REPEATED TRIAXIAL CHARACTERIZATION OF GRANULAR BASES

Granular bases are utilized extensively in Illinois flexible pavement construction. Typically, the granular materials are dense graded with a maximum size of 1.5 in. and 4 to 12% passing the No. 200 sieve (fines). The Illinois specification gradation designation is CA-6. More "open-graded" granular materials with reduced fines content are less moisture sensitive and generally provide improved granular base performance. In recognition of those factors, an additional granular base course gradation (CM-06) has been proposed. The current CM-06 gradation limits only differ in the permissible amount passing the No. 200 sieve. The CM-06 limits are 2% plus or minus 2%, and the CA-6 limits are 8% plus or minus 4%. As part of an overall evaluation of the proposed gradation modification, a comprehensive triaxial testing program (repeated loading and rapid shear strength tests) was conducted to establish the pertinent properties (resilient modulus, permanent deformation accumulation with loading repetitions, and shear strength characteristics (cohesion and friction angle)) of typical crushed stones, crushed gravels, and gravels meeting CA-6 and CM-06 gradation requirements. Seven materials were included in the program. Results of the testing program indicated the following: (a) For near maximum dry density and optimum moisture content conditions, friction angles show little variability and cohesion values ranged from 9.6 to 24.4 psi. (b) The resilient modulus parameters K and n fell within expected ranges, and only limited differences were noted among the various granular materials. Resilient modulus is not a good property for ranking granular base performance potential. (c) The rapid shear strength of "conditioned" (previously loaded) specimens was always greater than the "unconditioned" strength (increases from 34 to 217%). This is a very significant finding. (d) Permanent deformation under repeated loading varied considerably. In fact, four of the materials (all gravel products) did not survive the standard conditioning loading (45 psi deviator stress/15 psi confining pressure).