In May, Specialized Services Co. (SSC) began a horizontal auger boring project at the City of Glendale, Ariz.’s Oasis Water Treatment Plant as part of an expansion of the plant.
SSC’s scope of work consisted of digging and shoring a bore pit and then auger boring for 310 ft to install a 54- x .750-in. wall steel casing that would carry a 36-in. ductile water line. The bore would cross underneath both directions of busy Grand Avenue, an active BSNF mainline railroad track and into the Oasis Treatment facility. The purpose of this new line is to carry water from multiple well sites in the area into the treatment plant to be treated and then distributed as potable water to the growing community.
Having completed hundreds of bores around the Southwest, SSC is accustomed to performing auger bores such as this one to install lines where open-cut is not feasible. Each of these bores has its own unique challenges, and this was no exception. One of the owners of SSC, Aaron Veidmark, took on the role of project manager and used his nearly two decades of boring experience to meet the challenges of this project head on and deliver a successful product.
This project had extremely tight tolerances for line and grade. In a bore of this diameter and length, it would be acceptable and expected that the finished bore alignment would be within 12 in. of specifications. This project had strict requirements of being +/- 2 in. from specified line and grade. Not only did the bore have to finish in this tight window, but the bore path had to be within this window the entire way. To meet these criteria, SSC’s standard bore procedure had to be altered to include pulling auger after every 20-ft joint of casing was installed to check line and grade measurements. While this did allow for adjustments to be made along the way, it doubled the amount of time the bore was expected to take because of this time-consuming, added step. SSC also incorporated a water level into the bore setup so that the grade could be monitored continuously while the joints of casing were installed. All of the extra effort paid off when the bore popped out perfectly in line and only 5/8 in. lower than planned specifications. That means the entire bore dropped less than the thickness of the casing.
Another challenge was boring for 310 ft with this size of auger and casing, in soil that was a hard-packed, moist clay. The further out a bore gets, the more weight in auger that the bore machine must spin and the more thrust required to push forward the added weight in casing. Add to this soil that can stick to the inside and outside of the casing and the auger, creating added weight and friction, and you can run into a situation of too much force for even the large 60/66 Barbco auger boring machine that was powering this project to handle. To prevent any issues, polymer lines were installed at the beginning of the job — two for the exterior of the casing and two for the interior. A drilling polymer was pumped through these lines, which lubricated the exterior of the casing to enable to move through the soil with less resistance, and on the interior it surrounds the clay spoils, which prevent them from sticking inside the casing or to the auger, allowing for effective spoils removal. The drilling polymer used worked effectively to bore the full distance well within the bore machines power capabilities and with minimal water usage compared to if water alone had been used.
The final challenge came after the bore was successfully completed. A specification was presented by the engineer on the project stating that all exterior voids needed to be grouted. The exterior voids created on this project were minimal — between ¼ in. and 3/8 in. — and considered acceptable on all prior SSC projects. The engineers stood firm on wanting the outer diameter of the casing pressure grouted to fill these voids. SSC worked to show that pressure grouting was not feasible and the engineers ended up agreeing for a cellular grout mix to be used. SSC proceeded with this cellular grouting of the outer diameter, pumping 14 yds of the cellular grout and satisfied this requirement.
Throughout these technical challenges on the job, SSC was also required to produce a myriad of reports on a daily basis. The onsite engineers required reports each morning to show the prior day’s line and grade measurements, jacking pressures, bore track alignment/elevation and soil removal volume. The engineer used these reports as assurance that there were no unseen problems being encountered in the bore path.
While all of these challenges and added requirements lengthened the duration of the project beyond SSC’s original estimates, the bore was ultimately completed in time to meet the contractor’s construction schedule causing no delays on the project. The fact that it was so accurate prevented any extra time to be spent on their part to complete tie-ins of their lines placed via open cut, and the project was viewed as a success by all parties involved.
Michelle Walker is vice president of finance and administration at Specialized Services Co. (SSC), Phoenix, Ariz.