In underground utility construction, having the right team is just as important as having the right tools. That was evident recently when Brent Scarbrough & Co. (BSC) of Fayetteville, Georgia, teamed with Barbco Inc. and Trenchless Rental Solutions (TRS) to complete a quartet of challenging bores for a Mosaic Co. phosphate mining plant in central Florida.
The bores traversed challenging ground that included sand in addition to partially weathered coral rock, while crossing under Florida Department of Transportation rights of way and in proximity to a high-pressure 36-in. gas main.
The difficulty of the bore was demonstrated by the fact that the initial contractor hired to build the job was stopped in its tracks 100 ft into the first bore, leaving behind a large void at the surface – luckily just outside the zone of influence for a department of transportation (DOT) roadway.
With the problems they were having, the owner, Mosaic, in conjunction with FDOT, terminated the contract. The owner had to inject grout into the ground around the end of the casing to stabilize the ground and prevent further settlement and potential damage to the highway and potential interruption of traffic.
At this time, the owner approached Brent Scarbrough & Co. about revisiting the project, and the company was brought on board to attempt the project. The challenges were evident immediately, beginning the abandoned 100-ft section of 48-in. casing that was left uncompleted.
“We met with Mosaic and they wanted to try to salvage the 48-in. casing that was in place,” said BSC superintendent Chad Cox. “We collectively decided that attempting to simply start again with the 48-in. casing was not a good idea because it has been in place for several weeks at this time. So, it was determined that the best approach would be to install a new 42-in. casing through the existing 48-n. – and into and through the grout fill that was placed to mitigate the ground settlement.”
BSC consulted with Barbco Inc. representatives including Tony Barbera, Michael Leonatti and Scott Fisher, and decided to use an unsteered 42-in. bore. “We felt like although it was a little risky to attempt a 500-ft bore without any steering capability, but with the circumstances we were dealt, this was the best approach. We did encounter some dense, rocky material, but this bore came in very close to line and grade,” Cox said.
When it came time to do the second 500-lf, 48-in. diameter bore, the project team elected to use the same method since it was very successful. While performing the second bore, however, the crew encountered rock at about 280 ft in, past the highway ROW. After several hours of grinding on the rock and running very high pressures on the rig, it was determined that a new approach was needed. The crew stopped the bore and pulled all of the auger back to install a new cutting head. To its surprise, the crew found that the head was completely missing.
Since they were past the road ROW, the crew dug to the end of the pipeline, and upon exposing the line located the head near the end of the casing. With a new head in place at the launch pit, BSC resumed the bore and was able to push the old head into the temporary pit where it was retrieved. The pit was then quickly backfilled and the bore back track. BSC completed this second bore, but it was rather difficult due to the very dense coral rock encountered. This bore came in several feet off of line and grade, but was deemed acceptable and usable by the owner.
With two more 500-ft bores to go, the project team re-evaluated its options. “We had some decisions to make regarding the last two bores. We completed two bores, one that was very close to line and grade and one that was off by several feet. We knew that we were going to encounter this same dense coral rock material again on these other bores, so we elected to utilize the Barbco Pathfinder 500 pilot tube system for the final two bores,” Cox said.
The Pathfinder combines pilot tube and auger boring technology to deliver accurate casing installations up to 60 in. in diameter. Typically, bores start with a 6-in. pilot bore which is them reamed out to the final diameter. In this case, the 6-in. pilot bore was not a practical solution with the existing 48-in. casing.
Using the Pathfinder’s pilot tube and conventional head, modified with some additional rock bits, crews successfully negotiated the third bore until they hit the dense coral rock. At this point, they pulled the augers and installed a steerable roller cone head to penetrate the rock and complete the bore very close to the intended target.
To complete the reaming process, BSC utilized a 24-in. flat-face rock head with a 48-in. reamer trailing about 60 ft behind with a custom-made 24- to 48-in. transition knife reamer built by Barbco. The “knifes” were fabricate from high-strength 2-in. thick steel.
Although it was a little slow grinding through the rock, BSC was successful in getting this bore done. BSC repeated the process – and the results – for the fourth and final bore. The final two bores were both completed within 1 mm of line and grade, much to the approval of the owner.
Name of Project: Four Corners Corridor Project – Hwy 62 Jack & Bores
Contractor: Brent Scarbrough & Co.
Equipment: Barbco Inc., Trenchless Rental Solutions
Location of Project: Bowling Green, FL
Owner: Mosaic Fertilizer
Project Duration: September 2018 – April 2019