Cut and Cover Tunnelling

Cut and cover is the oldest method of tunnelling: the basic concept involves the digging of a trench, the construction of a tunnel, and then returning the surface to its original state.

As such, it is a disruptive technique, but it is also usually the most economical construction method.

WSP has designed and supervised construction on numerous cut and cover tunnel structures including tunnels for transport facilities, transit stations, underground structures, deep excavation for buildings, and water conveyance facilities.

Urban tunnels

WSP is a specialist in urban cut and cover construction. Tunnels beneath cities present several challenges unique to their environment, such as:

• Congested sites

• Historic areas

• Crisscrossing utility and transit lines

• Special underpinning, structural support, and building protection

• High groundwater levels

• Groundwater contamination

• Maintenance of traffic during construction

Our teams have designed and supervised the construction of various types of excavation support systems including soldier piles & lagging, temporary/permanent slurry walls and diaphragm walls, soldier piles in tremie concrete (SPTC) systems, jet grout walls, temporary secant pile walls, soil mix walls, and element walls. Protection schemes for adjacent buildings, tunnels, and utilities designed by our engineers have ranged from conventional underpinning to innovative solutions like micro piles, chemical grouting, jet grouting, compaction grouting, compensation grouting, and ground freezing.

We also have experience designing underground structures using the top-down method of construction. This technique is an effective means of reducing the impact of construction in urban areas and can expedite the construction schedule so that surface facilities and traffic can be returned to service sooner.

Supporting excavation

To successfully implement cut and cover construction with minimal impact on surface facilities, structures, and near-surface utilities, support of excavation is critical. Support systems design needs to consider a variety of factors, including ground and ground water conditions, adjacent environment and sensitive buildings/properties, legislative and consenting requirements, constructability and risk assessment. WSP is practised at addressing these factors.

Our expertise covers all forms of support, both temporary and permanent.

Temporary support generally consists of soldier piles and lagging, sheet pile walls, secant piles or contiguous piles. When support is permanent, these supporting elements are a part of the final structure and are designed to be left in place after the construction is complete. These include techniques like diaphragm (slurry) walls, secant piles, or contiguous piles. The excavation supporting walls, temporary or permanent, are often braced using temporary props or permanent beams which form part of the permanent structure roofs and slabs. The excavation bracing is installed sequentially as excavation is carried out in stages.

In order to successfully implement cut and cover construction with minimal impact on surface facilities, structures, and near-surface utilities, support of excavation is critical. Designing such support systems involves the consideration of a variety of factors that could affect its performance and impact the tunnel structure itself. These excavations supports are either temporary or permanent.

Temporary supports do not contribute to the final structure’s load bearing support. In general, they consist of soldier piles and lagging, sheet pile walls, secant piles or tangent piles. When supports are permanent, these supporting elements are a part of the final structure, and are designed to be left in place after the construction is complete. These include techniques like diaphragm (slurry) walls, secant piles, or tangent piles.

A recent example of extreme cut and cover construction work is the Asia Transition Structure of the Eurasia Tunnel in Istanbul, Turkey in which the excavation was 168 m long by 35 m deep and with variable width, all constructed within a few meters from the free water body of the Bosphorus Strait. The work was completed successfully using two rows of secant pile system and several layers of tie-backs.

Ground water control

Controlling ground water is also a significant consideration during the construction of cut and cover tunnels. The implementation of proper techniques can minimize the impact of potential settlement and the effect of ground water draw-down on the adjoining structures and facilities.

These measures are assessed during the design phase and are implemented during construction. Popular options include dewatering, watertight support of excavation system, permeation, jet grouting, and ground freezing.

Monitoring and instrumentation

Ground movement and groundwater monitoring provides very useful information regarding the performance of a cut and cover tunnel during construction. WSP provides ground movement and groundwater monitoring support.

Monitoring allows us to verify design assumptions, back analyses and calibrate design parameters. Trigger levels can be set to monitored ground movement so that appropriate actions can be taken, in the event of trigger exceedance. Mitigation measures are then implemented to control and reduce adverse impact on surrounding buildings and infrastructure to acceptable levels.