A1.3.3 Cut and Cover
Cut and cover tunnels are shallow tunnels constructed in a trench. Prior to excavation, buried utilities and services crossing the route have to be protected or diverted. Permanent utility diversions are used to avoid the tunnel alignment where possible. When this is not possible these utilities may be temporarily raised over the alignment clear of construction and then reinstated. For gravity sewers this may involve temporary pump installation to pump over the excavation prior to reinstatement.
The cut is constructed in a number of ways, depending on the support requirements of the ground. In hard rock this may be vertical walls supported by rock bolts and sprayed concrete. In soft rocks and soils stable slopes may be created by excavating benches or batters. If surface space is restricted, or the disturbance caused by construction needs to be minimised then retaining walls can be used to stabilise the excavation. These may be permanent and incorporated into the final structure or temporary and removed after the tunnel structure has been completed.
In the conventional “bottom-up” method the excavation is progressed to full depth, using anchors or bracing struts to provide temporary support to the retaining walls until the tunnel floor and roof slabs are cast.
Alternatively a “Top Down” sequence can be adopted where roof slabs are cast as a strut for the retaining walls. Excavation beneath this slab is carried out through access holes until the base slab level is reached and cast. Generally where long lengths of relatively narrow running tunnel are to be constructed using cut and cover, the more conventional bottom-up technique is used.
Once a stable open cut has been constructed, the tunnel structure is fabricated in the trench. This structure is generally constructed from reinforced concrete using large forms. Once the tunnel floor and roof slabs are cast, temporary struts can be removed or temporary anchors de-stressed. The tunnel roof could also consist of bridge beams placed on the tunnel walls that act as abutments. The use of precast concrete arch roof structures is becoming more commonplace in C&C tunnels. Once again provision can be made to facilitate land redevelopment above the tunnel.
After construction, fill is used to reinstate the ground surface. Where possible reinstatement should be carried out with stored material from the cut stage. C&C tunnels by nature usually have sufficient self weight and overburden to prevent floatation. However, in ground conditions with a high water table the design would have to cater for the effect of flotation and uplift forces on the base slab, this can involve tension piling in some cases, or permanent dewatering as implemented for Stratford International Station box.
In some cases the retaining walls and cut and cover tunnel may form the foundations for new buildings to be constructed above the tunnel. As surface excavation and wall support are the key cost elements of construction where possible both running tunnels (and any escape provision) are generally included in a single structure.
Cut and Cover
This information is taken from HS2 Publications and becomes important to Burton Green The implications will vary depending upon the method to be used. Tedious to read but worthwhile
