Subgrade Preparation and Soil Assessment
What the candidate must know
Subgrade preparation is the foundation of every mud floor, and questions in this domain test whether you can judge a soil before any material is placed. Key concepts: soil classification (granular vs. cohesive), bearing capacity, moisture content versus optimum moisture content (OMC), compaction in lifts, proof-rolling, and removal of unsuitable material (organic silt, topsoil, root matter, soft spots). Understand that a mud floor transfers load to the subgrade, so any yielding, pumping, or frost-susceptible layer must be corrected before placement.
Traps
Questions often present a subgrade that "looks firm" but has high organic content or is above OMC - both reduce achievable density. Another trap is confusing compaction effort with moisture: adding passes cannot fix a soil that is too wet or too dry. Watch for items asking what to do with a localized soft spot - the answer is excavate and replace or stabilize, not simply roll harder. Also be ready to distinguish proof-rolling (a surface check for yielding) from density testing (a quantitative check).
Worked example / rule of thumb
Rule of thumb: compact granular subgrade in lifts no deeper than about 200 mm (8 in) per lift, and bring the soil to within roughly ±2% of OMC before rolling. Example: a 600 mm fill is placed as three 200 mm lifts, each moisture-conditioned and compacted, then proof-rolled. If a lift fails a density check, rework that lift - do not add the next lift on top and hope the average passes.