Helms Powerhouse

Project consisted of critical repairs to rehabilitate the bypass tunnel’s existing 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.

Other work performed at the tunnel location included stabilization of the existing shotcrete and the installation of 550 lineal feet of horizontally drilled drains and instrument holes. Mitchell removed existing loose blocky rock and shotcrete followed by the installation of localized flow mitigation drains and 156 cubic yards fiber reinforced shotcrete.

Mitchell Engineering proposed, and PG&E accepted, an alternate design for the new concrete liner and drainage system to improve the stability of the existing Helms PSP T3 Access Tunnel. The Mitchell proposed lattice girder / shotcrete system was further modified to a shotcrete and drainage system to stabilize and drain the T3 Access Tunnel. These modifications to the original design saved PG&E substantial time and money. Mitchell Engineering also designed and provided an upgraded ventilation system to support this and future work.