Performance of a deep excavation with the composite retaining wall using artificial ground freezing method

A novel composite retaining system that combines artificial ground freezing (AGF) and diaphragm walls for a real construction project is presented in this work. In this project, a deep excavation was conducted above a tunnel with a minimum clearance of 0.8 m between the final excavation surface and the tunnel crown. The diaphragm walls were constructed as retaining walls. However, the existence of the underlying tunnel impedes the integrality of the diaphragm walls around the tunnel and causes the problems of poor reinforcement and unblocked seepage paths. To address this conflict, several freezing pipes were drilled and installed outside the diaphragm wall to form a large-scale frozen curtain. Through the composite retaining wall, the generation of the dual effect of ground reinforcement and waterproof sealing was also accomplished. Field measurements were conducted to investigate the performance of the excavation including temperature distributions, deformation variations, and pore-water pressure developments. From the acquired field data, it was revealed that a 7 m-width x 3 mthickness frozen wall was formed for the subsequent excavation and the vertical displacement of the underlying tunnel was controlled at a distance of 9.3 mm to guarantee the safe operation. On top of that, during the AGF construction, the composite retaining wall met the requirements of high strength and stiffness, as well as practically zero permeability. From this case study, it is demonstrated that the proposed construction technology possesses comparative advantages in resolving the conflict between the adjacent excavation and the existing tunnels for controlling the excavation-induced deformation and decreasing the construction risk.