Missouri's directional-drilling crews must visually locate, or “day light” the location of, all utilities being crossed through hand digging or with a hydro-excavation system. But on a bore in July 2008 in St. Charles, Missouri, a utility contractor faced a plethora of challenges, making it impossible to go the traditional route to get the job done.
A number of obstacles stood in the way of JF Electric Inc. (Edwardsville, Illinois) during a boring project for AmerenUE- Missouri (St. Louis, Missouri). Firstly, Elm Street was capped with a 9-inch layer of concrete. Secondly, the utility contractor had to install a 5-inch conduit using directional drilling at a highly trafficked intersection. Traffic is always a challenge on street bores, but the more daunting challenge was avoiding the nine underground utilities along the bore path.
Project Power On
The crossing was part of Power On, a multi-year project to upgrade AmerenUE's delivery system for continued reliability. Crews are converting sections of overhead primary to underground across Missouri. The estimated $300 million initiative includes up to 1000 undergrounding jobs of various sizes. In addition to converting overhead lines to underground, JF Electric is installing redundant loops to sections of AmerenUE's distribution system using directional drilling.
The primary objective of Power On is reliability. In 2006, St. Louis experienced record-setting severe weather — more than any other office in the National Weather Service. More than 723 severe weather events were recorded, including two back-to-back storms in Missouri that affected service to nearly 646,200 customers. The following January, 350,000 customers in Illinois and Missouri lost power due to an ice storm. It took 4200 linemen and 1500 tree-removal/vegetation management personnel to restore power.
At first glance, burying the power lines is very popular with the AmerenUE customers for aesthetic reasons, but more important is the added protection of burying the section of the primary for when severe weather strikes again.
The HDD Fleet
As part of the Power On project, JF Electric began adding workers and equipment to its fleet in late 2007 when the project began. JF Electric, which performs electrical utility work in five states, has 16 directional-drill rigs working exclusively on the conversion project. The horizontal directional-drilling (HDD) fleet ranges in model size from Vermeer (Pella, Iowa) models — 7×11s, 16×20s, 20×22s and 24×40s — to Ditch Witch (Perry, Oklahoma) models — JT2020 and JT3020 all terrains.
The field crews use smaller drill rigs for short bores and residential underground projects. The larger rigs are for road bores, more difficult soil conditions and water crossings. On the St. Charles crossing, the crew deployed a Vermeer 24×40 self-contained rig.
In addition to the drill rig, each JF Electric HDD crew is equipped with a tandem axle equipment trailer and a flat-bed truck, both carrying extra tooling and water tanks. A rubber-tire back hoe with a hydraulic breaker attachment is also available to open starter pits and exit pits.
The terrain and soil conditions in AmerenUE's Missouri territory vary anywhere from soft, easily compactable soil, which is ideal for boring, to hard, abrasive rock conditions, found south of St Louis and around Lake of the Ozarks. Fortunately at the Elm Street crossing, the soil conditions were favorable. The challenge was verifying the utilities located under the pavement.
Core Saw Innovation
Every utility imaginable was buried under the street. In the past, the crew would use a diamond-blade circular saw to cut the pavement at each utility. After the bore, the crew would replace each slab of concrete with a new patch of asphalt or concrete. Instead, JF Electric chose to use its core saw, a relatively new piece of equipment in the directional-drilling industry.
The core saw is designed to quickly make a 4-inch to 18-inch circular opening through concrete or asphalt pavement. Because of the concise size of the opening, these holes can be made in advance, leaving a round plug that can be reinstalled in the original hole and patched with epoxy. If done properly, this restoration procedure meets Department of Transportation requirements, and in this case, the municipality's street-restoration specifications.
The core saw drastically reduced the cutting and restoration process. For example, cutting a 12-inch hole in the 9-inch-thick pavement took less than 15 minutes. As the saw cut the concrete, water was supplied to lubricate the blade and remove cuttings. The workers applied hydraulic pressure from a vertical piston device attached to the core saw and mounted on a portable trailer.
The trailer also carried a water tank, making the system a self-contained unit that could be easily transported from job to job. With the plugs removed, the potholing crew used a portable vacuum excavation system to non-intrusively remove dirt to daylight each of the buried utilities.
Vacuum excavation is an integral part of JF Electric's directional-drilling operation. In many cases, it's a faster and safer method for digging around existing pipe and cable. HDD crews, organized in clusters of five or six, are assigned one or two hand-digging crews and a vacuum excavation unit from Vac-Tron (Okahumpka, Florida). The Vac-Tron units are trailer-mounted vacuum systems with a handheld wand used to vacuum up slurry mud behind a directional-drilling machine. On this project, crews used it to remove dirt to locate the underground utility lines at the Elm Street crossing.
Once the crew members located the utilities across the intersection, they laid protective 4-inch by 8-inch sheets of construction-grade plywood to protect the existing landscape of mature landscape surface along the easement. The 24×40 bore rig was then anchored in place and connected to the water supply tanks.
Crossing Underneath Elm Street
The drawings called for crossing the intersection diagonally from corner to corner. Adding to the challenge, turning lanes were present at all four corners of the busy four-lane street. The JF Electric crew used a DigiTrac locator to safely monitor and navigate the drill head. The overall length of the bore was less than 250 ft. Once the drill head cleared the buried utilities at a depth of 7 ft, the bore path followed an upward climb to the exit pit at the opposite corner of the intersection.
Once the crew completed the pilot bore, the workers attached a larger reamer to the drill string to expand the bore hole. By also attaching a pulling swivel, they were able to pull back the 5-inch conduit successfully. The workers used a mixture of bentonite drilling fluid to maintain the bore opening and remove excess spoils from the bore. Crews recycle the drilling fluid during the bore on larger rigs that pump higher volumes of drilling fluids. This mud-screening and recirculating process helps reduce valuable time and material costs.
By relying on the latest boring technology and devising an innovative approach to a daunting problem, JF Electric completed the boring project. By tackling the challenges at the St. Charles crossing, the crews helped AmerenUE to improve its reliability through its multi-year Power On project, both now and in the future.
Ray Pour is construction superintendent for JF Electric, where he oversees the field operations for work in Missouri. He was a former construction superintendent/lineman for AmerenUE for 21 years. [email protected]