What is the recommended Factor of Safety for bearing capacity?

A minimum factor of safety of 2.5 is recommended, though a value of 3 is commonly used in practice (Clause 5.2).

How does the water table affect bearing capacity?

It reduces bearing capacity by reducing the effective unit weight of the soil in the terms involving width (B) and surcharge (q). Clause 6 provides the water table correction factors (W' and W).

Where do I find the bearing capacity factors Nc, Nq, and Nγ?

They are provided in Table 1 of the code, corresponding to the soil's angle of shearing resistance (φ).

When should local shear failure be considered?

For loose sands and soft clays. For this case, modified shear strength parameters c' = (2/3)c and tan φ' = (2/3)tan φ should be used in the general bearing capacity equation (Clause 5.1.1.2).

IS 6403

: 1981

Code of practice for determination of bearing capacity of shallow foundations

CurrentEssentialCode of PracticeBIMGeotechnical · Soil and Foundation

Download PDF BIS Portal Google

This code provides a methodology for calculating the ultimate bearing capacity of soils under shallow foundations based on shear failure criteria. It covers both general and local shear failure conditions and includes modifying factors for foundation shape, depth, load inclination, and the effect of the water table. The code also gives guidance on estimating bearing capacity from in-situ tests like SPT and plate load tests.

Provides methods and guidelines for calculating the ultimate and safe bearing capacity of shallow foundations on various types of soils.

Quick Reference Values

Minimum Factor of Safety for bearing capacity2.5

Permissible total settlement for isolated foundations on sand50 mm

Permissible total settlement for isolated foundations on clay75 mm

Permissible total settlement for rafts on sand75 mm

Permissible total settlement for rafts on clay100 mm

Key Formulas

q_net_ultimate = c Nc sc dc ic + q (Nq-1) sq dq iq + 0.5 B γ Nγ sγ dγ iγ — Net ultimate bearing capacity for general shear failure

Practical Notes

The position of the water table is critical; its effect on the soil's unit weight in the second and third terms of the bearing capacity equation must be carefully calculated.

A distinction must be made between general shear failure (for dense/stiff soils) and local shear failure (for loose/soft soils), as the latter requires modifying the soil's shear strength parameters (c and φ).

The calculated safe bearing capacity (after applying a factor of safety) must be cross-checked against the allowable bearing pressure based on settlement criteria, and the lower of the two values should be adopted for design.