Large Scale Permeameter Tests for Internal Erosion Assessment of Widely Graded Materials in Embankment Dams

Internal erosion through and under embankment dams poses one of the greatest threats to the satisfactory performance of embankment dams. Some of the embankment dams in Canada consist of widely graded fill materials that are sourced from soil deposits of glacial origin with significant non-plastic silt-sand-gravel particles. Those widely graded soils are potentially internally unstable which may explain the occurrence of sinkhole incidents in some embankment dams. Engineers commonly rely on empirical screening criteria and non-standard permeameter tests to assess internal erosion induced by internal instability. However, these criteria were developed from limited laboratory tests on materials that may not be directly applicable to those widely graded materials. To address this issue, BC Hydro and its subsidiary Powertech Labs Inc. have recently developed two large diameter rigid-wall permeameters (300 mm and 500 mm) to assess internal instability of widely graded materials with large particles (>1.5″ (37.5 mm)). This paper describes the newly developed testing apparatus and test specimen preparation methods. A new two-stage discrete placement technique was proposed to reconstitute test specimen consisting of large particles. The two well-known internally unstable gradations from Skempton and Brogan (1994) and Kenney and Lau (1985) were tested to commission the test apparatus and develop test methods. The observed typical erosion behaviors are also presented and discussed in the paper.