International Journal of Geomechanics

This paper presents a reliability-based approach for the three-dimensional analysis and design of the face stability of a shallow circular tunnel driven by a pressurized shield. Both the collapse and the blow-out failure modes of the ultimate limit state are studied. The deterministic models are based on the upper-bound method of the limit analysis theory. The collapse failure mode was found to give the most critical deterministic results against face stability and was adopted for the probabilistic analysis and design. The random variables used are the soil shear strength parameters. The Hasofer-Lind reliability index and the failure probability were determined. A sensitivity analysis was also performed. It was shown that (1) the assumption of negative correlation between the soil shear strength parameters gives a greater reliability of the tunnel face stability with respect to the one of uncorrelated variables; (2) FORM approximation gives accurate results of the failure probability; and (3) the failure probability is much more influenced by the coefficient of variation of the angle of internal friction than that of the cohesion. Finally, a reliability-based design is performed to determine the required tunnel pressure for a target collapse failure probability.

A theoretical expression for clay soil-water characteristic curves (SWCC) is developed for cases in which the osmotic potential is small relative to the matrix potential, for the range of water potential lower than the cavitation value, based on the following foundations. Soil structure consists of saturated soil pods containing parallel soil particles, and angular interpod macropores possibly containing both air and water. The pod water is adsorbed to the particle surfaces, and subject to adsorption while the macropore water is subject to capillary potential (commonly referred to as suction). It is suggested that the capillary potential cannot be lower than the cavitation potential of the capillary water; at lower potentials, the macropore water drains into the pods and the matrix potential is represented solely by the adsorption potential. The paper examines the hypothesis that the adsorption potential is dominated by van der Waal forces, and this forms the basis for the theoretical SWCC. While the theoretical curve reasonably fits experimental data from several soils, kaolin soils depart from it, and pure montmorillonites fit only over a range of low potential, indicating that van der Waal adsorption forces may control adsorption potential for some, but not for all soils.

This contribution presents a so-called “multiphase” model for soft grounds improved by rigid vertical piles, aimed at capturing the global constitutive behavior of the reinforced ground. A special emphasis is put on the soil-inclusions interaction phenomena, which play a decisive role in the way the applied loads are transferred to the inclusions, and thus clearly affect the performance of the reinforcement scheme in terms of settlement reduction. It is shown in particular how the identification of appropriate coefficients, relating to the lateral as well as pile head interactions, performed from numerical simulations on a simple reference configuration, makes it possible to implement such a model in a linear elasticity-based finite-element simulation. The so-obtained numerical code is then applied to the settlement analysis of an embankment and the correlative reduction of this settlement which could be expected from the incorporation of rigid piles into the soft ground layer. The results derived from this simulation are favorably compared with those obtained from a direct and thus considerably more computational time consuming, three-dimensional simulation.

A foundation system may involve more than a single footing with an arrangement such that the footings are sufficiently closely spaced that adjacent footings interact. Vertical, horizontal, and moment capacity of multifooting foundation systems are potentially dependent on footing interaction, and in particular overturning capacity can be significantly enhanced by the structural rigidity of a multifooting foundation system. There is no established procedure for the calculation of bearing capacity of a shallow foundation system comprising cojoined footings and ad hoc approaches may simply involve summing the ultimate limit states of the individual footings as if they acted independently; neglecting additional capacity of the system available from the kinematic constraint provided by the structural connection between the footings. Plane strain finite-element analyses have been carried out to investigate the undrained ultimate limit states under general loading of skirted two-footing foundation systems at various normalized spacing compared to a single skirted footing of equal bearing area and embedment ratio.