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Technical paper: Unique cofferdam construction: Pt Marion Lock and Dam, Pennsylvania

Donald A Bruce, Brian Greene and Andrew Schaffer. This paper was first published in GE’s April 1996 edition.

When it was first attempted to incorporate an existing lockwall into a new cofferdam, in 1961 at Wheeler Lock, Alabama, the result was a failure. A major portion of the land wall slid about 9m into the dewatered excavation which was being used to construct an adjoining lock, and there was loss of human life. The reported cause of the failure was sliding on an undetected weak clay seam in the foundation rock. No stabilisation measures or instrumentation systems had been implemented.

So, when a similar project was recently conceived to replace an existing navigation lock that had been in service on the Monongahela River in Southwest Pennsylvania since 1926, the Pittsburgh District of the US Army Corps of Engineers was clearly determined to avoid a repetition of the same problem. The solution involved nearly 500 high capacity prestressed rock anchors, constituting one of the largest single uses of prestressing strand in North America, and a structural and geotechnical instrumentation program reckoned by the suppliers to be one of the most intense ever installed in their long experience. The data from the instrumentation were monitored in real time through on site and off site computers to help optimise ongoing construction activities by eliminating potential problems at the earliest stage. 

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