Well Design and Construction
Well Design and Construction
This domain tests the choices that determine whether a well stays sanitary and productive for decades: casing size and depth, screen or open-hole completion, filter pack, and wellhead completion. Most items are framed as "what is wrong with this design."
Casing and diameter
- Casing must be new, undamaged, and sized to pass the pump with clearance: a 4-inch submersible needs about 4 inches of clear ID, so 6-inch casing is the practical minimum for most residential wells. Four-inch casing limits pump choice and yield.
- Seat casing into competent rock or below the overburden contact, and grout the full annulus. Casing that stops in unconsolidated material invites collapse and surface-water entry.
- Casing must extend above finished grade (commonly about 12 inches) and carry a sanitary seal, a screened vent, and a concrete pad sloping away from the well.
Completion in unconsolidated formations
- Use a screen and filter pack. Select slot size to retain the filter pack, and size the pack gradation to the formation. Open hole in sand produces sand-laden water and eventual collapse.
- In consolidated rock, an open-hole completion below a cased and grouted upper section is standard practice.
Wellhead details
- Use a pitless adapter through the casing wall; it is watertight and frost-safe. Avoid well pits, which collect water and are a confined-space hazard.
- All openings must be vermin-proof; vents must be screened and turned down.
Worked example
A 200-foot domestic well in Coastal Plain sand: 8-inch borehole; 6-inch PVC casing to 60 feet, grouted 0–60 ft; 40-slot screen with gravel pack from 60 to 200 ft. Check the reasoning: 6-inch casing leaves room for a 4-inch pump; grout seals the upper annulus against shallow sandy layers and surface water; screen slot retains the filter pack rather than the fine formation sand. If the driller had set 4-inch casing, pump selection and achievable yield would be limited - a design failure, not an operations failure.
Traps
- Specifying screen slot from the formation gradation directly instead of from the filter pack.
- Casing that stops above the water table or in unstable material.
- Leaving the annulus ungrouted "because the clay seals it."
- Omitting the vent and pad, which an inspection will catch immediately.