**6.5.2 Unbalanced Force**

A grid point in a numerical model is surrounded by the adjoining zones that contribute forces to the grid point. At equilibrium, the algebraic sum of these forces is almost zero (i.e., the forces acting on one side of the grid point nearly balance those acting on the other). Unbalanced force approaching a constant non-zero value indicates elastic equilibrium and /or plastic flow occurring within the model (figure 18). A very low value of unbalanced forces indicates balance of forces at all grid points, however, steady plastic flow may occur without acceleration. In order to distinguish between these two conditions, other indicators of slope failure should also be examined.

Figure 18: unbalance force solving slope stability by FLAC based on finite difference method