Experimental evaluation of tensile stress states in fiber-reinforced fine-grained soils via triaxial extension testing

The reinforcement provided by discrete fibres is widely reported to be negligible under triaxial extension, a result drawn almost entirely from sands prepared by moist tamping or pluviation, methods that produce a sub-horizontal fibre fabric which unloads as the specimen extends. The results reported here suggest that this finding depends on that fabric rather than on the loading path alone, and the difference in fibre orientation is quantified. Consolidated undrained triaxial compression and extension tests were performed on slurry-consolidated kaolinite reinforced with 2% polypropylene fibre by dry mass, at 18- and 48-mm fibre length and effective confining stresses of 50, 100 and 200 kPa. Each fibre length was consolidated as its own block, and the 48 mm specimens were cut from one block in two directions, with the sampling tube advanced normal to the bedding and within the bedding plane. The fibre orientation distribution, measured on a transparent soil analogue, has a mean inclination of 21° from the horizontal. Extension testing drove effective stress paths beyond the tension cut-off line in every reinforced specimen, a state that undrained compression cannot reach, providing direct evidence of tension within the composite.