A backstop is a reinforced portion at the rear of a tunnel boring machine’s (TBM) entrance pit wall that is located directly behind the set of longitudinal rails (which are mounted on cross members, like a railroad track, for stability and support) that provide support and initial directional guidance for the TBM. Like the thrust block in pipe ramming, the backstop ensures even distribution of the forces resulting from the TBM’s initial efforts.
Trenchlesspedia Explains Backstop
As the TBM moves forward, Newton’s third law of motion – essentially, that for every action there is an equal and opposite reaction – begins to apply. However, the force from the drilling effort is transferred to the rail system and to the ground. This opposing force, coupled with the TBM's own motive power, successfully opposes the tendency of the TBM's cutting operations to push the machine backward and, in fact, drives the TBM forward.