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HDD provides solution for force sewer main river crossing.

Installing a force sewer main to serve residents in a new subdivision is a big job. This was especially true for an investment group developing a subdivision southwest of Houston in Sugarland. Nestled in the third fastest growing county in the United States, the new development is located across the Brazos River from a sewage treatment plant situated in the 5,000-home, upscale, master-planned community of New Territory.

Given the environmental conditions of the site and the impact any spill incident would place on the community, the Brazos River Authority specified that the required piping linking the subdivision and treatment plant would have to be installed under the river through horizontal directional drilling (HDD).

Realizing the difficulty of the work ahead, the Houston-based engineering firm of Brown & Gay was brought in to provide engineering design and perform core tests at the site.

As noted by Pate & Pate LLC's, Directional Drilling Division Manager, Grady Bell, his company had constructed a lift station and installed piping at this location about a year earlier. The installation of the 14-inch diameter HDPE force sewer main under the Brazos River was part of the second phase of the project.

Pate & Pate's directional drilling division was contracted to design the final bore, provide pipe weld up, install the 1,100-foot HDD river crossing and final line tie-in, while the company's water and sewer division was contracted to install several thousand feet of PVC through open cut and complete a roadbore on the project.

Challenges

Bell noted that instead of a conventional HDD bore to install the 14-inch HDPE crossing that could be pulled back through the hole, the Brazos River Authority required that the HDPE be installed inside an 18-inch steel casing.

The original plan was to fabricate the casing for a single pull. However, this changed when the right-of-way proved to have a steep bend at such an angle that it became necessary to fabricate the casing string in three separate sections.

"This also changed our plans to install the HDPE inside the steel casing on land and pull both lines into the hole simultaneously," he said.

Bell noted that crews were hampered by a number of large cottonwoods trees along the planned route. "Ultimately, the right-of-way was adjusted to avoid damaging the many older trees in the river bottom."

The number of trees and the size of the root systems along the route proved to be obstacles that became a consideration during the bore as well. One of the problems was the tree stumps and trying to avoid the root systems of the trees.

Crews also had to deal with a 16-foot change in elevation that required the 365-foot river crossing to be completed with an 1,100 foot bore." We wanted to be far enough away from the river's edge to be able to geometrically set up and drill down to the 65-foot depth required under the river and then come back up," Bell explained.

For the directional drilling portion of the project, Pate & Pate completed the pilot bore and pulled in the 18-inch diameter steel casing with an HRE-300 class rig. Prior to this project," Bell said, "we had only completed one job with the HRE-300 that was recently purchased from Horizontal Rig & Equipment of Conroe, TX."

The 1,100-foot pilot bore was jetted with an 8 5/8-inch diameter roller cone bit and back-reamed with a 30-inch cutter to enlarge the hole.

Bell said, "Brown & Gay did a good job engineering the project. The core samples taken indicated fine silty sands and a level of gravel that the bore passed through successfully."

Trouble in Arkansas

Bell said that shortly before the pilot hole was complete, a contractor in Arkansas who was having trouble completing a rock drill, called and asked to use the HRE-300.

Pate & Pate was willing to help, so shortly after the steel casing was pulled into place, the HRE-300 was rigged down and transported to Arkansas.

"We were able to leave our auxiliary equipment in place," Bell said. "The contractor now using the HRE-300 just needed a rig with enough power to handle the rock and gravel conditions at the Arkansas location."

Meanwhile, Pate & Pate's crews were finishing up testing of the HDPE pipe and rigging up the company's 140,000 pound American Auger's DD140 to complete the HDPE pull-in.

"With the steel casing installed, there was no chance of the hole collapsing," Bell said, "which gave crews ample time to rig up and prepare for the final HDPE sewage line pull-in."

Successful tie-in

Once the DD140 was in place, crews tripped the drill pipe back through the 18-inch casing and fabricated a 14-inch HDPE pullhead for the final line pull-in.

At the exit location of the earlier HDD casing installation, a backhoe was used to dig down and expose the casing at a depth of eight feet. Next, the casing was severed and the DD140 pulled the hydrotested HDPE pipe into place without the need for lubrications or drilling mud.

Bell said the line was then filled with water for a second hydrotest. With this accomplished and because the river bottom installation site is prone to flooding, the first few feet at each end of the line was sealed by pumping an expanding chemical foam into the annulus between the casing and HDPE to make it water tight.

In describing some of the more challenging aspects of the project, Bell said that while the bore was fairly straight forward they had a very narrow window at the exit location and had to hit it at a specified depth of eight feet.

He also cited steep elevations at the rig site location as posing problems for crews trying to mobilize the equipment and machinery needed to complete the bore. And, crews found pipe stringing particularly difficult because of the steep bends in the right-of-way.

Despite the many challenges, the trench-less installations of the steel casing and force sewer main as well as some 3,000 feet of open cut PVC installation on the New Territory side of the river by Pate & Pate's crews were completed on schedule.
Reader service card numbers:

Utility Contractor 148
HDD Rigs 149
HDPE Pipe 150
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Author:Tubb, Maretta
Publication:Underground Construction
Date:Jan 1, 2002
Words:1032
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