Geotechnical Design and Instrumentation of an Anchored Cofferdam, Point Marion Lock, Pennsylvania

Point Marion Lock was designed to replace an existing navigation lock that has been in service on the Monongahela River near the Pennsylvania-West Virginia border for some 67 years. As a result of the close proximity of the planned excavation for the new lock, the land wall of the existing lock required extensive stabilization to permit it to be utilized as the river arm of the project cofferdam. Installation of 471 prestressed rock anchors was closely linked with each progressive stage of soil and rock excavation required to build the new lock. Adverse geologic conditions affected cofferdam stability in that the land wall of the existing operating lock was founded on weak sedimentary rocks exhibiting very low shear strength. Critical to the successful performance of the cofferdam was the design and installation of an extensive instrumentation program. A vast array of movement detection systems including inclinometers, alignment pins, shear strips, tilt plates and load cells were interfaced with both on-site and off-site computers to provide real-time monitoring of cofferdam performance throughout the excavation process. Data from instrumentation yielded invaluable information needed to make necessary adjustments in on-going construction activities including excavation and rock anchor installation.