Excavation Calculator

The swell (or bulking) 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. As a rule of thumb clay swells by about 25% (factor 1.25), sand/gravel by around 10% (1.10), and rock by up to 40% (1.40). These are typical values only — published earthworks ranges are wide (sand/gravel ~10-20%, clay ~20-40%, chalk ~30-80%, topsoil ~5-50%, blasted rock ~40-70%) and the actual figure depends heavily on moisture, compaction and dig method, so confirm against site conditions. Swell matters because you transport and dispose of the swelled volume, not the in-situ volume.

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.

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. There is no fixed Building Regulations figure for general working space — the 150mm often quoted comes from Approved Document A in the context of minimum foundation thickness and the working space needed either side of reinforcement, not a mandated excavation allowance. In practice 300-600mm each side is typical, set by the depth, the trade following on (e.g. external waterproofing or drainage) and temporary-works/CDM safe-working requirements. This adds significantly to the total excavation volume.

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.

In 2026, clean (inert) soil disposal typically costs about £25-£45 per tonne as a gate fee at a licensed tip, with London and the South East often higher. That gate fee already includes Landfill Tax, which from 1 April 2026 is charged on inert waste at the lower rate of £8.65/tonne (more than double the previous £4.05) — so gate fees sit well above the tax itself. Contaminated soil costs significantly more (£50-£150+ per tonne, depending on contamination type and level, plus the £130.75/tonne standard landfill-tax rate where applicable). Haulage is additional. Always get a current quote from your tip, and reuse spoil on site where possible to cut disposal costs.