Taishi KANEKO

Eccentric Vertical Loading Bearing Capacity of Direct Foundation in Sandy Ground

Satoru OHTSUKA

In the stability verification of direct foundations subjected to eccentric inclined loads in the 2009 Specification for Highway Bridges, the relationship between the combined load of vertical force V, horizontal force H, and bending moment M and the ultimate bearing capacity is evaluated by the bearing capacity surface formula based on macroelement theory, and the bearing capacity and displacement of the foundation and soil are evaluated from a macro perspective. The results were verified based on the results of monotonic static loading tests, cyclic loading tests, and vibration model tests. However, there are many problems in the scope of application of the bearing capacity equation, such as the lack of clear applicability to sandy soil, lack of quantitative study on the dimensional effect of foundation width on bearing capacity, and the fact that only simple cases of combined loads were studied. The bearing capacity formulas in the specifications for road bridges are proposed based on large-scale model experiments, so their applicability to complex loading systems and ground conditions is not clear, and the number of experiments for direct foundations subjected to eccentric loading, in particular, is significantly smaller than that for vertical loading. In this study, the authors focused on numerical analysis, which can be applied to complex loading conditions, and investigated the ultimate bearing capacity of foundations subjected to eccentric vertical loads in sandy and cohesive soils using a two-dimensional analysis based on the rigid-plastic finite element method (RPFEM). The influence of the eccentric vertical load on the ultimate bearing capacity and the failure mechanism for varying the eccentricity were analyzed. However, the applicability of the bearing capacity equation proposed by the numerical analysis using RPFEM needs to be verified by model experiments. Therefore, this study aimed to compare the ultimate bearing capacity by small model tests (centered vertical loading test and eccentric vertical loading test) and numerical analysis using RPFEM. The model tests were conducted in loose and dense ground, and the numerical analyses were conducted under the same conditions as the model tests.
Comparison between the numerical analysis and the small model tests showed that the centered loading did not give a clear peak strength in the experimental results, and the increase in soil overburden pressure due to foundation settlement caused a problem in that the yield point could be defined, but the failure point could not be defined. On the other hand, in the case of eccentric loading, peak strength was obtained and bearing capacity was clearly defined. The numerical analysis showed that the bearing capacity was generally reasonable regardless of the relative density. Because the small model test is relatively stressful and the test method has a large influence, further improvement of the test accuracy is needed in comparison with the numerical analysis.