Earthmoving Fundamentals and Soil Characteristics
Three volumes, one material
The same soil is measured three ways, and confusing them is the most common error in this domain.
- Bank cubic yards (BCY): material in its natural, undisturbed state.
- Loose cubic yards (LCY): material after digging, when it has swelled and aerated.
- Compacted cubic yards (CCY): material placed and compacted in a fill.
Swell is the percentage increase from bank to loose: swell % = (LCY − BCY) ÷ BCY × 100. Shrinkage is the percentage decrease from bank to compacted. The load factor is BCY ÷ LCY and lets you convert truck or bucket volumes back to bank measure.
Soil behavior
Cohesive soils (clays) hold a slope but are sticky, swell a lot, and are moisture-sensitive. Granular soils (sand and gravel) drain well and compact easily but will not stand in a steep cut. Silt is erodible and unstable when wet. Organic soils and topsoil are unsuitable as structural fill. Rock may need ripping or blasting and shrinks little.
Moisture and density govern compaction: each soil has an optimum moisture content at which a given compactive effort produces maximum dry density, established by a Proctor test. Too dry or too wet, and the roller cannot reach spec no matter how many passes it makes.
Traps in questions
Watch for swell applied to a compacted volume, shrinkage treated as identical to swell, and organic material accepted as fill. Another trap is assuming more roller passes fix wet soil - they do not.
Worked example
1,000 BCY of soil at 25% swell becomes 1,000 × 1.25 = 1,250 LCY. At 25 LCY per truck, that is 1,250 ÷ 25 = 50 loads.