding block in the yielding support is relatively difficult,further practical exploration is required.