Barnhill 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.

Settlement modelling

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.

Making in-roads

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.

The fill body was built up in 1m thick layers and compacted using High Energy Impact Compaction (HEIC). The use of thick layers provided a rapid means of placing the fill to the required 5% air voids. The fill mix also needed to allow for ultimate inundation of the fill body that will occur on cessation of adjacent quarrying and associated groundwater lowering, whereupon the water level will thus rise by about 24m above the quarry floor. Prevention of collapse compression was therefore a significant consideration both in the soil grading specification and quality control during placement.

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.”

Fill Facts:

  • Maximum depth of fill   30m
  • Overall volume of fill  720,000m3
  • Contract value£2.55m

Engineers: Clarkebond
Compaction Contractor: Landpac
Earthworks Contractor: Sanctus
Site Owners: Chelverton Deeley Freed
Developer: Bloor Homes (Swindon) Ltd