BS 1377-7:1990 Soil Type and condition vs STAAD Foundation

With reference to the picture, I am assuming that STAAD.Foundation relates cohesive soil only to undrained condition and cohesionless soil only to drained condition.

Unless there's something I technically misunderstand, according to BS 1377-7:1990, cohesive soil can undergo drained AND undrained tests while cohesionless soil can only undergo drained tests. Accordingly, it is not right to tie the parameters of cohesive soil types only to undrained parameters.

For your reference, in the next image I tabulated from BS 1377-7:1990 the type of test (which is the clause number in the first column) stating the specimen condition in the second column, and which soil type is permitted to be tested for each type of test (third splitted column). Written in pencil is the clause number for reference of the extracted information.

Another question, 'shear strength' is the term that is interpreted by cohesion c and angle of shearing resistance ɸ. That being said, what value am I expected to enter in the shear strength field if cohesionless soil was chosen?

Thanks in advance

  • Some background.

    The terms

    Cohesionless Soil
    Cohesive soil
    Cohesion
    Shear Strength

    are used only for calculating the resistance to uplift forces. If there is a footing under uplift for one or more load cases, and one or more of these terms is switched on, the contribution to the uplift resistance will come from three terms:

    the selfweight of footing and pedestal (if any)
    the weight of soil on the footing
    the frictional resistance along the side faces of the footing if Cohesion or Shear Strength are provided. This resistance is reported in the output under the heading "Effect due to adhesion".

    If you still need the details of how adhesive forces are calculated, we have to get back to you after an investigation.
     



  • In SFA, Cohesive soil takes Cohesion value and Cohesion-less soil takes Shear Strength value. You are right in the statement that Cohesion-less soil can only go to drained shear test (Not un-drained). So, we must modify the statement here.

     

    Now, SFA uses Cohesion value for cohesive soil or shear strength value for cohesion-less soil only for two reasons-

    1. uplift resistance calculation is done when there is a pure uplift and cohesion, or shear strength imparts resistance against that. Otherwise they have no use.
    2. In lateral earth pressure calculation, (For sliding and overturning check) we consider cohesive resistance and resistance due to phi value. Though this feature is yet to come and cohesive resistance for lateral instability would be implemented only in Indian code. Lateral resistance from phi value would be available for all country code from coming release.