Settling down on a quarry
Work has started on the construction of new homes on top of a former quarry in the South West.
Just behind the high street in the medieval market town of Chipping Sodbury, near Bristol, construction work has started on the first phase of a new development of 2, 3 and 4 bedroom houses. Contractors have moved onto a flat level site and will in the main be using raft foundations for the development. This straightforward approach on the site of the former Barnhill Quarry belies the extensive engineering expertise that has been required to address the challenges of filling a quarry, in places 35 metres deep, to create a level development platform of over 30,000m2.
Understanding the scale and time of settlement was an essential requirement for the development management. Preconstruction assessment of the fill performance and prediction of settlement characteristics was carried out in order to define the point at which settlement would effectively be complete and construction could begin. This enabled the client to confidently negotiate the sale of the finished platform to the house builder.
The pre-construction model indicated that a period of 2.5 months was required between completion of filling and start of construction. Whilst the magnitude of post-construction settlement was very small, the final out-turn was a period of 3 months following completion of filling, and confidence that construction could begin.
The former quarry faces extend for up to 11m above the east and west side of the finished development platform. A detailed risk assessment was undertaken of potential trajectories in the event of rockfalls from the cliffs. This enabled efficient design of a catch trench at the toe of the slopes to minimise landtake within the development platform.
One of the first challenges on the project was to construct a new access road for an adjacent supermarket development at the south end of the quarry. The access road was required at original ground level before quarry filing could begin, requiring a 10m high reinforced earth slope formed at 70 degrees and constructed on a bench in the quarry face.
Additionally, a haul road was required along the floor of the 1.3km long and 125m wide quarry in order to deliver fill material from the adjacent operational quarry, to the platform area. The steep climb from the quarry floor also required an engineered access ramp to be built. The access ramp passed over a former tailings lagoon containing saturated silt soils. Engineers formerly engaged on the project recommended a design incorporating costly wick drains to relieve excess porewater pressures caused by the ramp construction. Through provision of a longer route on the ramp, at shallower gradient, Clarkebond provided an alternative design where porewater pressures were confined within the lagoon under increased normal loads, thus controlling the destabilising effect and removing the requirement for expensive additional drainage.
Immediately following completion of the temporary slope, construction started to fill the main quarry.
What lies beneath?
The fill was derived from several sources. These included waste soils remaining in the quarry when it was abandoned; quarrying an area of poor quality rock from the adjacent operational stone quarry, which also created a new platform from which quarry plant could operate, and a minor volume of imported waste soil from other local developments, via a treatment hub.
Arrangement of various soil types, including cohesive and granular portions, within each layer ensured short drainage paths facilitating rapid self-weight and consolidation settlement. Prediction of settlement characteristics were made ahead of the construction based on parameters derived from laboratory testing. It was recognised that this would provide a conservative assessment of both rate and magnitude of settlement.
Settlement monitoring was undertaken throughout the fill placement using gauges installed at various levels within the fill body. This provided quantification of actual magnitude and rate of settlement as the fill body was constructed, enabling continual refinement of the settlement model to provide accurate prediction of the post-construction settlement in order to define the finished fill level.
Jon Palmer, Clarkebond’s Ground Engineering Director, who has managed the geotechnical aspects of this project from day one, commented, “Achieving the residual settlement targets within 3 months of completion of filling was key to the financial viability of the project and, as far as we are aware, without precedent. The success of the project is testament to the compaction technologies employed, and rigorous control of the onerous specification. This was only achieved through close collaboration between client, contractor and designer.”
- Maximum depth of fill 30m
- Overall volume of fill 720,000m3
- Contract value£2.55m
Clarkebond: Clarkebond is a multi-disciplinary engineering and management consultancy providing civil, structural, Ground Engineering and management and implementation services across the UK. Integrated, multi-disciplined teams provide individual and team expertise across all sectors with areas of specialist expertise including geotechnical and geo-environmental engineering, flood risk, waste management and contaminated land assessment and utilities - asset design, resilience and recovery including blast analysis, and facade engineering.
In 2014, Building Magazine ranked Clarkebond as the 2nd fastest growing consultancy in its list of the top 150 Consultancies in the UK.
For further information, please contact Jane Stanbridge, 07790 888321, email@example.com