AREA WAS UNDER CONSTRUCTION DURING DAYTIME HOURS. CONTRACTOR FORGOT TO TACK WELDING 3 PIECES OFWALKWAY PLATFORM. EE WAS PERFORMING AN INSPECTION OF PUGMILL, WHEN A PIECE OF PLATFORM GRATING SLIPPED AND EE'S LEGS WENT THROUGH HOLE OPENING.EE SUSTAINED LACERATIONS AND CONTUSIONS TO THE LEGS.