The Dalles Dam Bay 8/9 Spillway
The Dalles, Oregon
The Portland District Corps of Engineers evaluated several design concepts to improve juvenile fish survivability at The Dalles Dam, including a Behavioral Guidance Structure (BGS) and two training wall alignments below the spillway. The BGS was proposed to be constructed in the forebay to direct migrating fish away from turbine intakes and toward bays 1-6 of the spillway. The conceptual design of the BGS consisted of a floating guidewall anchored to the floor of the reservoir by large-diameter drilled shafts. The two training walls evaluated consisted of 10-foot-wide walls anchored to the bedrock at the base of the channel with high capacity rock anchors. To further develop the designs, the Portland District retained Cornforth Consultants to complete over-water explorations, laboratory testing, engineering geology, and foundation design studies.
Cornforth Consultants (CCI) and their subcontract driller completed a total of 42 borings at the project under challenging conditions including water depths up to 110 feet and currents up to 15 feet/second. All exploration work was completed within permitted in-water work period without a spill or injury. To utilize exploration funds efficiently, crews used survey-grade GPS tools with real-time data correction to locate the floating plant. The GPS tools saved the cost of mobilizing a survey crew and enabled the floating plant to be positioned more quickly. In situ testing was performed in borings using single- and double-packer setups to measure permeability of bedrock. Selected zones of borings were tested with a Goodman Jack to measure the in situ deformation modulus of the rock. Borehole imagery was completed in all borings to measure the strike, dip, and aperture of fractures and the size of voids.
Laboratory testing was completed on samples of rock core collected during subsurface explorations. The testing primarily focused on determining the compressive strength and laboratory deformation modulus of intact rock. The results of laboratory and in situ rock engineering parameters were used to characterize the rock mass for different basalt flows underlying the proposed structures. Once the Spillway Bay 8/9 design concept was selected by the Portland District, CCI assisted the design team by completing foundation design for the spillwall. Foundation design included allowable bearing capacity for the spillwall, estimated total and differential settlements, and calculating stability of bedrock subjected to high uplift loads. Cornforth assisted the structural designers with evaluating potential construction issues including feasibility of underwater rock excavation, benefits of foundation grouting, and several aspects of high-capacity anchors with 40-strand tendons. CCI assisted Corps structural design teams with preparation of plans and specifications.