Expansion Index % 0-20 21-50 51-90 91-130 130+
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Plasticity Index % 0-10 10-15 15-25 25-35 35-100
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Expansion Potential Very Low Low Medium High VeryHigh
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From Robert W. Day - Geotechnical Engineer's Portable Handbook, 2000, McGraw-Hill Companies Inc. Table 12.2.
These values are a general guide. Individual sites may have varying clay minerals and silt contents that will vary from these values. These values assume typical distributions of kaolinite vs. montmorillonite in clay deposits.
Your best bet is to test for Expansion Index (ASTM), and recognize the limitations of laboratory results with respect to field behavior, like nearly all geotechnical testing.
Very nice. Thank you very much. The design will be based on an EI developed by a geotech firm. Here in the Cnetral Coast of CA. local professionals use a prescriptive design method (bolied down from the UBC and experience) for slabs on grade that is less rigerous analytically than UBC 1815 wire reinforcing institute method, but has been proven over many years, produces lighter designs that work fine in practice. For heavier buildings the more rigerous wire reinforcing inst. method should be used. That method relys on the weighted PI, but local soils companies typically only develop EI's....This correlation helps a lot. Thanks again...
