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.
WSP is a specialist in urban cut-and-cover construction. Tunneling beneath cities presents several challenges unique to the environment, such as:
Crisscrossing utility and transit lines
Special underpinning, structural support and building protection
High groundwater levels
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.