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In recent years natural water content and density determinations have been carried out in the stiff clay deposits on Montreal's Island by means of nuclear depth probes. The probes are lowered down aluminum access tubes that are permanently inserted into the ground, thus providing a means of measuring the volumetric water content and the density of the soils surrounding each tube, at any convenient time and depth. Access tubes have an external diameter of 50.9 to 57 mm and a wall thickness of 1.3 to 1.9 mm, and are sealed at the bottom to prevent entry of water. The tubes are pushed into holes of the same diameter which have been reamed out of the soil by a mechanical auger. Tubes have been inserted to a maximum depth of 6 m. Readings in each access hole are taken at 100–200 mm depth intervals along the length of the tube. Values of volumetric water contents and densities are determined by using field calibration curves. The paper gives the results obtained on several projects, reports on the precision of the predicted values and presents typical problems which were encountered during installation of the access tubes and the long-term (up to 8 years) monitoring programs. It is also shown that factory calibration curves furnished with the nuclear probes cannot be used in high-water content clay soils. It is further indicated that even though the water content calibration curves show a high degree of correlation, the neutron scattering technique will not effectively detect changes in soil water content for periods ranging from a few days to about one week. Finally, it is shown that voids around the access tubes adversely affect the nuclear gage readings.
water content, density, field, nuclear depth gages, long-term program, precision, clay
Professor, Ecole Polytechnique, Montreal, Quebec