Backreaming Through Rock

Taking on a challenge is nothing out of the ordinary for the experienced trenchless installation experts at Midwest Mole. Since its founding in 1982, the Indianapolis-based contractor has earned a reputation for installation innovation, overcoming obstacles and defying the odds. So, these trenchless pros didn’t hesitate to take on another challenge despite the recognition that they would likely encounter a few unknown and unexpected twists and turns along the way.

Midwest Mole was hired by general contractor Dudley Construction on behalf of project owner Northern Kentucky Sanitation District No. 1 (SD1). SD1 is responsible for the collection and treatment of Northern Kentucky’s wastewater, as well as regional storm water management and is the second largest public sewer utility in the state. The district’s service area covers approximately 220 sq miles in three Northern Kentucky counties, serving more than 30 municipalities and several unincorporated areas. SD1 maintains more than 1,600 miles of sanitary sewer line, more than 140 wastewater pumping stations, several treatment plants and approximately 17,800 storm sewer structures.

This project, located just outside of Florence, Ky., was to install a 506-ft section of 18-in. DR11 HDPE material that would eliminate a pump station and convert the existing sewer from a forced main to gravity flow. It was to be installed at 1.3 percent grade using auger boring as the installation method. The greatest challenge, however, was neither the grade nor the material, or even restricted access to the site — which would necessitate traversing over hilly, heavily-wooded terrain — but as is often the case, it would be ground conditions posing the bigger concern. After reviewing the plan, however, Midwest Mole project manager David Howell had an idea for an alternative approach.

“This was in a residential area with a lot of trees, on a hill and very limited access,” Howell says, “and it was also solid rock. SD1 had designed it to basically be bored through the hill with the auger boring method. We had used an AXIS laser guided boring system on a project before and thought it might be a good fit here. Our only real concern was backreaming through rock.”

How It Works

The AXIS system consists of a vacuum power unit, vacuum tank and the rack power unit, along with drill casing and the drill head. Located outside the launch pit is a high-performance liquid ring vacuum power unit that removes excavated material from the bore. Upon reaching the launch pit, the excavated material transitions from the drill casing to a 6-in. diameter hose that runs up the launch pit to the vacuum tank.
Outside the pit is a self-contained power unit that features a 140-hp diesel engine and hydraulic pumps that connect to a rack (located in the launch pit) that powers rotation, thrust and pullback functions of the drill head and casing.

The operator console is designed to centralize control of multiple components into an easy-to-use interface and consists of the target display monitor, parameter display, drill head steering control, thrust/pullback control, rotation control and vacuum controls. Drill casing segments, measuring 6 1/2-ft long, are placed in the rack carriage and feature a laser sight channel and a vacuum channel for removing excavated material.
The guidance system includes a laser that determines the line and grade of the bore, enabling on-grade accuracy throughout the bore process. The drill head uses a flat-face cutter that when combined with the laser guidance system is capable of completing bores with a strict and precise degree of on-grade accuracy. In standing ground it can be retracted in mid-installation, allowing the contractor to change the cutter bit to adapt to changing soil conditions.

Eliminating a Step Helps Enhance Efficiency

The rock was composed of layered shale with veins of limestone formations ranging from 1 to 5 in. thick and measuring upward of 12,000 psi in density. The overhead power line and an existing pump station located within proximity of where the launch pit area would need to be situated were also factors. However, after verifying the location of the sewer lines currently running in and out of the pump station and sizing up the final measurements for positioning the various components of the AXIS system, Howell was confident that this laser guided boring method was indeed the best approach … and officials with SD1 concurred.

“While we agreed that it certainly would have been possible to complete the job using auger boring, we were able to identify several advantages that favored the AXIS system and presented these findings to officials at our longtime customer SD1. They weren’t all that familiar with this method, but we had worked with them on several prior projects and delighted they trusted our recommendation and gave us the green light.”

Howell cited three important factors that helped tip the scale in favor of the AXIS system, beginning with the rock. The inconsistency of the formation and the variation in limestone and shale layers make it more difficult to maintain an accurate line and grade using auger boring. According to Howell, maintaining a precise grade — often less than 1 percent — is a major advantage for AXIS since the laser guided system allows the operator to continuously monitor the boring process and make instantaneous adjustments when changes in rock density and hardness alter the path of the drill head, albeit ever so slight.

Another important advantage for the AXIS system given the rocky Northern Kentucky terrain, coupled with the smaller-diameter material that was to be installed for the Florence project, is the ability to complete it in one pass.

“Auger boring would have necessitated a two-pass approach,” Howell explains. “The casing pipe would have to be installed first, followed by the new carrier pipe threaded inside that. And likely this approach would also have required that we grout the material in place. Not so using the AXIS system, however, as the carrier pipe could be installed directly in the bore minus the steel casing. So, in essence, we eliminated a step and that equates to less time and reduced expense.”

Midwest Mole used a 13 1/2-in. tri-cone bit for the pilot bore and achieved an average production rate of 14 ft per hour through this complex compilation of rock formations. With the HDPE pipe fused and ready on the receiving end, it was time to face the unknown and begin the backreaming phase of the project.

The one unknown, however, was whether the AXIS system would be able to effectively backream through this difficult formation of limestone, shale and high-density rock, having only pulled product back through the bore on previous installations. Howell’s crew selected a 19 1/4-in. tri-cone cutter head for navigating the 506-ft bore with roller cones — a tooling configuration considered to be effective for removing rock and best positioning the reamer during pullback.  

“This was the first time the AXIS system had been used to ream rock, so there was obviously some trial and error involved at first,” Howell says. “We made some adjustments along the way to get the cutter spacing set with the correct over-cut to allow the reamer to work most efficiently. We had a pulling head attached to the back side of the reamer on a swivel, which helps to clear the bore, so we were pulling the product pipe back in as the hole was reamed. We used the standard Vermeer reamer design custom built with roller cone cutting heads.”

Howell admits there were some nervous moments prior to pullback. “We knew that we had thought the project out as thoroughly as possible, but there were still some tense nerves as we approached backreaming time,” Howell says. “Vermeer was there to support us, and yes, there was nervousness doing something for the first time, but all in all things went very well. We had to make some adjustments, but at the end of the day, it actually reamed the rock quite well and we were very pleased with it. We averaged about 8 ft an hour on the backream.”

Randy Happel is a features writer for Two Rivers Marketing, Des Moines, Iowa.

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