Use and misuse of the isotache concept with respect to creep hypotheses A and B

Abstract

Time-dependent settlements of thick in situ clay layers are normally analysed based on results of thin laboratory specimens. However, the time used to complete primary consolidation is significantly different for laboratory specimens and in situ soil layers. Two totally different cases, referred to as creep hypotheses A and B, have been used as a basis of discussion to assess the effect of creep during the primary consolidation phase. Several laboratory and field experiments have been conducted to study the effect of soil layer thickness on the time-dependent compressibility of a soil layer. Some of these tests seemed to support hypothesis A, others hypothesis B, and in some cases showed a behaviour between the two. As a result this question has continued to be a controversial topic among researchers, and remains to be an issue that needs to be resolved. In this study, some relevant experimental investigations from the literature are thoroughly studied and critically reviewed, and also explained consistently using the isotache concept. This work indicates that the isotache approach can capture the main characteristics of the time-dependent compressibility of clays during both the primary and secondary consolidation phases. It is also shown that the misuse of the isotache concept, as reported in the literature, may give a confusing picture of reality. Based on the considered data, it is demonstrated that the measured time-dependent compressibility of clays agrees well with hypothesis B.

Examination on Time-Dependent Soil Models in One-Dimensional Consolidation

“…These results highlight that predictions using the SSC model vary significantly with the initial strain rate (this finding has also been noted by Degago et al [10]). In this case we face a dilemma in using SSC to predict field consolidation.…”

Predictions of settlement in peat soils

“…Some researchers consider that EOP is independent of sample thickness.A summary of some of the discussionscan be found in Mesri (2003) and Leroueil (2006).However, for clay soils many engineers now accept that EOP is not a unique parameter and depends on various factors such as applied stress and sample / layer thickness (Degago et al, 2011).…”

Numerical modelling of the improvements to primary and creep settlements offered by granular columns

“…Degago et al [19] have noted that time-dependent settlements of thick soft clay layers in the field tend to be analysed based on the behaviour of thin laboratory soil samples, despite the significantly longer primary consolidation times associated with the thicker field layers. Ladd et al [20] were the first to raise the issue of whether creep acts as a separate phenomenon in parallel with primary consolidation.…”

“…Isotache models (Hypothesis B) are based on the principle that the prevailing creep rate at any time is defined by the current state of the soil only, namely the current void ratio and effective stress, e.g. [19].…”