Calling all sports fans! The City of Tulsa is about to open the doors of an impressive new arena for sports and entertainment. A mere two months from completion, the $178 million, 18,000-seat venue is an essential component of the publicly funded revitalization strategy for Tulsa, Okla. And, an exciting setting to continue the winning traditions of its Tulsa Talons Arena Football and Tulsa Oilers Hockey teams!

Building support
Tulsa County citizens not only support the expansion of this business development and attraction for visitors, but they actually contribute to the cause. Vision 2025, voted on and approved in 2003, was a proposition to increase the sales tax rate by one penny for thirteen years to help fund capital improvements, like the arena and convention center.

When the bids rolled in for the project however, the original plan of $68.9 million required some enhancement and budget saving savvy. Bonds, corporate sponsorships, and private donations increased the city’s funds substantially, and the Bank of Oklahoma bought 20-year-term rights to the arena’s name, now known as the BOK Center.

Structural stress
When ground broke in August of 2005, sub-contractor Williams Construction Co. needed to keep costs within budget. Williams went out of their way to find a "better, faster, cheaper" alternative for the design of the structural bracing that supported the wall between the arena floor and first level decking that surrounded the perimeter of the BOK Center. The innovative method they found allowed multiple construction tasks to be undertaken and required less labor than typical construction, benefiting both the project schedule and financial plan.

"Typically, cast-in-place walls need to be braced before they can be backfilled against," Project Manager for Williams Construction Co. Adam Oglesbee said. "Bracing would have to be applied to the inside of the walls, minimizing access to construction on and around the arena floor." After seeing a segment on the Discovery Channel about geogrid employed in mechanically stabilized earth (MSE) projects, Oglesbee researched alternative methods. He thought, "What about something to relieve the pressure instead of bracing the wall in front?"

Oglesbee’s research turned up Tensar International Corporation, a manufacturer of biaxial and uniaxial geogrids for earth reinforcement. He contacted them to determine if a MSE alternative could be provided to replace the original, traditional bracing scheme.

"Building below-grade can often be time-consuming and expensive," noted Tensar’s Project Manager Luis Velez. "The structures themselves are often massive, concrete and steel behemoths designed not only to hold up buildings, but to resist the earth’s pressure as well."

One way to reduce the cost of these structures is to remove the earth pressure completely with a permanent below-grade pressure relief wall, but occasionally temporary MSE walls can be economically constructed to augment or replace bracing, as in the Tulsa arena’s case.

Taking the pressure off
Tensar provided a conceptual, preliminary design and cost estimate. Williams estimated the construction of a pressure relief wall would take the same amount of time as installing the traditional bracing, yet would be less expensive and would allow access to the arena floor.

The Tensar Pressure Relief Wall System—consisting of 18 inch by 18 inch by 10 foot-long welded-wire facing forms, Tensar Uniaxial (UX) Geogrid, a soil retention fabric, and full engineering plans—was approved for construction along a 1,300 foot section (which is approximately three-quarter of the arena’s perimeter) as a less expensive substitute to the bracing option; installation began in April of 2006.

The welded-wire facing forms, or "baskets", allowed the contractor to maintain a reasonable facing alignment, while the Tensar UX Geogrid provided the soil reinforcement. Together, they created a simple gravity retaining wall system.

"This was the first time we had ever even dreamed of a system like this," Oglesbee remarked. "There was a bit of a learning curve at the beginning, but by the end of the week we were rolling. We even used less labor than we anticipated; a huge cost savings."

The stiff, UX Geogrid is designed to carry stress along one principal axis as loading of the soil reinforcement occurs perpendicular to the wall face. The geogrid interlocks with fill material to carry these large tensile loads. Allowable geogrid stress is calculated in accordance with ASTM and AASHTO standards to ensure long-term performance. As an added benefit, the high-density polyethylene (HDPE) resins that comprise the geogrid resist degradation from chemicals.

A combination of either Tensar UX1500HS or UX1700HS Geogrid was installed with every basket to reach a height of 24.5 feet. Native soil comprised the bulk of the fill material, placed in 9 inch lifts and compacted to keep within the project specifications. A drainage layer of No. 57 crushed stone was required by the concrete wall designer and was incorporated into the MSE wall construction to help with alignment during construction. Click here for an illustration of the typical cross-section of a pressure relief wall.

Crane load reinforcement
At the top of the wall, where the first level of the arena began, Williams Construction needed crane access to drive piles for the arena bowl and seating supports. Heavy construction loads from this type of equipment transfer massive amounts of pressure on the wall below.

"The minimum embedment length of a reinforcing geogrid is typically 70 percent of the wall height," commented Velez. "Even with the extra construction equipment weight, we managed to use less than an 85 percent embedment length. The Tensar Geogrid did an excellent job supporting the required crane loads and allowed the piles to be driven directly through the wall mass."

The 30,000-square-foot wall installation was completed in just six weeks. And the best part, according to Oglesbee, was that the temporary wall did not have to be removed. It was simply left in place, and buried, adding extra cost savings.

Awarding success
The pressure relief wall design didn’t go unnoticed. "Build Oklahoma Awards" were presented for the "Best of the Best" 2007 projects by the Associated General Contractors’ (AGC) Oklahoma Chapter, the leading educational and trade organization for the commercial industrial construction industry, made up of general contractors, sub-contractors, suppliers and other industry professionals.

They presented Chris A. Ball, vice president of Williams Construction Co., the 2007 Oklahoma Builder of the Year Award for the BOK Arena Group IV: Cast-in-Place Concrete project.

Save the date
Tulsa sports fans will be ready to rumble the walls when basketball and hockey take center stage later this fall. And the groundwork laid will be sure to "take the pressure"!

With the completion of the BOK Center, the Vision 2025 initiative will be well on its way to meet its citizens’ goals to provide "economic and community infrastructure’ for present day and future generations of Tulsa residents to enjoy.


Joseph S. Bailey II is the Grade Separation Systems Manager—Southeast at Tensar International Corporation. He has authored several papers on Mechanically Stabilized Earth construction and holds three retaining wall patents. He can be reached at 866-284-0601 or jbailey@tensarcorp.com.