Las Vegas, host for the 2003 World of Concrete, is a showcase for concrete construction. Nearly every casino on the strip begins construction with a structural concrete frame and finishes with the installation of decorative concrete hardscapes, lakes constructed with structural concrete bottoms, and exotic features such as artificial concrete mountains, statues, and countless other themed features requiring artistic concrete. Excavation and caisson construction are now in progress for two projects on the strip: Steve Wynn's Le Reve Casino, located on the old Desert Inn site, and a new 42-floor tower for the Mandalay Bay Resort and Casino owned by the Mandalay Corporation.

Todd Nisbet, executive vice president for Wynn Design and Development, says, "The Le Reve Casino is an all-new project covering 55 acres and is projected to take 30 months to complete. It will be the pre-eminent luxury hotel and casino resort in Las Vegas, featuring a 3-acre lake, an 8-story mountain on a structural concrete frame, a 1-million-gallon aquatic theater, and a cast-in-place parking garage." With a 36-foot spacing between columns to accommodate larger guest rooms, it will be a heavy building with thicker-than-usual columns and floor slabs. Nisbet adds that the focus throughout the project will be on cost, constructibility, good use of materials, time management, and high utilization of concrete. Marnell Corrao, Las Vegas, is the general contractor for the low-rise structure and the tower and will perform its own concrete work.

In contrast to Le Reve, at Mandalay Bay, Lee Monford, senior project manager for general contractor Mandalay Development, Las Vegas, notes that for construction of the tower, "Our focus will be fast-track, lightweight construction." Mike Dean, president of M.J. Dean Construction, Las Vegas, the concrete contractor for the project, said that its plan is to place one floor every 4 days. The building is scheduled for completion in November 2003.

Soil conditions and caissons

Lochsa Engineering, Las Vegas, is the structural engineer for both Mandalay Bay and the tower at Le Reve. Jess Haldeman, a partner in the firm, states that bedrock isn't available in the Las Vegas valley at a depth to support buildings, only occasional beds of caliche--a cemented soil very common to the Las Vegas area. Given the weight of the structures, Lochsa decided to use caissons as the primary foundations for both buildings instead of "mat slabs," the traditional foundation method for casinos in the valley. When caissons aren't bearing on bedrock, they depend on "skin friction" between the concrete and the rough edges of the caisson hole to carry the load.

At Mandalay Bay, the owner and the engineer decided to use a standard "Kelly Bar" caisson construction method. Drills for this system have auger lengths of 3 or 4 feet. In the Mandalay tower, caissons will be 4 feet and 6 feet in diameter. To install each caisson, the operator of the rig drills 3 or 4 feet into the ground, pulls the bit up, clears the dirt from the auger, lowers the bit back into the hole, and repeats the procedure until the proper depth is reached--as much as 100 feet for this project. If soft material is encountered, the operator inserts a steel sleeve (casing) to prevent collapse. When the borehole has been drilled to the required tip elevation, a reinforcing steel cage is lowered into the hole, and the hole is filled with 8000-psi concrete. Haldeman states that 120 caissons will be needed to support the structure. Each pile cap will be connected to other pile caps by a 2x2-foot reinforced concrete grade beam. Lochsa approved M.J. Dean's request to incorporate a "rat slab" with the grade beams--needed for the start of construction work.

On the Le Reve project, much heavier than the Mandalay tower, original specifications called for the construction of 100 caissons with an average depth of 100 feet. Sixty of these would be 4 feet in diameter, and 40 of them, 6 feet in diameter. After the completion of the shafts, a 31,000-cubic-yard concrete mat foundation would be cast over the caissons to complete the high-rise foundation. The mat foundation was designed to be 9 feet thick, 90 feet wide, and 900 feet long.

As an alternate to the base bid foundation design, Morris Shea Bridge Co., Birmingham, Ala., a contractor specializing in design-build foundation construction techniques, submitted a value-engineered proposal (at considerable expense to itself). It proposed a system of pile caps and grade beams using "continuous flight auger" (CFA). Its proposal would shorten construction time, eliminate the need for a mat slab, and greatly reduce construction costs for the owner. It was awarded the contract after supplying extensive supplemental geotechnical analysis from Western Technologies, Las Vegas; Professional Services Industries, Denver; and STS Consultants, Chicago. It also funded a full-scale static load-testing program conducted on the jobsite.

According to Richard Shea III, the company's vice president, the CFA piles at the Le Reve project will be 1 meter in diameter with lengths ranging from 35 to 50 feet and allowable working loads up to 1500 tons per pile. The CFA pile system requires the use of special drilling rigs. The operator installs the CFA piles by drilling the bit to the proper depth in a single pass. In a process very similar to extracting a cork from a wine bottle, the auger bit is twisted in and extracted without rotation. Simultaneously, 8000-psi concrete is pumped under pressure through the hollow stem of the auger to fill the hole, which ensures that there will be no collapse of the pile walls. An on-board computer monitors the concrete injection to ensure that the proper amount is delivered into the hole. As one might imagine, special rigs are required for this work. As much as 250,000 foot-pounds of torque and 120 tons of extraction force are used to screw in and withdraw the auger.