IMPROVED DESIGN METHODS FOR THE BEARING CAPACITY OF FOUNDATION PILES

Pile foundations are often used for civil structures, both offshore and onshore, which are placed on soft soils. Nowadays, there are many different methods used for the prediction of the pile bearing capacity. However, the resulting design values are often different from the values measured at pile load field tests. A reason for this is that there are many pile installation effects and (unknown) soil conditions which influence the pile bearing capacity. Another problem is that for many pile load field tests in the past, the residual stresses at the pile after pile installation, have been ignored unfortunately. This ignoring leads to a measured tip bearing resistance which is lower than the real tip bearing resistance (capacity), and a measured pile shaft friction which is higher than the real pile shaft friction.
The main aim of this thesis is, to come to a better understanding of the pile performance and especially the pile bearing capacity. In order to achieve this aim, many numerical loading simulations were computed for small displacements with the Finite Element Model Plaxis and many existing pile design methods have been studied. The pile installation process itself was modelled and simulated with the help of the material point method, MPM, which is able to handle large displacement numerical simulations. The used version of this MPM method was recently developed at the research institute Deltares in the Netherlands.
The results from the MPM simulations showed that there is a big difference between the bearing capacity of a pre-installed pile (no installation effect are taken into account) and the bearing capacity of a pile where the installations effects are taken into account. This proves in a numerical way the importance of the pile installation effects on the pile bearing capacity. However, the MPM numerical simulations were done only for jacked piles. Therefore, impact piles, vibrated piles etc., were not simulated. For this reason, there is not a detailed numerical study for the effect of each installation method specific on the pile bearing capacity. The fact that the installation effects, in general, has an important influence on the pile bearing capacity was already proven by field tests and centrifuge tests, and has been published before by several authors.
The performed numerical simulations show that during the loading and failure of a pile, a balloon shaped plastic zone develops around the pile tip, which is in fact the failure mechanism. A better understanding of this zone could lead to a better estimation of the pile tip bearing capacity because the size and position of this plastic zone are directly related to the pile tip bearing capacity. Therefore, this plastic zone has been studied for different soil and pile parameters. Also, the influence of each parameter has been studied and discussed. A similar balloon shaped plastic zone was found for both small and large displacement simulations.
The tip bearing capacity of a pile is regarded to depend only on the soil in a certain zone around the pile tip. This zone is called the influence zone. The influence zone is found to be similar to the plastic zone of a pile tip. Therefore, the influence of a soft soil layer, near the influence zone of the pile tip, has also been studied. The numerical results have been validated with laboratory tests made by Deltares. The influence zone is roughly from 2 times the pile diameter, 𝐷, above the pile tip, to 5 or 6 times 𝐷 below the pile tip.
Laboratory tests, using the direct shear test machine, have been performed in order to define the difference between the soil-pile friction angle and the soil-cone friction angle. The tests were done for different surface roughnesses and for three different sand types. The results were compared with the roughness of the sleeve of the Cone Penetration Test (CPT) apparatus.
Based on the numerical simulations and the laboratory tests of Deltares, a new design method has been proposed for the estimation of the pile bearing capacity. This method has as main input value, the CPT results, therefore it is a CPT-based design method. The proposed method has been validated using pile field tests that were performed in Lelystad in the Netherlands.
During this research, several axial and lateral pile field tests were performed at the West Coast of Mexico. Their results have been reported and discussed in the appendices.