August 2009
Stabilisation of steep unstable slope requires difficult soil nailing programme
Following a significant landslide at Barbadoes Woods in the Wye Valley above Tintern, a major ground stabilisation project has now been completed for the Forestry Commission Wales.
Albion Drilling Group undertook a difficult soil nailing programme, designed by the Halcrow Group for main contractor, Dean & Dyball Civil Engineering, on an extremely restricted site. This provided a long term solution to the instability of the upper hillside in this Area of Outstanding Natural Beauty.
The area affected by the slope failure had a typical gradient of 1:1.5, was some 100m wide and extended between 25m and 80m down the hillside. Securing the slope involved the installation of some 400 soil nails, varying in length from 7m to 21m depending on the ground conditions, through the clay and into the underlying rock.
With no access to the site from below and only a narrow road at the top, conventional drilling equipment and long reach excavators could not be used, mainly because of safety fears that heavy equipment would increase the risk of further ground movement on the unstable slope.
Where possible a 13t soil nail rig was operated from the access road while over much of the area an 8t crawler rig was winched down the hillside, anchored to an excavator on the top road. This system was used to install the longer 21m soil nails where higher drilling power was required. On the steepest sections a spider rig, fitted with an Albion drill mast, was able to negotiate the slope.
Following successfully installation of the soil nails, the slope was covered in steel mesh that was secured to the ground to improve surface stability and provide erosion protection. Improved drainage and earthworks to reduce loads at the head of the slope further assisted in stabilising this severe incline in the Barbadoes Woods near Tintern Abbey.