Foundations · Footing settlement
Settlement on sand: Schmertmann (CPT) calculator
Strain-influence settlement from CPT qc layers with E = 2.5qc (square) or 3.5qc (strip), depth and creep corrections.
Standard: Schmertmann (1978) strain influence method
Inputs in the worked example
The live calculator starts with these values; change any of them and every result, check and diagram updates instantly.
| Input | Value | Unit |
|---|---|---|
| Footing width | 2.5 | m |
| Shape | Square / circular (z to 2B) | |
| Applied pressure | 180 | kPa |
| Embedment | 1 | m |
| Unit weight | 18 | kN/m³ |
| Time | 10 | years |
| Layers below base: thickness (m), qc (MPa) | 1.0, 4.5 1.5, 6.0 1.5, 8.5 1.0, 10.0 |
Worked example — results
| Result | Value | Unit |
|---|---|---|
| Overburden at base γDf | 18.00 | kPa |
| Net pressure Δq | 162.0 | kPa |
| Depth of peak Iz | 1.25 | m |
| Influence depth | 5.00 | m |
| σ′v at peak depth | 40.50 | kPa |
| Peak Iz = 0.5 + 0.1√(Δq/σ′vp) | 0.700 | |
| Iz at base | 0.100 | |
| C1 = 1 − 0.5 σ′0/Δq (≥ 0.5) | 0.944 | |
| C2 = 1 + 0.2 log(t/0.1) | 1.40 | |
| Settlement C1 C2 Δq Σ(Iz h/E) | 25.21 | mm |
- ✗ Settlement ≤ 25 mm
- ✓ Layers reach influence depth
Method, formulas and assumptions
- Overburden at base γDf = 18*1 = 18 kPa
- Net pressure Δq = 180-18 = 162 kPa
- Depth of peak Iz = IF(1=1,0.5,1)*2.5 = 1.25 m
- Influence depth = IF(1=1,2,4)*2.5 = 5 m
- σ′v at peak depth = 18*(1+1.25) = 40.5 kPa
- Peak Iz = 0.5 + 0.1√(Δq/σ′vp) = 0.5+0.1*SQRT(162/40.5) = 0.7
- Iz at base = IF(1=1,0.1,0.2) = 0.1
- C1 = 1 − 0.5 σ′0/Δq (≥ 0.5) = MAX(0.5,1-0.5*18/162) = 0.94444
- C2 = 1 + 0.2 log(t/0.1) = 1+0.2*LOG10(10/0.1) = 1.4
- Settlement C1 C2 Δq Σ(Iz h/E) = 0.94444*1.4*162*SUM([t.s]) = 25.207 mm
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For preliminary design and education. Check every result against the current standard and your own engineering judgement before use.