Drop shafts and vent shafts are a common means of providing access or ventilation into a large diameter, horizontally bored tunnel through bedrock.
The most common types of these shafts and tunnels are part of Combined Sewer Overflow (CSO) projects, where untreated or partially treated waste, debris, and storm water is collected and stored during large weather events that would otherwise lead to flooding.
Using specialized drilling equipment, Keller excavates overburden and sets a permanent steel casing on top of the underground bedrock layer. This casing is often grouted in place. Then, the rock is removed below it to a specified elevation, usually coinciding with the top of the tunnel to be bored below it.
For larger diameter drop shafts exceeding 12 feet in diameter, a secant pile wall can be constructed to support the excavation instead.