ABSTRACT:

The typical challenge of deep excavation analysis in undrained conditions, is to choose a constitutive model or a set of input parameters that provide a safe, and in the same time economic design. One of two approaches are usually adopted; the first approach utilizes the effective stress shear parameters and accounts for the induced pore pressures, while the other uses total stress shear parameters. The paper applies the concept of undrained strength to the analysis of deep excavations, which was previously used for embankment design. The concept is applied by introducing a newly defined friction angle (?un) rather than using the effective friction angle (?eff). The paper uses this new parameter in a numerical simulation of a prototype deep excavation in a reference clay. The behavior of the reference clay is modeled after the North-West Sinai Clay which was thoroughly tested previously. The numerical analysis is carried out using the two-dimensional finite element software PLAXIS. The simulations show the difference in the predicted performance when effective stress, total stress, and undrained strength parameters are used. The paper also presents the numerical simulation of the Nicoll Highway deep excavation. The analysis compares between sophisticated constitutive model results (MIT-E3) and using the undrained friction angle in the Mohr-Coulomb Model. The comparison shows that the new concept give reliable predictions for the deep excavation performance, considering the limited effort required in the calibration process.

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