DWR_3391241 - Laserfiche WebLink

2-13 The combination of the sand boil and increased pump -back rate could be caused by breakthrough of a preferred seepage path beneath the dam. The partial closure of such a seepage path would, however, have had to occur since the pump -back rate returned to a lower level. An additional explanation may be due to the infiltration and runoff effect of a heavy snowfall which occurred on November 29, 1982. Previous temperatures had been relatively warm, and significant melting of the snow due to warm earth temperatures may have occurred. Since the pump -back system collects surface runoff as well as sdbsurface seepage, increased natural groundwater and surface water flow could contribute to water entering the pump -back system. Continued observations and correlations would be required to definitively establish a relationship. The occurrence of the boil, its subsequent decrease'in apparent flow rate, �acid''�ielated increase and decrease in pump -back rate . sa t1TYTr"Y« ",cvt',i:-:e i+��•,""•- . warrant continued observation of the area downstream of'the`dam. 2.3 LABORATORY TESTING RESULTS Laboratory testing was conducted on samples obtained from the grout holes and from the borings advanced for piezometer installation. The samples from the grout borings were tested for classification purposes and to determine strength parameters. Tests were also conducted to evaluate the susceptibility of dispersive type piping in the dam or dam foundation. The samples from the piezometer borings were tested primarily for classification and natural water content. The results of the classification tests are presented in Table 3 and in a plasticity chart as shown in Figure 6. Natural water contents for samples from the piezometer borings are shown on the boring logs in Appendix A. The results of the classification test show the dam fill along the axis of the dam is mostly a silty or clayey sand (USCS SM or Y IF