A Discussion on valley slopes and cliffs in southern England: morphology, mechanics and Quaternary history - The history and stability of two Lias clay slopes in the upper Gwash valley, Rutland

Abstract

Two detailed valley side sections are described. Both are of landslipped slopes cut in Upper Lias at the point where the Inferior Oolite escarpment is breached by the east­ ward flowing river Gwash, a few kilometres east of Oakham, Rutland. Field mapping suggests that the slopes were cut in Ipswichian times either shortly before or during the deposition of the Second Terrace of the river Welland, of which the Gwash is a tribu- tary. One of the slopes (Barnsdale) was oversteepened by fluvial erosion which stripped the cambered Inferior Oolite and frost-disturbed Lias from the escarpment face, leaving, at the slope foot, a prominent erosion surface (the £Hambleton Surface’). After cessation of erosion Head accumulated on the Surface and the slope degraded, much of the degradation possibly occurring in the Middle Devensian, though land­ slide movement continued to the end of the Late Devensian. The other slope (Hambleton) which had formerly been cambered, suffered only slight steepening contemporaneously with the Barnsdale slope and the large, shallow rotational landslide that developed in the frost-disturbed Lias clay was subsequently partly obscured by Late Devensian solifluction. Stability analyses of these two landslides, together with three other previously pub­ lished case records, show that the field values of residual strength of Upper Lias clay are strongly stress dependent, with the magnitude of p'T (4 = 0)falling to 10° as the normal stress increases. Laboratory measurements of residual strength using the ring-shear apparatus are similarly stress dependent, but show considerable strength variations between different samples with similar index properties. The ring-shear strengths underestimate the field strengths by up to 11%.