Groundwater engineering: In which type of tube well is shrouding (gravel packing around the screen) specifically provided to control sand entry and improve yield?

Introduction / Context:
Shrouding, also known as gravel packing, is a standard development technique in well engineering. It improves well efficiency by stabilizing the aquifer formation around the screen and filtering out fine particles that would otherwise be carried into the well.

Given Data / Assumptions:

• Well construction involves a well screen or strainer to admit water.
• Formation contains sands/silts prone to migration under pumping.
• Objective is to minimize sand entry and maintain higher specific capacity.

Concept / Approach:
In strainer (screen) type tube wells, a graded gravel envelope (shroud) is placed between the borehole wall and the screen. The gravel gradation is selected to retain formation fines while allowing water to pass, reducing head losses and wear of pumping equipment due to sand.

Step-by-Step Solution:
Identify well type: presence of screens/strainers indicates suitability for gravel packing.Shrouding design: choose filter gradation satisfying empirical retention criteria (e.g., D15(filter) / D85(formation) ratios).Performance: gravel pack increases open area and reduces entrance velocity, improving yield and longevity.

Verification / Alternative check:
Field experience shows significant reduction in sand pumping and improved efficiency after proper gravel packing in screen-type wells compared with unshrouded installations.

Why Other Options Are Wrong:
Cavity type wells rely on developed cavities in coarse formations; shrouding is not typical.Generic slotted or perforated pipes without designed screens may not employ engineered gravel packs.

Common Pitfalls:
Using poorly graded or too fine gravel leading to clogging; omitting a centralizer, causing uneven pack thickness; ignoring uniformity coefficient in filter design.

Final Answer:
Strainer (screen) type tube wells