Earthmoving, Grading, Trenching, and Compaction Techniques
What the exam tests
These questions test whether you can plan a cut-to-fill operation, hold grade, install a trench to specification, and compact to a density specification, and explain your reasoning.
Core rules and concepts
- Mass balance: compare cut volume with fill volume and account for swell (loose volume exceeds bank volume, commonly 20 to 30% in granular soils) and for shrinkage once compacted. Haul distance and cycle time set production: cycle time is load plus haul plus dump plus return plus spot.
- Grade control: work from the benchmark and the plan, not from the last pass. Set hubs or offset stakes outside the work area, use a laser or grade checker, and check grade behind the blade regularly.
- Lift thickness: match the compactor. Typically 150 to 200 mm loose for cohesive soil under a padfoot, 200 to 300 mm for granular under a smooth drum. Thick lifts leave soft layers that fail proof rolling.
- Moisture: compact near optimum moisture (Standard Proctor). Too dry, add water and mix; too wet, aerate or scarify and dry. Typical specifications are 95% of Standard Proctor for subgrade and 98 to 100% for base.
- Compaction: several passes with overlap, backfill both sides of a pipe evenly to avoid lateral shift, and proof-roll the finished surface to find soft spots.
- Trenching: width is pipe outside diameter plus working room (often 300 to 600 mm); spoil at least 1 m back from the edge; slope or shore as the regulation requires; keep ladder access within the distance your jurisdiction allows.
Worked example
A 2.5 m deep fill must be built in 250 mm loose lifts that compact to 200 mm. Reasoning: 2,500 mm divided by 200 mm is 12.5, so plan 13 lifts and confirm the finished elevation by survey, not by counting buckets. If a lift fails density, rework that lift; do not cover it with the next one.
Traps that cost marks
- Confusing loose lift thickness with compacted thickness.
- Backfilling one side of a pipe first.
- Compacting frozen soil or frozen clods.
- Grading to the stakes without checking the benchmark.
- Cutting trench walls back to a safe-looking slope with no soil classification.