Study On Load Action Mode Of Shield Tunnel In Hard-Soft Heterogeneous Ground

With the continuous networking and scale of urban rail transit in China,coupled with a vast territory,the surrounding environment of subway construction is becoming more and more complex,and various new situations and new problems are constantly emerging.Among them,it is common occurrence that the construction of urban subway shield tunnels encounters various soft and hard uneven strata.It is urgent to explore whether the loading system of traditional modified routine method and the existing shield tunnel load calculation theory can better fit the actual project of subway shield tunnel in this kind of stratum.In view of this,this paper studies the load mode of subway shield tunnel in soft and hard heterogeneous strata.The main research contents and results are as follows:(1)Based on the actual project of Nanchang subway shield tunnel,a theoretical load model of subway shield tunnel in soft and hard heterogeneous stratum was proposed,which systematically considers the relationship between tunnel depth,slip surface form,lateral pressure coefficient and stratum.For shallow buried tunnels,the ellipsoid theory was introduced to derive the lateral pressure coefficient and vertical loose earth pressure expression of elliptical sliding surface under limit state,and then the lateral pressure calculation formula was derived from the mechanical equilibrium equation.For deep buried tunnels,the width of vertical slip surface in Terzaghi theory was reasonably corrected according to the mutation of hard-soft heterogeneous ground.(2)Relying on the Nanchang subway shield tunnel,three sections(soft and hard heterogeneous strata,homogeneous sand layer,homogeneous rock layer)were selected to carry out field tests.The test content was the earth and water pressure acting on the segment and the internal force of the segment.The load test results show that for the soft and hard heterogeneous strata,the influence of shield tail synchronous grouting on the soil pressure of the upper soft soil layer is greater.The measured soil pressure of the test segment is basically stable after 20 rings of ring detachment,and the measured water pressure is affected by the grouting pressure within about 15 rings of ring detachment.For the homogeneous sand layer,the measured soil pressure changes have been relatively flat after 8-9 rings of ring detachment.For the homogeneous rock layer,the measured soil pressure at the bottom of the tunnel is smaller than that at the top of the tunnel,and the water and soil pressure outside the ring of the segment is most significantly affected by the synchronous grouting at the end of the shield within 5 rings of ring detachment,and the influence of the secondary grouting pressure on the water pressure is limited to 3 rings of forward advancement.(3)The results of surrounding rock pressure calculated by the proposed method were compared with the measured data.The results suggest that for soft and hard heterogeneous strata,the load model of the modified routine method overstates the bottom reaction force,while the results calculated by the method of this paper have less deviation from the measured values in terms of quantity,while the measured lateral earth pressure near the stratigraphic boundary shows a large abrupt change,indicating that the load model proposed in this paper is more consistent with the actual loading of shield tunnels in soft and hard heterogeneous strata.Compared with the modified routine method,the accuracy of the vertical earth pressure at the top and bottom of the tunnel calculated by the method proposed in this paper is improved by about 24.6% and 70.9%,respectively.(4)A refined numerical model of shield segment considering circumferential,longitudinal bolt joint,bolt preload force and bolt contact with hand hole was established.Based on the load-structure method,the stress characteristics of segment under different strata were analyzed by comparing with the measured internal force of the segment.The results show that for the homogeneous sand layer,the bending moment of the arch bottom of the segment is significantly larger than that of the vault,the difference between the axial force at the top of the arch and the bottom of the arch is not large,and the axial force of the arch waist is larger than that of the arch bottom.For the soft and hard heterogeneous strata,it is verified that the internal force of the segment calculated by the load mode proposed in this paper is more realistic.The bending moment and axial force of the arch bottom are smaller than those of the vault.And the development of the internal force of the segment in the lower hard stratum area is greatly constrained,resulting in stress concentration of the segment in the upper neighborhood of the stratum interface.