Grindstone Canyon Dam is a 123-foot-high gravity roller-compacted concrete (RCC) structure in the Village of Ruidoso, New Mexico. Grindstone Reservoir is impounded by the dam and stores up to 1,635 acre-feet of water. Yeh and Associates provided engineering services for the Grindstone Canyon Dam Drain Cleaning project that was completed in May 2022. The primary purpose of the project was to restore the functionality of the dam's internal seepage gallery drains and mitigate the potential for excess hydrostatic pressure that could destabilize the dam. The New Mexico Dam Safety Bureau noted in historical inspection reports that many of the gallery drains were clogged due to mineralization (e.g., calcification).
The dam construction was completed in January 1988. The gallery foundation drains were drilled two years prior in 1986. The as-built depth of the uncased, vertical foundation drains was shown as 50 feet below the gallery floor. The drain-cleaning contractor encountered unanticipated blockages at lesser depths in many of the foundation drains. The blockages could not be cleared with high-pressure cleaning tools, indicating the blockages were more resistant than typical mineral deposits. The composition and thickness of the blockages was difficult to discern based on video footage collected after the cleaning attempt.
A review of historical documentation, including as-built geologic maps and construction photos, provided some possible explanations for the blockages, which may consist of intact (undrilled) rock, fragments of fractured rock collapsed from the drilled hole, or grout placed after the drain holes were drilled. The drain-cleaning project was considered a success despite the blockages. Seepage rates measured at many individual drains and at the gallery entrance increased from the start to the end of the project, even as the reservoir’s water surface elevation was lowered. The seepage rate measurements also apparently reflect geologic structure. Relatively high flows were typically measured where as-built geologic maps showed rock discontinuities such as faults and joints, and mapped contacts between younger, intrusive volcanic dikes and older, interbedded sandstone, siltstone, and shale.
Lessons from the drain-cleaning project that may apply to future dam projects include construction sequencing relative to grouting and the drilling of seepage relief drains, the need to case seepage relief drains, and the potential for hydraulic connection (via rock discontinuities) between grout holes and seepage relief drains.
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