Repeated Load Triaxial Tests — Unbound Granular Aggregates

Unbound granular materials, which are continuously graded materials containing fines, are also sensitive to moisture. Examples of influence of moisture on the

resilient modulus and on the permanent deformation of 3 French unbound granu­lar materials, of different mineralogy (hard and soft limestone, micro-granite) are shown in Fig. 10.10. All 3 materials present a decrease of their resilient modulus as the water content approaches the modified Proctor Optimum (wOPM), but the sen­sitivity to moisture is much more important for the limestone than for the igneous rock material (micro-granite). The permanent axial strains become very large for all 3 materials when the water content approaches wOPM.

Other examples, showing the influence of water content on the permanent de­formation and modulus of elasticity of several unbound granular materials from Slovenia, are presented in Fig. 10.11. Again, the permanent strains appear to be more sensitive to moisture than the resilient modulus.

Several studies carried out in France on a large number of different unbound granular materials have shown that their sensitivity to moisture is strongly related to their mineralogical nature, and is particularly important for soft limestone materials. This is illustrated in Fig. 10.12 which presents values of resilient modulus obtained for different natures of granular materials and different water contents.

The igneous materials present relatively low resilient moduli (generally between 300 and 500 MPa), but are not very sensitive to moisture. The soft limestone materi­als present significantly higher moduli at low water contents (up to 1000 MPa), but these moduli drop when the water content approaches the optimum (wOPM).

Ekblad (2004) investigated the influence of water on the resilient properties of coarse unbound granular materials in the saturated as well as the unsaturated state. This study was limited to one type of aggregate of different gradings (with

I In situ water content

Fig. 10.10 Influence of water content, w, on the resilient modulus, Mr, and permanent axial strains, A1c, of 3 French unbound granular materials: hard limestone, soft limestone and microgranite (Hornych et al., 1998)

Note: A1c is a level of strain anticipated once plastic strain has stabilised.

maximum particle size 90 mm). The aggregate comes from Skarlunda in Ostergotland in Sweden. Ekblad’s tests on unbound granular materials of differ­ent granulometric curves showed that the influence of water content on resilient properties depends on the material grading.

First, the dependency of resilient modulus, Mr, on confining stress was estab­lished (Fig. 10.13). Increased confining pressure leads to a substantial increase in resilient modulus. Confining pressures of 100 kPa were reached by Ekblad. Triax­ial tests at different water contents were also performed. The water content was successively increased from an initially low water content to a soaked condition (representing full saturation) and then the sample was allowed to drain freely. All these triaxial tests were performed at a confining pressure of 40 kPa (Fig. 10.13).

Finally, to achieve a summary comparison, the resilient response for a mean nor­mal stress of 100 kPa at a confining pressure of 40 kPa was calculated as a function of the degree of saturation. From Fig. 10.14 it can be observed that the relative reduction in modulus seems to depend on the grading coefficient, with a lower

Fig. 10.12 Sensitivity to moisture of unbound granular materials of different origin (Hornych et al., 1998)

grading parameter (i. e. a higher proportion of fine particles) yielding a larger mod­ulus reduction upon saturation.