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Finite element analysis of a deep excavation: a case study

Aktaş Engin, Tuğçe.
The optimum use of safety and economy in deep excavation design is possible with the selection of the appropriate system, and modelling of the selected system and soil properties properly. Therefore, soil parameters selection has a crucial effect in deep excavation analyses. The realistic estimation of the displacements with the finite element software is only possible by using the realistic deformation modulus values during analyses. However, in stiff clays for which undisturbed sampling is very difficult, displacements calculated with laboratory deformation modulus parameters may be higher than the measured values. Objective of this study is to determine the constant that shows linear relationship between SPT-N and deformation modulus parameter of Ankara clay by using three constitutive soil model of Plaxis-2D, namely Mohr-Coulomb (MC), hardening soil model (HS) and hardening soil model with small strain stiffness (HSsmall). For this purpose, back analysis of a 25 m deep excavation was performed by using inclinometer measurement results. To be more precise in numerical analysis, instead of using the idealized soil profile the soil is divided into layers according to SPT-N60 measurements. Additionally, each displacement measured by the inclinometer along the depth is compared with the analysis results to minimize the error. In case trial-error method is used in the study, time loss and the possibility of not reaching the correct result were taken into consideration; therefore, the analysis was done by writing a Python code. As a result of analyses, the soil models were compared with each other and it is concluded that displacements curves obtained from the MC model could not converge to the real displacements. HSsmall model results are closest to the real displacements. Moreover, displacement curves obtained from HS and HSsmall models are very close to each other, and the linear correlation formula is determined as E50ref=780xN60 kPa for this excavation of the case study in Ankara clay.