Case Study: Pollokshields
Client: Network Rail
Main Contractor: BAM Nuttall
Location: Pollokshields, Glasgow
Services on the Cathcart Circle line on the south-side of Glasgow were suspended between 22nd July and 14th August 2008 to allow for Ritchies to undertake urgent treatment of the old mine workings four metres below surface causing subsidence in the Pollokshields East area of Glasgow.
Prior to the blockade some perimeter grout holes were drilled behind the retaining wall running adjacent to site compound. These holes were inclined at various angles so as to hit the pavement level at the correct locations and terminated at rock head in the initial pre-blockade operations. Temporary steel casings of 131mm diameter were drilled to rock head and an 89mm plastic casing inserted.
During the blockade a crane lifted one of Ritchies’ fourteen tonne drill rigs down onto track. Perimeter grout holes at south end of site were drilled with the rig working westwards. The perimeter holes were followed by the infill holes, which were drilled using an 89mm diameter steel casing. For the infill holes a 55mm diameter plastic pipe was installed prior to withdrawal of the steel casing.
The other rig completed the drilling of the perimeter holes (started prior to the blockade) one metre into the pavement from the south end of the site. Once done; it too was lowered to track level for infill drilling.
Grout was mixed using Ritchies’ eight cubic metre mixing plant. Grouting commenced on the perimeter holes at the south end of the site. Four air powered Mono pumps were used fed from the grout holding tank. Those perimeter holes, which continued to take large quantities of grout, had pea gravel added via a hopper system.
In total fifty perimeter and 126 No. infill holes were used together with 21 No. Pressure/Test holes. 560 tonnes (dry weight) grout was injected. The treatment area covered approx 75m x 21m, of which 41m x 21m was found to be worked. The voids varied from 0.3m to 1.2m.