By the late 1880s, the city had become too dense with buildings, sewers, and electric cables for the " cut and cover" technique, so a new system had to be devised.

十九世纪八十年代末，伦敦因楼房、污水管、和电缆外露而变得过于拥挤，原因是随挖随填技术，因此，人们需要设计新的设备。