Soil and Material Selection
Soil is a graded system
A mud wall is only as good as its soil. The ideal is a well-graded sandy loam: sand and gravel form the load-bearing skeleton, silt fills the voids, and clay binds the particles together. For unstabilised rammed earth, a useful starting range is 70–85% sand and gravel, 10–20% silt, and 5–15% clay. Adobe mixes often contain more clay and rely on straw to control shrinkage. Avoid topsoil, roots, leaves, and other organic matter, which decompose and weaken the wall. Excess soluble salts can cause efflorescence and surface breakdown, and expansive clays cause severe cracking.
Candidates should know the difference between clay and silt. Clay is plate-shaped and plastic when wet; silt is fine but lacks clay's cohesion. A soil that feels smooth and ribbons strongly is clay-rich, but that does not prove it is suitable. Sample several pits across the borrow area because a single test hole can mislead. Remove oversize stones or use them only where the wall design permits.
Traps in questions include equating dark colour with high clay content, treating a single field test as a full classification, and assuming that more clay always means a stronger wall. In fact, too much clay increases shrinkage and reduces durability.
Worked rule: in a jar sedimentation test, shake one-third soil with water and a pinch of dispersant. Sand settles within a minute, silt within about an hour, and clay over many hours. If the sand fraction is below about 50%, blend in sharp sand before making trial blocks.