DWR_2801686 - Laserfiche WebLink

5 Case Studies – Repairs Fish Lake, CO –This dam has a history of seepage along the downstream toe, believed to be due to the open matrix character of the foundation rock. A sinkhole recently occurred on the upstream side of the dam near the left abutment, presumably due to voids most likely at the interface between the dam embankment and the foundation. The sinkhole was first observed during a routine inspection in July 2011. Further inspection and dye testing by a geotechnical engineer in October 2011 confirmed the location of the entrance and exit point of the seepage flowing from the toe of the embankment. The use of urethane grout was recommended to fill the sinkhole. Grouting took place in August 2012. The sinkhole was first cleared of rock and debris and then flushed with approximately 5 gallons of water to obtain thorough pre-wetting for the grout application. Photo 5: Flushing of sinkhole at Fish Lake, CO The application of the grout into the sinkhole was aided by a steel pipe and tarp to funnel the material. Stratathane ST-504 Vari-Gel Injection resin was used. Lake water was used for flushing and mixing the grout. The grout was applied in stages with three different mixes, becoming more viscous with each application. The mixes were: • 15 water to 1 grout: 30 gallons of water applied, grout completely absorbed, poured easily. • 10 water to 1 grout: 30 gallons of water applied, grout completely absorbed, some backing up during pouring. • 5 water to 1 grout: 5 gallons of water applied, grout was not entirely absorbed, pipe containment was half full. Photo 6: Final application of grout, contained within pipe (Fish Lake) The most fluid mixture took longer to set up before the next application could be poured, but obtained the greatest penetration into voids or cracks related to the sinkhole and seepage path. The second and third application took less time to set up but obtained less penetration. All mixtures expanded upon application and the last mixture served to cap the sinkhole. After the grout had set up and cured, the pipe was removed and the area was backfilled with rock to provide protection for the grout. An inspection 6 months after the grout application found some shrinkage of the grout at ground surface, but the sinkhole appeared to be sealed. No issues have been reported around the sinkhole or in other areas of the dam since the repair. Lake Ann, CO – Seepage 400 feet downstream of the dam has been a longstanding issue at Lake Ann. Evidence indicates water was flowing through the foundation of the dam from upstream. A sinkhole from the 1960s was repaired with a plastic liner. Deteriorated plastic was found and removed in the excavation of a more recent sinkhole in the same area. The recent sinkhole was observed at the dam approximately 15 feet upstream of the intake and 100 feet right of the outlet. The flow was approximately 0.3 cubic feet per second when initially measured in October 2014. The reservoir was immediately drained