Extensive experience in tunnel construction and rehabilitation, including small diameter jack-and-bore construction. The below sample Projects exemplify the capabilities of MITCHELL ENGINEERING.
Pulga, California The Poe Tunnel is a 6-mile, 24’ diameter tunnel that provides water from the Feather River to the Poe Powerhouse at the upper reaches of Lake Oroville. Scope of work required the stabilization and repair with shotcrete to two (upper and lower) unlined sections of the tunnel. Access to the upper repair area was through an intake ventilation shaft that also required the installation of 1,200 lineal feet of a concrete invert slab. Access to the lower repair area was through an access adit tunnel located five miles downstream of the intake; all material and equipment deliveries to the lower work area were delivered by helicopter. Owner: Pacific Gas & Electric (PG&E) Contract Amount: $2 million
Helms — Penstock Tunnel Plug and Shear Zone Rehabilitation
(near) Shaver Lake, California Project consisted of critical repairs to rehabilitate the bypass tunnel’s concrete plug and mitigate the impact of leakage into the Helms Pumped Storage Powerhouse Complex. Scope of work included drilling 120 holes (totaling 3,980 lineal feet) and installing high pressure grout using the Grouting Intensity Number (GIN) method; sealing of weep holes and cracks in the Penstock Access Tunnel; drilling and re-grouting of existing grout holes in the concrete tunnel plug; drilling and installing new piezometers and horizontal drain holes; and the chemical grouting of weep holes in the main penstock tunnel. Owner: Pacific Gas & Electric (PG&E) Contract Amount: $4.5 million
Beldon, California
Project consisted of drilling 50’ holes in 2 stages and subsequent grouting to create a grout curtain around the tunnel. Added work included: a) Reparation of cracked surface areas by chipping out loose material, forming and grouting, and b) Removal of 650’ of an original 8” slick line along the crown of the tunnel to make room for a PVC membrane liner. Owner: Pacific Gas & Electric (PG&E) Contract Amount: $573 thousand
Yuba County, California
Project required that a flow bypass system be constructed for the purpose of allowing the continuance of flow in the Yuba River while the Narrows II Powerhouse was shut-down for maintenance. Tunnel work included drill & blast excavation of the bypass tunnel, bypass structure and discharge channel; supply and installation of 114” steel tunnel liner and a 90-degree tee connection to the existing penstock. Owner: Yuba County Water Agency Contract Amount: $11 million (all inclusive)
Sacramento, California
Project consisted of installing over 11,500 feet of reinforced concrete pipe with diameters ranging between 108″ and 84″ diameter. Approximately 384 linear feet of 108″ pipe were installed by tunneling underneath an environmentally sensitive creek and an existing highway. Owner: Sacramento Regional County Sanitation District Contract Amount: $45 million (all inclusive)
Tuolumne County, California The Cherry Tunnel is a dual purpose water supply and power generation tunnel. This project consisted of a calculated power shutdown during low demand for the purpose of cleaning out the unlined portion of the tunnel. Work consisted of removing rock and gravel from the existing rock trap (and restoring to its original dimensions) in order to keep debris from the turbines at the power house. The rock trap was approximately 8,000 LF upstream from the penstock. Interestingly, though built in the 1950’s, this tunnel had received no maintenance until Mitchell Engineering began its work. Owner: City and County of San Francisco (Hetch Hetchy Water and Power) Contract Amount: $1 million
Sly Creek Tunnel Upgrade
Butte County, California
Project consisted of inspecting the Sly Creek outlet tunnel ceiling, walls and rock bolts; removing loose material as needed; and repairing or replacing rock bolts as needed. Required work included protecting existing instrumentation, utilities and spillway while scaling down the back and sides of the tunnel of all loose rock; drilling and setting two dozen new #8 Williams resin bolts including added wire mesh to protect the soft seam running across the top of the tunnel. Owner: South Feather Water and Power Agency Contract Amount: $47 thousand
