Which of the following engineering design criteria and construction standards are required for deep irrigation tubewells?
Options:
(A) Well screen slot openings must be selected based on the grain-size distribution ($D_{50}$ or $D_{60}$) of the aquifer or gravel pack material
(B) Entrance velocity of water through the well screen slots should generally not exceed $3.0\text{ cm/s}$ ($0.030\text{ m/s}$) to minimize frictional head losses and prevent screen encrustation
(C) Thickness of the surrounding gravel pack envelope typically ranges from $7.5 \text{ to } 15.0\text{ cm}$
(D) Well development (using air-surging, over-pumping, or high-velocity water jetting) is mandatory to remove drilling mud cake and fine sand from the near-well zone
Answer Key:
A, B, C, D
Step-by-Step Solution & Explanation
All four criteria are canonical tubewell design standards (Todd & Michael):
- A: Slot sizing prevents sand pumping while maximizing open area.
- B: Limiting entrance velocity below $0.03\text{ m/s}$ prevents laminar-to-turbulent transition and slows chemical scaling.
- C: A $7.5 - 15\text{ cm}$ pack stabilizes formation without bridging.
- D: Surging breaks up mud filter cakes and establishes a graded natural filter pack.