May 2002 Meeting Abstract
The new Braddock Dam replaces the existing 100-year old Monongahela River Dam 2 located 12 miles upstream from Pittsburgh. It is being built without the use of cofferdams using innovative in-the-wet construction techniques. In-the-wet technology was initially developed for offshore drilling platforms and immersed tube tunnels. The work at Braddock is the first time that a navigation dam on an U.S. inland waterway is being constructed using in-the-wet technology. The dam's construction procedure calls for fabricating two large concrete segments (reinforced concrete shells) at an off-site casting basin on the Ohio River. The segments were then floated from the fabrication site to the construction area near Braddock, PA. Once a segment is precisely positioned, it is sunk onto a preinstalled drilled shaft foundation. After each dam segment is set into place, concrete will be pumped into its internal hollow chambers, displacing water from the interior of the segment.
The foundation for this project involves the construction of eighty-nine reinforced concrete drilled shafts, which carry the weight of the dam and transfer loads into the bedrock. Each shaft is 78-inches in diameter and about 30 feet in length. Almost 16 feet of each shaft was drilled into bedrock to assure a secure connection and efficient transfer of loads. The rock units that encompass the rock socket include a soft clayshale and an underlying more competent siltstone layer. The tips of all shafts will bear on siltstone.
Prior to completion of the foundation design, an innovative load test of two prototype-drilled shafts was performed in the river along the axis of the proposed dam. The purpose of the test was to determine the axial and lateral loads that the shafts would take as well as to optimize the design size, spacing and proper depth of embedment of the production drilled shafts.
Segment One of the new Braddock Dam was successfully set down in early December 2001 and has received infill concrete. Segment Two was successfully launched and towed to the Braddock Dam site in late February 2002. The project is attracting attention nationally and internationally.
Dr. Brian Greene is currently Chief of the Geology Section for the Pittsburgh District, Corps of Engineers. However, he was recently selected as an Engineering Geology Regional Technical Specialist for the Great Lakes and Ohio River Region. He has 24 years of experience in the field of engineering geology having worked for the Corps' Buffalo District, the Ohio River Division Geotechnical Lab formerly in Cincinnati, and the Pittsburgh District. He has authored a number of publications dealing with broad aspects of engineering geology including: cofferdam foundations, dam failures, and engineering properties of weak sedimentary rocks. He is past Chairman of the Allegheny-Ohio Section of AEG and a past President of the Buffalo Association of Professional Geologists. Brian is currently a member of the National AEG Dams Committee and is a registered Professional Geologist in the Commonwealth of Pennsylvania. Brian was the second BAPG President and still has ties with the Buffalo geological community. Brian is a personal friend and it is a pleasure to have him come back and share with us his experience with the construction of Braddock Dam. So, I invite you to join us in attending the last meeting before summer and catching up with Brian and the rest of the BAPG members
See you there.
Lea Cervi
Executive VP