Crusader Avenue, Glasgow – Mine Shaft Investigation

Project Summary

Ground subsidence at Crusader Avenue, located on the north western outskirts of Glasgow, was reported in August 2011 at the position of two recorded former coal mining shafts. The ground collapse had caused significant damage to 2 of 5 No. detached garages as well as extending beneath and adjacent to two pairs of semi-detached dwellings. These four residences were immediately evacuated by the Coal Authority and Soil Engineering (SEGL) were subsequently instructed by The Coal Authority (Client, Engineer and CDM Co-Ordinator) under a framework contract to initially investigate and then make safe the dangerous collapse.

Soil Engineering Solution

The Coal Authority established a 20 m x 30 m exclusion zone around the unstable ground and zone of potential further failure and tipped 670 tonnes of stone into the surface void prior to any works being undertaken. Meanwhile research and decisions in relation to the treatment strategy were undertaken by The Coal Authority and Soil Engineering Geoservices Limited.

Research undertaken by The Coal Authority suggested the presence of 2 No. abandoned mine entries in close proximity to each other, one to 93 m depth and one to 147 m depth, which were abandoned in 1884. Workings were recorded in the Knightswood Gas Coal (46 mbgl), the Upper and Lower Garscadden Ironstones (99 mbgl & 116 mbgl) and the Garibaldi Clay Band Ironstone and Garibaldi Coal (123 mbgl) seams.

Initial investigation works comprised cable percussion and dynamic sampling boreholes to confirm the nature of the superficial soils. These were found to be circa 20 m in thickness and to comprise boulder clay over granular drift deposits. The dynamic sampling holes were required at the back of the site where the Exclusion Zone limited access and a safe system of work including safety harnesses and a running line had to be developed.

Following demolition of one pair of semi-detached properties indicated to be immediately adjacent to the two shafts, a geophysical survey was developed and undertaken. The Ground Probing Radar equipment was traversed across the affected area by pulling the equipment with ropes from outside of the Exclusion Zone to and fro over the unstable area.

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