Modeling the Fiber Addition Influence on the Small Strain Shear Modulus of Sand

The mechanical behavior of fiber reinforced soils has been extensively studied in the last decades. Previous studies have shown that inclusion of fibers increases the shear strength of the reinforced soil. However, the presence of fibers can reduce, in some cases, the stiffness of the composite material. In this paper, we study the change on the initial stiffness in alluvial sand reinforced with polypropylene fibers. A model based on Hertz elastic contact theory is developed in order to explain the trends of shear wave velocity and maximum shear modulus in the fiber reinforced sand as the fiber content varies. The model assumes that the shear wave is transmitted through elastic distortions at the contacts, so the stiffness of the contacts governs the initial shear modulus, which in turn is affected by fiber additions. Furthermore, the ratio between the amount of grain to fiber contacts and the total of contacts on the shear wave path influence the maximum shear modulus. An experimental testing program involving confined compression tests with shear wave velocity measurements of unreinforced and fiber-reinforced sand specimens was undertaken to validate the proposed model trends. The model predictions were found to be in good agreement with the experimental results.