Recently, the foundations of structures are becoming larger and larger, but an adequate method of determining the bearing capacity of such foundations has not been established yet.
This paper points out that the Terzaghi's equation of the bearing capacity is insufficient in the above cases, and recommends to use the graphical solution of plastic equilibrium which can take account of inhomogeneity of a rock mass.
A rock mass of weathered granite was chosen to show the usefulness of the graphical solution of plastic equilibrium. The weathered granites were sampled and tested both in the field and in the laboratory. The triaxial test results indicate that the undrained strength is 4.0t/m2 and the rate of strength increase by consolidation is 0.32. This means that the strength of the rock mass increases with depth, and in the stability analysis this fact must be taken into account. The value of the bearing capacity of a foundation (30m wide, for example) on weathered granite determined by the graphical solution of plastic equilibrium was compared to the results by the Terzaghi's equation and the slip circle method. It is found that the latter two methods give 10-20% higher value than the former and this discrepancy becomes bigger when the foundation becomes wider.
