Finite Element Analysis of Building Deformations Due to Deep Excavations

Abstract

Over the past decades an increase in underground construction is observed. Deep excavations are among the structures used for underground construction. The construction of such structures often affects nearby existing structures and causes possibly even damages.
The prediction of these damages is usually done by following these steps:
1. Determining the free-field ground displacements
2. Imposing the displacements on the structure
3. Determining the deformations of the structure
4. Assess potential damages following these deformations
The ground and the structure are often modelled separately taking no (LTSM) or a factor (Relative Stiffness Method) into account for interaction between soil and structure. Taking no interaction into account is particularly conservative when looking at vertical displacements. The non-linear behaviour of structural elements is neglected by these methods. This could lead to errors when assessing deformations of a masonry building due to the extreme non-linear behaviour of masonry structures.
The objective of this thesis is to get a better understanding of the effect of deep excavations on adjacent (piled buildings) by using integrated 2D modelling. This is done by remodelling the construction of the building pit in PLAXIS.
Based on the analysis performed in Section 4 the load on the pile is the largest contributor to the vertical and horizontal displacements of the pile relative to the soil around the pile. The diameter and stiffness of the pile are also very important factor determining the vertical movement of the pile.
The difference in the soil between a free-field situation and a situation with foundation piles (and a building) the piles stiffen the soil around the piles. Especially, when these piles are in a group. This effect increases when the piles are coupled by a structure. Vertical displacements are more smeared out over the length of the pile. This is less the case for horizontal displacements. Foundation piles undergo negative skin friction as a result of the settlements of the upper soil layers leading to stress in the soil beneath the pile tip. The stress in the soil beneath piles increases with increasing load and skin friction on the pile. This increased stress results in more horizontal stress, which in close proximity to a retaining wall could lead to increased deformations in the retaining wall and therefore the soil.
The results of the numerical calculations and the analytic calculations are analysed and compared in Section 6. In the numerical models both linear elastic as non-linear material properties have been assigned to the masonry building façade in different models. The numerical model with linear elastic material properties for masonry showed stiff response of the building to the induced soil deformations, underestimating the potential damages. The numerical model with non-linear material properties assigned to masonry showed more realistic results. These results are in line with the established analytical models.
Integrated numerical modelling could be a viable solution for future projects where potential building damage is assessed next to deep excavation. it could help to identify weak points within a structure which need addressing during the construction of an adjacent deep excavation. This study has shown that it is possible to do damage assessment in a fully integrated model. The important factors in assessing such projects are the overall soil deformation. Secondly, the relation between the soil and foundation of the structure needs to be analysed. Thirdly, the interface between the foundation and the structure is important.