Construction Overview
Construction of the proposed project is expected to begin in 2026 and last approximately 115 working days and take one construction season between May 2026- December 2026.Standard construction equipment would be used, including but not limited to: excavators, graders, scrapers, loaders, etc. Portable construction trailers would be placed on-site. Dewatering equipment would be used. Construction activities would be generally limited to the hours between 7:00 a.m. and 6:00 p.m. Night work would be allowed to ensure projects can be constructed prior to the rainy season.
Staging and Access
Staging areas have been proposed and evaluated for environmental resources. Staging would take place at the culverts inlet and outlet. Access to the inlet is available through the fire access route and access to the outlet is available through Old San Pablo Dam Road. Additional staging area is available along Old San Pablo Dam Road, and the boat ramp located 0.3 miles north of the outlet. San Pablo Dam Road will remain open for traffic throughout the project. Standard traffic control measures would be employed during construction activities, and emergency vehicles will have access at all times. Temporary construction easements from East Bay Municipal Utility District (EBMUD) would be required for access to the storm drain system, proposed staging areas, and other construction access.
Site Preparation
Tree removal will be limited to what is necessary to construct the proposed project.
Approximately 27 trees, including California bay, Arroyo willow, Coast live oak, and Blue elderberry will be removed around the inlet to create space necessary for pipe ramming, and at the outlet where the concrete flume will be reconstructed. Vegetation within the work area will be cleared and grubbed as needed. In addition, a cattle fence will be installed along the fire trail to allow grazing to continue while keeping cattle away from the project site.
Demolition
Construction would first require demolition of the existing inlet and outlet headwalls. The existing inlet headwall size is approximately 40 x 10 and the existing outlet is approximately 30 x 10. Demolition of the existing roadway will occur to depth needed for stabilization of the roadway prior to restoration of the roadway.
Installation and Repair of Culvert System
The two existing 72-inch corrugated metal pipes would be replaced through pipe ramming with 66-inch steel casing pipes through the existing alignment. A pneumatic hammer delivers percussive blows to the rear of the steel pipe, driving it forward. The steel pipe is equipped with a cone-shaped head to help navigate angular deflections, reduce friction, and expand the deformed CMP into a more circular cross-section through outward diametral pressure. To maintain consistent alignment, the cone-shaped head will also be pulled with a pulley system from the downstream end. This project involves ramming approximately 410 linear feet in total (205 feet in each direction). The steel casings, fabricated in 20-foot sections, are expected to be installed at a rate of one section per day. After each section is driven, another is positioned, welded to the preceding casing, and the process continues
until the full length is completed. Since the existing dual 72-inch system will be reduced to a dual 66-inch system, an additional 48-inch culvert will be installed by jack and bore on the south side to compensate for the reduced hydraulic capacity.
The third culvert would be constructed using the jack and bore method where a boring
machine would jack (push) a 48-inch casing pipe into the ground in segments while
simultaneously removing soil using a rotating auger within the casing.
Grouting
The County performed geotechnical borings around the existing culverts, which revealed severally compromised soils surrounding the system. To ensure that the newly installed pipes are properly supported and embedded in competent material, contact grouting is required around the exterior of the pipes. This grouting will be performed from within the installed pipes, using port holes specifically manufactured for this project, through which the contractor will pump the grout material. Additional vertical compact injection grouting from the roadway will be required to stabilize compromised soils between the culvert and roadway surface.
Concrete Work (Headwall, Flume, Abandonment of 36 Overflow CMP)
The existing inlet and outlet headwalls will be removed to install the three culverts. Once the three pipes are placed, the proposed headwalls will be constructed around them. Additionally, the existing concrete flume will be reconstructed due to structural damage caused by tree root intrusion. The proposed flume will be designed to direct the flow of the three culverts into the existing inlet, which conveys the flow through Old San Pablo Dam Road. Lastly, the existing 36 CMP overflow pipe will be abandoned and capped in place with concrete slurry to prevent future use or void formation.
Roadway Rehabilitation
The roadway will be excavated and replaced with a new aggregate base (AB) and hot mix asphalt (HMA). The reconstruction effort will rehabilitate the roadway section and fill any underlying voids contributing to the settlement.
Site Clean-up and Restoration
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 50 trees.
