The Performance of a Timber Crib Founded on Soft Very Sensitive Lacustrine Clay

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.