Strain rate behaviour of Saint-Jean-Vianney clay: Discussion

Abstract

The paper by Vaid, Robertson, and Campanella is notable because (a) the Saint-Jean-Vianney (SJV) clay is considered to be particularly susceptible to effects; (b) the measurements were made with special care; and (c) the results can be used to examine some central questions concerning in situ behaviour of sensitive clay soils. Crawford (1964) performed oedometer tests on samples of sensitive Leda clay in which the dmation of each load increment was varied from about 1 h to 1 week. He concluded that compressibility was strain-rate dependent and that the quasi-preconsolidation pressure (Leonards and Ramiah 1959) was strongly dependent on the loading procedure. Bjerrum (1967, 1972~) developed hypotheses to explain these phenomena and proposed (Bjerrum 1972b) that the quasi-p,' would actually vanish at field rates of strain. Although these views have been vigorously opposed (Leonards 1968, 1972, 1977), the prevailing opinion is to accept their validity. From the authors' Fig. 2, the change in void ratio per unit change in effective stress was calculated for stress increments of 900-1000, 1500-1 600, and 20003000 kPa, at different rates of load application. The results are shown plotted in Fig. 1. It is seen that at stress levels above the quasi-p,' compressibility is time independent (actually decreases somewhat with increased load duration), whereas for the stress level that straddles the quasi-p,' compressibility increases markedly with increasing load duration. Now, it has long been recognized that exceeding the quasi-p,' causes a breakdown in soil structure, which is accompanied by a marked increase in time-dependent secondary compression^.^ A manifestation of this phenomenon is an anomalous point on the otherwise continuous plot of secondary compression/consolidation settlement versus load increment ratio (Leonards and Girault 1961). Figure 1 of the authors' paper shows that at stress levels below about 800 kPa the effect of time on compressibility (indeed, the compressibility itself) is very small. Tests on artificially sedimented clays (Leonards and Altschaeffl 1964; Leonards 1972)