Analysis of a vertical segmental shaft using 2D & 3D finite element codes

Abstract

This computational research investigates the method of supporting underground cylindrical openings using prefabricated elements. The "Underpin" method is used to simulate the excavation and support of a vertical cylindrical shaft constructed in dry low cohesion soil layers. The primary objective of this study is to determine the stress distribution around the shaft due to three dimensional loading conditions. To this end the excavation is initially simulated as an axisymmetric problem using the "PLAXIS 2D" finite element program. The aforementioned method of analysis is subsequently verified against the equivalent "PLAXIS 3D" finite element analysis. Attention has been directed towards recognising the crucial parameters affecting the analysis. It has been found that the constitutive model adopted to simulate the soil behavior ("Mohr-Coulomb" or "Hardening-Soil" model) is of paramount importance and that the shaft diameter also has some influence. However, variations of the shear strength parameters (c', φ') at the soil-structure interface as well as in the value of "Unloading-Reloading Modulus" (Eur) have shown only a minor influence on the results of the analysis. Floor uplift due to unloading, the development of an excessive plastic zone around the bottom of the vertical shaft, significant surface settlement around the shaft and most importantly the high values of tangential hoop stresses observed in the circular segmental rings, are the main issues raised in the calculations.