Kristina Thomassen

Bio: Kristina Thomassen is an academic researcher from Aalborg University. The author has contributed to research in topics: Tension (physics) & Medicine. The author has an hindex of 4, co-authored 12 publications receiving 32 citations.

Experimental Investigations of Tension Piles in Sand Subjected to Static and Cyclic Loading

Abstract: For offshore wind turbines, the design criterion regarding tolerated deflection of the wind turbine structure is important in order to maintain the desired energy production. Unwanted permanent deflections of the wind turbine are usually caused by poor design regarding the effect of repeated, cyclic loading occurring from wind and waves. Due to the growing demand on renewable energy, many wind farms are planned for construction in the near future. With limited space for additional onshore and near-shore installations, these wind farms will be located on sites further offshore and with deeper water depths greater than existing wind farms. This implies new requirements for the foundations and support structures. Previously, the monopile has been the most commonly used type of foundation, but with increased water depths other foundation concepts such as the jacket pile foundation have become popular. The jacket pile foundation is widely used in the offshore oil and gas industry, in which the piles are mainly loaded in compression. However, wind turbines have relatively low self weight compared to the forces from wind and waves which provide great overturning moments. Hence, tension regularly occurs in one or more of the piles within a jacket structure supporting an offshore wind turbine. Over time, repeated cyclic tension in the piles can lead to accumulated displacement upward, resulting in critical permanent tilt of the turbine. The current design methods do not adequately account for the cyclic loading effects on the bearing capacity and the accumulated displacements. This thesis presents a state-of-the-art review concerning the shaft capacity of piles installed in sand. The review shows the factors influencing the pile shaft capacity and the methods that have been used to find and analyze these factors. The most commonly used methods for evaluating the shaft capacity are small-scale pile load tests. However, the current design methods are based on rather few full-scale pile load tests.

Small-Scale Testing of Laterally Loaded Non-Slender Monopiles in Sand

Abstract: In current design of o shore wind turbines, monopiles are often used as foundation. The behaviour of the monopiles when subjected to lateral loading has not been fully investigated, e.g. the diameter e ect on the soil response. In this paper the behaviour of two non-slender aluminium piles in sand subjected to lateral loading are analysed by means of small-scale laboratory tests. The six quasi-static tests are conducted on piles with diameters of 40 mm and 100 mm and a slenderness ratio, L/D, of 5. In order to minimise scale e ects, the tests are carried out in a pressure tank at stress levels of 0 kPa, 50 kPa, and 100 kPa, respectively. From the tests load-de ection relationships of the piles at three levels above the soil surface are obtained. The load-de ection relationships reveal that the uncertainties of the results for the pile with diameter of 40 mm are large due to the small soil volume activated during failure. From the load-de ection relationships normalised as H/(L2Dγ′) and y/D indicates that the lateral load, H, is proportional to the embedded length square times the pile diameter, LD. Furthermore, by comparing the normalised load-de ection relationships for different stress levels it is seen that small-scale tests with overburden pressure applied is preferable.

Safety Instructions in the AAU Geotechnical Laboratory: (Large Yellow Box)

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Test Procedure for Axially Loaded Piles in Sand

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Calibration chamber size effects on penetration resistance in sand. discussion and closure

Abstract: A discussion of an earlier paper with the aforementioned title by R. Salgado, J.K. Mitchell, M. Jamiolkowski, published in this journal (Volume 124, Number 9, September 1998), is presented. The discusser commends the original authors for developing a method to estimate calibration chamber effects on cone penetration resistance with size effect curves based on yielding at the lateral boundary. However, size effects caused by constant stress top and boundary conditions were not considered, which the discusser believes can cause a very significant reduction in the values of cone penetration resistance. Closure by the original authors on this topic is also provided.

Closure of "Mechanisms of Shaft Friction in Sand from Instrumented Pile Tests"

Abstract: Comprehensive measurements of the effective stresses developed during the installation, equalization, and load testing of displacement piles in a loose to medium dense quartz sand are presented. The results shed new light on the mechanisms that control shaft friction in sand. First, it is demonstrated directly that the stresses developed at any given soil horizon depend strongly on both the distance of that horizon from the pile tip and the soil's initial state. Second, pile loading is shown to induce radial effective stress changes associated with the soil fabric set up by installation and dilation phenomena at pile‐soil interface. Thirdly, the operational angles of interface friction are found to be constant volume values that correlate well with the results from laboratory interface shear tests.