Benjaminn Nordahl Nielsen

Advanced Laboratory Setup for Testing Offshore Foundations

This paper describes a test setup for testing small-scale offshore foundations under realistic conditions of high pore-water pressure and high impact loads. The actuator, used for loading has enough capacity to apply sufficient force and displacement to achieve both drained and undrained failure modes for small-scale offshore foundations. Results from trial tests on two small-scale bucket foundations, subjected to transient or cyclic loading, are presented. Tests showed that cavitation limits the undrained bearing capacity. Hence, a high pore-water pressure is important for simulating offshore conditions.

Preconsolidation of Søvind Marl - a highly fissured Eocene clay

Determination of the preconsolidation stresses is a key tool in geotechnical engineering used to evaluate and estimate the behavior of soils. However, it has proven difficult to accurately estimate these stresses in highly fissured, overconsolidated clays, due to the influence of the fissured structure. In the current study, oedometer tests were performed on Sovind Marl, a plastic Eocene clay containing a large number of fissures and slickensides. Four incremental loading oedometer tests and four continuous loading oedometer tests were performed in order to determine the preconsolidation stresses. Multiple assessment methods were used to assess preconsolidation stresses based on the oedometer test data, results of which are presented and discussed on this paper. All of the test and interpretation methods yielded very similar results, with two possible values calculated as the preconsolidation stresses. However, only the upper stress bound is actually identified as being associated with preconsolidation; the lower bound is generated as a result of the collapse of the fissured structure in response to increasing stresses that were applied during testing. The upper bounds of the preconsolidation stresses of Sovind Marl were between 6300 and 8900 kPa, whereas the lower were between of 600 and 800 kPa.

Response of cyclic-loaded bucket foundations in saturated dense sand

AbstractOne of the major concerns of designing offshore wind turbine foundations is to address the effects of cyclic loading coming from, especially, wave loading. This paper describes the results ...

Transiently Loaded Bucket Foundations in Saturated Dense Sand - Demonstration of the Boot Effect

The mono bucket foundation is a cost-effective foundation for offshore wind turbines. During a storm, these foundations are exposed to large wave loads of short duration. This paper investigates the effect of increased loading rate on the bearing capacity of two mono bucket foundations installed in dense sand inside a pressure tank. The foundations had aspect ratios (skirt lengths [L] relative to the diameters [D]), L/D, of 0.5 and 1.0, respectively. Foundations were brought to failure with varying loading rates, resulting in drained, partly drained, or undrained behavior. Increases in bearing capacity were observed as the loading rate increased. This behavior was caused by a combination of dilative soil behavior and suction created by upward movement, known as the boot effect. For mono bucket foundations, the boot effect resulted in an increased bearing capacity that was 18 to 25 times higher than the drained capacity. Furthermore, the boot effect also led to an increase in stiffness. The stiffness of the partly drained response is measured up to four times higher compared with drained behavior.