Gravity sewer main installation presents a challenge just by virtue of the accuracy needed to keep pipe on-grade.
With surface and subsurface obstructions such as numerous 14-ft. (4.2 m) driveways and a shallow maze of existing utilities buried in four different types of soil conditions, a precision job can quickly become even more complicated.
That’s the set of circumstances Dave Wampler, owner of Jackson Creek Enterprises, encountered this past summer in a sump drain project in Knoxville, IA.
Knoxville is a community of approximately 8,200 known as the “Sprint Car Capital of the World.” The city’s public works officials have a wastewater infiltration and inflow removal plan including an aggressive project to provide storm sewer capabilities to every household over the next 30 to 35 years.
Specifically, where storm sewer doesn’t exist in front of a property, a sump drain is being installed to provide residents with infrastructure for future connections of their sump lines or footing drains to the main pipe.
Sanitary Sewer Solution
This relatively new storm sewer alternative, primarily installed during new construction, was employed for the first time in existing residential areas to correct issues associated with routing sump pump lines out into residential lawns and streets, or worse, directing their clear-water flows to Knoxville’s sanitary sewer system.
This extra clear water enters the plant at a rate of 12 to 15 million gal. per day (mgd) during a rainstorm, and burdens the city’s plant, which is designed to treat flows of 4.5 mgd. For instance, the city currently averages 1 to 1.5 million gal. of wastewater flow per day. With normal wastewater flows and clear water entering the system during an average rainstorm, the flow increases to 15 mgd, creating a greater risk for sewer backups and other problems to occur, said Knoxville Public Works Director Jeff May.
“Eliminating the clear water from our sanitary sewer system will give us extra capacity, prevent backups and keep us from having to expand our wastewater plant prematurely,” said May.
The first phase of the sump drain project involved putting in 5,000 ft. (1,524 m) of 8-in. (20 cm) polyvinyl chloride (PVC) Certa-Lok restrained joint pipe achieving grades ranging from .4 to 2.8 percent depending on varying elevations.
The project also included sanitary sewer point repairs and utility removal and replacement. Installed along the parkway of five different streets, the line will serve the immediate needs of approximately 30 to 40 households that regularly operate sump pumps.
Subcontracted through Van Hauen and Associates in Clive, IA, the project was originally bid as an open-cut job using Jackson Creek’s 10,000-lb. (4,536 kg) excavator and a backhoe to bury the pipe, as well as the use of horizontal directional drilling (HDD) only under driveways, said May.
“When we set the project up, the idea was to bore the driveways to reduce surface restoration, and the inconvenience of blocking people out of their driveways and parking on the street,” he said.
Going the Distance
As a normal operating procedure, Jackson Creek Enterprises uses a vacuum-excavation unit to pothole and visibly verify any existing utilities along the bore path. The unit uses water for “soft” excavations, while the vacuum capabilities remove the cuttings and also the drilling fluids that collect around entry pits.
On this job, however, challenging soil conditions, plus a 12-in. (30 cm) pipe coupling that required the use of a 14-in. (35 cm) backreamer, all combined to create a very high volume of drilling fluid buildup.
The mixture of blue gumbo clay and dirt, along with a yellow, silty clay that tended to reconstitute itself, was producing 500 gal. (1,892 L) of drilling fluid returns an average of every 20 minutes. Unfortunately, the 25-minute round trip to haul the materials to the approved dumping site outside the city limits was reducing any efficiencies gained by Jackson Creek’s Vermeer D24x40A NAVIGATOR horizontal directional drill.
However, Wampler soon discovered a unique drilling approach that would allow him to ease the drilling fluid buildup, and complete the majority of the installation using HDD.
“It’s not economically feasible to bore a 14-foot driveway, then move your drill 60 to 70 feet to bore another 14-foot driveway, and try to be on grade with an 8-inch pipe.”
The reason for the shallow bore paths under some of the driveways, Wampler pointed out, is to catch the amount of fall necessary to tie into the existing storm sewer. To eliminate the multiple setups and accommodate placements made in unusually shallow depths ranging from 2.5 to 6 ft. (.76 to 1.8 m) of cover, Wampler’s crew went the distance by positioning their D24x40A up to 500 ft. (152 m) away from the outer edge of a targeted driveway.
