The Melbourne Underground Railway Loop

A most interesting engineering solution to a difficult tunnelling problem occurred at the site of Flagstaff Station. The problem was a geo-technical one, where the existence of a layer of silty clay, between the overlaying basalt and the weathered silurian red-rock, precluded the construction of the lower and upper platforms in four separately driven tunnels of psuedo-elliptical shape, as was done in Parliament Station. The existence of this weak compressible band of silty clay, some three metres thick, varied in location between the crown of the lower tunnels and the invert of the upper tunnels, including the vulnerable horizontal pillar between the two tunnels.

An unusual scheme was worked out, whereby only the bottom and top drifts (or small tunnels) for the walls were driven and a series of one metre diameter concrete columns, at three metre centres, were constructed by raise-boring between the upper and bottom drifts, as shown diagrammatically in Fig. 47. These columns, when concreted into the upper and bottom drifts, achieved the purpose of by-passing the compressible weak layer for vertical loads when the arch crown was taken over. This system was repeated for the other two platforms. The excavation then became a straightforward problem, being taken down from the top and cross strutted with temporary struts until the final structure was in place.

47 Upper and bottom drifts and construction of columns by raise bore method, Fagstaff Station, Melbourne

Australian Academy of Technological Sciences and Engineering