Workers complete 84-inch siphon section of large Houston wastewater project

HOBAS pipe was recently included in a contract for the siphon section of a large sanitary sewer project in Houston, Texas.

July 21, 2003 -- Houston, Texas, is home to more than 4.7 million people and growing. The metroplex's downtown area is being revitalized and a new light rail system is being constructed. The infrastructure is under constant expansion.

Near downtown, the installation of an extremely large siphon was specified at depths of up to 110 feet. It is part of the North Side Relief Tunnel section of the sanitary sewer that passes directly under Buffalo Bayou, one of more than 10 waterways that wind their way through the city.

For this City of Houston project, HOBAS pipe was included in the specifications by the project engineer, the Houston office of Black and Veatch. Installation contractor, Boyer, Inc., headquartered in the city then chose it for the siphon section.

Tunnel Advantage

The extreme depths and close proximity to downtown streets and buildings made this a challenging installation for Boyer. They chose the pipe on this installation for its light weight and lower installed cost. The 84-inch diameter HOBAS pipe has a thinner wall compared to concrete pipe of similar capacity. This allowed for an oversized inside diameter (ID) with a relatively small outside diameter (OD).

One of the greatest benefits of using HOBAS in tunnels is that it saved 10 to 15 percent in primary tunnel liner costs because of the smaller size required. Also, 20 to 30 percent less excavation and haul-away were needed. The 84-inch nominal diameter pipe had an ID of more than 85 inches with an OD of only 88.6 inches.

The entire 645-foot siphon was installed from one small shaft using wooden skids and a winch system. The pipes were placed in the tunnel which descended at a grade of 8.25 percent to navigate under the bayou. Once the pipes were properly located within the tunnel, they were blocked to maintain line and grade and to facilitate grouting. Twenty-four special, 6.5-foot short sections of HOBAS pipe were supplied to accommodate curves in the 112-inch ID ring beam and wooden lag tunnel.

Corrosion Resistant

Even though the installation was a tough one, the HOBAS siphon pipe was installed in less than two weeks. Tommy Mazingo of Boyer was pleased with the HOBAS pipe's performance, and especially liked the inherent corrosion resistance. "Once the pipes were installed, that was it. There was no secondary lining to apply," he said.

Other material options for this project, such as PVC lined concrete pipes, would have required welding of the secondary lining to provide the corrosion resistance inherent in HOBAS pipe.

HOBAS Challenge

After completing the pipe installation, Boyer proceeded to grouting preparations. The grouting was accomplished by filling the siphon full of water and pumping the annulus full of low-density cellular concrete.

Considering the size and complexity of this project, the contractor felt that the HOBAS installation was relatively easy and user friendly.

HOBAS pipe is manufactured in sizes from 18 inches to 108 inches in pressure and non-pressure classes. For more information, please contact HOBAS at 800-856-7473, 281-821-2200 or e-mail at Facts are also available at

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