Soft Ground Tunnelling

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 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.

Cut and cover tunnelling is a common, well-proven technique for constructing shallow tunnels. This technique consists of an in-situ cast concrete structure in an excavated trench, which is covered afterwards. This method accommodates changes in tunnel width and non-uniform shapes and is often adopted in the construction of underground transit stations. To minimize surface disruption, cut and cover tunnelling can be accomplished using the traditional bottom up method or as a top-down construction.

Tunnel Boring Machines

For deeper, longer tunnels in urban areas, or for a tunnel crossing major bodies of water, a pressurized-face Tunnel Boring Machine (TBM) is the best fit, because it is capable of handling the full range of expected ground conditions. A single pass, precast concrete segmental lining forms the tunnel behind the TBM. The selection and design of the precast segmental liner is critical for successful application. The segments are equipped with waterproofing gaskets and act as the structural support system and water barrier.

Compared to the cut and cover approach, TBMs significantly reduce the disturbance of traffic and the associated environmental impacts in urban areas. WSP has been a pioneer in the advancement of single pass liners, including its first application in the USA with the use of fibre reinforcement and double gaskets. We also introduced the use of special seismic joints in TBM driven segmental liner tunnel, outside of Japan.

There are two major TBMs used in soft ground tunnelling: Earth Pressure Balanced (EPB) and Slurry Type Shield Machines. An EPB TBM will perform better where the ground is silty and has a high percentage of fines. A slurry TBM is ideal in loose water-bearing granular materials. However, with the application of appropriate ground conditioning agents, the range of ground conditions for each machine can be extended. TBM technology has advanced significantly in the last 15 years, allowing for the construction of larger, deeper, and longer tunnels in more difficult ground conditions.

New Austrian Tunnelling Method

For shorter tunnel sections, non-circular tunnels, or tunnels with variable geometry, the New Austrian Tunnelling Method (NATM) – also known as the Sequential Excavation Method - provides another cost effective, flexible, and safe tunnelling option. The tunnel is sequentially excavated and support is provided by shotcrete, in combination with fibre or welded-wire fabric reinforcement, steel lattice girder arches. Ground improvement methods such as jet grouting, dewatering, ground freezing, and grouted pipe spilling are also available to stabilize the face.

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 minimize 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.