What Does
Cut And Cover Mean?
The cut and cover method is a traditional form of tunneling that involves opening up the ground surface and excavating to the required depth. Once the construction is complete, the excavation is backfilled. The method is used when excavation is possible and economical from the surface, and environmentally acceptable.
Shallow tunnels, tunnels for underpasses, and in flat terrain are built using this method. A section of the earth is excavated and then covered with a support system.
Unlike trenchless construction techniques such as horizontal directional drilling (HDD) and microtunneling, cut and cover method requires traffic diversion for the duration of the project. The surface of the excavated site is restored to its original condition once the project is complete.
The cut and cover method is also known as the open-cut method.
Trenchlesspedia Explains Cut And Cover
In the cut and cover method, a shallow excavation and grading are performed after which a subsurface concrete structure is built which acts as a retaining structure. This structure or vault provides protection to the construction activities taking place within it. Cut and cover method can be of two types: bottom-up method and top-down method.
The Bottom-up Method
In the bottom-up method (figure A), the site is first excavated and proper ground support is provided. The tunnel is then constructed of precast concrete, in-situ concrete, steel rings, etc. Once the tunnel is completed, the trench is backfilled over the tunnel roof and the ground surface restoration is carried out.
The Top-down Method
In the top-down method (figure B), cap beams and side support walls are constructed starting from the ground surface and moving down with slurry walls or secant piling. The tunnel roof is then made of precast beams or in-situ concrete from a shallow excavation. Leaving an opening for access, the ground surface is restored allowing restoration of traffic services. The remaining construction is then carried out under the tunnel roof from the access openings.
Choosing Between Methods
Every project is unique and has many variables and constraints that need to be evaluated prior to selecting a particular method. Some conditions like clear right-of-way (ROW), no necessity to limit sidewall deflections, and no obligation to restore surface permanently, favor selecting the bottom-up construction method. Projects having conditions that require the opposite fulfillments are better performed using the top-down method of construction.
Preference for Cut and Cover Method
In a paper published in the Electronic Journal of Geotechnical Engineering, Professor A. Mouratidis points out this method is preferred over other similar methods when there is:
- Significant risk of falling rocks or surface slides parallel to the road alignment.
- Significant risk of deep slides in the transverse direction.
- Significant risk of deep landslide due to extended excavation perpendicular to the road alignment.
- Obstruction by existing installations to the extended open excavation.
- Prohibition due to ecological constraints.