Typically grade beams are not checked for sliding because they are assumed to span between supports (usually piles or other grade beams). In other words, they are assumed to not make contact with the soil since they are spanning as beams. As for lateral earth pressure, usually grade beams are surrounded on soil by all sides, so lateral earth pressure cancels out on both sides of the grade beam. If one side of your grade beam is exposed, then simply apply lateral soil load to the vertical face of the beam and design accordingly.