An overall value of the coefficient of permeability of a soil deposit for a large area may be determined by:

Q29: An overall value of the coefficient of permeability of a soil deposit for a large area may be determined by:

A. Constant head permeability test

B. Variable head permeability test

C. Pumping out tests

D. Pumping in tests

Correct Answer: C. Pumping out tests

📝 Detailed Explanation

For a soil deposit spanning a large area, a small laboratory sample’s permeability result may not represent the overall, real-world behaviour of the whole deposit — particularly if the deposit has significant layering, fissures, or other large-scale structure. A different class of test method is needed to capture this larger-scale, “overall” permeability value.

✅ Why “Pumping out tests” Is Correct

Pumping-out tests determine an overall coefficient of permeability representative of a large area of soil deposit, by extracting water from a well and observing the resulting drawdown at surrounding observation wells (typically three, drilled radially at 120° intervals around the test site) — directly averaging the behaviour of a much larger soil volume than any lab sample could.

❌ Why the Other Options Are Wrong

Constant head permeability test: a laboratory method, testing only a small, controlled sample — not representative of a large-area soil deposit.
Variable head permeability test: also a laboratory method (the falling head test, under an alternate name) — again testing only a small sample, not a large area.
Pumping in tests: a genuine field method, but typically used for more localized permeability assessment near a single borehole, rather than specifically for determining an overall value across a large deposit area (the role pumping-OUT tests are specifically noted for here).

🔑 Key Point

When a question specifically asks about an “overall” value for a “large area,” that phrasing points to a field method (pumping-out test) rather than any laboratory method, which can only ever characterize the small sample actually tested.

💡 Key Concepts for Students

This is the same pumping-out test already introduced as a field method in Q18 — here, its specific advantage (large-area representativeness) is the focus.
Three observation holes at 120° intervals is a specific, standard field arrangement mentioned directly in the source material for setting up a pumping-out test.
See Q20 and Q21 for two fully worked numeric pumping-test calculations (unconfined and confined aquifer, respectively).
See Q18’s Going Deeper section for the complete laboratory-vs-field method comparison.

📚 Going Deeper: Why Field Tests Give a More Representative "Overall" Permeability

Natural soil deposits are rarely perfectly uniform — they often contain layering, lenses of different soil types, fissures, root channels, or other large-scale structural features that can dramatically affect how water actually moves through the deposit as a whole.

A small laboratory sample, however carefully tested, can only ever characterize the permeability of that specific small volume — it may happen to capture (or miss) these larger-scale features entirely by chance, especially if they’re spaced further apart than the sample itself. A field pumping test, by contrast, draws water from and through a much larger volume of the actual in-situ deposit, naturally averaging over any such larger-scale heterogeneity and capturing their combined effect on the water flow — which is exactly why pumping-out tests are specifically valued for determining a single, overall, representative permeability value across a large area, rather than a possibly-unrepresentative point value from one small sample.