Construction, following an act of parliament, began in 1784. Twenty-six shafts were dug along the line of the tunnel and workfaces dug in each direction eventually joining up before the diameter of the tunnel was expanded. Difficulties were encountered related to strata of Great Oolite (solid limestone) and Fuller's Earth clay. These caused roof falls and narrowing of the channel which continued after the tunnel opened in 1789 and required frequent maintenance. The stone portals at either end are listed buildings.