Abstract

PARROCK, A L; STRYDOM, J S  and  RIEGER, V J. Why stabilise? Using triaxial tests for determining pavement stiffness and shear strength parameters of mechanically modified layers. J. S. Afr. Inst. Civ. Eng. [online]. 2008, vol.50, n.1, pp.2-12. ISSN 2309-8775.

Triaxial testing of naturally occurring, slightly silty, medium-graded, coarse sand, derived from completely weathered granite (with some gypsum), compacted to 95 % of Mod AASHTO density generated unload/reload Young's modulus E-values of about 300 MPa at a representative confining stress of 90 kPa. This is some 80 % higher than what would usually be expected for this type of G5 material. Mohr-Coulomb shear strength parameters of c = 15,9 kPa and φ = 51,4° were obtained from the high-quality triaxial tests. Taking into account the variability of the materials, it is suggested that these be downgraded to c = 12,7 kPa and φ = 48,8° for use as design parameters. It is common practice to stabilise natural gravel materials to generate weakly cemented sub-base layers. However, the presence of naturally occurring gypsum within the in-situ granite generated concern as negative effects were observed on elements of past construction. The use of cement as a stabilising agent was eliminated by generating a nearly equally strong layer by mixing the naturally occurring gravels with varying quantities of crushed stone, crusher waste and dune sand, the latter to combat a high plasticity index. The best result was obtained by using a blend of 50 % natural gravel, 30 % crusher waste and 20 % dune sand. These blended materials, generated unload/reload Young's modulus E-values of some 560 MPa at 95 % Mod AASHTO compaction and a confining stress of 247 kPa. This E-value is very similar to what is thought would be attained for a cracked and hydrated cement-stabilised layer but without the disruptive effects of a lowered confining stress that would be the case when a stabilised layer shrunk and cracked on hydration of the cement stabilising agent.

Keywords : Triaxial testing; unload [reload Young's modulus]; confining stress; shear strength; airport pavement; mechanical modification.

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