An elasto-plastic solution of pile deflection and pile load capacity due to adjacent tunneling

Spherical and cylindrical cavity expansion/contraction analyses are employed for estimating the pile-tunnel interaction. Firstly, a cylindrical cavity contraction model was developed to simulate the process of tunnel excavation based on the Mohr-Coulomb yield criterion, and the elasto-plastic horizontal displacement of adjacent single pile was calculated based on Pasternak foundation model. Then, an elasto-plastic method for evaluating the pile load capacity affected by tunneling was proposed. The total load capacity of pile was determined as the summation of the contribution from the ultimate end-bearing capacity and the ultimate shaft friction. A spherical cavity expansion model in the infinite medium was used to account for the ultimate cavity pressure at pile tip, and then the ultimate end-bearing capacity of the pile was obtained. The equivalent average shear stress along the pile length was estimated using the modified β-method, and the ultimate pile shaft friction was obtained. The modified method considers the reduction of the shear stress of the pile shaft due to tunneling. On this basis, the elasto-plastic effect of tunneling on stress in surrounding soil was calculated. The distance effect of pile to tunnel on the pile load capacity was analyzed. Defining that the pile failure occurs when the pile capacity is reduced to 85%, the relationship between the distance of tunnel to pile tip and critical tunnel volume loss is analyzed. The parametric studies were performed to discuss the influence of various factors on this relationship, including the soil cohesion, internal friction angle, density, soil modulus and the pile diameter. Results show that, the elasto-plastic cylindrical cavity contraction model is capable in predicting the lateral displacement and bending moment of pile due to adjacent tunneling. A plastic zone is formed in a certain range after the tunneling. The effective soil stress reduction factor Rp in this area is less than 1, which indicates that the pile bearing capacity is reduced, while the pile capacity outside the plastic zone is not influenced by the tunneling. The distance of tunnel to pile has a significant effect on the load capacity of pile. If the tunnel-pile distance is a constant, the pile end-bearing capacity reduction factor Rqb will gradually approach 1 with the increase of the tunnel depth, which indicates that a deeper tunnel is more favorable for pile bearing capacity. There is a linear relationship between the critical tunnel volume loss at which pile failure occurs and the square of tunnel-pile tip distance. The load capacity of pile is sensitive to the soil modulus. © 2017, Science Press. All right reserved.