What is the minimum size hollow-stem auger, casing, or borehole used for installation of a typical 2-inch well?

• A. 3.75 in.
• B. 4.0 in.
• C. 4.25 in.
• D. 4.75 in.

Answer: (C)

When placing a typical 2-inch well with hollow-stem auger or casing, you must have enough space not only for the actual casing but also for the surrounding grout or gravel pack and for the tools to pass in and out. The outer diameter of a standard 2-inch well casing is about 2-3/8 inches, so you need several tenths of an inch of clearance on each side to allow for the annulus and to maneuver the casing and auger. This practical clearance pushes the minimum borehole diameter to about 4-1/4 inches. A borehole smaller than that would be too tight to install the casing plus grout and to retrieve the auger safely, while 4-1/4 inches provides a comfortable, workable space. Therefore the minimum size is 4.25 inches.