2000-12-01_REVISION - M1977342 - Laserfiche WebLink

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MINE ID # OR PROSPECTING ID#: M-1977-342 PAGE: 3 INSPECTION DATE: 11-14-00 INSPECTOR'S INITIALS: ACS In order to route 3-Dam seepage to the pipeline inlet, a collection ditch has been excavated into the tailing. The collection ditch lining, consisting of geotextile overlain by river rock, has been installed from the inlet works back some tens of feet. At the end of the rock placement, geotextile has been wrapped around the top of the rock and weighted in place. This forms a settling area to filter out fines that might otherwise enter the pipeline over the winter. Additional clarification of the water entering the pipeline may occur in the inlet structure since the lower elevation pipeline will be valved-off this winter to create a small settling pond. The north collection pond was inspected. Very little seepage water enters the north collection pond at this time. If the north pond were to begin to collect water, it would quickly spill to the south pond along the connecting road to the south pond located behind the natural rock knob. When the project resumes in the spring of 2001, the north pond may be dewatered to the south pond either by pumping or by using the ungrouted portion of the decant pipe that runs along the toe of 3-Dam. The plans call for an interconnecting culvert to be installed from the north collection pond to the south collection pond. This installation will occur during 2001. The decant tower SN location was inspected. The tower itself has been removed. The decant line has been grouted from tower 5N to 300 feet back into the pond between Dams 3 and 2. The plans included in technical revision TR-09 specified an ultimate compressive strength for the non-shrink grout of 3000 psi. However, the 28-day break test conducted on grout samples yielded strengths that were well below specification. The grouting contractor, Denver Grout, believes that substandard fly ash may have retarded curing times, and determined to resample the grout and test for ultimate strength. Denver Grout was on-site on the day of this inspection and collected a core sample of grout from the concrete decant pipe just north of tower SN. Prior to the writing of this report, the Division was provided with the results of two break tests conducted on the core sample. The tests yield compressive strengths of 231D psi and 2380 psi. Although these strengths are below the specified value, the Division agrees that these tests demonstrate that the grout has cured adequately to function properly and prevent flow in the decant pipe. The 3-Dam pump station was inspected. One of the two pumps had been removed and was being rebuilt. The pump station was receiving water from a temporary dewatering pump located in the south pond collection ditch. The rebuilt pump will be reinstalled and the 3-Dam pump station will remain in place over the winter to serve as a back-up Environmental Protection Facility to the dual HDPE seep water pipelines. Based on review of the grouting report, grout test results, discussions held with Henderson personnel and contractors, and observations made during this inspection, the Division of Minerals and Geology has made the following determinations related to the projects approved in technical revision TR-09: • The decant line grouting project has been successfully completed. In the unlikely event that there is reoccurring flow in the grouted pipe, it should be detectable in the south collection pond and could be further addressed at that time. The compressive strength of the slurry grout, although below specification, is adequate for the intended purpose. • Failure to complete the backfilling of the south dike to its preexisting elevation during this construction season does not result in any increased risk of loss of process water containment. Completion of the backfilling during the spring or summer of 2001 is preferable to trying to complete the job this year. By waiting