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 73 1001 "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"
- according to Danish standard DS/EN 1997-1 DK NA:2013
- according to Spanish standard CTE DB SE-C
In addition (with exception of Spanish standard CTE DB SE-C) 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 the 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 the soil is then presented in terms of total values of the angle of internal friction φu and the total cohesion cu (these parameters can be considered as the minimal ones). Depending on the degree of consolidation the value of the total angle of internal friction φu ranges from 0 to φef, the total cohesion cu is greater than cef. Owing to the fact that the choice of undrained conditions depends on a number of factors (rate of load, soil permeability, degree of saturation 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.