Project Sheet: Pentre Broughton Housing Estate
- Mytton Homes Ltd
- Dave Cottle Civil Engineering
- desk study
- ground investigation
- detailed design
- part time site supervision
Description of Project
This project, which comprised a housing estate with around 70 houses, involved the redevelopment of a site affected by the following geotechnical issues:
- Wide variation in ground levels
- 5m high colliery spoil tip on site
- Landfill gas and acid mine drainage associated with the tip
- Contaminated land associated with the colliery spoil tip
- Site underlain by old railway tunnel some 18m deep and 4m wide
- Four mine shafts and one adit entrance
- Shallow mine workings
- No adoptable drainage
A detailed desk study was carried out first, which amongst other research involved looking at mine plans held at the Coal Authority headquarters in Mansfield. This helped to identify the extras over normal construction costs, which enabled the client to get these costs taken off the selling price.
It also enabled a cost-effective ground investigation to be designed, which comprised the following:
- Machine dug trial pits and soakaway testing
- Boreholes on the tip and monitoring of gas standpipes
- Rotary percussive boreholes to identify shallow mine workings and the tunnel depth
- Physical and chemical testing
The ground investigation proved the general ground conditions to comprise colliery spoil in the tip, although the rest of the site was underlain by glacial till overlying mudstone bedrock, with a large fault crossing the site. A thin outcrop of sandstone bedrock was encountered at the extreme southern end of the site.
The variable ground conditions and mine workings meant that a variety of foundation types were required.
The re profiling of the colliery tip involved extensive earthworks, with the colliery spoil being excavated and recompacted beneath the building footprints. This then enabled the houses to be founded on raft style foundations.
The mine entrances and two old air shafts to the railway tunnel were found and treated. The shallow mine workings, which were at depths of 7 to 9m, were drilled and grouted, with the works designed, tendered and supervised by GroundSolve Ltd. Above the workings the houses were also founded on raft foundations in line with NHBC guidelines.
In areas where access roads were underlain by shallow mine workings a reinforcing geo-textile was used in the road construction.
In addition, GroundSolve Ltd applied for all of the permission to treat forms and completed the reporting of the works to satisfy the Coal Authority conditions
Elsewhere in those area not underlain by made ground or shallow mine workings the house was founded on conventional spread foundations.
The railway tunnel, which was found to still be open, was backfilled from surface where it affected the sites development. This work comprised drilling large diameter boreholes into the tunnel and backfilling with stone in case the tunnel was acting as a drainage conduit.
The properties on the remains of the colliery spoil tip had gas protection measures incorporated and these were inspected and validated by GroundSolve Ltd. In addition, minor heavy metal contamination was encountered in the tip material and Groundsolve Ltd provided the validation report to satisfy the planning conditions imposed by the Local Authority.
One of the main problems presented for the redevelopment of the site was the lack of adoptable drainage around the site. This was solved in two ways, firstly by drilling deep borehole soakaways. Secondly, the drainage was taken to the extreme southern section of the site where it was discharged into a large soakaway in the sandstone bedrock. This also enabled the problem of the acid mine drainage to be addressed, with the seepages from the tip being collected and fed into the main drainage system where it could be sufficiently diluted to negate the problem.
In conclusion, the undertaking of a detailed desk study and site investigation enabled a complex brown field site with an extensive industrial history to be developed in a cost-effective manner.