Tiger Creek Afterbay Dam Low Level Outlet (LLO) Dredging & Inspection

The Tiger Creek Afterbay Dam is a 120’ high, 448’ long variable radius concrete arch with a gate overpour spillway in Pioneer, California.  The original construction was completed in 1931 and is currently operated by Pacific Gas & Electric (PG&E). The dam contains a low-level outlet (LLO) discharge pipe at the base of the dam that has not been operated since the early 2000s due to damage.  The LLO gate required rehabilitation in order to ensure the water level behind the dam can be lowered in case of an emergency and to allow sediment transport during spill events. Ballard Marine Construction, LLC (BMC), recognized for their specialty construction in under water diving, was contracted by PG&E to inspect the condition of the gate.  The gate being imbedded within deposited sediment, Odin was subcontracted to BMC in July 2019 to remove sediment and debris in order to create safe ingress for divers to access and repair the LLO gate with water depths as great as 85’. Approximately 27,000 CY of sediment and debris was removed from within the dredge prism.

Odin is traditionally known for their specialty geotechnical construction capabilities specifically with construction of groundwater cutoff walls by the slurry trenching technique.  Odin utilizes specialty fabricated long booms and sticks to perform linear excavations up to 92’ deep. These specialty excavators can be disassembled for transportation as legal highway loads and mobilized to nearly any location and re-assembled at the project location. Tiger Creek Afterbay Dam is a discontinuous body of water with limited access via highway that prohibited equipment mobilization from conventional means by water and thereby required unique equipment selection to perform sediment removal in water columns as deep as 85’. Odin cross-utilized their geotechnical equipment using a Komatsu PC1250, a 300,000-lbs class excavator, to mount the long boom and stick and mobilized the rig to the Site to remove sediment from behind the dam.

Odin began excavating sediment using state-of-the-art mechanical dredging techniques and implementation of conventional Best Management Practices designed to minimize re-suspension, release, and residual sediments. 

In-water dredging was completed using a fixed arm excavator, mounted on-top of an equipment barge, that was equipped with a 360-degree rotating enclosed environmental bucket to limit sediment loss.  The fixed-arm excavator permitted sub-foot accuracy GPS-based system for accurate bucket positioning and improved bucket control on steeper slopes. Excavated sediment was placed in closed binned material barges constructed with filtered scuppers to promote gravity drainage of excess water.  Material barges were allowed to gravity drain for up to 8 hours within the dredge prism prior to pushing them ¾-mile upstream to a transload facility. Sediment was removed from the material barges using a Hyundai 290 long reach excavator and was directly loaded into highway dump trucks and the sediment was transported to a nearby quarry for use as reclamation soil.

Project Details

Client
PG&E Power Generation
Location
Pioneer, CA
Start Date
July 2019
End Date
October 2019
Service
Dam Rehabilitation, Sediment Removal
Performed As
Subcontractor

Project Highlights

  • Removal of 27,000 CY of sediment and debris from behind a 120-foot high, 448-foot long variable radius concrete arch dam including transportation and disposal
  • 5,926 manhours
  • Crew side of 6 craft personnel
  • Personal protection level D