Classical theories such as Terzaghi are still used for the bearing capacity of the ground. The validity of the bearing capacity theory has been verified by experiments. However, the bearing capacity of the actual structure has not been sufficiently examined. In this paper, the ultimate bearing capacity of the ground is estimated by finite element analysis. The failure mode of Terzaghi's bearing capacity theory is related to the residual strength of soft rock. The effect of the ratio of peak strength to residual strength on the ultimate bearing capacity of soft rock is clarified by the triaxial test.


In general, the relationship between average load per unit of area and settlement is used to evaluate the bearing capacity of footings classified as shallow foundations. It was noted (Terzaghi, K. and Peck, R.B, 1948) that in Terzaghi's bearing capacity theory, when the soil is dense or hard, the whole ground fails in the general shear failure mode, and when the soil is loose or quite soft, the load-settlement curve is called the local shear failure mode, which has a shape that makes it difficult to determine the ultimate load. In a ground subjected to a load, a limited area of soil will fail first. Since the fractured soil loses its resistance, the soil around the fractured soil will resist next. After that, the failure area expands to the surrounding area and the surrounding soil under the load approaches failure. This propagation of failure is the progressive failure of the local shear failure mode, which corresponds to Terzaghi's failure mode. It is reasonable to consider the bearing capacity problem and slope stability problem as the local shear failure mode (progressive failure). This is because it is unlikely that all the soils along the assumed slip line in the ground will resist at the maximum shear strength at the same time as in general shear failure.

Earth Quality Engineering Society (1965) proposed for the bearing capacity theory used in current design is based on the classical bearing capacity theory of Prandtl, Terzaghi and Meyerhoff. Although the validity of the bearing capacity theory has been verified by experiments, it cannot be sufficiently confirmed in actual structures. This is because the actual bearing capacity cannot be reasonably determined from the relationship between the load applied in the in-situ bearing capacity test and the settlement of the foundation. Architectural Institute of Japan (1974) proposed the long-term allowable bearing capacity of the current design method assumes a large safety factor value of 3 for the ultimate bearing capacity based on the classical bearing capacity theory. However, this may be an uneconomical design. It is considered inadequate for engineering purposes that the classification of soil for the two ground failure modes is determined by whether it is dense or loose, or soft or hard.

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