On January 22, 2010, Dr. Brian Greene a senior engineering geologist for the United States Army Corps of Engineers visited Kent State University once again, this time to give his talk: “Emergency Scour Repairs at Allegheny River Dams 6 and 5, Pennsylvania.” Dr. Greene served as the project manager for the dam repair projects mentioned in his talk’s title and currently serves as the Chair of the U.S. Association of Environmental and Engineering Geologists National Committee on Dams. For a longer biography of Dr. Greene, please see my previous post announcing this colloquium. The focus of Brian Greene’s talk was Allegheny River Dam #6, located approximately 35 miles upstream of Pittsburgh. The following is a synopsis of Dr. Greene’s talk. Following the synopsis, please take the time to read the announcement for the upcoming colloquium announcement.
The Allegheny 6 dam, built in 1928, is a 992 foot long fixed-crest dam. On one end of the dam is a single-chamber lock used for boat navigation. The dam itself was built on a foundation of wooden piles driven to refusal into glacial sand and gravel strata, and horizontal stone-filled timber cribs.
In 2008, the USACE began work to repair severe undercutting of the dam. An important step in repairing the damage is to determine its cause. During 1989, a hydropower plant was added to the east shore of the river, opposite the chamber lock. Construction of the hydropower plant required the use of coffer dams, which are typically temporary structures built to keep construction sites below water level dry until construction is complete. The USACE’s investigation revealed that a remnant coffer cell (section of a coffer dam) used to construct the hydropower plant caused turbulent flow on the downstream side of the dam, which in turn directed water flow back against the base of the dam where it began undercutting through the sand and gravel river bed. In addition to turbulent water flow, missing timber cribs in the location near the hydropower plant allowed the scouring to reach 160 feet long and undercutting the dam by as much as 2/3 of its width. Due to scouring of this severity, dam failure could occur at any time, resulting in loss of navigation and recreational boating in the river, water intakes going dry (especially to the hydropower plant), and local wetlands losing their water source.
Under Dr. Greene’s direction, the USACE was able to take actions to repair the dam before it inevitably failed. In order to make repairs, a steel sheet pile was driven into the ground in front (downstream) of the scoured area. Sand and gravel backfill was placed on the downstream side of the sheet pile wall and the void area beneath the dam was filled with concrete to support the dam and its wooden pile foundation. Finally, graded rock layers were placed over the backfill downstream of the sheet pile wall in order to give additional support to the toe of the dam, prevent the initiation of scour, and prevent the backfilled sand and gravel from washing away.
The situation at Allegheny 5 was much the same, though to a lesser extent. The notable difference at 5, however, is that the USACE more or less supervised engineering geology students from the University of Pittsburgh working on a senior engineering design project. As it turns out, the students identified the processes at work and came up with a similar and effective solution which the USACE agreed with and implemented.
In both cases that the initiation of scouring was caused by remnant coffer cells related to the construction of hydropower plants at both locations. At the time of construction, however, no regulations were in place which required specific design plans to minimize impact of hydraulic flow. The lessons learned in these cases were that hydropower developers should be required to perform detailed hydraulic model studies to predict project performance and future potential for scour; and hydropower construction should conform strictly to design details which may be confirmed by field inspections.
Thank you, Dr. Greene for taking the time to visit Kent State University!
UPCOMING COLLOQUIUM ANNOUNCEMENT:
This Friday, The Kent State University Department of Geology has the honor of hosting the 2010 Jahns Distinguished Lecturer, Dr. Paul Marinos (follow the link to view Dr. Marinos’ biography). A synopsis of Dr. Marinos’ talk “Geological Constraints and Geotechnical Issues in Mechanized Tunneling” will be posted next week.
January 29, 2010
Room 234 McGilvrey Hall
(Click here for a schedule of upcoming/past colloquium speakers)