T. R. Harper writes: Dr Nichol's discussion of time-dependent strains resulting from excavation in geological materials is a welcome publication. It seems particularly appropriate that this is presented before a United Kingdom audience, because perhaps the processes which are discussed are relatively unrecognised here. The apparent lack of reported in situ stress determinations in rock in the UK presumably reflects such an attitude. Yet, since it would be unreasonable to assume that the geological processes from which the rocks of the UK are derived differ from those which have pertained elsewhere, similar engineering behaviour can be expected. As stated by Dr Nichols, time-dependent strains must be expected whenever the equilibrium of geological materials is disturbed.
There are many points of interest in this paper. The following comments refer only to two pertinent factors. The first of these concerns the effects of residual stresses; the second refers to certain effects of intrap late geological processes which are active during the life of an engineering structure.
The author correctly states that the viscoelastic response (of geological materials) may be purely a response to pre-existing tectonic loads, manifested when the existing loads are removed. However, a greater emphasis is placed in this paper upon the possible contribution of residual stresses rather than the effects of relieving boundary tractions. Amongst other comments, Dr Nichols states that residual stresses may explain the very striking local variation of measured stresses observed by Gay (1975) and Her get et al. (1975). However, observations of strain