The Imperial Palace Casino was built on the Back Bay in Biloxi, Miss., in the late 1990s. It was a permanently moored, three-floor floating vessel that sat upon nine hopper barges, which were welded together. However, after 2005, when Hurricane Katrina prompted state legislators to reverse a law that required all casinos to float on water, the IP Casino sought a site on dry land.
Boyd Gaming Corp., owner of the IP Casino, put out a request for design-competition proposals in order to dry-dock its casino permanently without interrupting its 24/7 operations.
Thompson Engineering won the contract in late 2010, and contractor W.G. Yates & Sons Construction Co. began work in 2011.
“The solution we designed was extraordinary on many levels. It required the integration and execution of every engineering and architectural discipline in our organization,” says Henry Seawell, principal engineer at Thompson. “We were able to design and coordinate the construction without affecting the business of our client. In three years of construction, the commercial operations of the resort were never closed because of the project—not once.”
The dry dock itself had to accommodate extremely variable soil conditions, ranging from soft to firm, where the graving dock would soon be standing. Teams developed a first-of-its-kind solution that resulted in a long-term stabilized foundation to support the 16,000-ton vessel and adjacent facilities. Additionally, the support-system design features no piles or other conventional structural ground improvements, and there could be no long-term settlements beneath the graving dock.
“Thompson’s design incorporated intricate and well-conceived solutions that we could understand and build, even in the most challenging working conditions our marine construction group has ever faced,” says Ray Winters, construction project manager with Yates.
Dredging the area under and around the existing casino vessel to precise subaqueous neat-line profiles in zero-visibility conditions, divers removed 33,300 cu yd of silt, clay and sand. Next, 820 linear ft of cantilevered cofferdam—a combination of pipe and sheet piles—enclosed the area around the vessel.
To contend with a lack of direct construction access when placing the 2,000 tons of reinforcement required for the massive seal slab, the team created a series of floating pontoon sections. The slab was integrated into the initial stabilization mat, placed on soft subaqueous soils beneath the vessel. After divers positioned and bolted together reinforcing pontoon frames, crews shimmed them to the design elevation using remote pneumatic leveling controls. Soon after, the surrounding 18,000 cu yd of underwater seal-slab concrete was placed.
Along with supporting topside crews, divers, working under water in complete darkness with remote radio control, built a 615-ft-long by 130-ft-wide graving dock structure made of a structural-steel-blocking frame. It was encompassed by a concrete mat foundation that, at the time of placement, was the world’s largest continuous underwater concrete pour. Before the unwatering of the coffer basin, all subsea construction was done in zero-visibility conditions.
Below the slab, groundwaters were depressed through a peripheral system of shallow and deep wells. An induced load of a non-buoyant soil mass sufficiently presettled and precompressed the subsoils to prevent future movement.
Since completion in 2014, the graving dock slab, with 150 million lb of vessel and concrete atop it, has experienced no measurable settlement.
Yates’ safety program was built around behavior-based safety and an on-site process called “safety task assessment.” Teams worked 230,000 hours with no safety incidents, despite challenges above and below water.
The judges noted this project’s incredible level of difficulty. “To have no recordables and no lost-time accidents—that is a team that’s on top of their game, no doubt,” one judge said. “They absolutely reinforced the construction industry in building foundations of this magnitude.”
IP Casino Dry Dock
Biloxi, Miss.region Texas & Louisiana
Project team
Owner Boyd Gaming Corp.
Lead Design Firm/Structural/Civil Thompson Engineering
General Contractor W.G. Yates & Sons Construction Co.
MEP Engineer BayFour Design LLC
