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Synthesizing Bulk Density for Soils with Abundant Rock Fragments

Dep. of Geoscience, Univ. of Arizona, Tucson, AZ, 85721

Oliver A. Chadwick

Jet Propulsion Lab., California Inst. of Technology, 4800 Oak Grove Dr., Pasadena, CA 91109

*Corresponding author.

ABSTRACT

Bulk density is a fundamental soil property that is difficult to determine for gravelly to extremely gravelly soils because results vary significantly with sample volume. For such coarse soils, the representative volume (for whole-soil bulk density) should be large, but guidelines for selecting an appropriate sample volume do not exist. We evaluated the representative volume for a soil with abundant rock fragments, by comparing measured properties of samples ranging in volume from 0.03 to 410 L. For whole-soil bulk density determination, the representative volume is 4 L or larger for a soil horizon containing 34% (v/v) gravel and is between 5 and 50 L for a soil horizon containing 54% gravel. Intact samples of that size are prohibitively large, so we developed an alternative approach that starts with measurement of fine-earth bulk density. For fine-earth bulk density, the sample volume needed for representative results is between 0.2 and 1 L for gravelly to extremely gravelly soils. The alternative approach reliably synthesizes whole-soil bulk density using (i) fine-earth bulk density from modest-sized samples, (ii) mass-size distribution from large (>40 kg) representative disturbed samples, and (iii) rock fragment bulk densities. The mass and volume of rock fragments that should be in a sample are added to the mass and volume used to calculate fine-earth bulk density. This method allows integration of lateral variability in the soil without the consequence of averaging properties across a large depth range.

Received for publication December 2, 1992.

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