SUBSIDENCE OVER SOFT GROUND TUNNEL

Knowing the potential causes of and being capable of reducing subsidence during a tunneling operation is important, as well as means of forecasting the magnitude and distribution of settlements prior to tunneling. This paper describes the existing soil and ground water conditions encountered, as well as the construction procedures used in advancing a machine-mined tunnel in soft ground. Settlement data obtained from a comprehensive instrumentation program performed during tunnel construction are presented. A favorable comparision is made between the data obtained for the tunnel in question and that published for other soft ground tunnels. This comparison makes it possible to apply a mathematical expression (the error function) to approximate the subsidence profile. An overall look at soil responses to construction practices and shield dimensions further substantiates findings that the magnitude of subsidence obtained in soft ground tunneling is directly related to the soil encountered and techniques used to reduce the magnitude of source disturbance in the area of tunneling.