Tube-well construction – where is “shrouding” (gravel pack) essentially provided? Select the well type that primarily employs shrouding (a graded gravel pack around the screen) to prevent sand entry and stabilize the formation.

Introduction / Context:
Shrouding—also called gravel packing—is a critical construction feature of many tube wells. A properly graded envelope of gravel is placed around the well screen to filter formation fines, enhance entrance area, and reduce head losses.

Given Data / Assumptions:

• Unconsolidated alluvial formations with fine sands and silts.
• Well screens of the strainer/slotted type.

Concept / Approach:
Strainer (screen) wells rely on the screen plus gravel pack to control sand pumping. The gravel acts as a pre-filter and distributes flow uniformly into the screen. Cavity wells, by contrast, develop a natural cavity in coarse formations and typically do not require artificial shrouding. Slotted pipe alone without a designed gravel pack risks sand entry unless the native formation is self-supporting and well-graded.

Step-by-Step Solution:
Identify the well type whose design intrinsically uses a gravel envelope → strainer type.Confirm that cavity wells depend on natural development, not on gravel packs.Conclude that shrouding is essential for strainer type wells.

Verification / Alternative check:
Groundwater construction guides specify sieve analysis of formation samples to design the gravel pack (e.g., D30–D70 criteria) for strainer wells.

Why Other Options Are Wrong:

• (b) Cavity wells for coarse gravels do not require artificial gravel packing.
• (c) A simple slotted pipe without a designed pack is risky; when used with a pack it effectively becomes a strainer well.
• (d) “All the above” is too broad and not technically precise.

Common Pitfalls:
Using poorly graded gravel that bridges or allows fine migration; neglecting proper development after packing.

Final Answer:
Strainer type wells