Water Tank Support [Case Study]

A 120,000 gallon water tank installation on alluvial soil for a middle school in Touchet, Washington required a deep foundation system to properly support the load. 

Working together, these teams made the design and build a success:

• • Distributor: Rocky Mountain Steel Foundations 

  • Certified Installer: Montana Helical Piers 

  • Project Engineer: Strata Geotech; USKH Inc.

  • General Contractor: Chervenell Construction Co. 

PROBLEM:

Foundation system design and installation was needed to hold a new water tank in poor soil conditions. The construction was to take place while the school was in session.

THE SOLUTION:

Engineers prepared the design plans using 12-inch steel driven pipe. The general contractor, Chervenell Construction Co, contracted Montana Helical Piers in Kalispell, MT for the installation.

Montana Helical proposed an alternative solution utilizing CHANCE® helical piles. From a material standpoint and given the anticipated depth, helical piles were less expensive, utilized smaller equipment, and provided faster installation with readily-available material.

Speed of Installation was definitely a selling point. In this case, Montana Helical completed the job in two days, compared to a week or more for other solutions. A shorter installation time gives a general contractor more flexibility with the overall construction schedule.

Montana Helical installed (37) SS150 CHANCE helical piles, a solid steel square shaft with 1.5" dimensions. The lead sections had two helices each, 8 and 10 inches in diameter. The crew installed the piles using a ASV100 skid steer with a hydraulic drive-head to an average depth of 28 feet.

Of the 37 piles, 12 were battered at 40 degrees. The outer ring of piles were battered to address lateral loading needs. There were seismic considerations in the design. With a water tank of that size, if the ground shakes, there is potential for differential settlement without a deep foundation. Some CHANCE helical pile products are building code rated for seismic activity. Read more about which ones.

See how helical piles perform in seismic shake table testing. Watch the video.

The helical pile system was finished by cutting each pile and topped with a new construction cap. Each cap sits above grade to be casted inside the poured concrete slab the water tank will rest on. In addition to the torque-to-capacity correlation that could be monitored during installation, full scale load test per ASTM D3689 was performed to verify capacity. 

Read my article about load testing to learn more about what it is and why we do it.

KEY BENEFITS:

CHANCE helical piles outperformed the original plans for driven pipe. The piles were less expensive, had low to no vibration/noise and installed faster than driven piles allowing time for the general contractor to keep the project on schedule. Classes remained in-session nearby during the installation. I can't guarantee that the middle school students weren't captivated by the activity going on outside their windows, but, be assured, it was not the noise of installation that caused the distraction.