Ensuring an accurate pitch, grade was established on the pilot bores as they drilled several feet past the last drive with a 5-in. (12.7 cm) duckbill. The next step was to open-cut small relief pits. This not only allowed for backreamer hookups, but it also relieved pressure in the shallowest paths, further reducing the threat of pavement humping.
The crew then attached the sewer pipe to the open-face reamer, pulling it underneath the drive with the D24x40A’s 24,000 lbs. of pullback force. After completing 15 driveways, Wampler reported the paved surfaces showed no signs of stress or cracking.
“With this approach, we weren’t moving the machine and making six or eight different setups and trying to get on grade with each one of them. Once we’re on grade with the pilot bore, we stay on. Then, we pull the drill rod back to the next drive, open-cut to it, and put on the reamer, and ream the drive,” said Wampler who added that maintaining level sondes is one of the most critical aspects of on-grade boring.
Enhancing Jackson Creek’s ability to stay on-grade on a portion of the project, the crew replaced their conventional drillhead with the Vermeer Universal Transmitter Housing. The side-loaded housing’s indexing feature is designed to allow the transmitter to be aligned with the bit for more precise readings.
“With the housing, it’s possible to level your sonde to the head, so that you know exactly what your readings are. Of course, all conventional drillheads and sondes have the capacity to be off just a little bit, but with this housing it’s possible to manually adjust the sonde to the housing so that they fit each other correctly,” said Wampler.
According to Wampler, on a 1,000-ft. (305 m) section of a boulevard with nearly 20 driveways, the shallowest portion of the entire job, the transmitter housing helped increase stability and provide more level cutting downhole.
“The housing does not wander in the hole because it has a flatter duckbill, and the cutting bit is closer to the center line of the body. Once you get on-grade it will stay on-grade much better,” he said.
Tools of the Trade
Based in Allerton, IA, Jackson Creek Enterprises specializes in the underground installation of water, sewer, gas, electricity, fiber optics and CATV around the Midwest.
In addition to these applications of excavation and HDD, the company has the additional expertise to perform highly specialized environmental remediation projects. Experience with leading-edge technologies and drilling aids such as computer-aided tools are helping shape the way Jackson Creek works.
For instance, two additional tools Jackson Creek used to assist with the on-grade work included the Vermeer Atlas Bore Planner software system and Terrain-Mapping System. A state-of-the-art reflective laser surveying tool, the Terrain-Mapping System helped confirm manually gathered data such as elevation variances in terrain along a borepath, driveway dimensions and locations of utilities. Stored data was then downloaded from the mapping system’s palm-sized PC into Atlas Bore Planner software in a field laptop.
Along with the topography inputs, the Atlas Bore Planner system factored in drill stem and product pipe bend radius to determine proper machine set-back position. The system then plotted the optimal bore path in a graphic on-screen visualization, helping the crew see the job from start to finish.
According to Wampler, in order to accurately determine sewer line target depths in the bore plan, the crew input all relevant elevations and intersecting utility crossings.
“It would be quite a bit more difficult to do this job without the Atlas Bore Planner system. By laying the targets into the system, we are able to determine the elevation of the pipe that gives us the best and clearest shot with the least amount of interference. With the amount of utilities we’re working around, it helps us know how much of the existing water and gas lines need to be moved,” he said.
In the first 1,000 ft. (305 m) of the job, Jackson Creek had to lower two 30-ft. (9 m) sections of 4-in. (10 cm) sewer laterals to achieve the desired grade. The crew also hand-raised four 3/4-in. (1.9 cm) high-density polyethylene (HDPE) gas lines and lowered four 3/4-in. copper water lines to make room for the sump drain line.
With the HDD tools and installation methods used on this project, Jackson Creek met every challenge with a solution designed to ensure precise, on-grade storm sewer infrastructure that will meet the needs of this community for decades to come. (Debbie McClung is a technical writer in Knoxville, IA.)