Study on Three-dimensional Characteristics of Fill Failures for Safety Evaluation

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In order to study three-dimensional failure characteristics of fill and ground, centrifuge loading tests were first performed on level and slope sand foundations by changing the size of rectangular footing (zeta=L/B). Some remarks drawn in these tests were as follows ; (1) brittle failures showing sharp peak of load-settlement curve are observed as zeta increases, while the variation of the ultimate values of bearing capacity is small, (2) the shape factor reduces due to the decrease in zeta for level ground as proposed by DeBeer, whereas little change is seen in bearing capacity for slope foundation, (3) the size effect in the bearing capacity is observed in both level and slope foundations, (4) the existence of slope shows large difference in load bearing capacity, giving the value of 0.5 to 0.6 for the ratio between slope and level grounds ; (5) faiure surfaces observed in loading tests on sloping ground are clearly defined and they can be approximated mathematically in good accuracy by use of log-spiral curves.In another series of centrifuge model tests for three-dimensional failure aptterns of earthfills constructed on the sloping valley, model fills were forced to slide by moving the front suppporting wall during a constant centrifuge circulation, and sliding planes were determined after the circulation stops by surveying the positions of intersecting points of noodle-type strings embedded in the model fill. It was recognized in the tests that (1) the size of sliding bodies, their depth and horizontal extension, becomes large as the width of the valley increases because of the decrease in the effect of lateral constraint, and (2) similar tendency is observed in the conventional stability analysis for the observed sliding plane, the three-dimensional safety factors becoming close to the two-dimensional ones as the valley with increases.