Excavation, Backfill and Surplus Soil Calculation for Foundations

Excavation and backfill look simple until handover day, when the soil left on site turns out to be more or less than expected. The cause is usually that working space or the swell factor was left out of the calculation. Here is the structured way.

Excavation volume

You do not excavate to the exact footing size. You also need:

• Working space around the footing (commonly 0.5 m each side, or as specified) for formwork and waterproofing.
• Side slopes if the soil will not stand vertically or the depth is large, and shoring may be needed according to the soil report.
• Depth that includes blinding concrete and any soil replacement.

Example with vertical sides: footing 2.0 × 2.0 m, working space 0.5 m each side, depth 1.8 m:

(2.0 + 1.0) × (2.0 + 1.0) × 1.8 = 16.2 m³ per pit

For ten pits: 162 m³ (bank volume).

Backfill volume

Backfill = excavation volume − the volume occupied by concrete below backfill level (footing, column neck and blinding).

• Footing: 2.0 × 2.0 × 0.6 = 2.4 m³
• Column neck 0.4 × 0.4 × 1.2 = 0.19 m³
• About 2.59 m³ per footing, 25.9 m³ for ten
• Backfill = 162 − 25.9 ≈ 136 m³ compacted

Swell and compaction

Excavated soil increases in volume (swell) and decreases when compacted. Swell is roughly 10% to 15% for sand and can exceed 30% for clay, but rely on the soil report and local experience. When counting truck loads, use the loose volume, not the bank volume.

Surplus soil

Surplus (hauled off site) = excavation volume − required backfill volume, after considering whether the soil is suitable for backfill. If the backfill must be clean sand instead of excavated soil, all the excavated soil is surplus and you have to supply the fill material.

In the example: 162 m³ excavated in bank volume and 136 m³ compacted needed from suitable soil leaves about 26 m³ surplus in bank volume (more when loose, because of swell) if the soil is suitable for reuse.

Common mistakes

• Excavating to the footing dimensions without working space.
• Forgetting to deduct the concrete volume from backfill.
• Counting haulage trucks on bank volume.
• Ignoring the groundwater level and dewatering cost.

Ready tools

Enter the footing dimensions in the excavation and backfill sheet to get the volumes and surplus soil. Concrete volumes come from the concrete quantity sheet, then record the items in the bill of quantities. Numbers here are illustrative, and the soil report is the reference.