Newton Farm – Consolidation of Old Mine Shaft

Project Outline

To facilitate a scheme of residential development at Newton Farm in Cambuslang, GBLE were appointed by Johnson Poole and Bloomer on behalf of Taylor Wimpey to carry out stabilisation works to a deep old pit shaft within the site which represented a stability hazard. The works comprised infilling of the shaft with a granular material via a large diameter borehole, followed by grout injection to consolidate the infill.

The Mine shaft was known from historic records to reach a total depth of 280m below ground level. During initial excavations of the inferred location, a rectangular brick feature was exposed, indicating the lining of the mine shaft.  A specially designed shaft platform was constructed and installed over the mine shaft to allow the works to proceed safely. This platform was designed to span the calculated potential collapse zone of the shaft, particularly important in this instance since the shaft showed signs of surface collapse during the course of the works.  GBLE were then able to carry out initial investigative drilling of the shaft, the results of which indicated that it was in a void state from a depth of 60m down to its base.

The remediation methodology comprised the installation of 10-inch diameter steel casing to a depth of 100m within the shaft to allow the placement of gravel fill. The gravel was introduced into the hole using a 20m long conveyor belt with a hopper placed over the casing acting as a funnel for the material. During the course of the infill process it became apparent that the groundwater within the shaft was subject to tidal influence from the nearby River Clyde, with water levels rising to 18m at high tide and dropping to 43m during the low tide. The cyclic motion of water levels resulted in the gravel being forced up to surface and dropping continuously until the granular placement was completed, restricting the influence of the river Clyde within the previously voided area of the shaft. The total quantity of gravel used in the process was 3400 tonnes.

After completion of the gravel placement the steel casing was removed and a series of smaller diameter boreholes were drilled to allow the injection of a cementitious grout under pressure to bind the gravel and consolidate the upper reaches of the shaft. A total of 840 tonnes of grout was pumped into the shaft, followed by 16 pressure injection tests which confirmed that the old mine shaft had been successfully consolidated and no longer posed a risk to the long-term stability of the development.