Effects Of Construction-induced Disturbance On Post-construction Settlements Of Shield Tunnel And Ground In Soft Clavev Strata

At present,underground railway construction has been the predominant way to alleviate the problems causing by traffic jam,land shortage and environmental pollution in conjested cities.Shield-tunnelling technique is widely adopted for underground railway construction.In soft clayey strata,the post-construction settlements of ground and shield tunnel usually develop continuously,influencing the tunnel serviceability and even threatening its safety.Among the factors giving rise to the post-construction settlements,shield tunneling-induced ground disturbance and post-construction nearby excavation(mainly excavation and tunnelling)are believed to be the main reasons.This thesis carries out a comprehensive and in-depth study on the topic of effects of construction-induced disturbance on post-construction settlements of shield tunnel and ground in soft clayey strata.Firstly,the impacts of disturbance on the structure and compressibility of soft clay are investigated.Secondly,in terms of the reasons for the post-construction settlements,the influences of shield tunnelling-induced ground disturbance,nearby excavation and closely spaced paralleled tunnelling are examined.The findings can provide references for controlling the post-construction settlements of shield tunnel and ground in soft clay.The main works and findings of this thesis are drawn in the following aspects:(1)By considering the roles of additional void ratio and yield stress of compression curve,this study proposes 'structural index',i.e.,the intergral of additional void ratio times effective vertical stress ranging from zero to yield stress,to evaluate soil structure in structured clays.In addition,‘structural parameter'is developed by normalizing the structural index.Taking Hangzhou Xianghu clay as an example,the effects of disturbance on the compressibility and structure are examined.It is found that,with increasing disturbance degree,compression index,yield stress and structural parameter fall off linearly.It is also found that high plasticity implies strong structure and low compressibility.By quick direct shear tests,the soil disturbance degree evaluated by reduction of undrained shear strength is slightly higher than that by structural parameter.(2)This study proposes a method to quantify the soil disturbance degree on the basis of shear strain state.For typical structured clays including Ningbo clay,London clay and Bangkok clay,three dimensional finite-element analyses are performed to investigate the distribution of disturbance degree of soils around tunnel.The numerical results indicate that the transversal disturbed zones in Ningbo clay and Bangkok clay look like an 'eye' regardless of cover-to-diameter ratio,whereas London clay with high cover-to-diameter ratio a 'circular ring' owing to its much higher at-rest earth pressure coefficient.The soil disturbance degree drops down linearly with increasing distance from invert/crown in vertical direction within close range,whereas falls off exponentially as the distance from springline increaes in horizontal direction.Based on a case history of Shanghai Bund Sightseeing Tunnel,it is found that the disturbance degrees evaluated by structural parameter and shear strain are similar.(3)On the basis of compression curve of intact soil,the method for developing compression curve for disturbed soil accounting for disturbance degree is formulated.In addition,the methodology for calculating post-construction ground settlements is built on the basis of the modified compression curve.Based on the case history of Ningbo Metro Line 2,a suite of three dimensional finite-element analyses are carried out to investigate soil disturbance due to shield-tunnelling.The calculated post-construction ground surface settlements agree well with the field measurements.The influences of shield-driving parameters on the post-construction settlements of ground and tunnel are revealed.It is found that grouting ratio of about 200%and support pressure of 2.2-2.4 times of at-rest lateral earth and water pressure can most effectively reduce the post-construction settlements of ground and tunnel.(4)Based on a deep excavation nearby an interval of Ningbo Metro Line 1,the influence of nearby excavation on existing tunnel and the effectiveness of several protective measures are investigated.By building the correlationship between responses of retaining wall,surrounding soil and tunnel structure,the interaction mechanism of excavation-soil-tunnel is revealed.The results exhibit that the existing tunnels can inhibit the spread of excavation-induced disturbed zone.In addition,this study compares the effectiveness of divided excavation,soil improvement and cut-off wall on protecting existing tunnel.The results demonstrate that the longitudinal divided excavation can effectively alleviate the excavation-induced disturbed zone,as well as the internal forces and displacements of existing tunnel.(5)Based upon closed-form plasticity solutions for radial displacement toward tunnel(Mair&Taylor 1993),this study proposes a method to determine the shear modulus of soils around tunnel.By interpreting observations of model tests for twin-tunnelling(Chapman et al.2007),the radial displacements induced by the first tunnel(T1)and the second tunnel(T2)are compared.Due to T2 construction,it is interesting to find that the shear modulus of soils close to T1 degrades,while other side increases.Based upon the observations of combined axial-torsional tests;the rotation of principal stress axes is found to be the main reason causing the shear modulus change and the differences between settlements induced by closely spaced twin tunnels construction.