Optimization of modeling pile foundations

Abstract

In this thesis different models are considered which can deal with the interaction between pile foundations and structures. This to find the most efficient way to model pile foundation structures whereby both the efficiency of the design as building process are taken into account. This are the models D-Pile (geotechnical), Scia Engineer (structural) and Plaxis (geotechnical). Therefore a study is done to the effects which play a role by pile foundation structures and or the different models can deal with this. The study focused on displacement piles which is the most used pile type in the Dutch building practice. The reference project which is used for this study is the foundation structure of a power pylon (as shown in figure 1) To get trust in the models after this study a verification is done for single piles which were subjected to an axial (compression and tension) and horizontal load on top. For piles loaded by axial compression the load-settlement behaviour is compared with the behaviour as one can determine according to NEN 9997-1 (EC7). Hereby one can see that the fact that the numerical model Plaxis does not take into account the installation effects (densification and displacement of the soil around the pile) leads to a deviating load-settlement behaviour in comparison with the behaviour of the other models which are based on empirical relationship (pile tests). This behaviour can be improved by additional tricks like volume expansions of the soil around the pile, but this is not taken into account in this thesis. So therefore Plaxis was not taken into account furthermore. After the verification of single piles, the verification was expanded for pile groups which was subjected to the same loading conditions. Hereby variations were made in the centre-to-centre distances of the piles and the thickness of the cap on top. Hereby can be noticed that the fact that D-Pile cannot take into account the bending stiffness of the pile cap can lead to an unrealistic distribution of pile forces in case of a thin plate with large centre-to-centre distances of the piles. On the other hand the additional settlement of a pile group in comparison with a single pile cannot be taken into account with the chose way of modeling in Scia Engineer, but only by D-Pile. The behaviour of pile groups loaded by tension can differ significant from single piles This effect has to be taken into account manually in both models. For a horizontally loaded group there is a significant difference in results in the bending moments in the pile heads and the displacement of the pile cap, which is caused by the different horizontal springs used in the models. Combining all these knowledge leads to the conclusion that Scia Engineer seems to be the best model for the design of the foundation structure of the power pylon. Hereby both the efficiency of the design process and the structure are taken into account. The reassons for this are: • Besides D-Foundation which is a tool to determine the bearing capacity only one model is needed to analyse the behaviour of the piles and the foundation structure. This model is also used by the structural engineer to determine the distribution of forces and the reinforcement in the structure. • The load-displacement behaviour of (tension) piles is governed by springs which are easy to adjust and it is easy to check if the behaviour is in line with the applicable standards. This is especially the case for tension piles, for which the bearing capacity is dependent on the pile-to-pile distance and the alternation between tensile and compressive loads.