Profiling the in situ compressibility of cretaceous shale using grouted-in piezometers and laboratory testing

Grouted-in vibrating wire pressure transducers (VWPs) can be used to measure the in situ laterally constrained compressibility (mv) of deep claystone aquitards through measurement of barometric loading efficiency. Here, we present the results from 27 VWPs installed in overconsolidated argillaceous glacial till and claystone formations in southern Saskatchewan, Canada at two sites over depths ranging from 10-325 m below the ground (m BGS). The measured mv profiles at both sites produced similar trends of decreasing mv with depth. The trends in compressibility with depth were compared to the results from laboratory consolidation testing of core samples taken from the same Cretaceous shale profile. An apparent pre-consolidation pressure σc′ compression index (Cc), and the swelling index (Cs) were determined using 1-D consolidation testing. These tests yielded Cc values ranging from 0.12-0.41 x¯=0.27±0.11 andCs from 0.015-0.09 x¯=0.05±0.03. The theoretical depth profile for mv (during unloading) was calculated for a range of compression indices (Cc, Cs) and the in situ void ratio (e)estimated from the consolidation testing, the vertical effective stress σv′ calculated based on the effective unit weight of overburden and laboratory determined preconsolidation values σc′. Varying the values of σc′, Cc, or e in the hypothetical depth profile demonstrated minor influences on these profiles when compared to that of. The resulting theoretical profiles of mv with depth orσv′ exhibited a similar pattern to the laboratory and field observations; however, for the laboratory test data to replicate the in situ profiles, the laboratory measured values of Cs had to be reduced by an order of magnitude in order to compensate for both the applied strain increment differences, as well as sample destruction during recovery and testing. The good agreement between the theoretical and the in situ measured mv profiles with depth highlight the potential to combine in situ measurements of mv with laboratory consolidation test results to characterize the mechanical properties of deep claystone aquitards.