Water Well Construction and Drilling Methods
Domain 2 tests how geology dictates drilling method, hole stability, and the construction sequence. The exam expects you to match a method to a formation and to protect the borehole from contamination.
Common methods
- Cable tool: percussion; slow but gives reliable samples in glacial till and fractured rock.
- Rotary air: fast in hard bedrock; requires adequate air and can spread contamination if the hole is not cased.
- Mud rotary: circulates drilling fluid to cool the bit and hold unconsolidated sand and gravel.
- Auger: shallow, unconsolidated materials; not for boulders or deep rock.
- Jetting/driven: shallow sand points; limited diameter and depth.
Construction sequence
Set up on an approved site, drill, install casing and screen, place gravel pack, grout the annulus, develop, test, disinfect, and file the log. The method must not create a pathway between aquifers.
Worked example
A site has 40 ft of sand and gravel over shale, target depth 140 ft. Mud rotary holds the sandy hole; casing is set into the shale, the annulus grouted, and the shale is drilled open-hole. Air rotary from the surface would likely collapse the sand and make grouting unreliable.
Traps
Using air rotary in running sand, drilling ahead without casing, and failing to control drilling fluid are common failures. Do not assume a method that works in bedrock is suitable for unconsolidated deposits.