Some landslipping encountered during construction of the A40 near Monmouth

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Abstract
Summary: The A40 is a trunk road constructed in 1964 on sidelong ground beside the River Wye, about 1.54 km, north of Monmouth. The sidelong ground comprised a dip slope of 30-35° dip in the Brownstones of the Devonian which consist of sandstones with subordinate mudstones. On investigation it was found that three ancient landslips existed which the road had to cross. One, at Chapel Farm, was estimated to be probably first active in post-glacial times; another at Whipping Green, was estimated to have occurred at about Roman times; and the third, at Vaga, was estimated to have occurred sometime betweeen the other two. Conventional engineering had not succeeded in crossing the sidelong ground without re-activating these slides. Total failure had occurred at Chapel Farm Slip and the inter-carriageway slopes had failed where the road crossed Whipping Cross Slip. After a full investigation the road was re-designed more or less on its original route using light-weight fill for embanking and a massive berm at Chapel Farm, and chiefly by using replacement counterforts on the cut slopes at Whipping Green Slip. Sand drains were used through the Wye alluvium below the berm where it was founded over the alluvium. There was also a large area of ‘partly slipped rock’ which although it did not constitute a landslide, threatened the road from above and was far too large and complex a mosaic for treatment or removal. It was decided to build a rock fill revetment at the toe of this mass and then to monitor it for movement on the assumption that only one or possibly two parts of the mosaic could move at any one time. During the investigation a fairly exhaustive laboratory testing programme was carried out and a detailed study was made of the behaviour of groundwater. It is now 1984, some 20 years after construction of this section of road and there has been no trouble of any kind.

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