After Central Alliance (part of the RSK Group) completed ground investigation work ahead of schedule, it was commissioned by AmcoGiffen to work collaboratively with it to complete the drilling for grouting works at the mine site. The stabilisation works were required in order to stabilise the ancient mine workings that were identified during the investigations and posed a risk to a nearby railway.
The programme of works had to be completed on weekend nights, with only limited mid-week night-time working every six weeks.
The works had to be well planned and organised in order to maximise the very limited working window for the two Fraste PLG rigs employed on the project. Given site access constraints, road-rail vehicles were used to transport the drilling equipment to the site, each shift. The rigs and relevant drilling equipment and plant were lifted on to RRV trailers and mobilised to site, before being lifted on to a temporary drilling platform.
Temporary drilling platforms were constructed with rail sleepers to avoid any disturbance of the railheads (tracks) and to position the drilling rigs in the correct orientation for drilling operations.
A grid pattern of holes was drilled in accordance with the design and specification produced by WSP.
A combination of inclined and vertical holes were sunk in the cess, the 4ft and the 6ft to form a curtain wall, a total of 45 boreholes ranging from 6-18m were drilled at 3m spacings, to form the curtain wall. A total of 50 vertical infill holes were then drilled at 6m spacings inside of the 4ft and 6ft, to target the ancient mine workings and the high hazel seem.
Inclined holes were drilled to circumvent known services and drainage channels. Each borehole was drilled to 100mm in diameter, using open hole drilling techniques. Boreholes were installed with perforated 89mm UPVC pipe, to allow the AmcoGiffen team to inject liquid cemented grout into the mine workings and broken ground. In addition, three fully cored investigation boreholes were sunk to 30m to provide further information on the area surrounding the original site.
Validation boreholes for "permeability by grout acceptance testing" were also drilled to provide confirmation that the workings had been successfully infilled within the site.
Despite the restricted working time and access, the works were completed on programme and within budget - safely and without incident.
Central Alliance and AmcoGiffen have a long history of collaboration to deliver complex investigation and remedial works on railway infrastructure.