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During developmental work by the Strategic Highway Research Program (SHRP, 1987–1992) and the Federal Highway Administration (FHWA, 1993-present), it was recommended that knowledge and assessment of triaxial equipment and sample interaction be correctly applied to both test command and data reduction routines for the accurate measurement of resilient modulus properties of unbound materials using external instrumentation configuration. This concern is briefly addressed in the American Association of State Highway and Transportation Officials (AASHTO) test method T307-99, under paragraph 188.8.131.52. The paragraph refers only to calculations necessary to correct for the proper magnitudes of seating load based on known properties of the triaxial cell. The method does not contain information describing the methods and calculations needed to consider these parameters in the data reduction portion of the test. Further, the test method does not address other influences unique to a triaxial testing cell, namely frictional forces resulting from poorly manufactured or improperly designed seals, alignment issues and compliance.
This paper addresses the sensitivities of the results due to appropriate and inappropriate interpretation of the correction factors necessary, as well as influences of seal drag forces. The sensitivities have been calculated for two specific triaxial cells. Note that each triaxial cell and instrumentation configuration possesses unique values needed for the correction. Generally, the larger the triaxial cell (larger rod diameter, larger mass of rod), the greater the influences of the uplift and static weight components of load.
Users must be knowledgeable about their test equipment and the ability of the software to correctly apply proper loads and properly assess these loads for the correct calculation of resilient modulus. Further, physical tolerances must be developed for triaxial cells with respect to seal drag, alignment and compliance. These issues are extremely important when comparing results between laboratories, as the errors associated with miscalculation, misinterpretation or incorrect measurement of loads can result in variations outside the precision of the test itself.
resilient modulus, triaxial test, seal drag, compliance, subgrade
President, Boudreau Engineering, Inc., Norcross, GA
Branch Manager, GeoTesting of GA, Alpharetta, GA