Earthwork and Geotechnical Considerations
Earthwork and Geotechnical Considerations
Earthwork involves moving soil to achieve design grades. Candidates must understand soil classification (USCS and AASHTO), compaction (Proctor tests, relative density), bearing capacity, settlement, slope stability, shrink and swell factors, and trenching and shoring safety. Key tests include sieve analysis, Atterberg limits, standard and modified Proctor, and consolidation tests. Geotechnical reports provide recommendations for foundations, pavement subgrade, and slope angles.
Traps include: confusing shrinkage (soil volume decreases when excavated and compacted) with swell (volume increases when excavated and loosened). Shrinkage is common in cohesive soils; swell in rock or granular soils. Another trap is compaction specification: 95% of maximum dry density per ASTM D698 (standard Proctor) is common for fill, but heavy-duty areas may require 95% of D1557 (modified Proctor). Also, bearing capacity is not the same as allowable bearing pressure; the latter includes a factor of safety. Expansive soils can cause heave; proper drainage and over-excavation may be needed.
Worked example: A trench excavation requires 1,000 cubic yards (cy) of compacted fill. The soil has a shrinkage factor of 15%. How many bank cubic yards (bcy) must be excavated? Shrinkage means that after excavation and compaction, the volume is less than the bank volume. The relationship is: compacted volume = bank volume × (1 - shrinkage). So bank volume = compacted volume / (1 - 0.15) = 1000 / 0.85 ≈ 1,176 bcy. Conversely, if swell is 20%, loose volume = bank volume × 1.20.
Rule of thumb: For estimating, a 10% shrink for clay and 10-20% swell for rock are typical. Always confirm with a geotechnical report. For slope stability, a 2:1 (H:V) slope is often the steepest for permanent soil slopes without reinforcement; flatter slopes (3:1 or 4:1) are safer and easier to maintain.