Soft ground tunnelling

A tunnel built in soft ground – such as clay, silt, sand, gravel, or mud – requires specialised 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 tunnelling, 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 but not limited to ground conditions, length, depth, diameter, alignment geometry, and budget. Another important consideration is the risk and sensitivity of nearby infrastructure and buildings to ground movement.

At WSP we know that an important aspect of soft ground tunnelling is the protection of existing structures and utilities, as many soft ground tunnels are located in sensitive urban environments where settlement caused by tunnelling is a major concern. Protective measures such as dewatering, ground improvement, compensation grouting, and positive pre-support can be used to ensure successful tunnelling in soft ground is achieved. Above all, a comprehensive real-time instrumentation and monitoring system is essential. In Seattle, WSP was heavily involved with the SR-99 Alaskan Way Tunnel project, which was 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.

Tunnel jacking is a method for constructing monolithic, rectangular concrete box sections under surface areas with critical uses such as railways, major roadways, and airport runways. Due to its expense, it is generally only used when there is no other option, and so examples are rare. This method is generally applied in cases where the underground crossing is relatively short, is located in soft ground, and with shallow cover. Special construction techniques are required to minimise friction, reduce settlement, and maintain the tunnel alignment.

In Boston, WSP applied this approach during the construction of a section of the Central Artery/Tunnel, under a complex network of tracks leading into the South Station Railway Terminal. Three concrete tunnel box sections were jacked successfully under the track network, with typical cross sections of 11.6 x 24 m and lengths of 51 to 115 m.