Among common soil/rock materials, which has the lowest specific retention (i.e., the smallest fraction of water retained against gravity after drainage)?

Introduction / Context:
Specific yield and specific retention partition the total porosity into drainable water and water held by capillary and adsorptive forces. Knowing which materials retain more or less water is fundamental in aquifer evaluation and dewatering design.

Given Data / Assumptions:

• Comparison among common granular soils and clay.
• Gravity drainage after saturation is considered.

Concept / Approach:
Fine-grained soils like clays have large surface area and small pores, producing strong capillary forces and high specific retention. Coarse materials like gravel have large pores and weak capillary forces, so they retain the least water and therefore have the highest specific yield (and lowest specific retention).

Step-by-Step Solution:

Verification / Alternative check:
Typical data: specific retention may be ~0.05–0.15 for gravel and >0.5 for clays (ballpark), confirming the trend.

Why Other Options Are Wrong:

• Clay/Silt/Sand: Retain more water than coarse gravel due to finer pores and higher capillarity.
• Peat: Very high water-holding capacity; not the lowest.

Common Pitfalls:

• Confusing specific yield (drainable) with specific retention (held water); they are complementary portions of porosity.

Final Answer:
Coarse gravel