Time-dependent behaviour of the Bearpaw Shale in oedometric loading and unloading

Abstract

Time-dependent behaviour can have a significant influence on the compressibility characteristics of soils. However, most of the research on this topic has investigated the behaviour of soft soils. In this paper, the time-dependent behaviour of a hard clay shale (Bearpaw Shale) is investigated using both one-dimensional multi-staged loading (MSL) oedometer and constant rate of strain (CRS) oedometer consolidation tests conducted on 25.0 and 16.9 mm diameter specimens. The results show that soft clays and hard clay shales that share the same Cae/C Ã c ratio (where Cae is the secondary compression index and C Ã c is the incremental compression index) will show the same approximately 7% change in pre-consolidation pressure for an increase of one log cycle of strain rate despite the many orders of magnitude difference in pre-consolidation pressure. In the case of the Bearpaw Shale, this 7% change in pre-consolidation pressure corresponds to approximately 700 kPa. The time-dependent behaviour of the Bearpaw Shale during unloading (Cae/C Ã s , where C Ã s is the incremental swelling index) was observed to follow a similar ratio to that observed in compression (C ae /C Ã c ). While the exact nature of the compression and swelling events that have occurred over the life of the Bearpaw Formation is not clear, the influence of secondary compression cannot be ignored for interpretation of the geological history of this deposit.Résumé : Le comportement dépendant du temps peut avoir une influence significative sur les caractéristiques de compressibilité des sols. Cependant, la plupart des travaux de recherche sur ce sujet se sont penchés sur le comportement des sols mous. Dans cet article, le comportement dépendant du temps d'un schiste argileux dur (schiste de Bearpaw) est étudié à l'aide d'essais unidimensionnels de consolidation à l'odomètre avec chargement multi-étapes (CME) et avec un taux de dé-formation constant (TDC) réalisés sur des échantillons de 25,0 et 16,9 mm de diamètre. Les résultats démontrent que les schistes argileux mous et durs qui partagent le même ratio Cae/C Ã c présentent le même 7 % environ de variation dans la pression de préconsolidation pour causer une augmentation d'un log de cycle de déformation, malgré plusieurs ordres de magnitudes de différence dans la pression de préconsolidation. Dans le cas du schiste de Bearpaw, cette variation de 7 % de la pression de préconsolidation correspond à environ 700 kPa. Le comportement dépendant du temps du schiste de Bearpaw durant le déchargement (C ae /C Ã s ) suivait un ratio similaire à celui observé en compression (C ae /C Ã c ). Même si la nature exacte des événements de compression et de gonflements qui se sont produits sur la durée de vie du formation de Bearpaw n'est pas claire, l'influence de la compression secondaire ne peut pas être ignorée lors de l'interprétation de son historique géolo-gique.

Influence of effective stress and temperature on the creep behavior of a saturated compacted clayey soil

“…This divergence about the relationship between Cαe and Cc could be related to the applied vertical stress range values, i.e., it is more likely to observe a convex relationship at high vertical stresses. Whatever is the shape of this relationship, the Cαe/Cc ratio was found to be constant during the tests 5,17,20,[23][24][25] .…”

“…It should be pointed out that C * c differs from the compression index defined as the slope of the NCL. This approach was adopted by many other researchers 14,23,25 .…”

“…This observation was confirmed by many other researchers on various soils 18,19 . Later, it was found that Cαe and Cc increase with the effective vertical stress, reach a maximum value at a stress slightly higher than the yield stress, and then decrease and finally remain constant 14,17,[21][22][23] . This divergence about the relationship between Cαe and Cc could be related to the applied vertical stress range values, i.e., it is more likely to observe a convex relationship at high vertical stresses.…”

Experimental Study of Compression Behavior of Indian Black Cotton Soil in Oedometer Condition

“…In all the above tests, the loading is applied for a period in the range of 1-7 days. This loading pattern provides an equilibrium condition [32], as well as a long-term time-dependent behavior of the soil can be illustrated [28].…”

“…Loading increment occurred every 24 h or more than 24 h to predict the well primary consolidation during that period. Loading in this pattern also provides sufficient time to achieve the equilibrium of pore pressures [28,32]. Series of tests were performed in Casagrande's-type conventional Oedometer with different loading patterns named as E 1 , E 2 and E 3 .…”