Discussion on W. B. Evans

Abstract

E. J. Arrowsmith, A. G. Morris and L. Bradley (North Western Road Construction Unit) described slips and remedial measures in cuttings at three localities on the M62. With major slips occurring either during construction or after completion of excavation, and with the need rapidly to establish stable slopes, detailed investigations could not be undertaken. Subsequent observations showed that the slopes had certain features in common. The strata consist of alternating beds of sandstone and shale of the Millstone Grit and beds at the base of the Lower Coal Measures. Bedding dips gently into the cuttings at 6° to 20°. Modes of failure appeared to have been similar in all cases and were interpreted as translational slides on low shear strength materials in a geological setting of partially confined aquifers and potentially high seepage forces. The authors assumed a general form of wedge failure illustrated in Fig. 1, and presented simplified diagrams and geological cross-sections showing the slope profiles and the suggested limits of rock movement behind the face (Figs. 2, 4 & 5. Piezometers were installed in boreholes in the north batter of Windy Hill Cutting (Fig. 2) and the recordings from them gave some indication of the response of the several perched water tables to the incidence of rainfall (Fig. 3). They were used to check the effectiveness of the drainage measures in depressing the water-levels below critical values. Low strength materials encountered in the cuttings were probably clay mylonites and appeared in each case to have been the