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

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 C-alpha e/C-c* ratio (where C-alpha e is the secondary compression index and C-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 (C-alpha e/C-s*, where C-s* is the incremental swelling index) was observed to follow a similar ratio to that observed in compression (C-alpha e/C-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.