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. 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 20m, but rarely does it exceed 30m.
DKE 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.
WSP is a specialist in urban cut and cover construction. Tunnels beneath cities presents 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 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.
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.
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.
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.