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' Alana Scott <br />September 16, 1994 <br />Page 2 <br />STEFFEN•ERTSON AND KIRSTEN <br />• A pronounced erosion galley, as deep as 6 ft, was observed approximately 500 ift from the South <br />abutment on the upstream embanlm~ent face (Photo 5); ' <br />• Three aeeas of pronounced erosion were observed azound the impoundment perimeter, two on <br />the north side just west of the divider dike (Photo ~, and one on the south side east of the divider <br />dike; <br />• In these eroded areas, liner was exposed in a band as wide as 10 to 20 ft down to the tailings <br />pipeline bench; <br />• Ponding in the natural depression formed by the impoundment construction, approximately 650 <br />ft east of the embank®ent crest at the south abutmem, occurred and minor sedimtnt accumulation <br />existed in the diversion ditch and inlet approach area at the culvert structure (Photos 7, 8 and 9); <br />• Sediment accumulation in the solution channel at the tce of the embanlonent, above the solution <br />pond, was observed (Photo 10); <br />• BMRI had already begun responding to the situation by regrading the safety berms on the <br />embanl~ent crest to eliminate ponding of water on the crest (Photo 11); <br />• BMRI had excavated a shallow trough az either end of the embankment cresq, graded to the <br />downstream direction, to prevent runvn from the abutment areas (see Photo 1); <br />3.0 INTERPRETATION OF EVIDENCE <br />The observations listed above have been generally located on the as-built plan, provided as an attachment <br />under Figure 1. Based on these observations, it appears that the storm event created ponding of rainwater <br />on the crest between the safety berms inducing seWement, or possibly a minor slough, of the embankment <br />fill neaz the North abutme~. We believe the cracking observed oa the embankment crest was created <br />by differential settlement of the embankment fill induced by the infiltrating rainwater. The azea in <br />question constituted a slight low spot in the nest and considerable ponding in this azea is believed to have <br />occurred. The localized increase in the moisture of the embankrment fill beneath the pohding caused a <br />localized densification of the fill, creating a condition of differential settlement relative to the remainder <br />of the embankment. This differential settlement was expressed as the small crack seen in the field. <br />Cracking at this location permitted the rainwater to drain which enhanced the crack width locally due to <br />erosion during drainage. <br />