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Geotechnical: Plymstock quarry

Developing a Devon quarry has required a significant amount of slope stabilisation works.

Plymouth may be well known for its naval ­history, but probably less well known is the quarrying industry that exploited the Devonian limestone at many quarries in and around the city.

One of these was Plymstock quarry, on the eastern side of the city, where limestone was extracted and processed to produce cement. The quarry was acquired by Persimmon Homes for a major residential development.

Before construction work could begin, the base of the quarry had to be filled to create a platform for the new homes to sit on. This material was taken from the northern face of the quarry, which was cut to create a new face, generally at 80° to the horizontal.

Needed to be stabilised

This slope, along with the original quarry faces in other parts of the site, needed to be stabilised. “More than 2km of rock face, ranging in height between 10m and 30m, required stabilisation, including scaling of areas of loose or poor quality rock, installation of rock bolts and anchors coupled with either face netting or sprayed concrete, reprofiling of area of poor rock to slacker batters, and forming engineered soil buttresses to support the faces,” explains Laurence Tomlin, senior associate at Peter Brett Associates, which carried out the design of the rock stabilisation works, the reinforced soil slopes and assessed rock fall risk.

In addition to the stabilisation works, which were completed in early autumn, a reinforced soil slope is being built along the foot of the existing southern face to provide support to its lower part, and to act as a rock trap for falling rock debris from the exposed upper part of the face.

The main contractor for the earthworks was Cuddy Group. Its subcontractor Albion Drilling undertook the drilling and blasting to form the new face profiles, together with the drilling and installation of rock bolts to stabilise the slopes.

“We used observational techniques to assess rock mass characteristics,” says Tomlin.

“The designs of stabilisation measures were developed based on this data, to best suit conditions encountered at different locations. Rock quality, for example, changed during the year-long project, sometimes from day to day.”

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