Groundwater abstraction: A strainer-type tube well is generally unsuitable for which kind of aquifer material?

Introduction / Context:
Strainer-type tube wells use a slotted screen to admit water and exclude formation particles. Suitability depends on aquifer grain size and gradation, which affect screen entrance velocity, headloss, and sand control without excessive development efforts.

Given Data / Assumptions:

• Conventional slotted or wire-wrapped screens.
• Unconsolidated aquifers with varying sand/gravel sizes.
• No artificial gravel pack unless explicitly designed for.

Concept / Approach:

In very coarse gravels, slot sizes required to retain formation are large and hydraulic entrance velocities become unfavorable; moreover, maintaining adequate screen support and preventing structural damage is difficult. Strainer wells are best in fine to medium sands (often with a gravel pack) where slots can exclude fines at acceptable headloss and velocities.

Step-by-Step Solution:

Verification / Alternative check:

Well design manuals recommend properly graded gravel packs and specific screen types for coarse formations; straight strainer wells without tailored packs are not advised for coarse gravels.

Why Other Options Are Wrong:

• Fine sandy strata / well-graded medium sand: Common and suitable with proper slot size and development.
• Clean gravels: Similar issues as coarse gravels; however, “coarse gravels” is the clearest unsuitable option.
• None of these: Not correct since a clearly unsuitable case exists.

Common Pitfalls:

• Selecting slot size by nominal sieve size without considering uniformity coefficient and entrance velocity.
• Omitting gravel pack where needed, causing sand pumping.

Final Answer:

coarse gravels.