Utility contractor Planetary Subsurface Utility, Dallas, TX, spends most of its time working for Garland Power and Light, taking care of the utility’s high-voltage electrical distribution system.
And that’s fine with Planetary Chief Executive Officer Brian Sauer.
“Garland Power and Light is one of the best customers I’ve ever worked for,” said Sauer. “You couldn’t find a better client.”
However, Sauer keeps getting calls from other potential customers who have learned of Planetary’s success rate for completing unusual and difficult projects.
“We’re happy to stay home and work here,” said Sauer, “but some of the projects are so interesting and challenging, it is hard to turn them down.”
For example, Planetary recently made three unusual directional drilling installations in Austin, TX.
The work was for the Shoal Creek Wastewater Improvements to the 29th to 34th Street Tunnel, a part of the $200-million Austin Clean Water Program (ACWP), which includes removing wastewater collection lines from several creeks and relieving other lines by constructing parallel lines outside creek boundaries.
Planetary installed three 24-in. (61 cm) casings for new laterals to a 100-in. (254 cm) tunnel buried 60 to 70 ft. (18.2 to 21.3 m) underground. The laterals serving the large tunnel were installed at severe grades — one of them with a pilot hole that at a 125 percent grade was almost straight down.
All three installations were made with a compact Ditch Witch JT2720 All Terrain directional drilling unit.
Sauer said the city planners, engineers and the general contractor, W.L. Hailey, Nashville, TN, were familiar with the benefits of horizontal directional drilling, but not for use in the way it would be employed on the Shoal Creek project.
“The big tunnel had been installed by micro tunneling, so micro tunneling was considered the preferred method for installing the laterals,” he explained. “But micro tunneling equipment would experience severe difficulty with the harsh inclines, and there would be safety hazards associated with spoil removal.”
Sauer said the key to the difficulties for using micro tunneling was having to start the laterals at the large tunnel — 60 to 70 ft. below ground — and work upward. Grade of the laterals was critical with permitted deviations required to be less than one percent.
Searching for a practical alternative to micro tunneling, Sauer said several contractors were approached about using directional drilling, and all responded that it would be impossible to make the installations by directional drilling. General contractor personnel discussed the issue with one of Planetary’s project managers, Russell Prater, who urged Sauer to consider the project.
“With our extensive experience drilling on grade by HDD, we were able to design a profile to go from a surface entry and exit in the tunnel wall,” said Sauer.
Working as a subcontractor, Planetary made three lateral installations of 24-in. (61 cm) diameter steel pipe:
• The longest was 288 ft. (87.8 m) at a 13.5 percent grade;
• One of 50 ft. (15.2 m) at a grade of 47 percent;
• One of 75 ft. (22.9 m) at 125 percent grade.
Obviously the unusually steep grades required special setup of the drill rig.
For the 125 percent grade lateral, the unit was positioned at the edge of a 20-ft. (6.1-m) deep entry pit with the rear end raised to achieve the necessary angle of entry.
The front of the machine was placed against the top of the trench box to prevent it from falling into the pit and the unit was anchored with elephant chain, dead man plates, and other safeguards.
For the 50-ft. bore with 47 percent grade, the drill rig was picked up by a crane and lowered into the entry pit.
For the shot with the least aggressive 13.5 percent grade, the drill unit was set back from the entry pit and shore box. The crew drilled into the pit and established the correct pitch, then proceeded to drill down and into the main tunnel.
Sauer said pilot holes were drilled with a 5.75-in. (14.6 cm) rock bit through a Del Rio formation, a blue-gray rock that drilled like blue shale.
The Ditch Witch JT2720 All Terrain model generates 27,000 lbs. (12,247 kg) of pullback, has a maximum spindle torque of 3,200 ft.-lbs., and uses a dual-pipe drilling drive: The inner rod to drive a rock bit and an outer pipe for steering the downhole tool during drilling.
The outer pipe also provides rotary torque for the hole opener during backreaming. The system operates on low volumes of drilling fluid, can drill through a variety of soils and many types of rock without the need to use a mud motor.
Tracking while drilling pilot holes was a challenge.
“We started with a walk-over system, and then switched to a wireline system as the drill head moved deeper,” Sauer said. “As we got closer to the main tunnel, metal rings that circle it caused interference and we first used an AC mat and then DC mat to overcome the interference.”
Backreaming posed difficulties not found on most HDD projects.
“For each installation, personnel had to climb down 70 feet into the main tunnel,” explained Sauer. “Reamers and tools were lowered to the tunnel in a basket. They then had to be loaded onto cars of a small train like those used in mines, then carried to the spot where the bit had entered the tunnel. Everything had to be done by manual labor, and lifting the reamers into position was very difficult since some of the tools weighed 500 and 600 pounds.”
To enlarge the pilot holes to accept the 24-in. (61 cm) casing, three backreaming passes were made for each installation. Reamers were custom designs built in Planetary’s shop.
During reaming, spoil and drilling fluid entered the main tunnel, and disposal was labor intensive and time consuming.
“We deposited the fluid and spoil mixture into muck carts, which were moved to where they could be picked up by a crane and raised to the surface. Carts were emptied and then returned to the tunnel to be refilled,” Sauer said.
The job was further complicated by the fact that two-way radios routinely used for job-site communications would not send and receive signals due to the depth of the main tunnel; the only way to talk to personnel on the surface was via mine phones positioned at intervals along the 5,500-ft.-long (1,676-m) tunnel.
“When tunnel workers needed to communicate with the drill,” said Sauer, “one would have to go to the nearest mine phone, talk to a crew member on the surface who would relay information to and from drill personal by radio.”
Product could not be pulled back through the pilot hole as it is on most HDD projects.
A bullet nose cone was placed on the pipe, and it was pushed down the shaft a section at a time with each 20-ft. length of pipe welded to the end of the previous length of pipe after each push. A track loader was used to push the pipe down the pilot holes. The holes remained open and there were no difficulties in placement of the casing.
Once 24-in. (61 cm) casing was installed, Planetary’s part of the job was complete. Twelve-inch (30 cm) carrier pipe was installed by the general contractor with spacers keeping the smaller pipe at the correct grade.
Installation of the three laterals took approximately 30 days.
“Pilot bores took two days each, reaming and pipe placement was time consuming because of access in and out of the tunnel,” said Sauer.
The city reported its ACWP program is well ahead of schedule and was approximately 40 percent completed at the end of 2005.
This portion of the Shoal Creek project consisted of the replacement of the existing interceptors that were partially exposed within the banks of Shoal Creek. These were replaced with a single interceptor installed via tunnel construction methods through shafts constructed at both ends of the project.
The tunnel connects to the Crosstown Tunnel Inlet at West 29th Street via a new wastewater line also installed by tunneling methods.
In addition to the three laterals installed by Planetary, others were placed by open cut and jack-and-bore, and one was rehabilitated by cured-in-place-pipe lining.
Existing wastewater interceptors will be abandoned in place with exposed portions to be removed. CEG
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