Trenchless Technology a Key Piece of Phoenix Drought Pipeline
Whereas trenchless contractors in other parts of the country have to contend with groundwater, in the dry Arizona desert, tunnels are being constructed in an effort to safeguard our limited water supply. Currently, residents in North Phoenix (400,000 citizens and growing) are served by two water treatment plants that get their raw water exclusively from the Colorado River. With the prolonged drought in the southwest states, supply from the Colorado has been diminished and so to protect the water supply to this growing region of the valley, the city embarked on this major project to provide a redundant supply from the Salt & Verde Rivers. The City of Phoenix’s Drought Pipeline project included 12 miles of new pipeline and 4 booster pump stations.
Starting in 2019, SSC contracted with both the design engineers and three separate contractors to perform subsurface utility engineering utilizing our vacuum potholing fleet and team. SSC’s consulting division, Trenchless Constructability & Design, also worked with the owner and contractors to provide design input for the required trenchless crossings. The work areas included major arterial roadways, freeway crossings, nature preserve, neighborhood streets and active water treatment plants. The ground conditions ranged from caliche, cemented cobbles, alluvium and hard dirt, meaning there was no “one size fits all approach” for the estimated 12+ trenchless installations required. TCD was able to guide designs to minimize disruption to the public while considering time, cost and risk.
SSC kicked off the tunnels with a 105’ installation of 66” casing for a 48” waterline under Glendale Avenue at the base of Piestewa Peak Mountain, using a motorized SBU from Robbins to perform this crossing as the ground was solid rock. Working for Kiewit, this crossing was a key connection as this section was the mainly through neighborhoods and had the most impact to residents, so a timely completion was key for public relations.
The second segment included six – 86” diameter tunnels under intersections along the 32nd Street alignment for a total of 834’ for a 66” waterline. These tunnels were constructed using an S70 Bobcat skidsteer equipped with either a hammer attachment or an Antriquip rotary head to do the digging in hard to extremely hard dirt conditions, production varied between 3’ to 9’ of installation per shift. As a subcontractor to Sundt Construction, SSC worked as a partner to sequence the tunnels in an effort to minimize traffic lane closures and keep access as open as possible to the many businesses along this corridor.
The final segment of SSC’s installations is for three 96” tunnels at the origination point of the pipeline, the 24th Street Water Treatment Plant for PCL Construction. Here, SSC is installing 96” casing with the same skidsteer method as along 32nd Street, but in extremely hard, solid rock conditions, averaging 18” per shift of tunnel advancement. These three tunnels – 35’, 115’ and 101’ in length – connect four pits for installation of a 72” waterline on a railing system. Due to the minimal space between existing underground lines and the need to use the pits after tunnel completion to tie together the line, and in the fourth pit, install valves and various connections to the plant, three of the four pits were constructed as semi-permanent structures to allow for the angles and elevations necessary to complete all of the connections.
This project has been a great example of the benefit of including a trenchless expert at the design phase of a project, especially one of this complexity, to ensure the successful and minimally disruptive construction of trenchless crossings.