Keep excavation dimensions separate from the finished structure

A footing's concrete dimensions and its excavation dimensions are not always identical. The dig may include working space or specified side slopes. Estimate the excavation from the approved excavation shape rather than assuming the finished structure fills it completely.

Start with a rectangular example

A straight, vertical-sided trench 12 m long, 0.6 m wide and 0.8 m deep contains 12 × 0.6 × 0.8 = 5.76 m³ in place. A drawing showing a narrower footing does not reduce that excavated volume if the trench itself is still 0.6 m wide.

Handle changes in depth

If a level-width trench deepens linearly from 0.6 to 1.0 m, its average depth is 0.8 m, so the same volume applies. Where there are steps, pits, junctions or irregular changes, split the length into sections and sum their volumes. Deduct overlaps at intersecting trenches once.

Keep slope measurements meaningful

A 0.3 m level difference over a 6 m horizontal run is 0.3 ÷ 6 × 100 = 5%. Use horizontal distance for this gradient calculation. A ground slope does not define a safe excavation batter or drainage design.

Record the exclusions

Services, temporary works, groundwater handling and safe excavation methods are separate planning matters. Confirm service locations and the excavation design before work. Keep the measured in-place volume distinct from the loose volume that will later be transported.

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