Mechanism And Countermeasures Of The Tunnel Deformation Induced By Adjacent Excavation

The unloading effect of excavations can cause deformation of adjacent existing tunnels,which may seriously influence the operation and safety of the tunnels and hence should be strictly controlled.At first,based on the field measurements of Jiahai Garden project in Tianjin,as well as a large number of case histories,the finite element method considering the small strain of soil was adopted to conduct parametric studies of the relationship between the deformation of the existing tunnels and the excavation construction.According to various deformation controlling criteria of the existing tunnel,the excavation retaining side was zoned with different depth of excavation,different deflection modes and maximum horizontal displacements of retaining structures.And then,two methods called design charts and design table to predict the influence of excavations on adjacent tunnels were proposed according to the maximum displacement contours of tunnels caused by excavation.Based on that,a series of finite-element models were conducted to investigate the coupling relationship between maximum horizontal displacement of the tunnel and excavation depth,relative location of the tunnel as well as the maximum horizontal deformation of retaining structures.A simplified calculation formula was proposed to predict the maximum horizontal displacement of the tunnel,which could accurately predict the maximum tunnel horizontal displacement caused by excavation and was verified by cases histories in Tianjin and other coastal soft soil area.Further studies were carried out on the methods of deformation control on existing tunnels.Base on a project which adopted the isolation piles to protect the adjacent existing tunnel,a parametric study of isolation piles using finite element method considering the small strain of soil was conducted to investigate the horizontal displacements of the tunnel and the soil below the ground surface adjacent to a deep excavation influenced by isolation piles.It was shown that isolation piles had both effects of isolation and drag on the tunnel adjacent to the deep excavation.The deformation of adjacent tunnel from deep excavation can be increased when the drag effect was more remarkable.The buried isolation pile could be used to minimize the effect of drag and the horizontal displacement of tunnel.Moreover,the isolation piles had higher efficiency in reducing the tunnel displacement when the piles were located closer to the tunnel.A further study focused on the effectiveness of compensation grouting control on the deformation of existing tunnel adjacent to an excavation was conducted after that.Two field trials of compensation grouting were conducted in a greenfield and a site adjacent to an existing tunnel nearby a deep excavation,respectively.Deformation of adjacent soil and existing tunnel and pore water pressure of soil were monitored.A sophisticated finite element model was established and verified by the field measurements.The comparison of various models show that it is necessary to consider the non-uniform expansion of condition zone and the fissures for the dissipation of excess pore water pressure.The light overconlidation has a slight influence.The parametric study shows that despite of different soil moduli and grouting volumes,the horizontal displacement of the soil reduces to 10%of the maximum value at a same distance of 3 m in silty clay.The rate of residual horizontal displacement(ξ)could be calculated by a hyperbolic model.Despite of different soil modulus and distance,ξreaches 90%of the maximum growth when the grouting volume reaches approximately2.3 m~3,which is the optimum grouting volume.The horizontal displacement mode of the existing tunnel affected by the adjacent compensation grouting can be described by a Gauss curve.The horizontal displacement of existing tunnel decreases to a stable value with the dissipation of the excess pore water pressure generated by the adjacent compensation grouting.At the condition of different distances between the injection point and the existing tunnel,the horizontal displacement of existing tunnel increases linearly with increasing grouting volume approximately.At the condition of different grouting volume,the horizontal displacement of existing tunnel decreases linearly with increasing distances between the injection point and the existing tunnel approximately.According to the micro-disturbed concept,less grouting volume and shorter distance should be chosen as the parameters of compensation grouting in practice.