was modern floodplain alluvial soils, composed of sand, clays and gravels, sometimes mixed with glacial tills and outwash. Some occasional layers of stiff, high-plasticity clays lower in the excavation were a concern because these soils had a tendency to grab the very wide (4.2 m) clamshells used for the project. The bedrock consists of shale/ sandstone/limestone with an average com- pressive strength of 50 MPa (7,250 psi). While the rock was generally of high quality, a layer of weathered and decomposed rock was encountered in many of the borings at the top of rock. Overburden varied from soft, fine grained river deposits to glacial till. The borings indicated a range of N Values from 0 in the river to refusal in the till near the bedrock interface. The tills and poorer quality top of rock were concerns, and much discussion centered on how much, if any, “key” into rock was necessary. In the end all parties agreed that if the proper quality control was with special attention to how the project quality objectives and client satisfaction were achieved. Stakeholders. To properly assess and address the risks a world class team was required, including AMP, its engineer MWH, the joint-venture partners, Kiewit’s Foundation Group (KFG), Mueser Rutledge Consulting Engineers (MRCE), Liquid Earth, the Federal Energy Regulatory Commission (FERC) and the USACE. The JV worked closely with MRCE to develop the plans and specifications for the cut-off wall. Part of the design review process on this design/build project required by FERC was a review by a Board of Consultants (BOC). This group of industry experts reviewed the design, plans, specifications and construction methodology proposed by the JV and its designers, and its approval was required for FERC acceptance and of the proposed cofferdam. Bringing Moretrench on the team gave the BOC a level of confidence Marine dike cross section Geology. The Geotechnical Design Report (GDR) included 38 borings, 5 of which were in the Ohio River. In addition, 11 test pits between 1 m (3.3 ft) and 6 m (20 ft) deep were completed. Rock cores between 3.4 m (11 ft) and 6 m (20 ft) were extracted, and, as part of the design/build contract, an additional 7 borings confirmed the information presented in the GDR. According to the GDR, the project site is contained within the Western Coal Field Physiography Region. The geology is unglaciated, but does include some glacial tills and outwash. Essentially, the ground implemented then a seal to top of “competent” rock would achieve the intended cut-off and no key was required. Risks. The risks associated with the cofferdam included: 1) completing the marine dike, cut-off wall and flood structure prior to the rising waters on the Ohio River, 2) the permeability and continuity of the cut-off wall, 3) the quality and suitability of the foundation for the marine dike, and 4) the stability of the cofferdam berms. Some significant project design features follow: the mix design for the cut-off wall and the construction aspects of the cut-off wall, that the dewatering system would serve its function and be constructed to a high standard of quality. Liquid Earth worked with the team to develop the mix design needed for the cut-off wall method. Design Marine Dike. The marine dike consisted of two outer containment berms of USACE Graded A stone placed in the river at the maximum depth of over 21 m (70 ft), with two inner transition layers of stone covered by filter fabric. The central portion of the dike consisted of sand (maximum 10% DEEP FOUNDATIONS • NOV/DEC 2013 • 83
