STATE OF THE ART OF SOFT-GROUND TUNNELING

Abstract

THE PRESENT TREND IS TOWARD USING SHIELDS THAT HAVE NOT SIGNIFICANTLY IMPROVED IN DESIGN IN RECENT YEARS AND EXCAVATING MACHINES IN WHICH THE GREATEST ADVANCES HAVE BEEN MADE IN MUCKING AND LINING SYSTEMS. ALTHOUGH SAFETY HAS BECOME OF PRIMARY IMPORTANCE, SOME OF THE SKILLS PREVIOUSLY USED IN SOFT-GROUND TUNNELING SUCH AS HAND MINING ARE NOT PRACTICED BY TODAY'S WORKMAN. BASED ON SCHMIDT'S METHOD (1969), THE SHAPE OF THE SETTLEMENT CURVE CAN BE DETERMINED BY PROBABILITY FUNCTIONS IF SUCH PARAMETERS AS THE GROUND SURFACE, TUNNEL DIAMETER, ETC. ARE CONSIDERED. THE SETTLEMENTS ASSOCIATED WITH CONSTRUCTION MAY INVOLVE SOIL MOVEMENT TOWARD THE WORKING FACE AND INTO ANNULAR SPACES SURROUNDING THE TUNNEL LINING THUS RESULTING IN THE REMOVAL OF THE ORIGINAL STRESSES WITHIN THE SOIL MASS. THE DESIGN OF TUNNEL LININGS IS CLASSSIFIED INTO (1) FLEXIBLE LINERS, WHICH INTERACT FULLY WITH THE SOIL, MUST BE DESIGNED TO REGISTER OVERALL STRESS DISTRIBUTIONS, TO WITHSTAND BENDING MOMENTS, AND TO PREVENT BUCKLING; AND (2) RIGID LINERS, IN WHICH THE COEFFICIENT OF EARTH PRESSURE AT REST CAN BE ESTIMATED ALONG WITH THE MOMENTS AND THRUSTS SINCE THE SOIL IS AN ASSUMED LOAD. A GENERAL METHOD FOR DETERMINING THE STIFFNESS OF A TUNNEL LINER SYSTEM THAT INTERACTS WITH THE SOIL IS PROPOSED. IN THIS METHOD OF BURNS AND RICHARDS (1964) AND HOEG (1968), THE COMPRESSIBILITY RATIO IS SEEN AS A PART OF THE EXTENSIONAL STIFFNESS; THE STIFFNESS MEASUREMENT IS OBTAINED BY SUBJECTING A SECTION TO UNIFORM EXTERNAL PRESSURE THUS RESULTING IN NO CHANGE OF SHAPE AND YET DIAMETRIC STRAINS ARE RECORDED BY USING YOUNG'S MODULUS AND POISSON'S RATIO. THE FLEXIBILITY RATIO, BEING THE SECOND PART OF THE ABOVE METHOD, REFLECTS THE RESISTANCE OF THE TUNNEL SECTION AND THE LINEAR TO A CHANGE IN SHAPE UNDER A STATE OF PURE SHEAR. FOR BOTH TYPES OF RATIO, SLIPPAGE BETWEEN THE SOIL INTERFACE, THE STRUCTURAL LINERS, AND THE TUNNEL SECTION MUST BE ACCOUNTED FOR: EQUATIONS FOR SLIPPAGE OF A DEEPLY BURIED TUNNEL ARE GIVEN FOR CROWN AND INVERT AND FOR THE SPRINGLINE. FOR MORE COMPLEX CONDITIONS, SUCH AS TUNNEL LINERS DESIGNED FOR OVERCONSOLIDATED CLAYS INCLUDING HIGH BENDING MOMENTS AND HIGH THRUSTS AT THE CROWN AND INVERT, FINITE-ELEMENT METHODS ARE APPROPRIATE.