Cut-and-cover is the oldest method of tunneling. 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. Where the tunnel alignment is beneath a city street, the construction may cause interference with traffic, utilities, businesses and other urban activities. The disruption, however, can be lessened through the use of proper staging, decking over the excavation to restore traffic or by implementing a top-down construction technique. While cut and cover is a technique usually reserved for relatively shallow tunnels, it is not uncommon to see it used at depths of around 60 feet (20m), but rarely does it exceed 100 feet (30m).

WSP has designed and supervised construction of 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. Tunneling beneath cities presents several challenges unique to the 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 slurry 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.

In New York City, WSP performed the construction management services for the Second Avenue Subway, a new transit line in a highly congested area of Manhattan, with numerous high-rise buildings, extensive utilities, historic and fragile buildings, and heavy traffic, including several bus lines. The work was accomplished using various support of excavation systems including diaphragm (slurry) walls, secant piles, tangent piles, and soldier piles and lagging.

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 it can expedite the construction schedule so that surface facilities and traffic can be returned to service sooner.

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 is the Asia transition structure of the Eurasia Tunnel in Istanbul, Turkey, in which the excavation was 168m long by 35m deep and with variable width, all constructed within a few meters of the Bosphorus Strait. The work was completed successfully using two rows of secant pile system and several layers of tie-backs.

Controlling groundwater 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 groundwater 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.

For the tunnels of the East Side Access Project in Queens, New York, the presence of contaminated ground water necessitated the use of rigid support of excavation using a combination of diaphragm (slurry) walls and jet grouting to limit the groundwater drawdown to less than 2ft (600mm).