What is trench fill foundation?

Trench fill is a foundation method where the entire trench is filled with concrete to within 150mm of ground level. Unlike strip foundations (where only the base is concrete, with blockwork above), trench fill uses more concrete but eliminates the need for below-ground blockwork. It's faster and requires less skilled labour.

How do I calculate trench fill concrete?

Trench fill concrete volume = Total Length x Depth x Width. For example, a 30m trench at 1m deep and 600mm wide = 30 x 1.0 x 0.6 = 18 m³. Add 10% for over-dig and wastage = 19.8 m³. Use C25 (GEN 3) or C30 (ST 4) concrete depending on soil conditions and structural requirements.

Is trench fill cheaper than strip foundations?

Trench fill uses more concrete but saves on blockwork materials and labour. For shallow trenches (under 600mm), strip foundations are usually cheaper. For deeper trenches (1m+), trench fill becomes more cost-effective because the labour saving on below-ground blockwork outweighs the extra concrete cost. This calculator compares both options.

How wide should trench fill be?

The minimum trench width for trench fill is typically 450mm for single-leaf walls or 600mm for cavity walls. The width must be at least 150mm wider than the wall it supports. In practice, the trench width is often determined by the digger bucket size — 450mm or 600mm being standard mini-digger bucket widths.

What concrete grade for trench fill?

Standard trench fill uses C25 (GEN 3 / ST 2) concrete. In sulphate-bearing soils, sulphate-resistant concrete (DC-2 or higher) is required. In contaminated ground, use designated concrete to BS 8500-1. The building control officer or structural engineer will specify the required grade based on a soil analysis report.

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Trench Fill Calculator — Concrete Volume & Cost Comparison

Calculate trench fill concrete volume and compare costs with traditional strip foundation alternatives.

Total Trench Length

Total perimeter of foundations

Trench Depth

To suitable bearing stratum

Trench Width

600mm for cavity walls

Compare With

Concrete Price

Block Price

For strip foundation comparison

How We Calculate This

This calculator determines trench fill concrete volume and compares the cost with a traditional strip foundation approach.

Trench fill

Concrete (m³) = Length x Depth x Width x (1 + Wastage%)
Full-depth concrete to within 150mm of ground level.

Strip comparison

Strip Concrete = Length x Strip Depth (225mm) x Width
Plus blockwork from strip top to ground level (10 blocks/m²).

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.