In 2005, the West Harris County Regional Water Authority (WHCRWA) began constructing a water transmission system to prepare the far western Houston area for mandated conversion from groundwater to surface water. One of the projects in this system, known as Contract 34, involved the installation of approximately 18,700 lf of 30- and 24-in. potable water lines from north of Cypress North Houston Road to Harris County MUD 371/374 Water Plant.
In January 2014, WHCRWA hired Lockwood, Andrews & Newnam Inc. (LAN), as the engineer-of-record to implement Contract 34 successfully. Fused Industries (previously known as Meiners Construction Inc.) served as the general contractor. For approximately 2,160 lf, the proposed water main crossed Cypress Creek and a surrounding area of protected wetlands. To comply with U.S. Army Corps of Engineers requirements, the project team designed the crossing to be installed using horizontal directional drilling (HDD). Fused Industries subcontracted TCH Directional Drilling, which specializes in HDD.
“The size of the identified wetlands area exceeded the typical length for conventional trenchless installation methods such as augering and microtunneling, which would have required multiple intermediate pits within a crossing of this length,” said Shelley Serres, P.E, LAN senior associate and senior project manager.
Due to the complexity of the HDD crossing, the design team had to take into account several critical issues. Chief among them was the pipe material needed for the HDD crossing. After evaluating a number of pipe materials, the team chose fusible PVC pipe.
“We considered HDPE but it has a much thicker wall, which would reduce the internal diameter, and in turn, the flow capacity,” said Serres. “Fusible PVC pipe gave us an additional 3 in. in internal diameter, as well as the flexibility that we wanted. Considering the diameter, length, depth and bending radius requirements, we felt Fusible PVC pipe was the best option.”
Underground Solutions, a subsidiary of Aegion Corp. and manufacturer of fusible PVC pipe, aided LAN in the pipe stress analysis and fusion services on the 30-in. DR21 water line. Based on the bore geometry of 10-degree entry angle, 5-degree exit angle, and 40-ft maximum depth, calculations were performed for the HDD pulling force. One of the critical considerations during this analysis was the need for ballasting the pipe during installation. As such, the HDD pulling force was calculated for both empty and filled pipe. The analysis showed that the pull force was three times less for a ballasted pipe, which increased the factor of safety by more than two-fold. As a result, the design team recommended filling the pipe with water as it enters the bore to control buoyancy and downhole friction during pullback.
“This installation was a 30-in. diameter water main at a depth of 40 ft in sandy soils,” said Ray Dudley, senior process engineer at Underground Solutions. “It was important to reduce the amount of force needed to pull the pipe through the finished bore. Ballasting the pipe with water reduced the estimated pull force by more than 200,000 lbs. Also, the use of pipe rollers to support the pipe above ground and minimize friction drag forces was beneficial in this regard.”
Special connection details were also included during design to provide a seamless transition from the Fusible PVC pipe to the rest of the water main, which was installed primarily by open-trench methods. The allowable pipe materials for the upstream and downstream portions of the 30-in. water line included standard pipe materials such as steel, PCCP, bar-wrapped steel cylinder and ductile iron pipe. To connect the Fusible PVC pipe on each side, various alternatives were considered such as ductile iron and steel adapters. Two details were provided, one for connection to a ductile iron pipe and the other for connection to steel or concrete pipe.
Another critical design issue was the size of the easement needed for installation on the entry side. The majority of the proposed 30-in. water line was designed to be placed within a 20-ft wide easement, with a 15-ft adjacent temporary construction easement. For the HDD, a minimum of 80-ft by 100-ft area was needed to accommodate the drill rig, mud system, vacuum truck, track hoe and the 18-wheeler carrying drill stem. To accomplish this, an additional temporary construction easement of 70-ft by 150-ft area was acquired for the drill pad, with the 20-ft easement and 15-ft temporary construction easements also being utilized. On the exit side of the crossing, the pipe could be fused in one long string, parallel to an existing drainage channel.
Drilling operations started in February 2016. TCH Drillers used an American Augers 440T drilling rig. A power flow mud control system mixed the drilling fluids, and separated and cleaned the solids from the mud so it could be recycled back into the process. A vector magnetics steering tool for guidance (Para-Trak guidance system) was used to track the pilot bore.
Once the rig and equipment was set up at the entry location, the contractor drilled the pilot bore along a predetermined path with a 9-10 in. tricone bit. A gradual hole enlargement process was used to keep the solids manageable. The next pass was a 24-in. diameter cut with trailing stem. The drillers added more stem behind the reamer in the event of a hole collapse so they could regain the path. The geotechnical conditions consisted of multiple layers of sand and lean clays. At the bottom elevation of 40 feet, which continued for approximately 1,500 ft, the layer was sandy.
After the 24-in. pass was completed, the contractor expanded the hole to 36 in., and then to 48 in. At all times, the drilled hole was filled with drilling fluid and cuttings. At the exit end of the crossing, the drilling fluids were trucked back to the entry side where the drilling slurry could be processed and re-used. While the hole was being reamed, the 2,160-ft pipe string was fused and placed on rollers. The McElroy T900 fusion machine was protected from rain and wind during the butt fusion process. Qualified fusion technicians faced the ends of pipe joints, heated in a controlled process including data logging, and completed the fusion process by holding joints in compression until cooled.
After the 48-in. hole was completed, a swab pass was done with a barrel-type reamer that spins through the hole and pumps drilling fluid through it to help flush the hole. This ensured that the path was clear for the pipe. From exit point to the rig, the driller connected to the pull head, which was attached to 2,160 ft of pipe. The drill stem and reamer rotated as pipe was pulled in with a rotary swivel between the reamer and pull head, so there was minimal torsional load transmitted to pipe. Following the design team’s recommendations, the contractor filled the line with water as it was being pulled in to control buoyancy, significantly reducing the pulling force required by the drill rig.
The project team faced quite a few challenges during the HDD installation. The biggest challenge was limited site space and access, said Colby Ellis, operations manager at TCH Directional Drilling.
“Normally, we would like to have a 300-ft by 350-ft setup area. Here, we had to work in an 80-ft by 100-ft area and we were beside a ditch,” said Ellis. “We had to organize all the necessary equipment and tooling in a tight, confined space. But we were able to get the job done.”
In addition to the confined layout, accessing the site also was not easy. There was no direct route between the entry and exit locations, requiring drivers to weave through different roads and adding time to the process. During the installation, the contractor also had to deal with rainy, wet conditions that made it difficult for trucks to drive within the easement without getting stuck.
Additionally, some mud ended up inside the pipe after pull back was completed. To deal with this challenge, the project team opened up a drain line valve on the south side of the creek and flushed water through from the north. This provided adequate velocity to scour the mud from the bottom of the crossing.
Despite these challenges, the project team successfully completed installing the HDD portion of the pipeline on March 3, 2016. The entire water main was completed in June 2017.
“This HDD crossing is one of the longest large-diameter pressure pipe trenchless installations in the state,” said Serres. “Thanks to this project, we are one step closer to expediting the switch from groundwater to surface water in western Houston.”