G. H. Child and D. R. Norbury write: From our investigation of the slip when active we would like to make the following comments. Although we did not find shear zones in borehole samples, overnight movement of drill casing in two holes and positive response from a slip indicator in another enabled us to identify the surface of sliding under the embankment side slope which the authors confirmed. We disagree with the authors on the location of the back of the slip, considering the crack in the road to be a secondary feature. In our investigation we considered the primary slip surface to be as shown on Fig. 1 Figures 2 and 3 show the top and toe of the slip on 25 October 1976. The casing of the borehole commenced on the verge and referred to in the paper can be seen in Fig. 2 At this stage major movement had occurred and was all downslope of the road. Figures 3 shows the surface ruckling that had already occurred in the field below.
Because of the large crack in the embankment side slope we question the authors’ conclusion regarding the significance of pore water pressures in the fluvioglacial deposits uphill from this crack. Pore water pressures to their line A or above seem most improbable since this would imply flooding of the old A55 road.
We agree that the embankment fill was relatively permeable; gradings indicated that it was probably more permeable than the fluvioglacial sand. Therefore, a feature
- © The Geological Society 1984