Bromhead's paper provides useful information, derived from the back-analysis of existing slips in the Herne Bay area, on the values of residual strength of the London Clay mobilized in the field. As the slips treated are generally rather deep-seated, the data extend our knowledge of the field residual failure envelope for the London Clay into a higher range of normal effective stress.
In his summary plot (Fig. 10) of this envelope, however, the author does not make use of some existing data in the low and medium normal effective stress range. Field residual strengths derived from the back-analysis of shallower slips in the London Clay are available from sites at Guildford, Hadleigh, Sud-bury Hill and Dawes Road. These data, plotted with Bromhead's values in Fig. 1a, permit the construction of a well substantiated field residual envelope for the London Clay up to a value of normal effective stress of about 160kN/m2. The shape of the residual failure envelope in the vicinity of the origin of the Mohr diagram and the magnitude of any small intercept on the shear strength axis remain, of course, uncertain. The field residual envelope shown in Fig. la can be approximated, up to a normal effective stress of about 70 kN/m2, by the parameters c′r = o, φ′r = 13.3° (Hutchinson & Gostelow 1976) or by c′r = 0.5 kN/m2, φ′ r = 12.9°. The envelope thus defined lies slightly lower than that shown by the author for this stress range. At higher normal
- © The Geological Society, London 1980