What this calculator does
Total settlement is a number that gets checked once. Differential settlement is what actually cracks buildings, and it is invisible in a total. A structure can settle 150 mm uniformly and be fine, or 25 mm differentially across a window and be unusable. This tool works in the terms that matter for that distinction: total, differential, and the angular distortion that governs cracking.
How to use it
- Enter your reference settlement and the net pressure it applies at. This pair is the anchor for the whole profile, and it should be your own measured or calculated value.
- Add one point per footing: its label, its bearing pressure, and its distance from the first point along the row.
- Distances must increase along the profile, because a settlement profile is read along a line and a reversed interval has no meaning for a distortion ratio.
- Read the worst local pair, not the end-to-end average. A profile whose ends settle identically but whose middle dips has a near-zero average distortion and a serious local one.
Governing method
s_i = (σ′_i / σ′_ref) × s_ref → Δ/L = |s_j − s_i| / L Where:
-
= Settlement at point i, mm -
= Net bearing pressure at point i -
= Net pressure at the reference settlement -
= Horizontal distance between the pair
Derived Equations:
Δ/L is dimensionless worst pair = argmax over adjacent points Assumptions & limitations
- The reference settlement, the reference pressure, every bearing pressure, every spacing and both acceptance limits are your inputs. This tool holds no soil database, no pressure-settlement curve and no code, and looks nothing up.
- Settlement is taken as linear in the stress ratio. Real soil is not linear, especially near failure, so this is a screening relation and not a substitute for a site-specific analysis.
- This is a serviceability check for cracking and distortion. It does not check ultimate bearing capacity — that failure mode is different and is not what angular distortion measures.
- Foundation interaction is not modelled. Adjacent footings influence each other, and each pressure bulb here is treated as independent of its neighbours.
- Construction sequence, soil consolidation over time, groundwater drawdown and adjacent excavation are all excluded, and each of those can change the differential after the building is complete.
Worked example — three footings, the ends equal and the middle loaded
| Reference settlement | 20 mm at 100 kPa |
|---|---|
| Point A | 100 kPa at 0 m |
| Point B | 150 kPa at 4 m |
| Point C | 100 kPa at 8 m |
| Acceptable distortion | 1/300 ≈ 0.00333 |
Settlements come out at 20, 30 and 20 mm. The two adjacent steps each give 10 mm over 4 m, which is an angular distortion of 0.0025 — inside 1/300. But the end-to-end difference from A to C is zero, because both ends settle identically. An end-to-end check would report perfect uniformity; the local step is what actually matters, and that is the pair this tool names.