BIG WALKER MOUNTAIN TUNNEL AND LITTLE WALKER MOUNTAIN GRADING

WORK ON INTERSTATE ROUTE 77 IN VIRGINIA THROUGH THE APPALACHIAN MOUNTAIN REGION IS DESCRIBED IN RESPECT TO BIG WALKER MOUNTAIN TUNNEL AND LITTLE WALKER MOUNTAIN GRADING. TOPOGRAPHIC MAPPING BY PHOTOGRAMMETRIC METHODS WAS USED TO STUDY AND EVALUATE ROUTES FOR INTERSTATE 77. AN OPEN CUT IS BEING EXCAVATED FOR LITTLE WALKER MOUNTAIN GRADING WHICH IS APPROXIMATELY 290 FEET ON CENTERLINE AND CONTAINS 5 MILLION 200 CUBIC YARDS OF EXCAVATION. THE ENTIRE SECTION IS PRIMARILY IN SHALE WITH A LESSER AMOUNT OF THINBEDDED SANDSTONE. THE EXCAVATION AND HAULING PROCEDURES ARE RATHER UNIQUE. THE CONTRACTOR DRILLS 4- TO 6-INCH HOLES APPROXIMATELY 40 TO 50 FEET DEEP AND LOADS THE HOLES WITH APPROXIMATELY 40,000 TO 80,000 POUNDS OF AMMONIUM NITRATE. FOLLOWING THE BLASTING OPERATION HE MOVES IN A CONVEYER-BELT TYPE OF LOADING EQUIPMENT AND, UTILIZING 3 LARGE BULLDOZERS TO FILL THE HOPPER, HE LOADS A 25-YARD EUCLID TRUCK EVERY 40 TO 45 SECONDS. THE HAULING PROCEDURE REQUIRES 10 OF THE EUCLID TRUCKS TO PROVIDE A CONTINUING OPERATION. THE GRADING IS APPROXIMATELY 80 PERCENT COMPLETED IN THIS CUT WHICH IS ONE OF THE LARGEST THROUGH-CUTS IN VIRGINIA. THE CUT AREA AT THE CREST IS 1100 FEET WIDE. BIG WALKER MOUNTAIN PRESENTED A MUCH GREATER PROBLEM SINCE OPEN CUTTING, GETTING AROUND OR OVER THE BARRIER WAS NOT PRACTICAL, THEREBY NECESSITATING TUNNELLING. THE GEOLOGICAL INFORMATION INDICATED THAT THE FULL TUNNEL LENGTH WOULD BE IN COMPETENT ROCK OR ROCK THAT WOULD NOT REQUIRE OTHER THAN NOMINAL SUPPORT PROBLEMS. THE BASIC SHAPE FOR THE TUNNEL CROSS-SECTION IS AN INVERTED HORSESHOE. THE SUSPENDED THIN SLAB OVER THE ROADWAY PROVIDES THE AIR DUCT IN THE UPPER SECTION FOR BOTH THE EXHAUST AND FRESH AIR. THE ROADWAY SECTION PROVIDES FOR TWO 13-FOOT LANES FOR TRAFFIC IN EACH TUBE AND A 16-FOOT 6-INCH VERTICAL CLEARANCE. SINCE THE PROJECT IS PARTIALLY IN NATIONAL FOREST SERVICE PROPERTY, IT WAS NECESSARY TO CONSTRUCT CHECK DAMS IN THE EXISTING STREAMS AND DRAINAGE CHANNELS TO CONTROL SILTATION AND TO MAINTAIN THESE FEATURES DURING THE CONSTRUCTION OF THE PROJECTS. THE ROADWAY PAVEMENT WILL CONSIST OF 10 INCHES OF COARSE AGGREGATE AS A SUBBASE, 10 INCHES OF CEMENT CONCRETE PAVEMENT AND 2-1/2 INCHES OF BITUMINOUS CONCRETE TOP OR WEARING COURSE. THE TUNNEL IS ON TANGENT WITH A 3.5 PERCENT UPGRADE. STEEL RIBS AND POSTS ARE BEING ERECTED AS A STEEL FRAME, INSIDE THE TUNNEL EXCAVATION, FOR SUPPORT AS WELL AS PROTECTION.