John Mitchell Lecture: Geophysics for Geotechnical Design-Paper

Paper

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The paper examines how in-hole geophysical testing supports the safe design and operation of the Zelazny Most copper-tailings facility in Poland, one of the world’s largest. Its central geotechnical concern is flow failure caused by static liquefaction. The facility is monitored by an International Board of Experts using the observational method.<br /><br />Measurements of shear-wave velocity (Vs), obtained using cross-hole, seismic CPTU and seismic DMT methods, provide complementary information on tailings stiffness and help verify the consistency of different testing methods. Compressional-wave velocity (Vp), measured at cross-hole stations, is particularly useful for locating the phreatic surface: it distinguishes fully saturated from partly saturated tailings. Monitoring at 18 stations showed that the saturation boundary remained broadly stable from 2011 to 2017, despite the dam crest rising by 9–12 metres. This demonstrates the effectiveness of the facility’s circumferential drainage system in keeping groundwater depressed beneath much of the beach.<br /><br />Partial saturation can substantially increase resistance to liquefaction, but field seismic measurements alone cannot determine the degree of saturation or quantify this strength gain. The authors therefore recommend combining in-situ Vp measurements with consolidated-undrained triaxial tests on fully and partly saturated specimens. Gel-Push sampling helps obtain high-quality, relatively undisturbed tailings samples for laboratory testing. Comparing field and laboratory wave velocities also helps assess whether test specimens represent field conditions.<br /><br />The paper concludes that integrated seismic testing and laboratory testing provide a practical framework for evaluating tailings liquefaction susceptibility under both fully and partly saturated conditions. Geophysical measurements are valuable for mapping saturation and characterizing the deposit, while laboratory testing remains necessary to assess its liquefaction resistance.

Keywords

tailings facility

static liquefaction

shear-wave velocity

compressional-wave velocity

phreatic surface

seismic CPTU

seismic DMT

cross-hole testing

partial saturation

liquefaction susceptibility