Laboratory R-value vs. in-situ NDT methods.
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2006-05-01
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Abstract:The New Mexico Department of Transportation (NMDOT) uses the Resistance R-Value as a quantifying parameter in subgrade and base course design. The parameter represents soil strength and stiffness and ranges from 1 to 80, 80 being typical of the highest strength for typical granular materials (theoretically R can vary between 0 and 100). Currently, a field empirical method allows estimation of this value by first determining the AASHTO Soil Classification and the Plasticity Index (PI), and then referencing the R-Value from a standard estimated table of values. This methodology often leads to overestimated R-Values and can be costly. Soil stiffness is a parameter more closely related to the R-Value than the PI and AASHTO classification of a soil. Therefore, it can be used to obtain a more accurate estimate of the R-Value. Three devices, the Clegg Impact Hammer, GeoGaugeā¢, and a Dynamic Cone Penetrometer are possible candidates for obtaining such an in-situ soil stiffness. By obtaining mathematical relationships between the stiffness values by using the above-mentioned devices and laboratory determined R-Values, a suitable replacement for the current R-Value estimation method may be chosen.
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