Crusader Avenue, Glasgow – Mine Shafts Platforms

Project Summary

Soil Engineering were requested to stabilise two abandoned mine shafts. The shafts were each approximately 3m in diameter and in close proximity to each other, circa 9m apart.. The site was located in a residential area on the outskirts of Glasgow and access was very restricted and problematic.

Soil Engineering Solution

Considerable attention was spent planning the works, given the residential environment which presented several issues in relation to access and potential inconvenience and disruption, as well as the sites physical constraints. It was agreed with The Coal Authority that the safest and most appropriate method to undertake the works was from two separate platforms, constructed across the already established exclusion zone to allow personnel to work safely using two rigs concurrently.

The considerable thickness of unconsolidated near surface soils required extensive safety platforms of some 24m length, longer and stronger than any held in-house. Fortunately the shafts were reasonably central within the site and the platforms could be suitably positioned to sit entirely within the site, without physically imposing on neighbours.

The bridge platforms and piers were designed (to SEGL specifications), assembled on-site and lifted into place by Mabey Hire Services Limited. Foundation bases were designed and constructed by Soil Engineering for the structural support of the platforms and the crane outriggers. All temporary foundation works were designed by Soil Engineering’s Resident Engineer, Rob Rogers.

The slope of the site, some 3m fall from the back of the site to the road, meant that some stripping of ground at the back / top of the site needed to be incorporated to restrict the height of the platforms at the road edge and to restrict the actual depth of foundations for the platforms bearings. The 24m length would have however placed the eastern support load for the southern platform over the footpath which contained, amongst other things, a cast iron gas main. The southern platform hence had to be shortened and the foundation at the road edge deepened to lower the bearing level below the assessed potential failure envelope and unstable ground.

Foundation pads for the bridge supports were 1.5m thick and required 66m³ of poured concrete. Further foundation pads were also constructed to accommodate the crane outriggers.

The front of the site was still more than 2m lower and required temporary piers to be constructed to provide reasonably level work platforms with some fall / drainage back into the site as opposed to on to the road. The stripping of ground from the back of the site, with a long reach excavator outside of the exclusion zone, also allowed the slope to be formed into a bowl to prevent run-off of rain, grout and wash out.

To facilitate construction and lifting into place, arrangements were made with Glasgow City Council to temporarily remove a street lighting column. Delivery of the platform sections, arrival of the cranes and the 72 tonnes of ballast also needed careful planning to allow unhindered access. Issues included avoiding rush hour periods (school route) which involved consultation with local residents and temporary parking restrictions implemented along key access route points.

The platforms were constructed sequentially using a 90 T crane over 3 days. They were built outside the Exclusion Zone, one on the road and one along the southern boundary, with less than a metre to spare in many directions and were then lifted into place with a 200 T mobile crane and finally fully decked.

Once erected the platforms were fenced to allow safe working at height, surfaced with visqueen and sleepers to prevent grout spills and metal tracks damaging the platforms. Stairs were also incorporated at the road edges to provide two access / egress routes from each platform. The bearing plates and piers were also bolted to the concrete bases to preclude movement when in use. The drilling rigs were lifted onto the platforms and the shaft drilling commenced.

On completion of the grouting works the platforms were removed in a similar manner to the emplacement, although both platforms were sequentially lifted on to the road and dismantled there to save demobilising and re-mobilising the grouting silos. However, the toilet facilities and the generator etc. had to be double handled to make room at this very restricted site.

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