The area south of Tucson, Ariz., is a rapidly growing area and is home to the residential communities of Sahuarita and Green Valley and the Sahuarita Mine. In 1995, the ACC approved Tucson Electric Power (TEP) to construct a new 138-kV transmission alignment to provide redundancy and reliability to the TEP’s system.
The final phase of the project was the Green Valley Transmission Upgrade, Canoa Ranch to Cyprus Sierrita. This portion of the project spanned approximately eight miles and included the requirement to bore under Interstate 19.
The bore would be 720 ft long and would connect the newly constructed Canoa Ranch 138-kV substation and provide access to an existing underground conduit for relocation of distribution lines. TEP’s original plans for the bore called out for the installation of 24-in. casing to carry 4- to 6-in. conduits.
Specialized Services Company (SSC), an Arizona-based trenchless technology construction company, was first contacted by TEP, along with other local general contractors bidding on the construction of this final phase of the project, to consult on the bore. Arvid Veidmark III, owner and senior estimator of SSC, began consulting on the constructability of this crossing and advising on trenchless methodologies to consider.
TEP’s initial plan for the 24-in. installation faced logistical challenges for successful bore of this length (720 ft) and multiple options were studied in combination with the owner’s most critical factors of consideration to determine the most feasible method. Ultimately, it was the requirement to not infringe on Arizona Department of Transportation (ADOT) right of way (ROW) that eliminated both directional drilling, which would have required setup in the ROW and any use of a tunnel head (SBU, TBM or BMTA), which contain the risk of needing to dig up the head in case of failure, which could happen at any point under the freeway or in the right of way.
The second factor considered was time — the substation was already completed, and this was the final component of a project that had been under way for 15 years. Because of the need to complete the crossing as quickly as possible, hand tunneling was eliminated as a choice. This left auger boring as the most practical option, however, this required upsizing to a 42-in. casing size in case of the need to transition to a hand tunnel operation at any point. TEP agreed to this design change and modified their plans to install 14- to 6-in. Schedule 80 PVC conduits for future use or sale/lease to another utility.
Getting to Work
Once the methodology was decided upon and plans revised, the project went out to bid. Tucson contractor Behan Excavating was low bidder and selected SSC as the trenchless contractor to perform the jack-and-bore portion of the project. Construction was originally scheduled to begin in September 2012, but delays in permits from ADOT delayed the start by several months, increasing the pressure on SSC to be successful using auger boring to make up time and keep the project on schedule.
SSC co-owner and vice president Abe Veidmark was named project manager and began work in November 2012. The first step for SSC was to dig and shore the bore pit. In preparation for a bore of this length and duration, Abe Veidmark spent extensive time designing the pit and layout of the site for best equipment access, most logical stockpile location for materials and spoils.
SSC began turning auger in December 2012 and initial production was good. Soil conditions proved challenging to start so a drift-ring bit was changed out for a Christmas-tree bit, and SSC’s crews were able to bore and install 180 ft of casing in December and another 50 ft in January 2013 before the bit broke. This created the first setback due to equipment failure, but crews quickly replaced the bit, and by mid-January had installed 300 ft of casing. At this point, the strain on the machine, turning 35,000 lbs of auger and pushing 66,480 lbs of casing became apparent, so Abe Veidmark directed the crews to begin using the slick lines that had been installed to pump polymer along the OD and ID of the casing.
At 517 ft of installation, the shear force exerted when turning this length of auger became a problem and caused the auger shaft to snap. This happened twice more — at 530 ft and at 580 ft — each time requiring all the auger to be pulled out, cutting out the broken piece and removing it and subsequent pieces using a hydraulic tugger — creating multi-day delays on each occurrence. In one case, SSC crews had to crawl through and cut the flighting inside of the casing in order to reach the broken piece.
Following the 580-ft break, all 4-in. hex auger was replaced with 5-in. hex in an attempt to mitigate this problem. This was successful for another 60 ft and by March 19, 640 ft of 42-in. casing had been installed using auger boring, at which point the auger broke again. The bore machine was also at its limit by this point, and SSC informed the owner that the balance of the crossing would need to be completed either by hand tunneling or open-cut. Since the bore had already traversed the freeway, and because time was as critical now as ever, TEP petitioned ADOT to allow the 80 ft of casing to be installed via open-cut. After two weeks of meetings and proposals, ADOT agreed and by April 5, SSC crews had dug the open-cut trench, pushed the full 720 ft of casing and backfilled the open-cut, returning ADOT right of way and access road traffic flow after only three days.
With the crossing completed, SSC crews worked for the next two weeks to weld a rail system inside the casing and build and push the conduit bundles into the casing. With the carrier lines fully installed and bulkheads built, the final step was the grouting of the annular space. Having grouted dozens of bores, Abe Veidmark knew that pumping this quantity (145 yds) of grout, would create extreme heating inside the casing as the grout cured, and the concern of damage to the conduits was real, compounded by the fact that once they were grouted, there was nothing that could be done to repair or replace them.
A result of months of brainstorming on how to deal with this situation, Abe Veidmark and his crew plumbed a cooling system, using the installed conduits to continuously pump water through the conduits and a 2,000-gal water truck tank to absorb some of the heat generated by the curing grout. The design proved successful and was run for 24 hours after grouting, pumping water the entire time, draining it into a tub in the bore pit and then pumping it out to ground level. The water temperature was raised more than 30 F to 107 F, even the day after grouting. This increase in the temperature of the water showed how critical this step was in preventing damage to the conduits.
By being able to bore 90 percent of the footage, SSC was able to complete the project in accordance with TEP’s schedule. The conduits were installed on line and grade, and thanks to the ingenuity of Abe Veidmark’s cooling design, the grouting operation was a successful conclusion to a challenging project.
Michelle Walker is vice president of finance and administration at Specialized Service Company, Phoenix.