RESEARCH ON THE BEHAVIOUR OF A CIRCULAR TUNNEL BUILT IN AN OPEN TRENCH

RECHERCHE SUR LE COMPORTEMENT D'UN TUNNEL CIRCULAIRE CONSTRUIT EN TRANCHEE OUVERTE

AN EXPERIMENTAL STUDY WAS CONDUCTED ON A REINFORCED CONCRETE TUNNEL HAVING A 10,50.M DIAMETER AND A CIRCULAR CROSS-SECTION; THE TUNNEL WAS BUILT IN AN OPEN TRENCH IN A NON-COHESIVE COMPACT SOIL (BRUXELLES SAND); BACK FILLING TOOK PLACE IMMEDIATELY AFTER REMOVAL OF THE FORMWORK. 90 PRESSURE CELLS MONITORED STRESSES IN THE SOIL IN THE IMMEDIATE VICINITY OF THE TUNNEL AT FOUR POINTS WHERE THE SOIL COVER WAS RESPECTIVELY 5M, 10M, 15M AND 18M THICK. MEASUREMENTS WERE ALSO TAKEN OF STRESSES IN THE CONCRETE AT THE KEYSTONE, HAUNCH OF THE ARCH, AND AT THE BASE OF A SECTION LYING UNDER A 15M THICK EMBANKMENT. DEFORMATIONS WERE ALSO MEASURED IN THE SAME SECTION. IN-SITU, AND LABORATORY TESTS WERE CONDUCTED WITH A VIEW TO DETERMINING THE GEOTECHNICAL CHARACTERISTICS OF THE EMBANKMENT BUILT OVER THE TUNNEL. MEASUREMENTS WERE MADE IMMEDIATELY AFTER LOAD APPLICATION; THEY WILL CONTINUE UNTIL THE PHENOMENA OBSERVED ARE SETTLED. THE STRESSES IN THE MASS IN THE VICINITY OF THE TUNNEL LED TO THE DETERMINATION OF THE LOADS, AND IT WAS OBSERVED THAT PRESSURE CONCENTRATION AT THE TOP OF THE ARCH EXISTED; THIS WAS EXPLAINED BY THE OCCURENCE OF TRANSFER ARCHES BETWEEN THE SIDE OF THE TRENCH AND THE TOP OF THE TUNNEL. THE PRESSURE AT THE HAUNCH WAS LIMITED TO A GIVEN VALUE; THIS LED TO THE ASSUMPTION THAT THERE WAS A GENERAL DECREASE IN THE LOAD. LOAD CONDITIONS WERE SEEN TO BE CLOSELY LINKED WITH THE CONSTRUCTION METHOD USED. CORRELATIONS BETWEEN STRESSES IN THE SOIL AND DEFORMATIONS OF THE TUNNEL DETERMINED WITH THE KOMMERELL CULMAN-RANKINE METHOD AND MEASURED WERE ALSO ESTABLISHED.