Employee 1 was setting up his survey instrument at the edge of a concrete apron surrounding an 11-foot-deep concrete utility vault with a 9x9 steel access cover/lid. Employee 2, then walked behind employee 1, stepping onto the steel vault access cover/lid. The cover gave way, causing employee 2 and the steel vault cover to fall into the vault approximately 11 feet below. Employee 1 fell 8 feet, striking an existing pipeline valve located within the vault. Employee 2 was hospitalized for fractures to the ribs on the left side. Employee 1 did not require hospitalization.