The HDD Company recently completed a high profile 7,400 foot (2,256 m) watermain installation. The project involved crossings of Napa Valley Memorial Park Cemetary, Napa City College, Kennedy Park Golf Course and Native American artifact sites. Trenchless International documented this critical project in their April edition.
The installation of a 7,400 ft section of watermain was an impressive feat for the City of Napa located in California. Here Project Engineer Megan Thomas talks with Trenchless International about the challenges of drilling under the Napa Valley Memorial Park Cemetery, Napa City College, Kennedy Park Golf Course, and Native-American artefact sites.
The Highway 221 Water Transmission Main Upgrade Project involved installing a watermain from the west side of Highway 221 from Napa Valley College, running down south to Kaiser Road. Using Trenchless Technology provided an answer to a project that was fraught with environmental constraints and location difficulties.
The City ruled out trenching for the installation for several reasons, one being the projected time-frames for the project that made lengthy trenching unfeasible. However, this was only one factor explaining the rationale behind the City's selection of Trenchless Technology.
Napa Valley has a rich history; at the time of the first recorded exploration into the area in 1823, the majority of the inhabitants were Native American Indians. Ms Thomas said past excavation for projects in the area had uncovered Native American artefacts of cultural value. Because of this, trenching would have been very destructive and potentially damaging to cultural heritage sites.
Another contributing factor in the decision to avoid trenching was the route of the watermains. The projected watermain route went under residential properties, and would have resulted in the purchasing of easements if trenched. At the cost of approximately $US15,000 for a 120 sq ft of one person's property, the option was not ideal. However, it wasn't just the human population the City of Napa had to take into consideration, but also the arboreal population. With many oak and redwoods species protected in the area, permission had to be obtained to remove these trees for trenching, or to remove any tree of a certain age/diameter. Ms Thomas said trenching would have resulted in removing at least 50 trees, making it a time-consuming and costly endeavour.
Additionally, the pipeline had to be installed under CalTrans infrastructure; the public transport system. Replacement stops would have had to be enforced, as well as road closures that would have been another difficult and costly inevitability. The watermain route was also adjacent to utility poles, with large reclaimed mains from other utilities (including a high-pressure gas line) in existing easement. If this weren't cause enough to settle on trenchless installation, the route also required installation under the local cemetery, and alongside of a mental correctional facility and college.
The City of Napa chose horizontal directional drilling (HDD) as the method of installation over pipe jacking or tunnelling because they had trialled the technology on two previous projects. The projects tested both the method of installation as well as the materials used. The previous projects had been installations of pipe diameter 4 inches and 12 inches. However, the Highway 221 Water Transmission Main Upgrade required installing their biggest diameter yet using HDD, 24 inches.
The pipe material and installation method was chosen based on the soil conditions. The elevation and topography of the areas was very unlevel, varying by 30 ft along the perimeter, and soil conditions in the area were extremely corrosive.
Ms Thomas said they wanted to use a non-corrosive pipe-material, and ruled out concrete because the diameter required would have been too large to suit the installation method, and concrete would still potentially corrode in the soil conditions. Ms Thomas said plastic pipes could safely handle the HDD installation, and Underground Solutions was contracted to supply the pipe. For every bend the City of Napa encountered, they had to add protection because of corrosive soil.
The $US3.1 million project began on 4 January 2012 with two drilling teams starting on the pilot hole. HDD Company Incorporated was contracted for the HDD component. Throughout the project, three different machines bored multiple locations at the same time; A 200 tonne American Auger – which was upgraded to a 400,000 pound thrust pull-back (by the addition of more motors to upgrade torque and thrust), an American Auger DD210, and an American Auger DD5.
On 20 January, sections of pipeline were delivered to the site to prepare for the fusion process. Over just a few days, more than 1,000 ft of pipeline was fused together and laid out in preparation for installation after completion of the first bore hole.
On 3 February, the first length of pipeline, approximately 1,700 ft, was pulled through the drilled hole, which was in front of the local College, and by the end of that week the 36 inch hole for the second length of pipe was also completed. This second length of pipeline, approximately 1,500 ft, was installed on 14 February, and was pulled through the drilled hole that runs under the Napa Valley Memorial Park Cemetery. Ms Thomas describes this part of the installation project of particular sensitivity.
"At one point workers had to stop because there was a funeral going on in the cemetery near our installation, and the crew wanted to be respectful of the service."
Though Ms Thomas said they didn't expect frac out to occur while drilling 1,700 ft under the cemetery, it could always be a possibility and they were constantly aware this was a high profile project that was highly conspicuous to the public eye. Crews waited for the funeral procession to end before resuming their work.
In the last week of February, the fifth length of pipeline (approximately 1,100 ft) was pulled through the hole, leaving only a little more than 1,000 ft to install.
Ms Thomas said the crews 'lucked out' that their core samples matched what their drillers ran into during the project. However, in some locations they had extreme variation of depth for drillers to work through, ranging from 40 ft to 20 ft in the same area.
Ms Thomas said "We got a weird one [frac out] at 40 ft – no one expected it. We had to be extremely fast to clean it up. Another frac out occurred 100 ft away from the alignment, and it occurred close to the public. Project workers didn't realise at first what was happening.
"We lost 300 gallons of mud before they could finally see the location. It was in this dip-island, so they couldn't see it and had to try and locate it, and at first they thought they were losing the mud in the gravel. In the end the frac out solved itself and the mud just filled up the gaps."
Once the drilling portion of the work was completed, the contractor commenced connecting the sections of pipeline. Each connection was slightly different from the next based on existing facilities that needed to connect to the watermain, including several water services and fire hydrants, as well as new valves needed to optimise the operation of the watermain.
A track hoe was used to guide the new watermain into the hole by lifting the middle so that the pipe could curve into the drilled hole without any unnecessary strain on the pipeline.
Ms Thomas said "A lot of the time we had people asking us how much our cost savings were versus trenching. We like to put it in costs of impact instead of costs of dollars."
With the project under a constant scrutiny from the public-eye and local media the crew remained aware and conscientious while working in the areas that ran alongside a college and mental health institute. Safety of the locals as well as crews was paramount, which meant vigilance keeping track of exactly who was onsite, and that no civilians had wandered in. The project allowed locals to go about their daily lives without interruptions, allowing for the installation of this impressive infrastructure to go virtually unnoticed.
Appeared in issue: Trenchless International — April 2012