Construction of the Kentucky Lock Addition project’s ongoing $67 million downstream cofferdam presents safety, geologic, and construction method challenges. Approximately 500 linear feet of the cofferdam utilizes new lift-in, in-the-wet techniques to construct a concrete segmental wall - a cofferdam wall that will eventually be incorporated into the permanent wall of the new 1200 foot x 110 foot navigation lock instead of being demolished. This new navigation lock is the focal point of the U.S. Army Corps of Engineers’ $1.25 billion project that is approximately 35% complete. The project site is at Kentucky Dam, the lowermost dam on the Tennessee River in western Kentucky, USA. Kentucky Dam impounds the largest reservoir east of the Mississippi River.
Kentucky Lock’s cofferdam concrete wall will be constructed in ten segments with plan dimensions of approximately 45 feet by 55 feet each. Stay-in-place concrete forms, or shells, for each segment will be cast on barges in heights up to 35 feet. These 12 to 16 inch thick shells will weigh up to 1.3 million pounds. Each shell will be lifted off its casting barge and placed on a prepared foundation on the river bed using a specially designed gantry barge system. This construction method is similar to one employed by the Corps for a cofferdam wall on the Chickamauga Lock project in the 2007 timeframe. However, construction of the Kentucky Lock wall is significantly different in many respects. While Chickamauga shells were placed on previously constructed 4 foot diameter concrete drilled shafts, Kentucky shells will utilize integral and adjustable spuds on all four corners to support the shell on an irregular rock surface. This approach is a component of a Value Engineering proposal ($3 million cost savings) by the contractor to eliminate rock blasting and excavation down to a design grade. Instead of underwater blasting, the rock surface is being scarified by a hydraulically powered drum cutter to remove the weathered rock zone to expose competent rock with compressive strengths in the 20,000 to 30,000 psi range. Each shell is being designed for the resulting top of rock grade and configuration. Lifting and lowering the Chickamauga shells was accomplished by a fixed gantry, strand jack arrangement that was supported on the drilled shafts and the previously placed shell. Kentucky’s method of lifting and lowering using a gantry barge eliminates the time consuming process of establishing and de-constructing the fixed gantry system for each shell.
The location of Kentucky Lock’s cofferdam wall in the existing lock’s immediate lower approach presents significant safety challenges and space limitations. An alternative design analysis chose this design in part to maximize the cofferdam wall’s distance from the edge of the navigation channel, but the edge of the wall is still within 23 feet of the edge of the existing channel. Since the existing Kentucky Lock is one of the most heavily utilized locks in the U.S., almost continual commercial vessel traffic presents a safety and logistical challenge in this confined environment. A key to minimizing safety risks and minimizing disruptions to construction operations is a dedicated helper boat with an associated marine safety plan (MSP) owned by all parties involved. The development of this MSP commenced during preparation of the plans and specifications and involved representatives of the U.S. Coast Guard and the towing companies transiting the lock. After award of the construction contract in September 2016, the contractor, Johnson Brothers Corporation (Roanoke, Texas USA), became involved and was responsible for drafting and maintaining this living document. Development of the MSP was enhanced through the use of a simple Table Top exercise. The Table Top employed plan views during various phases of construction in conjunction with scale two-dimensional models of all the vessel types and floating plant expected to be in use during construction. Port Captains of the towing companies could relate to this type of “modeling” and were instrumental in analyzing and selecting helper boat procedures. These procedures are now successfully being used by the helper boat and transiting vessels. In addition, the contractor has adjusted his work activities to minimize impacts to his operations because of passing vessels.
When this cofferdam’s initial design was completed in 2003, it was envisioned as a float-in structure that would require a 3 month closure of the existing lock during its construction, significantly impacting the towing industry. Based on lessons learned from the Chickamauga lift-in approach, that 3 months was reduced to 1 month. Then based on the value engineering proposal that eliminated underwater blasting and by incentives incorporated into the construction contract to minimize lock closures, the expected total lock closure is now approximately 10 days.
It is expected that other lock addition projects and projects utilizing in-the-wet construction methods in confined environments will benefit from the techniques and experience gained from the design, construction, and contracting methods employed by Kentucky Lock’s downstream cofferdam.