When the 70-foot-wide by 70-foot-deep Little Wolf Creek sinkhole opened up on the morning of January 11, 2017, in Grass Valley, California, the City Engineer called NV5 to meet him onsite to assess the damage.
We immediately deployed an engineer and a geologist, and our FAA-licensed drone pilot arrived soon thereafter to take aerial photo and video. This footage was used by city officials, the NV5 team, and a local contractor to quickly and safely determine that the extent of the failure was localized and did not impact the city’s wastewater treatment plant located only a few hundred yards downstream. It was determined that the sinkhole resulted from a broken 90-inch diameter culvert that drained surface water from nearly one-third of the City of Grass Valley.
To help restore the site and provide recommendations regarding safety setbacks, the city contracted with NV5 to provide geotechnical consulting services related to slope stability analysis and recommendation development for grading and culvert thrust block design, special inspection and testing of shotcrete used to repair the culvert, laboratory soil and compaction testing of fill material, and a storm water pollution prevention plan (SWPPP). We also conducted consistent drone flights to capture high resolution 4K video and geo-referenced 20 megapixel still images to document ongoing site conditions. Our HD video, 2D maps in the form of Orthomosaics, interactive 3D models, and elevation data illustrations were shared with stakeholders and the media.