U. S. Army Corps of Engineers
Sacramento, CA
Project Description:Kleinfelder characterized post-construction sediments and subsurface conditions at the heel of Folsom Dam, one of California's largest dams and reservoirs and the most critical segment of the Sacramento region's flood control system. Lake Folsom, located about 25 miles east of downtown Sacramento, is formed by damming of the South and Middle Forks of the American River. In the past year, significant Federal funds have been allocated for investigation and design aimed at increasing Folsom Dam and Reservoir's discharge and storage capacity. This five-year Folsom Modifications project will consist of two phases: 1) Increasing the pre-flood discharge capacity by increasing the outlet flows2) Increasing the reservoir capacity by elevating crest heights of eight dikes about the Lake's perimeter. Outlet flows are to be increased by essentially "reaming" the existing outlets from the dam face, requiring placement of temporary bulkheads over the inlets on the lakeside of the dam. Placement of these bulkheads requires excavation or dredging of accumulated lake bottom sediments above the invert of the bulkhead and ingress-egress for placement. This is expected to entail approximately 20-30 feet of excavation below the existing mudline.
Under USACE's on-call geotechnical contract, Kleinfelder performed drilling in more than 200 feet of water to characterize these materials and to assess the underwater stability of proposed cuts. Our scope included research of 1945-55 geologic investigation and as-built records, permit assessment, RFQ/RFP preparation, subcontractor selection and procurement, exploration, laboratory testing, analysis, and report and graphics documentation. Vast as-built records and photographs were consolidated and compiled using GIS, resulting in field GIS maps used extensively for real-time field interpretations. As-built data indicated that nearly 50 feet of post construction lake bottom sediments had accumulated over construction debris and unweathered rock in the foundation footprint. Performed permit assessment in close interaction with the USACE to avoid unnecessary delays and schedule/cost issues. This task identified strategic turbidity monitoring issues relative to multiple water purveyors drawing water from the Lake. Environmentally sensitive lakeshore issues became strategic to labor and equipment access. Three environmental borings were included in our study due to the potential existence of residual metals contamination derived from upriver, historic gold-mining. Environmental sampling/analyses were performed to chemically characterize lake bottom sediments proposed to be discharged back into the Lake or its shorelines. Spill prevention and worker safety issues were equally critical. Thirty geotechnical and three environmental borings were drilled using vibracore, mud-rotary, tricone, and BQ and NQ rock coring. Soil samples were collected using alternating SPT and Cal-modified samples as well as piston, Shelby tube, and core sampling. Early drilling encountered extensive cofferdam and construction debris containing large quantities of large, angular rock resulting in shearing of drill stem and casing. Gregg quickly adjusted to field conditions by improving anchor systems, and modifying drilling and casing procedures to accommodate the anticipated conditions. The first boring took 9 days and the remaining 21 borings were completed in an additional 37 days - well ahead of schedule.
Client Benefits
- Data was compiled and presented to the USACE in detailed logs as well as 2-dimensional (2D) and 3-dimensional (3D) graphical formats, along with surface contour maps showing mudline (i.e. lake bottom), lake bottom sediment-construction debris, and unweathered rock surfaces. Performed slope stability analyses to assist the USACE in designing stable excavations through lake bottom sediments, construction debris, and rock. Although the need for this expensive and time-consuming characterization step had been questioned initially, identification of large volumes of undredgeable rocky construction debris and underestimated unweathered rock surface elevation avoided changed conditions claims and potentially severe project delays.
|
Click here to find a Kleinfelder office location and contact information nearest you.