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Ultimate bearing pressure for footings on rock subgrade

 
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thomaskok
SEFI Member
SEFI Member


Joined: 23 Oct 2018
Posts: 1

PostPosted: Tue Oct 23, 2018 3:30 am    Post subject: Ultimate bearing pressure for footings on rock subgrade Reply with quote

Dear sirs,

Thank you for accepting me as a member to this forum.
I am a building structure designer from Malaysia.


  • I am interested in knowing the common values for allowable bearing pressure for isolated footing founded directly on surface rock subgrade, relative to the rock compressive strength (unconfined test). This is in absence of more defined methods such as shear wave tests. Is it common practice to take the allowable bearing pressure of rock subgrade, as a percentage of laboratory compressive test. Some RQD (rock quality designation) values from borehole explorations are available.

In our country most isolated pad footings are founded on soil. For low rise buildings, the ultimate bearing pressure is usually determined from a Mackintosh probe test. Typically, blow count per foot penetration from the probe test will correlate to an ultimate bearing pressure from an empirical correlative graph. Subsequently, a safety factor is applied to this value (typically 2.0) to yield an allowable working pressure.

We seldom have isolated footings founded directly on a bedrock subgrade.
For this project I am currently doing, I have obtained compressive strength values of the siltstone bedrock, ranging from 30MPa ~100MPa.

I would like to ask for the opinions of dear sirs, regarding to my first question above. Much appreciated.

Thank you.

Kind regards.
Thomas Kok,
Building structures
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