Breakthrough of a tunnelling scheme in the US may have set a new record for small diameter tunnelling in hard rock.
Work on the Corbalis to Fox Mill Water Main Project in Fairfax, Virginia called for the Robbins tunnel boring machine (TBM) to bore through diabse rock with a strength of up to 345MPa.
When initial work on the scheme using a Lovat Double Shield TBM failed a new contractor was brought in to complete the 2.7km long tunnel. Southland Contracting brought in a 2.2m diameter Robbins Double Shield TBM for the scheme which took 18 months to complete the remaining 2km of the bore.
Drill and blast was not an option, according to Southland project manager John Marcantoni because the tunnel route runs parallel to two petrol pipelines as well as being close to an airport.
“Our issue on this scheme was not water inflows, faults, or seams, it was actually the consistently hard rock,” says Marcantoni. “The TBM was working at its highest capacity just to cut the rock. We kept pushing and a lot of our success was in maintaining the cutters and the ability to keep pushing as hard as we could. We had a rigorous maintenance schedule.”
The strength of the rock not only tested the equipment, but also generated a lot of heat, making working conditions and cooling the equipment difficult. Despite the equipment challenges and much higher than normal cutter wear, the bore is now complete and work is underway to install the secondary lining. The tunnel is expected to be in operational use by the end of the year.