C HAPTER 1: G ENERAL I NTRODUCTION 1.1 I NTRODUCTION
Micro-piles are long and slender piles used as foundation elements and are applied in various situations. They regularly use as a foundation element in the form of tension piles. Micro-piles cast in place and have a diameter of maximum 300 mm. The piles consist of a grout column with a steel bar in the center. Micro-piles are used worldwide and different anchor pile systems are developed. GEWI anchor system is commonly used in the Netherlands. Figure 1 shows a visualization of this anchor pile system.
FIGURE 1: VISUALIZATION OF ANCHOR PILE
Micro piles or tension piles are commonly used in the Netherlands to stabilize the excavation floor of the basements. The main function of these piles is to balance the uplift forces. Two failure criteria are of importance in the bearing capacity design which are;
1. Friction capacity between the grout and soil
2. The cone of soil that can be mobilized around the pile
Based on recent experience the soil cone can become the governing factor in testing the production piles. An important aspect for this is the group effect, the reduction in bearing capacity of closely spaced tension piles, as this determines the testing load for the production piles.
1.2 P ROBLEM DESCRIPTION
The pile design parameters of tension piles are based on pile tests that are performed on site, in which the piles are loaded up to failure. Quality control is done by testing a number of production piles to a certain loading level, as is described in the Euro-codes.
The design of the production piles is based on the failure tests that are conducted just before the design is done. The failure tests are performed and interpreted according to CUR236.
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.
Chapter 1: General Introduction
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CUR 236 design procedures 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 (as is done during the acceptance test). To take this effect into account for the acceptance test, an additional load is added to the test load.
The maximum gross test load for acceptance test is calculated based on the following formula in CUR 236;
𝐹
𝑃= 𝑛. 𝐹
𝑑+ 𝑅
𝑠;𝑤𝑟Which:
Fp is the maximum gross test load for the acceptance test [kN];
Fd is design value of the tested tension pile [kN];
Rs;wr is the contribution of pile shaft friction that should be excavated at a later stage [kN];
n is the factor in which the design value of test load should be multiplied to compensate for:
- Influence group effect (f2) - Influence of load variation
- Influence of soil layers or de-watering during excavation
This procedure is questionable because the pile is loaded to a much higher level than it will actually experience during its lifetime.
It turned out at the Amsterdam car parking project that piles failed the acceptance tests due to this additional load which is needed to apply according to CUR 236 guide line.
To have a solution for the explained problem, an evaluation of the standards and project execution is needed. This research focuses on the improvement of design standards for a group of tension piles based on advanced numerical analyses and validates the results by evaluating to Boerenwetering (Albert Cuyp) car parking project in Amsterdam as a case study.
1.3 S UB RESEARCH TASKS
The main research question is to evaluate the Dutch design standards for group of tension piles based on the international methods used in other countries and advanced numerical methods.
In order to reach the main goal of this thesis, some steps need to be taken towards the main research question and therefore the following sub tasks should be included which are;
- Comparison of international design standards to the Dutch design standards for tension piles
- Evaluating Dutch design standards about design bearing capacity of single and group tension piles
- Finding an appropriate model for single and group tension piles by using Finite Element Method
- The importance of pile spacing within a group
- The effect of different pile spacing on failure mechanism in the pile group
(1)
Chapter 1: General Introduction
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1.4 G OALS AND OBJECTIVES
Objective of this thesis is to focus on both failure criteria from a design and testing perspectives and comparing the results to field test observations. The research tools will include analytical and FEM (Plaxis) analysis. The test results from previous projects are provided for the further analyses.
In this research the following points will be also considered and included:
- The effect of applied stress to the soil during installation of the piles - Shaft resistance between pile and the surrounding soil
- Effect of different soil types
The results of the thesis should give insight in the relation between friction capacity, group effect and the mobilized soil cone as well as recommendations for a trial to verify the results.
Chapter 1: General Introduction
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1.5 M ETHODOLOGY AND APPROACH
The plan of approach to this research is as indicated below;
FIGURE 2: PLAN OF APPROACH
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