How does mass concrete underpinning work?

Mass concrete underpinning involves excavating beneath the existing foundation in alternating sections (pins), filling each with concrete, and allowing it to cure before excavating adjacent sections. This 'hit and miss' sequence ensures the wall is always supported. Each pin typically 1m wide, excavated to 1-1.5m below existing foundation level, and filled with C25/30 concrete.

What is the correct sequence for underpinning?

Underpinning must follow a strict sequence: pins are numbered and excavated in a pattern that never undermines adjacent unsupported sections. Typically, alternate pins are done first (1, 3, 5...), allowed to cure for 3-7 days, then the intermediate pins (2, 4, 6...) are completed. Corner pins are usually done last. A structural engineer must specify the exact sequence.

How much does basement underpinning cost per metre?

Traditional mass concrete underpinning costs approximately £800-£1,500 per linear metre of wall, depending on depth, ground conditions, and access. This includes excavation, concrete, formwork, and backfill. Mini-piling can cost £1,200-£2,500 per metre but is faster and suitable for restricted access. London prices are typically 20-30% higher.

How long does underpinning take to cure?

Mass concrete underpinning pins must achieve adequate strength before adjacent pins can be excavated. Typically, 3-7 days curing time is required between sequences, using C25/30 or C30/37 concrete. In cold weather, curing takes longer and may need frost protection. The structural engineer will specify minimum curing periods based on concrete strength development.

Do I need a party wall agreement for basement underpinning?

Yes, if you are underpinning within 3 metres of a neighbour's foundation (or 6 metres if excavating below their foundation line), you must serve a Party Wall Notice under the Party Wall etc. Act 1996. This applies even if you are not touching the party wall itself. Allow 2-3 months for the party wall process before work can begin.

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Basement Underpinning Calculator

Calculate underpinning pin sequence, mass concrete volume per pin, total pins from wall perimeter, and sequential working schedule for basement lowering.

Wall Perimeter

Total length of walls to underpin

Pin Width

Width of each underpinning section (typical 1.0m)

Pin Depth

Depth below existing foundation

Pin Thickness

Thickness matching existing wall

How We Calculate This

This calculator determines the number of underpinning pins, concrete quantities, and working sequence for mass concrete underpinning to lower a basement floor. The calculation follows standard UK practice per BS 8004 (Code of Practice for Foundations).

Pin calculation

Pin count: Wall perimeter ÷ (pin width + gap between pins). Pins are typically 1.0m wide with 1.0m gaps, giving 2.0m centres.

Concrete per pin: Pin width × depth below existing foundation × wall thickness. Each pin is a rectangular block of mass concrete.

Sequence planning

Pins are divided into sequences (typically 3) to ensure the wall is never left unsupported for more than one pin width. Each sequence requires excavation, concreting, and curing time before the next sequence can begin.

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.