This paper reports the results of triaxial compression tests on sand reinforced with coir fibers and demonstrates that fibers are useful in increasing the shear strength of sand. An approach for considering the effect of random-oriented fibers in numerical analysis is proposed and the results of numerical simulations are reported. Numerical simulation results are compared with those obtained from laboratory triaxial compression tests. The mechanisms by which random fibers reinforce sand are explained in terms of microstructure that prevents the formation of distinct localized strain bands and increases pull-out resistance