Mechanism On The Stability Of Trench In Construction Of Deep Diaphragm Wall In Soft Ground

With the continuous development of urban underground spacein decades,the difficulties in the construction keep increasing and the surrounding enviroments become more complicated.In Yangze Delta metroplian region,the diaphragm walls(CDWs)perform as an effective support for deep excavation in foundation engineering with the advantages such as low noise,small vibration,and small disturbance to surrounding soils.During the construction of CDWs,the trench excavation releases the earth stress and probably triggers the trench instability.In global instability,the soil collapse initials at a certain depth and propagates to the ground surface and jeopardizes the surroundings.The local instability takes place just in the interlayer.The void subjected to pilloff of weak will cost more fluid concrete in tremie concreting.Both of global and local instability are considered in this research.The specific research work and main conclusions are presented as follows(1)A three-dimeiosinal mechanism for trench global instability is.establsihed.In this mechanism,the instability is involved in the movement of truncated conical block which separated by conical discontinuity surface.Through kinematic approach of limit analysis,the upper bound solutions of safety factors for the global stability are obtained.By the optimaziation,the upper bound solution gives out the critical depth of global instalbity.Through parametric analysis,the influences of factors such as soil shear strength,slurry level,weigth of slurry,length of trench,and surcharge on ground surface are quantitatively evalutated.(2)The trench local instability is investigated by establishing a three-dimensional kinematically admissible mechanism in weark interlayer.The mechanism indicates that the instability is involved in the momvent of rigid wedge block which separated by translational discontinuity surface from stational soil.Through kinematic approach of limit analysis,the upper bound solutions of safety factors for the global stability are obtained.The parametric analysis shows that the weak interlayer can be more stable subjected to the higher shear strengths,smaller vertical stress of overburden,shorter trench length,and higher slurry pressure.The influences of shear strength,vertical stress of overburden and slurry pressure is more significant than trench length.The instability impacted by the trench length is substantially attributed to the increase of vertical stress of overburden.(3)The validation of the current solutions is conducted by investigating the trench stability in the excavation of CDWs in the project of Zhuhui Road subway station of Suzhou.The results show the agreement with the field measurement.The current solutions are able to be applied in design and construction of CDWs.