A mechanical tailgate (3) for a dump body (1) of an underground truck, the purpose of the mechanical tailgate is to increase the loading capacity of the dump body, which mechanical tailgate is pivotably arranged between an opened and a closed position, comprising a tailgate sheet (4) attached to two lifting arms (5), which are pivotably arranged to the dump body (1), the dump body (1) comprises also a controlled tailgate (2) arranged to be controlled between opened and closed position of the dump body, the opening and closing movement of the controlled tailgate (2) is effected in the form of a circular movement. In the closed position of the mechanical tailgate (3) and the controlled tailgate (2), the tailgate sheet (4) and the controlled tailgate (2) are arranged to be positioned overlapping each other in such a way that a lower edge of the tailgate sheet (4) is placed on the inside of an upper edge of the controlled tailgate (2) for locking the tailgate sheet (4) towards the controlled tailgate (2).