In a ground working machine for working ground surfaces or traffic surfaces, in particular cold recycler or soil stabilizer as a self-propelled machine or attachment unit, featuring a machine frame, featuring a milling/mixing drum rotating transversely to the working direction of the machine frame, featuring a drum housing enclosing the milling/mixing drum and resting on the ground surface during the milling operation that forms a mixing chamber for crushing the milled material and/or mixing the milled material with additives, where the milling/mixing drum is height-adjustable for adjustment of a selectable milling depth, featuring a pivotable drum flap arranged, as a minimum, at the front end of the drum housing as seen in working direction, and featuring a controller for, as a minimum, the working process, it is provided for the following features to be achieved: the drum flap is, as a minimum, of two-part design and comprises no less than one upper part and one lower part, in which design no less than one part is pivotable and no less than one further part is adjustable in a telescoping fashion linearly or in the shape of an arc.