Pollokshields Treatment of Old Mine Workings

Background
Services on the Cathcart Circle line on the south-side of Glasgow were restricted between Tuesday, July 22, and Thursday, August 14, to allow for urgent repairs to subsidence in the Pollokshields East area caused by collapsed mine workings.

As a result, First ScotRail ran a revised timetable between Glasgow Central and Neilston and Newton over a three week period while Network Rail carried out repairs on the line at Pollokshields East station.

The works, which were successfully completed four days earlier than originally planned, resulted in no services at five stations along the Cathcart Circle line – Pollokshields East, Queens Park, Crosshill, Mount Florida and Cathcart.

The problems at Pollokshields East were first identified at the end of May 2008. A speed restriction of 5mph was imposed immediately and watchmen were assigned to the site to monitor the track condition after every train movement.

A full survey of the site by Network Rail identified the cause of the subsidence as ancient mine workings four metres below ground which had partially collapsed, causing the ground near the track to subside.

In order to fix the problem, Network Rail considered several options and all but one would have involved excavation of the site and long term disruption to train services and local residents for many months. The solution involved drilling bore holes and filling the gap beneath the ground with grouting material.

The ground treatment work was been undertaken by Ritchies, the specialist geotechnical division of Edmund Nuttall, with Edmund Nuttall carrying out the track removal and replacement works, all on behalf of Network Rail at a cost of approximately £2.4 million.

The Works
Following the establishment of the blockade and removal of the track in the effected areas by Edmund Nuttall, Ritchies established two drilling rigs and bulk grouting equipment on the site.

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. Steel casings of 131mm diameter were drilled to rock head and an 89mm plastic casing inserted. The steel casings were then withdrawn.

Following commencement of drilling Ritchies established a water supply for grout mixing and took delivery of the grout batching plant together with holding tank and pumps. These assembled on site. A materials storage area was established adjacent to the grout batching plant.
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 it had done this; it too was lifted down to track level for infill drilling.

Grout was mixed using Ritchies eight cubic metre mixing plant. Cement and pulverised fuel ash loaded using a telehandler with bucket attachment with water being provided using a diesel pump fed off of two large water tanks.

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.

Ritchies are one of the UK’s leading geotechnical contractors undertaking a variety of specialist works on the railway infrastructure including cutting and embankment stabilisation, installation of rock fall and debris flow barriers, piling and mini-piling, ground anchors, old mine workings treatment, tunnel and bridge repairs, tie-bars, grouting, concrete repairs and ground investigation works.

Ritchies operates nationally from four regional offices.

Article courtesy of The Rail Engineer - Sept 2008