Water Well Construction, Drilling Methods, and Equipment
This domain tests whether you can choose a drilling method and equipment string that fit Illinois geology and then build a well that seals out surface contamination. The exam favors practical reasoning: method versus formation, casing and screen design, and grouting as part of construction, not an afterthought.
Matching method to formation
- Mud rotary: best in unconsolidated glacial till, sand, and gravel; bentonite or polymer slurry holds the hole open while cuttings are circulated out.
- Air rotary: fast in stable bedrock; poor in running sand or wet gravel, where the hole collapses and fines plug the bit.
- Cable tool: bails cuttings and handles boulders and hard rock; slow but gives clean formation samples.
- Hollow-stem auger: shallow unconsolidated work; not suitable for deep wells or flowing artesian conditions.
Construction essentials
Casing must be watertight, extend at least 12 inches above finished grade (higher where flooding is possible), and be seated into a confining layer or bedrock so shallow groundwater is sealed off. Screen slot size is matched to the formation; a filter pack is used where the aquifer is fine and uniform. The annular space is grouted from the casing shoe to the surface.
Worked example
A 250-foot well passes through 40 feet of clay till, 60 feet of sand, then limestone. Choose mud rotary, set 6-inch steel casing to 105 feet (5 feet into rock), grout the annulus from 105 feet to surface, and drill open hole in limestone. Reasoning: the casing seals the sand and shallow water, the grout prevents a vertical leakage path, and open rock gives maximum yield.
Traps that cost marks
Assuming air rotary works in every formation; casing only through topsoil; sizing the screen to the pump instead of the sand; and treating grouting as optional when a confining layer is present.