Two of the first WR 2000 soil stabilizers in North America are working at the site of a massive liquid natural gas (LNG) receiving terminal south of Houston on the Texas Gulf Coast.
Zachry Construction Corporation is using the Wirtgen WR 2000s to lime-stabilize the site of the $600-million LNG facility, first to be constructed in the United States in more than 20 years.
“We’re building a marine dock to receive LNG tankers and the terminal to go with it,” said Phil Griffin, project director of Zachry. “The LNG will be sent underground via a pipeline system up to the main site, where two LNG tanks will receive the liquid gas, which will come in at minus 286 degrees Fahrenheit.”
The tanks are enormous, 280 ft. in diameter, and 180 ft. tall, with 3-ft. concrete walls with liners.
“They’re like two big Thermos bottles, each the size of a football stadium,” Griffin said.
But unconsolidated site conditions along the coast have raised a significant challenge in getting the base conditions right.
LNG For The Future
Freeport LNG Development LP is constructing the facility, designed with storage capacity of 418,544 cu. yds. (320,000 cu m) of LNG, and a vaporization send-out rate of 1.5 billion cu. ft. per day. The natural gas will be transported through a 9.4-mi. pipeline that will extend to Stratton Ridge, TX, which is a major point of interconnection for a number of Texas intrastate systems.
The LNG terminal, transfer lines and the storage and vaporization units are to be located on Quintana Island, southeast of Freeport, right on the Gulf. LNG tankers will enter the Intracoastal Waterway and dock right at the facility.
LNG is conventional natural gas that has been reduced to 1/600th its volume through a sophisticated refrigeration process that cools the gas to below minus 259 degrees Fahrenheit, until it liquifies. The liquid form allows for natural gas to be shipped over long distances safely and economically.
Siting of LNG terminals is difficult. A site must have a 45-ft. draft channel to open water; there must be a port facility sufficient to service the large LNG carriers; it must be located in a supportive state and local environment; and it must be near existing pipeline capacity. Freeport met those requirements.
On completion in early 2008, the Freeport LNG terminal will serve as a distribution hub for LNG that is shipped in supertankers to the United States. ConocoPhillips has an agreement with Freeport LNG to manage the project and will receive 1 billion cu. ft. per day of throughput capacity, while The Dow Chemical Company will receive the remaining 500 million cu. ft. per day.
Building on Dredge Spoils
The 86-acre site has a minus 3- to 4-ft. water table, but that actually is better than anticipated.
“We’re bringing the site up 14 feet, and we’re building levees that are 21 feet high that will totally contain the site,” Griffin said. “The entire site is located on a dredge spoil area, which includes fat clay, sand, lots of silt, and lots of organic material. For years they have been dredging the ship channel, and the spoils have been placed here. We’re reclaiming the dredge spoils, and bringing it back to life again and using for backfill.”
Other soil is coming in from an offsite borrow pit, 40 to 50 truckloads a day, Griffin said.
“We’re placing 6,000 to 7,000 cubic yards of soil per day,” he said. “In total we will move some 900,000 yards of soil for this site. We’re using all the soil we can possibly get off the island to build this area up, plus bringing it in off site from the borrow pit.”
Soil is being applied in 18-in. lifts, which is mixed into the top layer of the previous mix.
Lime is the stabilizing medium.
“We’re bringing in 12 to 16, 25-ton truckloads a day of pelletized lime from central Texas,” Griffin said. “The lime is pelletized to cut down on fugitive dust, for environmental issues. These trucks come in first thing in the morning, and they belly-dump windrows of lime in an area all laid out for it. A road grader spreads and levels the lime out, and we follow up behind the grader and slake it with water, which activates the lime. Then we bring the mixers in to go to work.”
Lime content is 5 percent by dry weight and is spread 24 ft. wide, at about 44 lbs. per square yard. Water is drawn directly from the gulf, and about 10,000 gallons are required for a 25-ton load. Water is applied through water wagons with cab-controlled spray nozzles. Curing time is two to three days.
“We’ll mix the soil back and forth a couple of times, and once the mix is where we want it, they start compaction with sheepsfoot and smooth drum compactors,” Griffin said. “Sometimes we have to go back and remix; if we don’t get the compaction we need we may go back and reapply lime and mix again.”
A materials lab tests the prepared soil prior to compaction.
“Pebble lime has very little dust,” said John Walsh civil site manager of Zachry. “When you slake it, it steams. There is a lot of moisture already in the soil, and we are trying to dry it out, and the lime reaction will take a lot of that moisture out. We are mixing 18,000 square yards earth per day.”
WR 2000s at Work
With an enormous site like Freeport, the Wirtgen WR 2000s have met their match. The machines can mix in one direction, then reverse-mix without doing a U-turn.
When the machine works from left to right, it operates in an up-milling mode. The wheels underneath the cabin constitute the front axle, and steering is affected via the steering wheel. The operator cabin is positioned either in the centre or on the far right.
When working from right to left, in the down-milling mode with front-wheel steer, the wheels behind the milling and mixing rotor act as the front axle and are steered via the joystick, and the operator cabin is positioned on the far right.
“They appealed to us right away because they are a lot faster, because you don’t have to turn them around,” Griffin said. “You can stabilize back and forth in forward and reverse.”
But getting operators to take advantage of the innovation is something else.
“Some of the guys don’t want to back up,” Griffin said, “but that operation is available to them.”
Inspectors, he said, also must be convinced that reverse mixing works, but that hasn’t been a problem for Zachry.
The powerful traction of WR 2000s is ideal for work in the borrow pits, preparing capped paths for haul equipment.
“The four-wheel drive on them is much better than on other makes, so they don’t get stuck in bad material,” Griffin said. “In a lot of these borrow areas we have to put a foot of material down just to get our equipment on the site. They will maneuver out there and do the job where the other big machines will break through.”
(This story appears courtesy of Wirtgen Technology.)
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