The investigation of and remedial works for unstable slopes beside the A40 Trunk Road near Monmouth

Abstract

The A40 improvement between Whitchurch and Monmouth entailed the construction of a dual carriageway road with trunk road standards from Ganerew Bridge at about 300 ft above N.D. down a steep sidelong slope beside the river Wye to the flood plain level of the Wye at about 60 ft above N.D. The course of the road approximates to the strike of the rocks which have an easterly dip of about 30° into the Wye valley and which consist of sandstone with subordinate mudstones of the Devonian Brownstones Group. The mudstones have marked clay mineral orientation parallel to the bedding which makes them particularly susceptible to bedding slips. In a length of cut on the up-dip side of the road near Ganerew slips occurred, were investigated, and successful remedial measures were designed and applied. On the sidelong slopes opposite the Wye the road had to cross three ancient landslips involving considerable cut and fill. The fill on one slip at Chapel Farm resulted in reactivation of the entire slipped mass and the ultimate destruction of the farm. On another, the Whipping Green Slip, the intercarriageway cut failed repeatedly. The geometry and piezometric condition of all the slips were investigated thoroughly and remedial measures were prescribed. These involved counterforts, lightweight embankments, and a large berm at the toe of Chapel Farm slip, below which sand drains had to be employed in the Wye alluvium. The object of these remedial measures was to at least restore if not improve the original Factor of Safety which was marginally > 1. Above the road there is a large area of partly slipped rock which it was decided to leave subject to the installation of a permanent monitoring installation. The installation of the monitoring devices is taking place this year.