Impact forces from full-scale granular flows on civil engineering structures exhibit huge fluctuations mainly caused by the co-existence inside the granular medium between a quasi-static zone, in direct contact with the structure, and a surrounding inertial zone. The aim of the current paper is to describe a time-averaged study and to show first results about those force fluctuations with the help of discrete numerical simulations on a two dimensional granular lid-driven cavity. A small-scale granular sample is trapped between one bottom wall, two side walls and one upper wall shearing the sample at constant speed under a given confinement pressure. We conducted a systematic study of the time-averaged force on the walls under different velocities, pressures and lengths of the sample relative to its thickness. Our study evidences a critical length above which the results are not influenced by the length and we observe a recirculation zone whose center mainly depends on the shearing velocity. Despite the strong inhomogeneities of pressure distribution, the steady forces experienced by the walls are governed by the macroscopic inertial number. We also provide a preliminary analysis of the force fluctuations.