GRANULAR EARTH PRESSURES ON STEEL TUNNEL LINING

A NUMBER OF FIELD MEASUREMENTS WERE MADE TO DETERMINE THE PRESSURES AND CORRESPONDING DEFLECTIONS FOR AN ARCH TYPE STEEL LINED TUNNEL UNDER STORAGE PILES OF CRUSHED STONE. THE STONE WAS 1 1/2 INCH MATERIAL AND UNIFORM THROUGHOUT. THE TUNNEL WAS CONSTRUCTED OF 1/4 INCH STEEL PLATE SECTIONS, FLANGED ON ALL EDGES INWARDLY. THESE SECTIONS WERE BOLTED TOGETHER THROUGH THE FLANGES ON THE INSIDE OF THE TUNNEL, THUS LEAVING THE OUTSIDE SURFACE FAIRLY SMOOTH AND UNIFORM AND MAKING THE TUNNEL QUITE FLEXIBLE. THE WIDTH OF THE TUNNEL, AT THE CONCRETE BASE SLAB, WAS APPROXIMATELY 7 FEET AND THE HEIGHT AT THE CROWN 7 FEET. GOLDBECK PRESSURE CELLS WERE CAREFULLY CALIBRATED AND MOUNTED UNDERNEATH THE BASE SLAB AND AROUND THE OUTSIDE OF THE TUNNEL DURING CONSTRUCTION. DEFLECTION GAGES WERE ALSO MOUNTED INSIDE THE TUNNEL AT THE SECTION WHERE THE PRESSURE GAGES WERE PLACED. MEASUREMENTS WERE MADE OVER A PERIOD OF SEVERAL MONTHS DURING WHICH TIME THE CRUSHED STONE OVERBURDEN WAS PLACED ON THE TUNNEL TO A MAXIMUM HEIGHT OF 70 FEET AND THEN REMOVED. IT WAS POSSIBLE, THEREFORE, TO OBTAIN MEASUREMENTS AT VARIOUS HEIGHTS AND CONFIGURATIONS OF OVERBURDEN LOAD. THE RESULTS INDICATED THAT THE TUNNEL LINING EXPERIENCED THE FULL OVERBURDEN PRESSURE AT THE CROWN AND THE PRESSURES AROUND THE REST OF THE TUNNEL TAPERED OFF TO A VALUE APPROXIMATELY 0.3 TIMES THE VERTICAL OVERBURDEN AT THE BASE. A RECOMMENDED LOADING IS GIVEN, BASED ON THE EXPERIMENTAL DATA. IT CONSISTS OF A TRAPEZOIDAL PRESSURE DISTRIBUTION OVER THE HORIZONTAL PROJECTION OF THE TUNNEL AND A DISTRIBUTION OVER THE VERTICAL PROJECTION WHICH IS RECTANGULAR ON THE UPPER ONE-THIRD AND TRAPEZOIDAL ON THE LOWER TWO-THIRDS. /AUTHOR/