Research On Influence Control Of Shield Tunneling Pile Foundation Construction Under Upper Soft And Lower Hard Composite Stratum

The composite stratum with soft upper and hard lower is a typical complex geological condition.The tunneling of shield machine in this kind of stratum is easy to lead to surface collapse,difficult to control the attitude of shield machine and serious tool wear.When urban subway shield penetrates deep foundation（structure）in this kind of stratum,it is easy to cause the bearing capacity of pile foundation to decline,resulting in the settlement of pile foundation and the cracking of superstructure.In view of this problem,this thesis,based on the shield tunneling project of Guangzhou Metro Line 8 through the West Cha pedestrian culvert of Beihuan Expressway in a short distance,studies the influence and control of shield tunneling through the existing bridge piles in the upper and lower hard composite stratum as follows: Firstly,the influence of shield tunneling under different pile top loads in the upper and lower hard composite strata with different hard rock height ratios is analyzed.Then,according to the actual situation of the supporting project,two kinds of pre-reinforcement measures are developed,and the construction process of shield tunneling through the culvert structure under the two reinforcement schemes is numerically simulated,and the two schemes are compared and analyzed,and the best scheme is selected for the actual construction.Then,the pre-reinforcement construction control design is carried out for the optimal scheme,and the corresponding field monitoring is carried out for the shield construction process.Finally,relevant deformation laws are obtained by analyzing monitoring data.The main conclusions are as follows:（1）A quantitative parameter,the hard rock height ratio β,is proposed and defined to describe the distribution of hard rock strata in the upper and lower hard composite strata.Through numerical simulation,the influence of shield tunneling under different height ratio of hard rock and pile top load on upper bridge pile and ground surface is analyzed.The results show that the ground surface of the existing bridge piles is more sensitive.The smaller the height ratio of hard rock in the face of the composite stratum,the greater the disturbance of underpass construction to the bridge pile above.The disturbance of shield tunneling in upper soft and lower hard composite stratum is stronger than that in single stratum.（2）The distribution of axial force and bending moment of shield tunneling friction pile in upper soft and lower hard stratum changes.The axial force of pile appears in a small range and changes in the whole pile during shield penetration.The maximum bending moment of the pile decreases during the downpass.The larger the hard rock height ratio of the palm surface is,the smaller the reduction of bending moment caused by underpass is.After the completion of shield crossing,the pile top and the following parts change back to axial pressure,while the bending moment of pile body further decreases.When the pile foundation is end-bearing pile,the displacement and internal force distribution of the pile foundation do not change significantly in the process of shield tunneling.（3）The results of numerical simulation on the supporting project show that: Without reinforcement,when shield machine through the culvert entrance,shield at the top of the culvert load of loading and unloading of soil,the surface will produce excessive differential settlement,ground subsidence,and two kinds of design in this thesis is given priority to with MJS methods of reinforcement scheme can significantly eliminate the settlement difference between the two places of the earth’s surface and maximum ground subsidence control;The reinforcement scheme can also reduce the disturbance of the trailing tunnel to the leading tunnel in vertical surface direction and horizontal direction.（4）In the double-track tunnel construction process,after the completion of the tunnel construction of the left line,the monitoring data show that the horizontal settlement value of the surface reaches the maximum at the position where the height ratio of hard rock is 0,and the surface settlement value is within the safe range.However,when the hard rock height ratio of the adjacent excavated face is in the range of 0-0.2 and 0.5-1,special attention should be paid to the possible uneven surface settlement caused by subsequent shield construction.（5）According to the monitoring data,the displacements of the surface and abutment do not exceed the warning value,which confirms that the reinforcement scheme in this thesis has a good effect on the reinforcement of the pedestrian culvert.Meanwhile,the shield construction parameters are set reasonably and the displacements of the surface and abutment are effectively controlled.The surface settlement of the tunnel vault in the numerical simulation of the supporting project is similar to that in the monitoring section with the same hard rock height ratio.In addition,the settlement of the lateral surface is greater than that of the preceding tunnel,which proves the accuracy of the numerical simulation in this thesis.