November 2, 2020
Seeking to strengthen its portfolio of groundwater resources, Springville City, with the support of Hansen, Allen & Luce (HAL), began plans for the 400 South Well No. 2 at the City’s public works complex. The complex was already home to one successful drinking water well and could accommodate another, to be located some 550 feet away.
However, because the underlying aquifer is so permeable, engineers were concerned that drilling fluids from a rotary drilling method might be transmitted into the existing well. But drilling with the alternative cable tool method with typical casing perforations could result in sand production which can damage pumps and other water system facilities. Instead, the team devised an approach that would deliver the advantages of both drilling methods.
An extra-large borehole was drilled with the cable tool method to advance a temporary 26-inch-diameter casing to the total depth of 570 feet. The final 20-inch-diameter well casing and well intake screen were set inside the 26-inch casing (shown below). While the outer casing supported the surrounding soil, the contractor placed gravel between the two casings as the outer casing was slowly retracted. This dual-casing setup enabled installation of an engineered gravel pack in direct contact with aquifer materials without the need for the difficult removal of drilling fluids from the well. Finally, a bentonite surface seal was placed on the outside of the casing at the top of the well to protect the well from surface contamination.
After relatively little well development, the well produced clear, sand-free water during a 24-hour test at a rate of 5,000 gallons per minute with less than 10 feet of drawdown. Very little interference was observed at the other well. A well house (shown above) was constructed on the site and the well was equipped to pump 4,000 gallons per minute. The City now enjoys another productive, high-quality well as part of its drinking water system.
To discuss well drilling and other groundwater issues, contact Lance Nielsen, P.E., at firstname.lastname@example.org.