A long distance horizontal directional drilling (HDD) project on its own is a big undertaking. Add in bore-path complications like two heavily vegetated ravines, a road, two restrictive railroad rights of way and a reinforced concrete parking lot, and there is the recipe for a jobsite that contractors are begging to leave. Not to mention, the path runs between two abandoned underground gasoline storage tanks that had been filled and alongside a mismarked fibre optic line that had to remain up and running.
This was the situation for a Greenville, South Carolina, USA fibre optic installation project. Magnetic field anomalies confounded utility locators and HDD guidance systems. Several regional contractors had either failed to complete the installation or refused to take the job altogether. The project was now more than a year overdue. Luckily for the customer, this is exactly the type of job Atlas Group’s HDD division pursues. “We are called in to finish jobs other contractors cannot finish. You could say that is our niche, drilling in hard rock, doing the most difficult jobs.” said Atlas Group President KJ Woody.
Founded in 2001 in Buckhannon, West Virginia, USA by KJ and his brother, Kyler, Atlas Group is a family-owned, family-operated company serving the needs of underground utility and construction customers throughout the East Coast region. HDD applications
typically involve fibre, natural gas and waterline installations.
Mapping The Route
The 672 ft (205 m) run went along a busy highway. It began directly beneath hightension power-transmission lines with a massive magnetic field that completely masked a typical drill string’s beacon signal. The path then descended beneath the heavily vegetated surface of a 40 ft (12 m) deep ravine before crossing deep beneath a side road.
Beyond the road, the surface descended again. Here the bore path crossed beneath a
decommissioned railroad track and then further on beneath two active railroad tracks. The railway company forbid any locating activity within 10 ft (3 m) of the rails. Once past this obstacle, the surface above the path rose to an elevation beneath an old gas station’s parking lot. The bore must rise beneath the parking lot to pass between two abandoned tanks lying just before the exit point. Adding further interference was an existing fibre optic line that lay parallel to the bore path along its full length, also passing between the abandoned gas tanks.
No Stranger To Energised Ground
Atlas was brought onto the job after finishing a different HDD project for the same
customer in similar energised ground conditions.
Since the two sites were located close together, KJ anticipated they would have similar
problems with signal reception and interpretation. The initial BPA using the company’s own Subsite TKQ receiver confirmed it. The TKQ is a four frequency tracker especially designed for larger rigs and longer bores with a locating depth range rated to 110 ft (33.5 m) and tracker-to-rig range rated at 2,000 ft (610 m).
Despite the receiver’s power and accuracy, in many of the places where Atlas crews could get signals, they could not consistently trust them. The shallow noise floor they discovered meant even after a successful start, portions of this job would have to be either drilled blind or exposed. Obtaining a visual verification at the depths specified for this run, and working on the slopes in rough terrain, would add enormously to project time and cost.
The Woody’s decided to see if they could get better signal reliability and brought in Subsite application specialist Brett Romer to demo the newest HDD guidance equipment.
Romer brought along a four-frequency Subsite TK RECON 4 receiver, Subsite 17T4 beacon and Subsite Commander 7 remote display. What is more, KJ said, Romer brought: “a wealth of knowledge” that further increased his and Kyler’s understanding of how interference affects a receiver’s interpretation and how to use that knowledge to their advantage.
Innovations in HDD guidance systems have made them so user friendly over the years that they are a cinch to learn and operate. However, interpreting what they say in actual field conditions entails a much deeper grasp of the underlying operating principles.
Measure twice, drill once
Romer hooked up the Commander 7 to Atlas’ Ditch Witch JT3020 All Terrain drilling rig and calibrated the receiver. Then, using both of the receivers on hand, the team initiated another BPA and compared the results to the first BPA. Comparisons are useful because they can show whether anomalous variations remain consistent from one BPA to the next or if they have changed with time of day or other variables. Both instruments recommended frequencies of 29 kHz and 12 kHz to mitigate ambient interference.
Next, the crew re-examined the noise floor for the run, determining a maximum depth of about 55 ft (16.8 m). Knowing they would not be allowed to perform locates near the railroad tracks while running at a depth of 35 ft (10.7 m), they set up the beacon for 29 B power and 29 X power.
The 17T4 beacon Atlas was using in its Ditch Witch Rockmaster housing on this job can emit its signal in four frequencies at three fieldconfigurable power levels. Pre-set frequency and power combinations can be switched from one to another on the fly by putting the beacon to ‘sleep’ and restarting within a designated period. Then they began drilling. One of the big benefits of the TK Recon 4 was its beacon compass. The crew no longer needed to track steering by having the receiver operator cross back and forth 20 ft (6 m) at a time
to verify nulls.
Another benefit was its pitch-assist tracker boot, which came into play because of the rough, irregular terrain and varying slope of the surfaces above the bore.
Finishing the job
The crew got more than halfway through the full length of the run on the first day.
Day two’s drilling would include the railway crossings and the pass under the rebarembedded concrete of the old gas station’s parking lot before exiting on target.
When the crew got to within 12 ft (3.6 m) of the active railway tracks, they switched
guidance techniques from the TK system’s walkover mode to its DrillTo mode. Despite
the distance to the receiver, the driller easily tracked bore progression to the target on
his Commander 7 display. The setup had a continuously reliable signal with the bore 14
ft (4.3 m) beneath the rails, progressing at about 2 ft/min (0.6 m/min) for each rod.
The final peak of the bore path topped out about 150 ft (45.7 m) away from run
completion. The trick here would be steering the bore precisely enough to avoid the
fibre optic line running alongside the bore path and the abandoned gas tanks on the
other side of the lot.
Even using a Subsite Utiliguard locating system, it took two hours to locate the fibre
optic line. The crew finally had success after running out an additional 50 ft (15 m)
of fish tape and grounding the transmitter to the air conditioner ground of a nearby
business. They first detected the signal by running the unit at 13,000 Ohms. Setting the
power to level 5 and using 8.01 kHz enabled the crew to get down to reliable reception
at 5,000 Ohms.
Despite being satisfied with their utility locator’s settings, the crew did not believe the
reading it was giving them. They knew the fibre as marked was mislocated, but they
were showing it at just 1 ft 1 in ( 305 mm) below the concrete.
Atlas Group called in a ground penetrating radar (GPR) contractor to determine the
fibre optic line’s true location, only to learn their readings had been nearly spot on. The
GPR showed the line to be on the exact path, but slightly deeper at 1 ft 9 in (530 mm).
Even with an excellent track record, KJ and Kyler acknowledge that they still walk away
from every job with valuable new knowledge and experience.
“Everyone gains from each locating experience. Maybe it is not something we use on
the next job, but it adds to our expertise.” KJ said. “The best advice we have to give from
this job is, why not use all your resources on hard jobs?”