A coupled fluid-solid numerical method is presented. The solid phase consists of a granular assembly where dynamics of each solid particle is described with the discrete element method. In the inter-particle space the fluid flow is solved with the lattice Boltzmann method. Interactions between the solid phase and the fluid phase result from the integration, over the solid particle boundary, of the momentum exchange between solid and fluid phases. Consequently, assumptions about fluid-solid interactions are very limited with such a coupling. To illustrate abilities and limits of the coupled method in the framework of internal soil erosion, a two-dimensional model of piping erosion is considered (inspired from the hole erosion test). Direct simulations of erosion of a cohesive- frictional granular matter have been performed for different values of water pressure gradient and inter-particle cohesion. As classically found experimentally, the mass rate of erosion follows a relation of the kind of Shield's law, with a linear dependency on the hydraulic shear stress. A critical hydraulic shear stress, below which detachment of particles does not occur, can be identified and is directly related to the inter-particle cohesion. On the contrary, this latter has no influence on the erosion coefficient.