Specific yield (drainable porosity) of a soil aquifer system depends primarily on which microstructural features?

Introduction / Context:
Specific yield, also called drainable porosity, is the volume of water that a saturated soil or rock releases by gravity drainage per unit bulk volume. In geotechnical and groundwater problems, specific yield governs storage change, dewatering performance, and aquifer yield estimates.

Given Data / Assumptions:

• Soil is treated as a granular, porous medium.
• Gravity drainage is the mechanism; capillarity and residual saturation reduce drainable water.
• Macro fabric (compaction) and microstructure (pore network) are important.

Concept / Approach:
Specific yield depends on the volume of pores that actually drain, which is lower than total porosity because some water is retained by capillary forces. Factors that reduce pore throat size or connectivity lower specific yield. Compaction reduces void ratio and porosity; particle shape and grading influence packing, and pore-size distribution controls capillary retention and drainage pathways.

Step-by-Step Solution:

Verification / Alternative check:
Empirical correlations show higher specific yield in well-graded sands and gravels, and lower values in silts and clays where capillarity traps water.

Why Other Options Are Wrong:
Each single factor (A–C) matters, but the most complete answer is the combined effect in option D.

Common Pitfalls:
Equating specific yield with total porosity; ignoring capillary water that does not drain under gravity.

Final Answer:
All the above