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The problem of estimating the bearing capacity of massive caisson foundations in frictional soil under combined vertical ( N ), horizontal ( Q ) and moment ( M ) loading is examined numerically by means of three-dimensional finite element analyses. The analysis is performed with due consideration to the foundation’s depth-to-width ratio ( D / B ), the magnitude of the vertical load and the caisson-soil contact interface conditions. The constitutive law for soil behavior is appropriately validated against experimental results from 1-g small-scale tests, available in the literature. The ultimate limit states are presented in the form of a bearing strength surface in dimensionless and normalized form, while detailed discussion is provided on the physical and geometrical interpretation of the kinematic mechanisms that accompany failure. A generalized closed-form expression for the failure envelope in M – Q – N space is then fitted to the numerical results with use of an appropriately trained artificial neural network. An upper-bound limit equilibrium solution for a certain failure mechanism (designated as the “sliding” mechanism) associated with maximum horizontal bearing capacity is also developed for verification purposes. One of the originalities of the paper lies with respect to the post-failure response of the caissons, where it is shown that the incremental displacement vector is accurately reproduced by assuming normality on the bearing strength surface irrespective of the considered plastic flow rule (associative or non-associative) at the microscale (soil element).

The seismic behaviour of caisson foundations supporting typical bridge piers is analysed with 3D finite elements, with due consideration to soil and interface nonlinearities. Single-degree-of freedom oscillators of varying mass and height, simulating heavily and lightly loaded bridge piers, founded on similar caissons are studied. Four different combinations of the static ( FS V ) and seismic ( FS E ) factors of safety are examined: (1) a lightly loaded ( FS V = 5 ) seismically under -designed ( FS E < 1 ) caisson, (2) a lightly loaded seismically over -designed ( FS E > 1 ) caisson, (3) a heavily loaded ( FS V = 2.5 ) seismically under -designed ( FS E < 1 ) caisson and (4) a heavily loaded seismically over -designed caisson. The analysis is performed with use of seismic records appropriately modified so that the effective response periods (due to soil-structure-interaction effects) of the studied systems correspond to the same spectral acceleration, thus allowing their inelastic seismic performance to be compared on a fair basis. Key performance measures of the systems are then contrasted, such as: accelerations, displacements, rotations and settlements. It is shown that the performance of the lightly loaded seismically under- designed caisson is advantageous: not only does it reduce significantly the seismic load to the superstructure, but it also produces minimal residual displacements of the foundation. For heavily loaded foundations, however ( FS V = 2.5 ), the performance of the two systems ( over and under designed) is similar.

A large number of landslides induced by the 2004 Mid Niigata Prefecture earthquake resulted in the closure of 233 segments of national and prefectural routes in Higashiyama mountain district, and 61 localities were completely isolated. Since railway and road facilities follow closely the motion of soils, damage to these facilities has to be discussed in terms of soil deformations that they experienced. The example of Kizawa tunnel shows that even relatively small soil deformations can be large enough to cause serious cracking of tunnel lining.[PUBLICATION ABSTRACT]