Field tests and interpretation of screw micropiles subjected to axial loading in cohesive soil

Author(s):  
Alex Sanzeni ◽  
Emanuele Giuseppe Danesi
Author(s):  
Johannes Labenski ◽  
Christian Moormann ◽  
Johannes Aschrafi ◽  
Britta Bienen

Open steel pipe piles are used for various applications in costal engineering and port structures and they are becoming increasingly more important for offshore structures. A plug formed during the installation of open steel pipe piles has an influence on the installation process of the steel pipe pile as well as on the final bearing behaviour and the pile resistance. Forming of the plug depends on different influences, e.g. the pile diameter, the soil conditions and the installation method. To obtain a better understanding of the formation of the plug and its consequences several experimental small-scale tests, experimental field tests and numerical simulations have been performed. But so far this phenomenon is not completely investigated yet. At the beginning of this paper a structured overview about the characterization of the plug as well as soil mechanical processes during the pile installation in non-cohesive soil are provided. Then, the results of a centrifuge study are shown. The focus of this paper is the numerical simulation of the installation process of impact and vibratory driven piles in non-cohesive soil to investigate the effect of plugging with regard to different installation methods. Centrifuge experimental results are utilized to validate the numerical model, which can provide detailed insights into the physical processes occurring in the soil but are difficult to measure in experiments. The numerical analyses hence illustrate zones of densification/loosening, which highlight the differences resulting from the installation methods.


Author(s):  
Donatas Urbaitis ◽  
Ieva Lekstutytė ◽  
Domas Gribulis

In order to evaluate overconsolidation ratio (OCR) of soil, the necessity to restore them as much as possible to in situ conditions appears, because sometimes when it is not taken into account, mistakes could be made while interpreting mechanical – strength properties of the soil. According to the work purpose, overconsolidation ratio of the investigated soil was set by performing odometer test and the obtained values were compared with the OCR calculated from cone and seismic penetration data. When the tests were performed and data analysed, it was found that OCR values depends on soil occurance depth, strength characteristics and stress conditions. The OCR values decreases with the declination of the depth. As many authors noted in the literature – the upper part of the soil is consolidated abnormally, thereby we can see that in our work. When results are evaluate, we can conclude that all of the analysed soils was overconsolidation. That was demonstrated by calculations according static penetration, oedometer test and seismic waves results. OCR values differences between the laboratory and field tests can show low quality of soil sampling, also due to correlations which was applied.


2018 ◽  
Vol 55 (1) ◽  
pp. 34-44 ◽  
Author(s):  
Zhengyang Guo ◽  
Lijun Deng

Field tests of full-scale screw micropiles with a diameter varying from 76 to 114 mm and a length varying from 1.6 to 3 m were undertaken to investigate the axial pile capacities, load-transfer mechanism, and end installation torques of the piles in cohesive soils. Forty tests were performed on piles subjected to axial compressive and tensile loads. Six tests were instrumented with strain gauges on the pile shaft. Results showed the piles reached the limit state before the displacement exceeded 10% of the shaft diameter. The majority of axial load was transferred to the threaded segment. The adhesion coefficient of the top smooth shaft at the limit state was less than 0.1. The failure mode along the cylindrical threaded shaft was cylindrical shearing along the edge of the threads; the threads increased the axial capacities of the segment. Axial capacities of the threaded tapered segment were 43% on average greater than that of a cylindrical segment with the equivalent volume. Compressive capacities of all test piles were estimated and the results agreed reasonably well with the measured capacities. A theoretical torque model was proposed to estimate the end installation torques based on the cone penetration test results; the theoretical results matched the measured end torques very well.


Author(s):  
Tobias Rapp ◽  
Georg Jacobs ◽  
Dennis Bosse ◽  
Tim Schröder ◽  
Ralf Starzmann ◽  
...  

AbstractAs a regenerative energy source, tidal energy can significantly contribute to greenhouse gas reduction, even though the potentially achievable energy output is lower than that of wind or solar energy. The decisive advantage of tidal turbines lies in the simply and reliably predictable energy output. However, their commercial use has so far been impeded by the fact that on the one hand complex mechanical systems are required to convert energy of tidal currents and on the other hand multi-axial loading conditions caused by turbulent ocean currents act on the turbine. For this reason, field tests on prototypes are an essential part of the development strategy to ensure operational reliability. However, in-field tests do not allow for accelerated lifetime testing, so that test bench experiments are becoming an increasingly important alternative. Today, established procedures for testing the turbines main bearings and gearing system are already available, both for setting up the required test configuration and for determining the corresponding test loads. However, the use of advanced calculation methods, such as the finite element method for stress calculation, requires a deep understanding of the examined components and hinders the transfer of the approaches to other components.To simplify the process of test loads determination, a general methodology is presented, which relies exclusively on standardized empirical calculation rules. Doing this, fatigue equivalent loads can be determined for any component in a simple process. It was shown that the achieved reduction in complexity opens further potential for test acceleration, since several components can be tested simultaneously.


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