Mechanical rock excavation today provides a much more cost efficient method of tunneling than drill and blast. The drill and blast method  is necessarily a sequential operation: drill small diameter holes, load with  explosives, blast, ventilate, remove muck and start the cycle again. Mechanical rock excavation is a continuous process whereby rock is "cut" from the tunnel face and removed by conveyor or train from the tunnel. Robbins' primary product  is tunnel boring machines (TBMs).