Study On The On-site Control Of Slurry Indices And Face Pressure For Very Large Diameter Slurry Shield Tunnel

With the rapid development of economy,underground tunnel engineering has increased very quickly due to the limited land resources in urban areas.For the tunnel construction,the slurry shield is widely used due to its unique advantages.Hence,the slurry shield is gradually developing with the characteristic of large cross section,boring in shallow soil and long distance.During the construction of large diameter slurry shield tunnel,the stability control of the excavation face is the key to ensure the safety and quality of the project.For the very large diameter slurry shield tunnel,the quality of slurry indices and the face pressure are the key to the stability of the excavation face.During construction,if the quality of slurry is poor,it will not be able to form dense slurry membrane,and thus the face pressure can not act on the excavation face effectively,which caused large fluctuation of face pressure and impacted on the stability of the excavation face.If the acted face pressure is too large,it will not only induce surface uplift,but also damage to the shield cutter.If the acted face pressure is too low,it will cause large settlement at surface subsidence and even lead to the collapse of excavation face.Under such circumstance,the safety will not be guaranteed.Therefore,it is meaningful to study on the on-site control of slurry indices and face pressure.For the demand of the on-site control of slurry indices and face pressure for very large diameter slurry shield at the project of Shanghai Beiheng Channel the control of slurry indices and face pressure in the construction is studied in this paper,the maincontents are as follows:(1)Based on the field test data of Beiheng,the engineering characteristics of different construction sections was firstly studied and summarized.Then the on-site control system of the slurry indices(i.e.viscosity and specific gravity of slurry)was proposed..(2)The ground deformation under different stress equilibrium in composite strata for very large diameter slurry shield was investigated.The Finite element model and the theoretical analytic model was established and used to calculate the face pressure at the excavation face.Then based on the theory of best stress balance calculation point and the effect of weight gradient of slurry,the improved method was proposed to optimize the face pressure from the traditional method.(3)The Finite element model was established and used to investigate the influence of different buried depth and different stratum conditions on the limit face pressure.Based on the index of stress ratio(the ratio between the pressure on the excavation surface and the horizontal earth pressure at the center of the excavation surface),the minimum face pressure corresponding to different working conditions was studied.Furthermore,the difference of the minimum face pressure in sand and clay was discussed.(4)With employment of the Hardening Soil with small-strain stiffness soil model and the method of equivalence of synchronous grouting,a three-dimensional finite element model is developed.The impact of the face pressure and main soil parameters on the surface deformation were studied.Based on the theory of sensitivity,the above parameters which had higher impact on the ground deformation were obtained.At same time,it was found that at the distance of(H+D/2)ahead from the excavation face,the ground deformation were largely influenced by the acted face pressure.Hence,it is necessary to strictly control the acted face pressure to avoid the disturbance of ground..(5)Combination of the previous research results and the analysis of the on-site monitoring data of Beiheng,an on-site control system for face pressure were given,which is of importance to practitioners in construction site.