Estimation Of Rectangular Jacking Pipe’s Jacking Force In Guangzhou Silty Stratum

In recent years,the rectangular pipe jacking construction technology has been widely used in the construction of underground projects such as subway,tunnel and utility tunnel due to its advantages of good safety,fast construction speed and high space utilization.However,many problems have also occurred in the construction process,among which the estimation of pipe jacking force is one of the key problems.If the jacking force is insufficient,the pipe jacking cannot be carried out normally.If the jacking force is too large,It will affect the design of the back wall structure and the layout of the relay room,resulting in the increase of the project construction cost and the extension of the construction period.At present,the calculation formula of jacking force in relevant specifications is mainly for circular pipe jacking,and the calculation value of jacking force is relatively conservative for the sake of safety;In some areas,empirical formulas of jacking force are summarized according to the properties of soil layers in the area and the measured data of Engineering jacking force,but their applicability is relatively limited.There is no unified calculation formula for the jacking force of rectangular pipe jacking,and the influence of the friction resistance between the pipe jacking body and the surrounding soil on the jacking force needs to be reasonably considered.In view of this,in this paper,combined with the underground utility tunnel and the road reconstruction project of Guangzhou Guanghua highway,aiming at the rectangular pipe jacking project in silty clay layer in Guangzhou,the shear mechanical properties of the interface between the pipe jacking body and the surrounding soil are studied,and the calculation formula of rectangular pipe jacking force applicable to silty clay layer is attempted to be established.The specific contents,research methods and results are as follows:Through the direct shear test,the effects of different water content and thixotropic mud content on the shear strength characteristics of silty clay were explored.When the water content is in the range of 20%-35%,the cohesion and internal friction angle of silty clay decrease with the increase of water content,and the decreasing range gradually decreases;Adding thixotropic mud can make the plastic characteristics of soil become obvious.The slurry soil ratio is used to express the amount of mud.The slurry soil ratio is the ratio of thixotropic mud quality to soil quality.When the slurry soil ratio is in the range of 0%-20%,the shear strength,cohesion and internal friction angle of silty clay(slurry soil mixture)decrease with the increase of the slurry soil ratio.The internal friction angle reflects the internal friction between soil particles.With the increase of slurry soil ratio,the reduction of internal friction increases,indicating that the addition of thixotropic mud has a drag reduction effect,and the larger the slurry soil ratio,the better the drag reduction effect.Using the discrete element analysis software MatDEM2.0,the direct shear numerical test of silty clay(slurry soil mixture)with different slurry soil ratio is carried out.Based on the results of the indoor direct shear test,a large number of simulation experiments are carried out by setting different parameter scenarios,and repeated trial calculations are carried out to obtain the numerical test curve that is consistent with the shear curve of the indoor direct shear test,and the physical and mechanical parameters of silty clay under the above conditions of different slurry soil ratio are obtained.On this basis,the shear test of silty clay concrete pipe contact surface with thixotropic slurry is further simulated to study the shear mechanical properties of the contact surface,and the friction coefficient of pipe soil contact surface under different slurry soil ratio is obtained.Combined with the actual pipe jacking project in Guangzhou,the calculation formula of the jacking force of rectangular pipe jacking is established.Considering the contact state between pipe and soil in the actual project,the suggestions for the value of the friction coefficient between pipe and soil in silty clay layer and shallow buried large cross-section rectangular pipe jacking project are given.For this project,the friction coefficient can be taken as 0.0305,and the calculated jacking force is in good agreement with the measured jacking force;Through similar projects,the rationality of the calculation formula of jacking force and the value of friction coefficient in this paper are verified.In practical engineering applications,in order to ensure the safety of the project,the calculated value of jacking force can be multiplied by a safety factor.