In Nov. 18 Issue
Last week the U.S. Army Corps of Engineers modified the method of construction in Critical Area 1, the earthen section nearest the concrete portion of Wolf Creek Dam, according to Lee Roberts, public affairs specialist with the Corps.
All work in that area was suspended in March of this year after an increase in movements in the range of one inch or less was observed primarily in monitored cracks along the crest of the dam, according to Roberts.
He said the initial concern was these movements might be caused by a deep slope movement in the upstream embankment.
A six-month study showed the movements were not the result of deep-seated sliding of the embankment, but rather shallow movements attributed to several other causes.
The causes included deflection of the sheet pile retaining the slope above the work platform, settlement of the work platform and rainfall runoff into cracks in the pavement and shoulder on the dam crest.
In February of this year, the Corps had already terminated the grouting in Critical Area 1.
The grouting was a pretreatment to improve the foundation to allow safe installation of the permanent barrier wall. Because the Corps was not seeing the degree of improvement wanted, they had begun developing alternative construction approaches in Critical Area 1 that did not rely on the grouting pretreatment to safely install the permanent wall. The Corps are taking a very conservative approach in Critical Area 1 to ensure dam safety during construction, Roberts said.
"The new method differs from the original in that the overlapping concrete piles that form the barrier will be installed through temporary steel casing advanced ahead of the excavation. This will isolate the cave and cutoff trench material outside the casing from the effects of the construction activities inside the casings," said Michael Zoccola, chief of civil design for the Nashville District. "Once the hole is drilled to the design depth it will be filled with concrete as the casing is removed. This approach will be slower but it satisfies concerns regarding dam safety and constructability. Public and dam safety, as always, is our first priority."
A request for proposal will be issued to the foundation remediation contractor within the next several days and the Corps will negotiate contract modifications over the next few months, officials said.
The contractor will begin to procure the necessary equipment and casing which will be fabricated and mobilized to the site over a period of five to eight months. Modified construction activities will begin once the equipment is assembled and tested.
Looking forward, the work platform in Critical Area 1 will be raised and widened while specialty equipment needed to handle and drive the steel casing is being fabricated. A test section will be completed using the revised process before actual barrier wall construction begins.
Schedule and cost impacts of this method for completion of the barrier wall in Critical Area 1 are being evaluated and may extend the completion of the project in this location.
Extent of cost impacts are not known at this time, according to the Corps.
Roberts said in keeping with existing Corps policy, raising the level of Lake Cumberland will not be considered until structural measures in Critical Area 1 are complete and post-construction risk assessments indicate the dam is safe.