Contractor employee was pulling a rubber tire supply car with a scoop. To avoid hitting a dewatering pump ee decided to set the scoop over against the right rib. As ee proceeded to tram the scoop, the back of the scoop deck undercut the rib causing some rib rock 30"" x 14"" and approximately 5"" in thickness to dislodge striking ee on the back of ee's hard hat and shoulders.