While the geology encounter typically comprised rocks of sufficient hardness for mechanical drilling to be used, those of exceptional hardness were often encountered, resulting in even the best drills being blunted and greatly slowing the rate of advancement, sometimes to as little as one meter per day. There was also a section of entirely disaggregated bedrock encounter, which could only be cleared using traditional manual techniques, which came with the risk of potential collapse. To attain sufficient clearance for the running of trains, the ceiling was vaulted following the completion of advanced boring.