North Marin Water District Relies on HDD as Part of Regional Recycled Water System Upgrades

North Marin Water DistrictThe use of recycled or reclaimed water continues to gain traction as water demand increases while potable water supplies dwindle. Recycling the water we ‘waste’ after domestic use makes good sense in terms of the cost, efficiency, scarcity and local control of our water resources.

A very popular use for recycled water is public park and landscape irrigation. One major issue with recycled water distribution piping networks is that the infrastructure required for a new dedicated distribution and delivery system must oftentimes be installed in areas of established communities and development. This type of construction requires additional easement space and is often performed with very tight project site constraints considering public access, traffic patterns and existing build-out. One potential way to help ease the installation pains in this situation is to employ trenchless methods. Trenchless methods, including horizontal directional drilling (HDD), minimizes excavation requirements, which go a long way to easing the impact of a construction project on the area of installation.

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The North Marin Water District (NMWD) is located in Marin County in the San Francisco Bay Area of California. In terms of raw water, NMWD relies on imported Russian River water for nearly 80 percent of its supply. However, this source has been impacted by limitations promulgated to protect fish habitat and the river environment. Reacting to these constraints in water supply going forward, NMWD officials have endeavored to reduce potable water demands within their service territory. They have done this primarily with two methods. First, they undertook an aggressive and successful water conservation program, reducing the amount of water waste in their system. Second, NMWD has been proactively developing recycled water for parks, schools, and common area landscape irrigation as another drought-proof water source.

In 2011, NMWD began advertising a series of recycled water expansion projects for its North and South Service Areas to be completed by the end of 2013. The Novato Recycled Water Phase 2 Expansion Project, which totals approximately 10 miles of recycled water pipe, is part of a regional conservation effort in partnership with the Novato Sanitary District and Las Gallinas Valley Sanitary District.

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NMWD is located in Marin CountyUnderstanding that these projects would be installed in communities of established infrastructure and development, NMWD designed them to allow the use of HDD as an alternate form of installation to the standard direct bury, or open trench methodology. The idea was that if the HDD installation methodology could save the contractor money and hassle in terms of construction practice, NMWD would realize both the financial benefit of a competitive price, and the socioeconomic benefit of less intrusive public impacts of the trenchless construction for the project.

Two of the recently contracted projects for NMWD’s system ended up going the way of trenchless construction and HDD installation for these very reasons. The first project was installed by Ghilotti Construction (Ghilotti) and consisted of approximately 5,000 lf of 8-in. recycled distribution waterline. The project site included areas of extremely ‘tight’ working environment, with two-lane streets and a crowded infrastructure easement, along with work on a very busy thoroughfare in Atherton Avenue. Ghilotti performed the work in 500- to 600-ft increments. The HDD installations would be angled to get to depth with the use of an insertion pit, and then run at the correct grade for the length of the run. Typical depths of bury were in the 7- to 8-ft range to limit issues with inadvertent returns of drilling fluid, but to keep the pipeline accessible for the future.

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The pipe material used for the installation was Fusible PVC pipe (FPVCP), which due to the thermal butt fusion used to make the joints, is capable of installation by being pulled into place. This is a requirement for HDD installation, which creates a bored hole along the desired alignment and as a last step of installation, pulls the new pipeline through the space created by the process. The 8-in. FPVCP was generally fused and staged in the lengths required for the installation at a location separate from the actual location of the final alignment. When the HDD sections were properly prepared the lengths of FPVCP were moved into position for the insertion and then installed. All crossing utilities were potholed prior to the installations to make positive identification and to ensure that the drilling operation would not result in a utility strike. HDD was very successful in limiting the amount of construction impact in these tight locations, keeping access available for the residences and businesses along the project route.

Fusible PVC pipe (FPVCP)Project No. 2
The second project was installed by Ranger Pipelines (Ranger) and consisted of more than 6,000 lf of 12-in. recycled distribution waterline. This project, like the previous one, took place in a very busy and congested area. The main route of the alignment was on Redwood Boulevard, with heavy traffic congestion as well as a high volume of commercial properties. As part of Ranger’s project, there was a 700 lf Sonoma Marin Area Rapid Transit (S.M.A.R.T) train crossing required. This crossing was accomplished with HDD by first installing an 18-in. FPVCP pipeline to act as a casing. This casing was then used to house the 12-in. FPVCP carrier pipe. The S.M.A.R.T. train HDD crossing was designed by Jacobs Associates Engineers of San Francisco.

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Another section of this project included the crossing of the pipeline alignment under a Hwy 101 overpass. This section was drilled to eliminate the need to try and dig under the overpass which would have been very difficult for excavators due to the minimal clearance of the bridge structure and the existing road. Much like the first project, pipe was fused in one location and brought to the specific installation locations for insertion. Pipe trolleys were used to move the pipe and keep it from dragging on the ground.

As water becomes more and more precious, recycled water systems will become more and more prevalent. The construction required for these systems will tend to be very disruptive both in scope and location, and trenchless technologies like HDD are helping to ease this disruption. For NMWD, two recent projects were bid with the option to install the new pipelines via open-cut or HDD with both contractors choosing the less disruptive HDD method as the most efficient and competitive. Trenchless methods are providing a viable means to limit the impact of these critical infrastructure installations as well as save construction dollars.

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Drew McIntyre, P.E., is chief engineer with the North Marin Water District, San Francisco.

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