Capillary rise in soils: how does particle (pore) size affect it? Select the statement that best describes capillary rise of water in soils with differing particle sizes and pore diameters.

Introduction / Context:
Capillary phenomena control how water ascends above the groundwater table into the unsaturated zone. The maximum height of capillary rise influences suction, moisture distribution, and heave potential in fine soils.

Given Data / Assumptions:

• Capillary rise depends on surface tension, contact angle, and effective pore radius.
• Smaller pores correspond to finer-grained soils (e.g., silts and clays).
• Consider water-wet soils with small contact angles.

Concept / Approach:

In a capillary tube model, the height h is approximately proportional to 1/r (inverse of pore radius): h ≈ (2 * γ * cos θ) / (ρ * g * r). Thus, for smaller pores (smaller r), capillary rise is higher. Fine soils sustain higher negative pore pressures (suction) and hence larger rises compared to coarse sands or gravels.

Step-by-Step Solution:

Verification / Alternative check:

Field observations: capillary fringe can extend meters in clays but typically only centimeters to decimeters in clean sands.

Why Other Options Are Wrong:

(b) reverses the true relation; (c) ignores key physicochemical parameters; (d) is ill-posed (saturated soil has no capillary rise above the water table); (e) contradicts theory and practice.

Common Pitfalls:

Confusing “wet” appearance with capillary height; ignoring contact angle effects; assuming a single pore size in real soils.

Final Answer:

Capillary rise increases as particle size decreases (smaller pores draw water higher).