Since 2014, under multiple MSAs, McMillen has completed a wide variety of task orders at this 70 MW hydroelectric facility. Sample task orders include the alternatives analysis and design of a Low-Level Outlet (LLO) sluice gate rehabilitation and spillway crane rail extension, electrical upgrades (transmission and fiber optic lines, load cells, controls, and GSU transformer), and upgrades for pier concrete and the debris management system.
McMillen has been providing dam safety services for this 140-foot-high, 700-foot-wide concrete gravity dam for two decades. Serving as the FERC Part 12D independent consultant and Chief Dam Safety Engineer (CDSE), McMillen performs inspections and prepares reports, including the facilitation of Potential Failure Modes Analysis (PFMA) and updates to the Supporting Technical Information (STID) documents. Our team also supported the preparation of the Owner’s Dam Safety Program (ODSP) and Emergency Action Plans (EAP) and developed annual Dam Safety Surveillance and Monitoring Plans/Reports (DSSMP/ DSSMR) and Standard Operating Procedures (SOP). As the CDSE, we have prepared, delivered, and shared dam safety training workshops and assisted with training and drills for Lewis County PUD staff.
As a result of dam safety findings, McMillen provided more detailed technical support from our in-house structural, geotechnical, and mechanical engineers to evaluate the stability and condition of the dam and its four radial gates. Using rope access, we physically inspected all four radial gates in compliance with FERC Chapter 14, Appendix L. Following the inspection, we performed a seismic analysis and evaluated the stresses in the gate strut arms of the radial gates. We did this by installing strain gauges and other monitoring equipment to the gate strut arms and trunnion hubs to test friction between the fixed steel pin on the concrete pier side and the interior bushing material on the rotating portion of the gate. Based on the results, McMillen recommended strengthening the arms, replacing parts of the gate, and changing the bushing material to a more modern design that lowers friction. Our team also developed a Finite Element Model (FEM) for the dam and completed a seismic stability analysis (upstream/downstream and cross-valley). Work included updates to the seismic loading, revisions to the uplift loading based on post-seismic drain system failure, and cross-valley seismic loading utilizing pseudo-dynamic and 2D finite element methods using SAP2000 software. The scope also included seismic analysis for the four spillway gates.
Sluice gate rehabilitation
Since commissioning the gates in 1994, significant and potentially harmful vibrations and cavitation have occurred during operation. After inspection, our engineers identified the root cause. We then provided an alternatives analysis, seismic stability analysis, CDF models, a physical model, and final design. Scope included the development of new entrance and exit geometry, reshaping the spillway ogee above the sluiceways, switching the two vertical lift sluice gates (12'x16') to new top-seal radial gates (17'x12’), new hydraulic gate operators and controls, a new emergency closure bulkhead, new air venting, and other ancillary sluiceway retrofits.
Spillway gantry crane rail
The gantry crane rail was not originally designed to extend the entire spillway length, resulting in the inability to move the 175-ton gantry crane over spillway bay 4 and the sluice gates. The only way to service these bays was to bring in a mobile crane that crowded the small deck area. Our team performed an alternatives analysis and investigated solutions to increase the allowable loads that could be applied. We also considered dam thermal movement and necessary electrical modifications to accommodate a new configuration. Our resulting design alleviates significant access challenges with a new bridge section to extend the rail with major retrofits to the existing bridge girders to support the new rail.
