Physical Modelling of Silt in Relation to Offshore Wind Turbines

Offshore wind energy is a sustainable form of energy that can help reduce the emission of greenhouse gases and increase the share of renewable energy. In order to ensure that offshore wind energy can continue to be a competitive source of energy compared to other types of renewable energy, it is important that the optimization of the technology continues. The foundation of an offshore wind turbine constitutes about 20-30 % of the total cost, so there is great interest in minimizing the costs associated with the foundations of offshore wind turbines. The designs of offshore wind turbines are often controlled by water depth and soil conditions, which is why it is essential to be able to determine the soil conditions with high accuracy. The North Sea is the preferred site for wind farms as there is shallow water with a relatively short distance to the coast. The subsoil in the North Sea area contains many meters of sand and silt sediments, wherefore focus is particularly set on optimizing offshore wind turbine design founded on sand or silt. Silt, especially causes problems when assessing soil condition to determine soil parameters.

This thesis concerns the determination of soil conditions in silty soil using the CPT (Cone Penetration Test) as the in situ test method, since the CPT is in situ method being utilized the most worldwide. The overall objective of this thesis is to improve the knowledge of interpreting CPT in silty soil, thus making it possible to determine soil parameters more precisely so that the competitiveness of offshore wind turbines can be improved.

Carrying out in situ tests offshore is very expensive. Therefore, all experiment regarding this project is conducted onshore. A field in Dronninglund was found useful for this purpose since the subsoil at the field mainly consists of silty soil and the water table is situated just below the surface so the field is comparable to offshore conditions. In connection with the project, CPT and SCPT (Seismic Cone Penetration Test) were conducted in the field, just like large soil samples were collected with the purpose of carrying out laboratory tests.

This thesis is based on a number of scientific papers that deal with issues within determination of soil conditions in silt. The thesis consists of three parts. Part I covers issues related to the classification of silt based on laboratory tests. Part II deals with the interpretation of the CPT in silty soil conducted under partially drained conditions while Part III deals with the interpretation of the SCPT in silt. Each part is based on an experimental programme which is described at the beginning of each part.

Classification of silt can be difficult because the contrast between silty sand and sandy silt as well as between silty clay and clayey silt is often not simple to define. In addition, there is no worldwide consensus on the definition of silt. Part I of this thesis is, therefore, exploring the disagreement in classification of silt with the aim of assessing how silty soil is best classified. In order to do so, classification tests are carried out, meaning water content, grain density, grain size distribution (sieving and hydrometer analysis), plastic limit and liquid limit (Fall cone and Casagrande cup). The results of the tests show that silt should not be classified in terms of plastic and liquid limit since the liquid limit (especially Casagrande cup method) are operator dependent. Instead, the grain size distribution is more suitable for classifying silty soils.

As mentioned, Part II concerns interpretation of CPT in silty soil conducted under partially drained conditions. Normally, CPTs are carried out at a penetration rate of 20 mm/s. This rate yields undrained conditions in clays and drained conditions in sands. In silt, a penetration rate of 20 mm/s yields partially drained conditions, giving rise to interpretation difficulties since existing empirical correlations are either developed for fully drained conditions (sands) or fully undrained conditions (clays). In connection with Part II, 15 CPTs with penetration rates of approximately 60, 20, 5, 1 and 0.5 mm/s (3 of each) have been carried out. The results show that when the penetration rate is reduced the cone resistance increases, the pore pressure decreases and the sleeve friction seem unaffected. This is caused by the fact that a reduced penetration rate generates lower excess pore pressure. In addition, the results indicate that if the cone resistance is measured under partially drained conditions, it is underestimated by a factor of approximately 1.4 compared to if it were measured under fully drained conditions.

When designing foundations for offshore wind turbines, the soil stiffness parameters are essential and as such it is important to determine shear modulus for the silt in situ. The shear modulus can be computed by means of the relative new Seismic CPT. SCPT is normally carried out with data collected with 1.0 m intervals at which the soil is assumed homogeneous between two measurements. Silt, however, is rarely homogeneous. In connection with Part III, five "down-hole" SCPT are conducted: three with 1.0 m intervals and two with 0.5 m intervals. This is done to examine whether data collected every 0.5 m yields more accurate and reliable results than collecting data every 1.0 m. In order to examine how reliable the results are, at least 8 measurements at each depth have been conducted. The results, unexpectedly, show that although the silt is inhomogeneous, a more closely spaced data interval increased the variability. For this reason, it is not recommended to carry out SCPT with an interval of less than 1.0 m. In addition, the results show that SCPT measurements are unaffected by the dissipation process that initiates each time a SCPT reading is carried out (because the CPT is stopped), and so it is irrelevant whether the readings are taken approximately 1 or 5 minutes after the CPT is stopped.

The three parts which form this Ph.D. thesis has led to a better understanding of conducting CPT in silty soil as well as the interpretation of it. The research contributes to more accurate determination of soil parameters of silts which may lead to increased economic advantages for future offshore wind.