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:
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Congested sites
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Historic areas
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Crisscrossing utility and transit lines
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Special underpinning, structural support, and building protection
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High groundwater levels
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Groundwater contamination
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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.
