In June 4 Issue
This week the U.S. Army Corps of Engineers allowed work to resume in Critical Area I, the area where the earthen embankment wraps around the end of the concrete section of Wolf Creek Dam, eight months after the Corps announced they were going to modify the method of construction in the area due to its instabilities.
All work in that area was suspended in February of 2010 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 the Corps.
Initial concern was that these movements might be caused by a deep slope movement in the upstream embankment but 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.
On Wednesday, vertical holes began being drilled to define the edges of the cave and core trench along the alignment of the proposed concrete barrier wall, according to David Hendrix, project manager for the Nashville Corps. After testing, the holes will be gravity backfilled with grout to enable safe installation of the barrier wall.
Subcontractor Hayward Baker Inc. mobilized their equipment midweek into Critical Area 1 where they began drilling. Once they have finished with their part Treviicos Soletanche JV, the dam's construction firm, will commence with installing their permanent barrier wall in that location.
"There is no change to the timeline. This is a good step forward for us to initiate this work," Hendrix said. "We've had some things that have been delayed by high water but this is not one of them."
Hendrix said the modification of how to proceed with work in Critical Area I took eight months, which was a time allotted for the process.
"The work they'll be doing is very important to ensure that we safely install that concrete barrier wall," Hendrix said. "Where we're working is the area where you always see the pictures of the really bad geology with all the big caves and we're taking some special precautions in that area."
Before work was suspended, 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.
Michael Zoccola, chief of civil design for the Nashville District, 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, according to the Corps.
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.
The work platform in Critical Area I was raised and widened while specialty equipment needed to handle and drive the steel casing was fabricated
The Corps of Engineers released a project update in April this time saying major rehabilitation work should be completed by December 2013 and Hendrix echoed that sentiment again this week.
According to this timeline, the unstable Critical Area I, where the earthen and concrete sections meet, could be made safe by that time. The Corps also said the remainder of the 4,000-foot-long barrier wall could also be in place by then, meaning the return of a normal lake level of 720 ft. by summer 2014 is possible.
The last work to be completed will most likely be that in Critical Area 1 due to the work suspension from February - November 2010 due to slope stability concerns and changes to the construction methods used on the tender portion of the dam.
The installation of a permanent barrier wall has continued in the other portion of the earthen section and as of April 1, the foundation remediation contract was 51 percent complete, according to the Corps' latest update on the work.
If the Corps' latest information goes unchanged and the dam is completed by Dec. 2013 it would mean that the lake would have been at around 680 ft. above sea level for seven years.
As always, public safety is the top priority and the Corps of Engineers said they will be monitoring this and all other construction activities closely.
For news and information about the Nashville District, visit www.lrn.usace.army.mil, and follow the district on Facebook at http://www.facebook.com/nashvillecorps or Twitter at http://www.twitter.com/nashvillecorps.