Geomarine Limited, based in Jersey, contacted Byland Engineering for specialist advice after inspecting a near vertical 6.5m deep rock slope excavation undertaken by a local builder to accommodate a new swimming pool complex at the rear of an existing house in St Aubin, Jersey.

In the absence of any site investigation data for the development, Byland Engineering visited site to log the exposed rock face including collection of discontinuity data for stability analysis and design.  The bedrock geology of Jersey Shale, a light to dark grey weak to medium strong Mudstone with closely spaced sub-vertical joints, was present over the full height of the slope except for localised weathered soil layer in one corner. 

Rock Slope, Jersey  - Jointed Jersey Shale

Sub-Vertical Jointed Jersey Shale

Analysis of the joint data using the Rocscience program Dips identified three critical joint sets which combined with the dip and dip direction of the face indicated planar sliding was the likely failure mechanism combined with circular failure in the localised upper weathered soil layers.  

Jersey Rock Slope Joint Data

Stereonet Plot of Rock Joint Data

Stabilisation of the rock face was by drilled and grouted passive rock dowels which being drilled at 15° below the horizontal were practically perpendicular to the critical joint set and thus designed in pure shear only.  Initially, soil nails were proposed for the upper weathered soil layers but sufficient room was available to temporarily batter back to slopes to a safe angle.  The rock face was finished with Maccaferri Matmat_R, a 2mm diameter double twist hexagonal wire mesh complete with extruded poly fibre structure which acted to contain any possible large or small loose debris on the face.  At selected location, the as-dug face was actually overhanging the excavation and Byland Engineering detailed simple concrete underpins to support the overhanging rock mass and restore the face angle as present elsewhere. 

The stabilisation works allowed the permanent works basement reinforced concrete walls to be constructed in a safe environment as well as reducing the thickness compared to that required to stabilise the slope, providing a significant cost saving to the client. 

Installed Passive Rock Dowels and Facing Mesh

Installed Passive Rock Dowels and Facing Mesh