Simplified simulation of the installation of vibro-piles in water saturated soil.

This paper presents a novel and simplified approach to simulate the effects of the vibro-pile installation on the deformation of deep excavation shoring walls at high ground water levels. Although commonly-used finite element simulations provide acceptable predictions of the wall displacements caused by underwater excavations, anchor installation, or pit dewatering processes, they could not explain the unexpected large wall movements measured in different excavations during the vibro-pile installation stage. Previous dynamic finite element simulations of the vibro-installation of a single pile showed that after few cycles the effective stress in the soil surrounding the pile reduces almost near to zero. The soil region affected was called "the liquefaction zone". Due to the waves emanating from the pile, the soil beyond the liquefaction zone is subjected to a (high-) cyclic loading, i.e. a large number of strain cycles with small amplitude. When the soil behavior is modeled with hypoplasticity or elastoplasticity, its response to these small strain amplitudes is nearly (hypo)elastic resulting in an erroneous accumulation of stress or strains, see Osinov (2017) [1]. Based on this observation, Osinov et al. (2015) [2,3] suggested that the accumulation effects caused by many cycles of small strain amplitude could explain the large wall displacements observed during the pile installation. Following this idea, in this paper we model the soil behavior with a combination of hypoplasticity (which can be applied to cycles of large strain amplitudes and monotonic paths) and the high cyclic accumulation (HCA) model, which is able to extrapolate the accumulation of stress/strain caused by a large number of cycles of small strain amplitude ( ϵ ampl < 10 − 3 ). The pile installation is not explicitly modeled, but its effects are incorporated in the numeric model by applying a field of cyclic strain amplitudes around each pile. Using this combined constitutive model and simulation technique, the behavior of the shoring of an excavation pit in Potsdamer Platz, in Berlin was simulated under quasi-static conditions in a finite element program. The obtained simulation results are in good agreement with the horizontal displacement of the diaphragm wall measured during the entire construction stages, including the underwater excavation and the pile installation. • During pile vibration, the soil is subject to many cycles of small strain amplitude. • Pile installation effects captured by a combination of hypoplasticity and HCA-model. • Simplified FE-simulations reflect the influence of the pile installation sequence. [ABSTRACT FROM AUTHOR]