Analysis of Single and Group Micropile Behavior

The Bearing Capacity of Single Pile and Pile Groups under Axial Tensile Loading using FEM

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This thesis investigates the behavior of single and group micro piles under axial tensile loading. Micro piles are small diameter piles consist of grout and steel rebar and they are capable of absorbing tensile loads. By constructing a group of piles the bearing capacity of each pile within a group is less compared to the single pile. The reason is due to the existence of group effect in the pile group which influences the bearing capacity. During the production of the piles for Amsterdam car parking project, load tests are performed to check whether the piles behave according as expected which can be considered as acceptance tests. However in this test, only individual piles are tested and not a pile group. CUR 236 design procedure states that the bearing capacity of a pile in a pile group is less than the bearing capacity of that pile when it is loaded not in a group. To take this effect into account for the acceptance test, an additional load is added to the test load. It turned out at the Amsterdam car parking project that piles failed the acceptance tests due to this additional load which is required to apply according to CUR 236 design guide. However this procedure is questionable because the pile is loaded into a much higher level than it will actually experience during its lifetime. Therefore an evaluation of the standards and the influence of group effects on micro pile behavior is needed. The initial plan of approach was to investigate the standards of other countries and compare them to Dutch standards to find the eventual existing gap and propose an improvement method which did not succeed. The reason was that the standards of other countries were all written in their national language and therefore it was not possible to study them. Seeking for the research on tension pile group behavior did not help so much because many researchers believe that including a realistic group effect in the design is not an easy task and there was no clear conclusion on micro pile group influence. The direction of approach is then changed towards the numerical modelling and based on it the influence of group effects on micro pile behavior is presented. First a single micro pile is modelled with Finite Element Method in Plaxis. The single pile is finally modelled in plane strain by using Embedded Beam Row element. Model parameters and properties are defined based on Amsterdam case study. The load-displacement behavior of the single pile model was comparable with the one from Amsterdam field failure test and is therefore validated based on field failure test data. After development of single pile model, the model for pile groups is made. This group model is made to represent the practical situation in a building pit for Amsterdam case study. Three models for the pile group have been made which differ in pile spacing and the impact of this parameter on the capacity per micro pile is investigated. The different used pile spacings are 5D, 10D and 15D which are equal to 1 meter, 2 meters and 3 meters. Also for each pile spacing, the dominant failure mechanism is determined. According to the results it was concluded that for 5D and 10D pile spacing, the failure mechanism is based on soil plug pull-out while for 15D, it is according to slip failure. By validation of the pile group model, an improvement for the space between the piles within a pile group can be proposed as 10D where the soil plug pull-out is dominant failure mechanism. It is recommended to validate the pile group model based on full-scale failure test on pile groups or by small-scale test using Geo-centrifuge models. Also by monitoring, a real data base of the group behavior can be obtained. Comparing the monitoring data to the design values based on CUR 236 could give an idea how well the design guide is formulated.