The proposed North Devon Link Road will connect the M5 with Barnstaple by way of Tiverton and South Molton. As a background to the geology, the rocks along the route are of Carboniferous and Permian age. The Carboniferous rocks are thinly bedded mudstones and sandstones, highly deformed and compressed into a synclinorium with a regional east-west strike.
These are overlain unconformably by relatively undisturbed Permian marls, sandstones, pebble beds and breccias which are horizontally bedded.
In the Permian strata the main consideration in route location has been to avoid the finer, partly cemented clay and silt-sized deposits which soften rapidly when handled. These are difficult to deal with as embankment fill or foundations. In some cases the less permeable layers enclose water under artesian pressure and this can be difficult to allow for in cutting or foundation design.
For route location a simple preference has been shown for the more granular pebble bed, breccia and some sandstone deposits. These form the higher ground, are free draining and consequently have less ground water problems. Cuttings in these deposits have steeper side slopes and are generally less obtrusive.
In the Carboniferous synclinorium, the synclines and anticlines have differentially weathered often leaving synclinal ridges and anticlinal valleys running east-west. Faulting has had the effect of stepping the ridges en echelon. The major geological consideration in route location has been slope stability. A proposed route trending linearly north-west may have to cut through the same ridge several times since the faulting may repeatedly transpose
- © The Geological Society, London 1977