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subgrade modulas of foundation

 
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subha_mukherjee
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Joined: 22 Jan 2025
Posts: 4

PostPosted: Thu Jan 23, 2025 5:25 pm    Post subject: subgrade modulas of foundation Reply with quote

Subgrade modulas of foundation can be calculated in two ways:-
1. By Bowels method: 40xFOSx Bearing Capacity of soil
2. IS Code method: Allowable Bearing Capacity/Allowable Settlement.
Which method should we follow?
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JVCSNL
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Joined: 26 Jan 2003
Posts: 173

PostPosted: Fri Jan 24, 2025 3:19 am    Post subject: subgrade modulas of foundation Reply with quote

The subgrade modulus of soil (ks) is a stiffness of soil and can be defines the pressure required for unit settlement.  

The allowable bearing pressure (SBC) can be decided either by settlement criteria or shear criteria.  Hence, unless we know the criteria that governs allowable bearing pressure, we can not make use of bearing pressure for determining the ks value.  

When SBC is governed by settlement criteria with required FOS and permissible settlement value for a type of foundation, ks can be determined by FOS x SBC / allowable settlement.  

For case where SBC is governed by shear criteria, the case where settlement is smaller than allowable, refer to the settlement calculations in soil report to know actual values for defined soil pressure and work out ks value.  

The equation given by Bowels 40 x FOS x SBC ( i think it is in metric unit) appears to be for allowable settlement value of 25mm, which is for isolated foundations and cannot be used for combined or raft foundations where allowable settlements are 40mm and 75mm respectively.

A final word of wisdom would be to consult a competent geotechnical engineer who will help you determine correct value of ks for the type of foundation you are analyzing.    

Best Wishes,

Jignesh V Chokshi
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