Carlsbad Caverns Visitor Center

National Park Service and DHM Design Corporation, Carlsbad Cavern National
Park, NM

The Carlsbad Caverns Visitor Center was being renovated, which included upgrades to the sanitary sewer system and addition of stairways for access. In addition to upgrading the sanitary sewer system to handle the increased flows, leakage from the sanitary sewer lines was of concern as the leakage may be forming an acidic leachate that was or would adversely affect the delicate features within the caverns. DHM Design contracted Yeh and Associates to conduct geologic and geotechnical investigations to provide design recommendations for the renovations.

The stairway addition was placed on conventional footing foundations due to the hard rock conditions. The portion of the sewer system to be renovated and realigned was through a topographically diverse portion of the National Park. The new sewer system from the existing Comfort Station lift station to the edge of the mesa escarpment was to be constructed below grade, the portion of the line from the mesa escarpment to the base of the mesa would be located above ground and anchored to the native bedrock using micropiles. Along the base of the mesa and to the existing sewage lagoon system the line would again be placed below grade.

The site consists of three geologic formations that are Permian in age; the Capitan Limestone, the Tansil Formation of the Carlsbad Group, and the Castile Formation. Each of these units is susceptible to solution by water, particularly acidic waters. These units are also hard enough to readily transmit vibrations with little attenuation. These geologic conditions combined with the delicate nature of the caverns presented a significant challenge to the design of the sewer line renovations. Placing the line below grade required excavating into the hard limestone bedrock, which would generally mean blasting for the excavations, however, blasting could damage the cavern features. Alternate methods for hard rock excavation use significant amounts of water, which would seep into the underlying caverns.

Yeh’s key personnel that worked on the project included Shan-Tai Yeh who was the QA/QC reviewer, Rick Andrew was the project manager and Ben Arndt was the project engineer for the investigation and design.