San Pablo Dam Road - La Honda Storm Drain and Slide Repair Project

The San Pablo Dam Road–La Honda Road Storm Drain and Slide Repair Project (proposed project) involves the stabilization of an embankment failure, the repair of the northern side of San Pablo Dam Road, and replacement of an existing culvert approximately 200 feet southeast of La Honda Road in unincorporated El Sobrante, Contra Costa County, California. The proposed project would install a stitch pile wall 50 feet north of San Pablo Dam Road between the roadway and San Pablo Creek with an approximate length of 150 feet. The wall consists of cast-in-drilled-hole concrete piles about 50 feet in depth that are spaced six feet apart. The slope between the proposed stitch pile wall and San Pablo Dam Road would be excavated to a maximum depth of 24 feet. The slope would be rebuilt with the excavated material reinforced with layers of geogrid placed at 2-foot vertical spacing to provide long-term stabilization. The 18-inch existing culvert outfall within the embankment failure area would be relocated west of the proposed stitch pile wall with the installment of two manholes and 75 feet of reinforced concrete pipe intended to convey stormwater to the outfall location. The existing 18-inch culvert under San Pablo Dam Road would be repaired using cured-in-place-pipe lining through the installation of a flexible liner inside the culvert to restore previous hydraulic conditions while the damaged segment towards San Pablo Creek would be removed. The 18-inch relocated culvert outfall would be reinforced concrete pipe enhanced with a concrete flared end section discharging to a rock slope protection pad to disperse flow and to prevent bank erosion around the inlet. Approximately 200 feet of San Pablo Dam Road would be repaired along the embankment failure including restriping and placement of a 170-foot guardrail. Construction of the proposed project is expected to begin in late 2026 or early 2027 and would last approximately 100 working days. Construction activities would involve excavations 50 feet in depth for the installment of the cast-in-drilled-hole concrete piles associated with the stitch pile wall and excavations 24 feet in depth between the stitch pile wall and San Pablo Dam Road, minimal grading in front of the stitch pile wall, the removal of approximately 67 trees, and roadway repairs including restriping and placement of approximately 170 feet of guardrail. Construction activities would require the establishment of three temporary staging areas with the first on the northbound lane on La Honda Drive, a second on the north shoulder of San Pablo Dam Road, and a third smaller staging area south of San Pablo Dam Road directly across the embankment failure. Standard traffic control measures would be employed during construction activities, and emergency vehicles will have access at all times. All disturbed areas will be stabilized with best management practices following construction and restored to pre-project conditions through hydroseeding with a native seed mix and replanting approximately 67 trees.