Influence of water and lubricant fluids on peak strength of Queenston shale from southern Ontario

The strength of Milton Queenston shale (MQS) before and after soaking in water and lubricant fluids (LFs) was examined. The investigated LFs (i.e., bentonite and polymer solutions) are utilized in the microtunnelling technique (MTT) to facilitate the installation of tunnel or pipe sections. To investigate the relevant mechanical properties of MQS under different wetting conditions, a series of laboratory tests including Brazilian split, uniaxial compression, and triaxial compression were carried out. These tests were performed on vertically and horizontally cored specimens with respect to the rock bedding. Fresh specimens (i.e., intact) and specimens soaked for 100 days in LFs and in water were examined. The 100 day period was selected, as it may reasonably represent the construction period where some swelling of the Queenston shale can occur. It was revealed that the strength of MQS substantially decreased after soaking. With greater impact in the vertical direction, both water and bentonite solution caused similar decrease in MQS strength of 23%-70%, while polymer solution caused a smaller decrease of 10%-57%. Accordingly, strength envelopes of MQS were established for vertical and horizontal directions before and after soaking.