# Underpinning Calculator Concrete and rebar for underpinning beams in one go. - Page: https://jianzhujisuan.com/en/underpinning - Category: Construction (https://jianzhujisuan.com/en/c/construction) - Chinese version: https://jianzhujisuan.com/zh/underpinning.md - Data checked: 2026-10-25 ## Quick result With the default inputs (Beam length per segment 3 m, Beam width 0.5 m, Beam depth 0.7 m, Number of segments 6 strips): **Concrete 6.30 m³** ## Inputs | Parameter | Type | Default | Unit | Note | |---|---|---|---|---| | Beam length per segment | number | 3 | m | Excavated in alternate bays, usually 1-2.5 m each | | Beam width | number | 0.5 | m | | | Beam depth | number | 0.7 | m | | | Number of segments | number | 6 | strips | | ## Formula - **Concrete per segment**: V₁ = L·B·H - **Total concrete**: V = V₁·segments - **Rebar**: W = V·110 (kg) ## Worked example 3 m × 0.5 m × 0.7 m × 6 segments: 1.05 m³ per segment, 6.30 m³ total, ~693 kg rebar. ### Quantities by number of segments (3.0 × 0.5 × 0.7 m needle beam, 110 kg/m³) | Segments | Concrete (m³) | Rebar (kg) | Per segment (m³) | Where it fits | |---|---|---|---|---| | 4 | 4.20 | 462 | 1.05 | Short wall, one-sided underpinning | | 6 | 6.30 | 693 | 1.05 | This tool's default - a typical run of wall | | 8 | 8.40 | 924 | 1.05 | Longer wall with more open segments | | 10 | 10.50 | 1155 | 1.05 | Long wall, or where the segment has to be shorter | ### Choosing the underpinning method - a needle beam is not the only answer | Method | Fits | Advantage | Main risk | |---|---|---|---| | Needle beam in open segments | Brick or block strip footings, low-rise | Cheap, simple, done in place | Wrong sequence removes the support | | Pile underpinning (pit or pier) | Insufficient capacity, load must reach good ground | Big gain in capacity | Deep pits that need their own support | | Widening the footing | Footing too narrow, ground still sound | Keeps the underside level | The old-to-new concrete interface can debond if not prepared | | Grouting (compaction / fracture) | Sand and gravel, settlement already under way | No work to the superstructure, fast | Variable results - trial and testing are essential | ## Questions ### Why must underpinning be done in alternate bays? Because the existing footing continuously carries the structure above. Undermine it all at once and the support vanishes - walls and columns lose bearing, cracking or even collapsing locally. Alternate bays keep most of the footing supported at all times; once a new bay is cast and cured it takes over, and the next bay is excavated. This is the crux of underpinning, not an optional sequencing choice. ### When is underpinning needed? Two cases: a nearby excavation (deep pit, metro, utilities) destabilises or de-rates the existing footing, or the building shows settlement cracks from an under-strength footing. The trigger is the zone of influence - once a nearby excavation goes below the existing footing level, underpinning or support should be assessed, not left until cracks appear. ### Why must underpinning be done in alternate open segments? The needle beam has to take over from the piece of footing you have removed. Open the whole run at once and the wall above loses every support point: a brick wall cracks immediately and, in the bad case, collapses. It is the most common and most lethal underpinning failure. The sequence is alternate bays: dig 1, cast 1, wait for strength, then dig 3 and cast 3, and only come back to 2 and 4 when the rest is done. Segment length follows the integrity of the wall and the load, typically 1.5-3.0 m. ### Where does the 110 kg/m³ reinforcement figure come from - can I quote on it? It is an order-of-magnitude figure for needle beams, not a code value. The beam is shallow and short but the reinforcement ratio runs high, usually governed by detailing rather than by bending, because the depth is limited by the footing. Real values scatter between 90 and 140 kg/m³ depending on depth, span, seismic detailing and whether it carries a concentrated wall load. Use 110 for a budget; price the settlement off the reinforcement drawing. This tool estimates only and does not replace structural design. ### Underpinning or grouting - how do I choose? Ask whether the load has to be carried down. A needle beam is a structural member: it takes the load around the removed or inadequate piece of footing and passes it to either side or to better ground, and it solves a capacity problem cleanly. Grouting improves the soil - it raises strength and stiffness and reduces settlement - but it is an areal improvement, not a load path, and its results are variable and hard to verify. So: footing too narrow or too shallow means underpinning or widening; loose ground with settlement already under way means grouting; both means pile underpinning, taking the load straight to the deeper stratum. ### What should be monitored during underpinning? Four things, none optional. Settlement points on the wall being underpinned and on both sides, read daily, and stop work for analysis at 5 mm of differential. Crack monitoring on the existing wall, with tell-tales or gauges, to see whether old cracks are moving. Settlement of the adjacent footing, because underpinning redistributes load and the neighbour can go down instead. And observation of the open face for soil boiling or seepage. The readings must be taken and reviewed the same day - next-day data is useless, because underpinning failures happen within hours. ## Related tools - [Plaster and Render Calculator](https://jianzhujisuan.com/en/plaster-coat): Mortar, cement and sand by layer for internal plastering. - [Mortar Mix Design Calculator](https://jianzhujisuan.com/en/mortar-strength): Trial strength, cement, water and sand, and the mix by mass. - [Labour Days and Crew Calculator](https://jianzhujisuan.com/en/labor-productivity): Man-days, crew size and labour cost from output rates. - [Line-of-Balance Duration Calculator](https://jianzhujisuan.com/en/schedule-flow): Rhythmic flow duration with overlap and break adjustments. - [Material Storage Area Calculator](https://jianzhujisuan.com/en/material-storage): Net footprint, total area and a suggested bay size. - [Dewatering Calculator](https://jianzhujisuan.com/en/dewatering): Pit inflow by the equivalent-well method, then well count. --- Generated by BuildCalc. All calculations run in your browser; no data is uploaded. Formulas follow national standards and common trade practice; local codes govern.