Bearing capacity on undrained subsoil

One of the following approaches is available to assess the bearing capacity of a foundation if undrained conditions are assumed:

- standard analysis

- according to CSN 731001 "Základová pùda pod plošnými základy" approved 8.6. 1987

- according to Indian standard IS:6403-1981 "Code of Practice for Determination of Bearing Capacity of Shallow Foundations" from year 1981

- according to EC 7-1 (EN 1997-1:2003) "Design of geotechnical structures – Part 1: General rules"

In addition the coefficients due to Brinch – Hansen are used to account for inclined footing bottom (see standard analysis).

In case of undrained conditions it is assumed that during construction the spread footing undergoes an instantaneous settlement accompanied by shear deformations of soil in absence of volumetric changes. When the structure is completed the soil experiences both primary and secondary consolidation accompanied by volumetric changes. The influence of neutral stress appears in the reduction of soil strength. The strength of soil is then presented in terms of total values of the angle of internal friction φ_u and the total cohesion c_u (these parameters can be considered as the minimal ones). Depending on the degree of consolidation the value of the total angle of internal friction φ_uranges from 0 to φ_ef, the total cohesion c_u is greater than c_ef.  Owing to the fact that the choice of undrained conditions depends on a number of factors (rate of loading, soil permeability, degree of saturations and degree of overconsolidation) it is the designer's responsibility to decide, depending on the actual problem being solved, if the effective parameters should be used. Nevertheless, the total parameters are generally used for fine-grained soil.