INSTALLATION OF DEEP FRENCH DRAINS IN THE 'DRY'

摘要

This paper presents a case history of the construction of a deep (up to 12 m) French drain atan eastern Saskatchewan potash mine. The 1450 m long drain was constructed through relativelypermeable fractured Floral Formation till and keyed into low permeability, overconsolidated, unfracturedFloral Formation till. While deep French drains have been around for some time, excavation of a deepopen trench in the "dry" presents a number of challenges, and successful installation is not guaranteed. Inorder to reach the desired depth, the excavator was placed in a shallow open cut to further complicate theinstallation. Construction took place using a hydraulic excavator to excavate to the desired depth. Nopolymer slurry or other method of trench support was used. Drains themselves have had a mixed historysuccess. As a result, special care was taken in the design and set up of the QC/QA programs to ensurereasonably high probability of successful performance of the installed drain.Investigations at the potash mine site found that the primary medium for migration of brine from theTailings Management Area (TMA) was through the oxidized and fractured portion of the Saskatoon Grouptills and associated intertill and intratill stratified deposits. Spigotting of salt tailings within the TMA hasresulted in a large brine mound within the tailings pile. This brine mound acts as a hydraulic driving forcefor the transport of brine from the TMA through the shallow till deposits. A multi-year study was conductedto examine alternative passive and active barrier alternatives. The results of the study found that theinstallation of a deep French drain in the "dry" was the most economical alternative; however, theinstallation of this type of drain in these conditions involved an elevated level of uncertainty over whetheror not the drain could be physically installed. This case study presents the design considerations,construction methodology, project risks, and a summary of the challenges encountered duringconstruction. The successful installation of this drain and the experience gained may prove valuable forthe construction of similar drains for other civil engineering applications.