Well Construction and Casing
Construction questions test the physical choices that keep surface water out of the well: casing depth, casing material, hole diameter, screen selection, and the wellhead finish.
Casing fundamentals
- Michigan requires a minimum of 25 ft of casing, seated into competent confining material where it occurs shallower than that, and grouted for its full length.
- Casing must be new pipe meeting the code, fitted with a drive shoe when driven, and sized for the pump and drop pipe it will carry.
- The borehole must be large enough to leave an annular space for grout, typically at least 3 in. of clearance around the casing.
- Casing terminates at least 12 in. above finished grade with a sanitary, vermin-proof cap; a pitless adapter keeps the discharge below frost without a pit.
Screens and intakes
Match slot size to the formation: openings should retain the bulk of the aquifer sand while passing the fine fraction during development. In fine uniform sand, use a smaller slot or a gravel pack; in fractured bedrock, an open borehole is normal.
Worked example
A log shows sand from 0 to 48 ft, clay from 48 to 70 ft, and sandstone below. Reasoning: the shallow sand is a direct path for surface contamination, so the driller sets casing through the sand and 5 to 7 ft into the clay, grouts the full annulus, and completes in the sandstone rather than screening the shallow sand.
Traps
- Measuring casing length from a pit floor rather than from grade.
- Forgetting the drive shoe or reversing it.
- Drilling a hole only slightly larger than the casing, leaving no room for grout.
- Screening fine sand with an oversized slot, which then pumps grit and collapses the formation.