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A major landslide of the flow variety took place in the fall of 1924 in the Lake St. John region of the Province of Quebec, Canada. The slide involved 2.5 million cubic yards (2.0 × 106 m3) of clay and resulted in loss of life and considerable damage to property. A review of the circumstances preceding failure, the geotechnical properties of the clay and the mechanism of the slide movements is presented in the paper.

As part of the development of a site in northern Quebec for a paper mill and associated townsite, a separate water intake structure and pumping works were required to serve the townsite. A timber crib intake structure was constructed approximately 200 ft. offshore in a lake, founded on very soft to soft clay, of high moisture content and sensitivity.The clay is a late glacial freshwater sediment which has been consolidated by pressures probably not exceeding the existing overburden pressure. Careful sampling and testing of the clay, however, indicated a two-part consolidation curve, with the break occurring at a point about 400 lb./sq. ft. in excess of the in situ overburden pressure. Beyond the break in the oedometer curve the compression index was up to 30 times greater than in the initial portion, being a maximum of 3.0 to 3.5.The crib was founded on a sand and gravel mattress, keyed into the lake bottom, the resulting net stress increase on the clay being slightly within the inferred range of apparent preconsolidation established by laboratory tests. Performance readings on the crib for a period of about a year after construction have substantiated the field existence of an apparent preconsolidation of about the order of magnitude predicted by laboratory tests.

A long overpass structure with 32 piers between abutments was founded on strip footing and friction pile foundations in a deep marine clay subsoil of firm consistency. The piles were steel H-sections which were provided at the construction stage at 6 pier locations in lieu of footings because of unfavorable soil conditions exposed during excavation. A load test to failure on a single pile indicated the pile–soil adhesion to be considerably lower than the undrained shear strength of the clay.Post-construction observations indicated the settlement of the pile-supported piers to be about 50% that of the adjacent footing supported piers. The settlements of both types of foundations were small and within tolerable limits. The design approaches used for prediction of footing settlements and the group action of friction piles in bearing capacity and settlement are presented in the paper.