Allowable Bearing Pressure(ABP) in Well Foundation

Hi, this post is the continuation of the lecture notes for the subject "Advanced Foundation Engineering".Read the last post: Settlement of the Pile GroupIn this post, I am going to lay down the criteria for finding out the Allowable Bearing Pressure in Well Foundations.

For well foundations, the allowable bearing pressure can be estimated by satisfying the 2 criterias:

1. Shear Failure criteria
2. Settlement criteria

As per Indian Standard Code the allowable bearing pressure can be found by the following equations:
    
 Q = 5.4.N^2.B + 16(100+N^2).Df
           
Here, Q = Allowable bearing pressure in kg/cm^2
          N = Corrected N value from SPT.
          B = Smaller dimension of the well cross section
          D = Depth of foundation below the scour level.

Points to be remembered:

• In case of cohesive soils, undisturbed samples need to be taken to determine the shear and consolidation characteristics of deposit.
• The settlement of the well foundation can be obtained by Terzaghi's theory of one dimensional consolidation.
• If the bed is resting into the hard rock then it can be obtained by crushing strength or UCS of the rock core samples taken from the field and accordingly the various corrections for fissures, joints, presence of water table, dip and strike & presence of fines in the joints etc. can be obtained from the field and  the corrections can be applied accordingly. The RMR values from the ABP values can be obtained using IS: 17270.   IRC: 78 says, normally ABP exceeding 2 MPa shall not be adopted.

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