Arizona’s Varying Soil Conditions Make Water Transmission Project Interesting
As part of a large water transmission distribution project for the development of the Superstition Vistas community in Apache Junction, Arizona, SSC Underground was contracted to perform several bore crossings of box culverts and a dry flood control channel.
The Phoenix-based contractor was originally contracted to perform 18 crossings on the project. Several bores were added as part of this original project, as well as several others for sewer and power utilities. By the time this project is complete SSC will have performed more than two dozen bores ranging in from 20 to 200 ft in length.
The Phoenix Metro area, located just across the county border in Pinal County, is experiencing rapid growth with homebuilders taking advantage of the expansive raw land to add housing for the ever-expanding population. As a previously undeveloped and somewhat remote area, the infrastructure development is critical to enabling the building to take place.
This project was being driven by both the City of Apache Junction and the homebuilders and had many levels of complexity including five different segments, designed by three different engineering firms, all to be built as one overall construction manager at risk (CMAR) project. An advantage leading into performing the bores on the project was that SSC’s vacuum potholing teams had performed all the utility potholing and mapping for the final design of the project.
Four of the bores under the flood control channel proved to be more challenging than the others. A pair of 36-in. bores for 24-in. ductile iron pipe water lines, a 48-in. bore for a 36-in. ductile iron pipe water line and a 42-in. bore for an electrical duct bank. For two of the bores, SSC’s crew setup on the north and bored southward, and the other two started on the south and bored northward. What made what sounds like a fairly typical project challenging is the soil conditions.
Arizona soils, specifically in the central area of Maricopa and Pinal counties, are made up of alluvial fill – layers upon layers of sediment washed down off the mountains – resulting in a wide variety of soils in a span of as little as 10 ft.
Imagine auger boring through dirt, sand and cobbles, sand and hard pan in a single 160-ft long shot. Add to this the fact that these four bores are all within 200 ft of each other, cross under the same flood control channel and all encounter each of these soil types, but in completely different spots.
Challenge No. 1 – Sand in the Path
The first bore at this channel location was a 36-in. bore and started after completing at least a dozen of the other box culvert bores. Feeling like the crews had a pretty good grasp on the soils in the area, the bore pit was excavated in average soil that stood, allowing shores to be set and secured with no issues, and giving a sense of optimism for the soil conditions for boring.
However, once the auger bore was launched a sand pocket was immediately encountered creating concern for crown collapse. To minimize this risk, the crew transitioned to a hand tunneling operation to get the first 20-ft joint of casing installed on line and grade, with the anticipation to transition back to auger boring. However, at 20 ft, the flowing sand turned to sand and cobbles, so hand tunneling continued until 40 ft, when augerable dirt conditions returned. After 40 ft of boring – at the 80-ft mark – another sand pocket was encountered.
At this point, with the bore 50 percent complete and on line and grade, the decision was made to bore out, which crews were able to do successfully, eventually getting back into good dirt at about 140 ft and finishing the bore.
Challenge No. 2 – Sand at the Pit
For the second bore, the operation was flipped to the north of the channel to launch a 48-in. casing. When the bore pit excavation started, it was revealed that it was on a sand pocket, resulting in an excavation much longer than the intended size of the pit. With extreme caution crews were able to get the shore boxes placed, backfilled and locked in for safety.
Looking at what was encountered on the 36-in. bore – only 10 ft to the west – the same soil conditions in reverse were anticipated. The thinking was that at least the first 20 ft would be in good dirt allowing the bore to get locked in to start. This was not the case and the bore started off as a hand tunnel due to the sand conditions at the bore pit. After 20 ft and now in good dirt, the crew transitioned to auger boring, but with the anticipation of encountering more sand and cobbles. To everyone’s great surprise, this alignment proved to be good dirt for the remaining 140 ft, making the greatest challenge at this location not the bore, but the building of the pit.
Challenge No. 3 – More Sand in the Path
For the third bore – a 200-ft shot for a 42-in. casing for the electrical duct bank – crews launched from the south to bore north and prepared for another challenging bore pit excavation. The crew was pleased to encounter good, standing dirt that made digging and shoring a safe pit no problem. Luck ran out 20 ft into the bore where sand was encountered. The operation transitioned to hand tunnel for another 60 ft until dirt once again made auger boring possible to finish out.
With no better handle on what soil conditions to anticipate after three bores, the crew plans to head into the fourth with open minds and ready to deal with whatever challenges are encountered.
With all the construction taking place in the area, each subsequent bore has the added complexity of new utilities in the area creating shifting alignments, varying depths and changing pit locations. There’s never a dull moment when working in Arizona’s soils.