What is a soil swell factor?

The swell factor accounts for the increase in volume when soil is excavated and loosened. In-situ soil is compacted; once dug out, it occupies more space. Clay swells by about 25% (factor 1.25), sand by 10% (1.10), and rock by up to 40% (1.40). This is critical for calculating lorry loads and disposal quantities — you need to transport the swelled volume, not the in-situ volume.

How many lorry loads for a typical house foundation excavation?

A typical strip foundation for a 3-bed semi (50m² footprint, 1m deep trench, 600mm wide) excavates approximately 15-20 m³ of soil in-situ. With a swell factor of 1.25 for clay, this becomes 19-25 m³ (30-40 tonnes). Using 8-wheel tippers (20t capacity), you need 2-3 lorry loads. A full basement (50m² × 3m deep) produces 150m³ or more.

What is working allowance in excavation?

Working allowance (or working space) is the extra width excavated beyond the foundation or structure dimensions to allow room for formwork, waterproofing, drainage, and safe working. Building Regulations Approved Document A recommends a minimum of 150mm, but 300-600mm is typical practice. This adds significantly to the total excavation volume.

Do I need to consider batter angles for deep excavation?

Yes, for excavations deeper than 1.2m, the sides should be battered (sloped) or supported with shoring to prevent collapse. Safe batter angles depend on soil type: 45 degrees for clay, 30-35 degrees for sand, and steeper for rock. This significantly increases the volume of soil excavated. The trenching and excavation must comply with the CDM Regulations 2015.

How much does soil disposal cost in the UK?

Clean (inert) soil disposal costs £10-£30 per tonne at a licensed tip. Contaminated soil costs significantly more — £50-£150+ per tonne depending on contamination type and level. Landfill tax applies at the lower rate (currently around £3.25/tonne for inert waste). Haulage costs are additional. Consider reuse on site where possible to reduce disposal costs.

TradeCalculator

Excavation Calculator

Calculate cut volume from plan area and depth, apply soil swell factors, and determine lorry loads and disposal costs for UK excavation projects.

Plan Length

Length of the excavation area

Plan Width

Width of the excavation area

Depth

Depth of excavation from ground level

Soil Type

Affects swell factor and density

Lorry Type

Vehicle for spoil removal

How We Calculate This

This calculator determines excavation volumes, adjusts for soil swell, and calculates the number of lorry loads needed to remove spoil from site. It accounts for working allowance and optional batter angles.

Calculation method

Cut volume: (Plan length + 2 × working allowance) × (plan width + 2 × working allowance) × depth. For battered excavations, the prismoidal formula accounts for the sloped sides.

Swell factor: The in-situ volume is multiplied by the swell factor for the soil type. Clay = 1.25, sand = 1.10, chalk = 1.20, rock = 1.40.

Weight: In-situ volume × soil density (clay = 1.8 t/m³, sand = 1.7 t/m³). Weight is used for lorry load calculations alongside swelled volume.

Lorry loads: The constraining factor (weight or volume) determines the number of loads. Lorry payload and body volume are both checked.

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.