How to Compact Subgrade and Base

How to properly prepare and compact subgrade and aggregate base to achieve specified density for slabs, pavements, and foundations.

Step-by-Step

1. 1

   Test existing subgrade

   Proof-roll the subgrade with a loaded tandem dump truck. Any area that deflects more than 1 inch under wheel load is soft and must be overexcavated and replaced. Mark soft spots with paint before the truck finishes the pass.

2. 2

   Remove soft material

   Excavate soft, organic, or wet material to undisturbed soil. If the over-excavation is deep, fill with engineered aggregate and compact in lifts. Get geotechnical engineer approval before proceeding if conditions are unexpected.

3. 3

   Condition the moisture

   Soil compacts best at optimum moisture content (from the Proctor test). If the soil is too dry, add water with a water truck and allow time to absorb. If too wet, scarify and dry before compacting. Never try to compact saturated soil.

4. 4

   Compact in lifts

   Place fill in loose lifts of 6–8 inches for cohesive soil or up to 12 inches for granular material. Compact each lift before placing the next. Running a roller over a 24-inch lift does not give you the same result as two 12-inch compacted lifts.

5. 5

   Make overlapping passes

   Overlap each compactor pass by 6 inches. Make at least 4–6 passes on each lift with a vibratory roller. Check for visible heaving on adjacent passes — that means the soil is saturated and cannot be compacted.

6. 6

   Test density

   Test compaction with a nuclear density gauge after each lift. The spec is typically 95% of maximum dry density per standard Proctor, or 98–100% for road base. Failing tests mean more passes or moisture adjustment — not moving on.

7. 7

   Place and compact aggregate base

   Place aggregate base in lifts per spec (typically 4–6 inch lifts). Compact with a vibratory roller to 98% Proctor. Shape the base to finished grade before paving or forming.