SPT Bearing Capacity Calculator

Allowable bearing pressure for a spread footing from a borehole log — settlement-controlled in sand, three published Nc methods compared in clay — with the seismic site class from the same data.

BNBC 2020

Profile

Borehole to design from

Footing

ShapeWidthBftFounding depthDftLength units

Method

Soil modelBearing capacity factorcᵤ per blowkkPaFactor of safetySoil unit weightγkN/m³Pressure units

Everything you have entered is in the address bar, so the link reproduces this exact calculation.

q_a = 2.89 ksf · N̄ = 9.9

Net allowable pressure

2.89 ksf

Load on a B×B pad

463.5 kN

Gross allowable

3.47 ksf

Undrained strength

59.4 kPa

Seismic site class

SC (ASCE C) — Dense to medium-dense sand, gravel or stiff clay

N̄ₕ = 15.9 over the upper 50 ft

All three methods

Method	Nc	Net qa
Skempton (1951)	7.00	2.89 ksf
Meyerhof (1963)	6.72	2.78 ksf
Hansen (1970) / Vesić (1973)	7.47	3.09 ksf

Borehole log

depth in ft

Depth	BH-1		BH-2

If the footing were wider

A wider footing reaches deeper soil and spreads its load further, and the two do not move together. Same founding depth, same method.

B (ft)	N̄	qa
3	8.5	2.84 ksf
4	8.9	2.79 ksf
5	9.4	2.83 ksf
6	9.9	2.89 ksf
8	11.1	3.13 ksf
10	12.3	3.39 ksf
12	13.8	3.75 ksf
15	16.2	4.33 ksf

Calculation

1Influence zone below the footing
$z_{top} = D - \tfrac{B}{2}, \quad z_{bot} = D + 2B$
$z = 2.00 \text{ to } 17.00$
$h_{zone}$ 15.00 ft
The band of soil that actually carries the footing. Samples outside it do not enter the average.
2Average blow count through the zone
$\overline{N} = \frac{1}{n}\sum N_i$
$\overline{N} = 9.900 \quad (\text{31 samples})$
$\overline{N}$ 9.9
3Undrained shear strength from blow count
$c_u = k\,\overline{N}$
$c_u = 6 \times 9.900 = 59.4$
$c_u$ 59.4 kPa
Stiff by the Terzaghi and Peck description. A correlation, not a measurement.
4Bearing capacity factor — Skempton (1951)
$N_c = 5\,s_c\,d_c$
$N_c = 7$
$N_c$ 7.000
Factors: shapeFactor = 1.200, depthFactor = 1.167.
5Net allowable bearing pressure
$q_{a,net} = \frac{c_u N_c}{FS}$
$q_{a,net} = \frac{59.4 \times 7}{3} = 2.89$
$q_{a,net}$ 2.89 ksf
6Gross allowable pressure, including the soil replaced
$q_{a,gross} = q_{a,net} + \gamma D$
$q_{a,gross} = 2.89 + 0.57 = 3.47$
$q_{a,gross}$ 3.47 ksf
7Harmonic average blow count for site classificationBNBC Table 6.2.13
$\overline{N}_{h} = \frac{\sum d_i}{\sum d_i / N_i}$
$\overline{N}_{h} = 15.900 \text{ over } 50$
$\overline{N}_h$ 15.9
Harmonic, not arithmetic: one soft stratum should pull the classification down, and averaging the reciprocals is what lets it.

Warnings

The log reaches 50 ft, short of the 30 m the classification is defined over. The class is indicative until the boring is extended.

Notes

The influence zone logs as 84% clay and 16% sand, so the clay method is used.
The three methods differ by 11% on this footing. That spread is the honest measure of how precise the answer is.
Undrained, short-term bearing. Consolidation settlement in clay is a separate check and often governs.
Blow count separates three site classes only. Rock needs a shear-wave velocity, and the softer and special classes turn on layer geometry, plasticity and liquefaction potential.

Assumptions

Field N as recorded, uncorrected. No overburden, energy, rod-length or water-table correction is applied.
The footing bears on the soil described by the log, on a level site, with no adjacent excavation, slope or nearby footing influencing it.
Loading is vertical and concentric. Eccentricity and inclination are not considered.
The influence zone is taken from half a footing width above founding level to two widths below it.

What this calculator does not check

Bearing pressure only. It sizes nothing: no punching shear, no bending, no reinforcement.
Does not check the water table. A table at or above founding level reduces bearing capacity substantially and is not modelled.
In sand the result is settlement-controlled, not a shear-failure capacity, and the two are different checks.
In clay the result is undrained and short-term. Long-term consolidation settlement is a separate calculation and frequently governs.
Correlating undrained shear strength to N is unreliable in stiff and hard clay, and is no substitute for a laboratory or in-situ strength test.
The site class uses blow count alone, which separates three classes only — it cannot identify rock, soft soil, or a site needing special study.

Results are for preliminary sizing and educational use. They are not a design and must be reviewed, verified and sealed by a licensed or registered engineer before being used in construction.

spt-bearing-capacity v1.0.0 · BNBC 2020 · input 288fdd99e640bca8