Grant Lake Outlet Valve Replacement Geotechnical Evaluation

The Los Angeles Department of Water and Power is preparing to conduct a geotechnical investigation at Grant Lake to support the replacement of the malfunctioning outlet valve and the outlet tunnel gate. The project involves drilling 14 sonic boreholes to assess subsurface conditions, including soil types and bedrock characteristics, which are critical for the engineering design of the replacement structures. Two of these borings will be positioned over the reservoir using a barge or watercraft, while the remaining twelve will be located along the dam access road and adjacent to the existing shaft and shaft house on the western side of the lake. Of the 14 boreholes, two may be repurposed as long-term groundwater monitoring wells to provide ongoing data after the initial investigation. Drill cuttings generated during the process will be collected in drums and transported offsite for proper disposal. The fieldwork is scheduled to commence in September 2026 and conclude by the end of November 2026, aligning with operational seasonal constraints to minimize environmental and operational disruption. The investigation is a necessary step in ensuring the structural integrity and long-term functionality of the outlet system.