September 2017 Dinner Meeting
September 28 @ 5:30 pm - 9:00 pm
Swift No. 2 Dam Failure (Cowlitz County PUD Washington)
Why Geology Matters, How Even Geotechnical Giants Can Get It Wrong, and How we can Design and Build Rapidly when High Costs are at Stake.
Presented By: Ken Green, CH2M
Abstract: We are pleased to announce that Ken Green, a geotechnical engineer and geologist with CH2M practicing in the Puget Sound area and beyond for over 40 years, will describe the 2001 investigation to discover the reasons for failure of the 80-foot high Swift No. 2 earth embankment and the rapid design and construction to re-establish power generation on the Swift system.
Additional Detail: The foundation of the 80-foot high Swift No. 2 dike embankment failed suddenly after 40 years of service on 21 April 2001, resulting in breaching of the embankment and destruction of the Swift No. 2 powerhouse and related facilities. An extensive investigation was launched to determine the cause of failure. Investigations included large open pit excavations, borings, aerial and ground surveys, geologic mapping, laboratory tests, smoke testing of the foundation and video observation of boreholes.
Results revealed that failure began when alluvial deposits forming the bottom of the forebay piped into a large open basalt lava tube in the foundation. The loss of material by piping formed a sinkhole in the forebay and provided a direct hydraulic connection to distribute large volumes of water to cavities and flow paths leading under relatively thin basalt flows upon which the dam was founded. Foundation failure under the downstream toe of the dam occurred when the water pressure and flow from the hydraulic connection to the reservoir caused the development of significant erosion paths in granular soils under the basalt, leading to eventual disruption and total collapse of the rock foundation under the dike. The failure then rapidly progressed headward into the embankment to cause complete breaching of the dike. The sudden release of water resulted in the severe downstream destruction and loss of the Swift No. 2 powerhouse, and related facilities. Although the possible mechanism of failure was recognized by the dam’s original designers including Arthur Casagrande, the field work failed to recognize the important role that a detailed understanding of the geology played in recognizing the potential for failure of the dam.
Failure of the Swift No. 2 dam and powerhouse resulted in the immediate need to bypass all flow through the emergency spillway at the upstream Swift No. 1 dam and powerhouse to avoid use of the failed canal system. To get the Swift No. 1 powerhouse back on line as soon as possible, a temporary earthen dam in the canal upstream of the failure was field designed. Modifications of the canal to form a side channel spillway capable of diverting up to 12,000 cfs with velocities exceeding 40 fps back to the Lewis River from the canal were also field designed and tested.
Swift No. 2 canal and powerhouse was rebuilt as soon as practical once the failure mechanism was thoroughly understood.