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Allowable Bearing Pressure for Soil or Rock
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Type and Condition of Soil or Rock
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Maximum Allowable Bearing Pressure, kPa (psf)
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Dense or compact sand or gravel
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150 (3,130 psf)
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Loose sand or gravel
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50 (1,044 psf)
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Dense or compact silt
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100 (2,088 psf)
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Stiff clay
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150 (3,132 psf)
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Firm Clay
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75 (1,570 psf)
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Soft clay
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40 (835 psf)
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Till
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200 (4,177 psf)
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Clay shale
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300 (6,265 psf)
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Sound rock
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500 (10,442 psf)
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Sand or gravel may be classified by means of a picket test in which a 2"x2" picket bevelled at
the end 45° to a point is pushed into the soil. Such material is classified as "dense or
compact" if a man of average weight cannot push the picket more than 200mm (8") into the soil
and "loose" if the picket penetrates 8" or more.
Clay and silt may be classified as "stiff" if it is difficult to indent by thumb pressure, "firm"
if it can be indented by moderate thumb pressure, "soft" if it can be easily penetrated by thumb
pressure, where this test is carried out on undisturbed soil in the wall of a test pit.