Methods of determining bearing capacity

Bearing capacity of soil for design is determined by the following methods.

1. Presumptive values from codes

Safe bearing capacity (SBC) is read from tables in IS 1904 (also used in Nepal): for example, hard sound rock about 3240 kN/m², soft rock 440, gravel 440, medium sand 245, medium clay 245, soft clay 100 kN/m². These values are used for small, light buildings.

2. Analytical (theoretical) methods

These use the soil properties $c$, $\phi$ and $\gamma$ from lab tests. Terzaghi's equation for a strip footing:

$$q_u = cN_c + \gamma D_f N_q + 0.5\gamma B N_\gamma$$

and $q_{safe} = q_u / FS$. Other methods are those of Meyerhof, Hansen, and Skempton (for clay).

3. Plate load test

A steel plate (300 to 750 mm square) is loaded in stages in a pit at the foundation level, with the settlement measured by dial gauges, until failure or a settlement of 25 mm; a load-settlement curve is then plotted. It gives a direct field estimate, but tests only a shallow depth (about 2 times the plate width).

   Load frame / kentledge
       |  jack  |
   ====[ plate ]====  ground
    dial gauges

4. Standard Penetration Test (SPT)

A split spoon sampler is driven 450 mm by a 63.5 kg hammer falling 750 mm; the number of blows for the last 300 mm is N. SBC is correlated with N (Terzaghi and Peck charts, Meyerhof).

5. Static / dynamic cone penetration test (SCPT, DCPT)

A cone is pushed or driven into the soil, and the resistance gives its strength; this is good for a continuous profile.

6. Other

Laboratory tests on undisturbed samples (triaxial, direct shear, unconfined compression), pile load test, and geophysical methods (seismic refraction, resistivity).