A tunnel built in soft ground—such as clay, silt, sand, gravel or mud—requires specialized techniques compared to hard rock, to compensate for the shifting nature of the soil. At WSP, we have extensive experience with soft-ground tunnels on every continent, and we pride ourselves on developing innovative strategies to deal with even the most complicated soil situation while preserving stability. There are a wide variety of techniques for soft-ground tunneling, and the best fit is ultimately determined by ground type, timeline, budget and surrounding structures.
In order to select the most appropriate method for a particular tunnel, several factors need to be taken into consideration, including ground conditions, length, depth, diameter, alignment geometry and budget. Another important consideration is the risk to—and sensitivity of—nearby infrastructure and buildings to ground movement.
At WSP we know that an important aspect of soft-ground tunneling is the protection of existing structures and utilities, as many soft-ground tunnels are located in sensitive urban environments where settlement caused by tunneling is a major concern. Protective measures such as dewatering, ground improvement, compensation grouting and positive pre-support can be used to ensure successful tunneling in soft ground is achieved. Above all, a comprehensive real-time instrumentation and monitoring system is essential. In Seattle, WSP contributed planning and design services to the Alaskan Way Viaduct Replacement Program (SR 99 Tunnel), which is being constructed in challenging ground conditions, under more than 150 buildings. The tunnel has a diameter of 17.5 m and is the second-largest tunnel of its kind in the world.