Thalle’s role on the HHD rehab includes replacement of several 1940s era culverts that provide irrigation water and flood control to surrounding areas.
August 2015 will mark the tenth anniversary of Hurricane Katrina’s deadly and costly assault on a large part of this nation’s Gulf Coast region. While the devastation inflicted on the coastlines of Alabama, Florida and Mississippi was historic, it’s the images of the breached dikes of New Orleans that have been imprinted most on our minds.
About 600 mi. (965 km) to the southeast, a similar threat for dike failure, with an even more catastrophic prospect for loss of life, has been looming for decades. Ongoing work to reconstruct a 143-mi. (230 km) earthen dam surrounding south central Florida’s Lake Okeechobee will help eliminate that risk. Playing a key role in those efforts, Thalle Construction is using GNSS-based systems and equipment to reconstruct a series of dams and culverts which will not only provide peace of mind to area residents, it will help ensure their safety well into the next century.
A Dike Is Born
Located roughly midway between Ft. Myers and Port St. Lucie, Fla., and at 730 sq. mi. (1,174.8 sq km), approximately half the size of Rhode Island, Lake Okeechobee is the nation’s largest freshwater lake situated completely within one of the lower 48 states. In 1928, a powerful hurricane crossed over the lake, causing a storm surge that topped the 6-ft. (1.8 m) earthen dike that surrounded it, flooding adjacent areas. By storm’s end, at least 2,500 local residents had died, prompting subsequent construction of more than 83 mi. (133.5 km) of levee structures at key locations on the lake. After additional storm damage, the Flood Control Act of 1948 helped initiate major levee work, resulting in what would eventually be a 25-ft. (7.6 m) high, 143 mi. (230 km) long levee system called the Herbert Hoover Dike (HHD).
“It’s hard to believe, but many parts of the current dike system date back 60 years or more,” said Tim Pernsteiner, project manager of Thalle Construction, one of the general contractors performing work on the levee rehab. “As a result, several parts of the dike system are suffering wear issues — particularly in times of high lake levels — and showing signs of weakness evident through visible leaks. The U.S. Army Corps of Engineers has crews that ride the dike every day looking for just such signs and, by nature of its condition, have rated the HHD a Level 5 concern, indicating it as one of the highest risks for a catastrophic breaching.”
Thalle’s role on the HHD rehab includes replacement of several 1940s era culverts that provide irrigation water and flood control to surrounding areas. The culverts, 10-ft. (3 m) diameter, 111-ft. (33.8 m) long corrugated steel structures, are in a serious state of disrepair and are being replaced with a trio of 10 ft. diameter reinforced concrete culverts equipped with slide/flap combination gates on the lakeside, which open automatically based on landside water levels.
In order to get to a point where work could even begin on the culverts, Thalle had to construct a cofferdam to isolate each structure, then de-water the area for excavation. Constructing the cofferdam was, in itself, a challenging effort considering a good portion of the work took place as much as 20 ft. (6 m) under water.
“For the cofferdam to happen, we essentially moved the dike out into the water to isolate the work area,” said Pernsteiner. That required us to fill in a good portion of the canal immediately in front of the structure with more than 100,400 cubic yards of material, create a 4:1 slope and then armor it, first with 4,000 tons of bedding stone and then with about 16,000 tons of armor stone or rip rap. The water in those areas ranges anywhere from three feet to about 20 feet deep — given those parameters, we were looking at roughly 64 to 65 feet of slope all underwater.”
Working through Lengemann Corporation, Altoona, Fla., and North Carolina-based Benchmark Tool & Supply Inc., both dealers of Topcon Positioning Systems, Thalle secured systems and equipment designed to enhance the cofferdam segment as well as subsequent facets of the overall project.
Veteran general contractors, Thalle Construction was already GPS savvy, so they knew the benefits the technology could provide at the Lake Okeechobee site. For initial sloping of the cofferdam, the company used a Komatsu PC-600 equipped with a Norris long-reach boom and a Topcon 3D-MC2 machine control system, which allowed the operator to see the slopes which needed to be created for the dam.
“The stick, boom and bucket on that Komatsu PC-600 provide about 64 feet of reach; almost exactly what we needed for the sloping part of the job,” said Catherine Harris, Thalle’s survey manager. “The operator has a surface model loaded into the machine control system and he simply has to look at the in-cab screen, see exactly where his bucket is and make his cuts.
“The alternative would have been a surveyor on shore with stakes and another surveyor in a Jon boat reaching down with a grade pole into the water and telling the operator that he was — or wasn’t — where he needed to be.”
Pernsteiner added that, because the HHD project was a Corps of Engineers job, with tight specs, GPS availability was all the more valuable.
“We probably would have been in a world of hurt without the GPS system out there,” he said. “One of the cofferdam’s many complexities included a muck layer on the lake bottom which, as our operator built the dam out, rolled out in front him. Eventually we had a huge curl of mud into which the slopes needed to intersect. Without GPS, he would have had no way of knowing where that point in space was. With it, however, he knew exactly where the new toe-to-slope was and adjusted accordingly. Generating as-builts was also a breeze: all the operator had to do was push a button to measure and save a point, measure and save a point, and so on. The time savings in that respect alone is impressive.”
For the actual culvert excavation and reconstruction, Thalle augmented the PC-600’s capability with a Komatsu PC-360 — also equipped with Topcon 3D-MC2 — and a Topcon-ready Komatsu PXi D61 dozer, boasting the company’s “intelligent grading” capability.
“With the cofferdam constructed and the culvert area de-watered, we were free to excavate to the existing structure,” said Pernsteiner. “From the center line to the culvert was 300 feet on either side. It was a massive excavation to get down to the minus three elevation before we could replace the structure, build the new system up with structural fill and tie it into the slurry walls. Having 3D-MC2 on every machine was excellent for grading those 3:1 and 4:1 slopes. A job of this size and complexity could have been littered with stakes but GPS literally made that unnecessary.”
Every GNSS unit on site — including Tesla controllers used for periodic grade checks and equipment back checks — worked off a single Topcon HiPer Lite base which Harris said had been recently upgraded.
“Company wide, we are going through a rigorous program of either upgrading or outright replacing older equipment,” she said. “So we had two HiPer Lites that we converted from their original 915 MHz to UHF to bring them current. We had one of those semi-permanently situated on pole at the culvert site and it was ideal at handling all our GNSS needs. This was a project that was just made for GPS and the Topcon systems we had in place did not disappoint.”
Pernsteiner said, “This is an outstanding project that, when completed, will not only result in a much stronger HHD, it will help ensure that the people of this area will no longer have to fear when lake levels get too high or severe weather approaches. It’s been a great job to be as part of.”
The current $36 million contract has Thalle handling two culvert replacements with an overall December 2016 completion date.