PhD Project: Physical and numerical modelling of silt with focus on offshore wind turbines

  • Holmsgaard, Rikke, (Project Applicant)
  • Ibsen, Lars Bo (Other)
  • Nielsen, Benjaminn Nordahl, (Project Participant)


In these days, offshore wind turbines are being constructed in the North Sea where the subsurface contains a large part of silty soil. Silt is an ‘intermediate’ soil, which has a grain size that lies between sand and clay. The behaviour of silt is often considered to be close to that of clean clays or sands, depending on its grain size distribution. However, silt has its own characteristics and as little guidance in silty soil exists, silt is leading to conservative and uneconomic designs.

In intermediate soils, such as silty soils, standard cone penetration tests may vary from undrained to partially or fully drained conditions. This means that use of standard correlations developed for clean sand or clay will not work for soils where penetration takes place under partially drained conditions. Most design problems occur as drained over long time scales, which makes the soil strength interpretation more difficult in case the cone penetration process is partially drained or undrained.

For sand and clay, soil properties are well known due to triaxial tests. For silty soils however, there exist only little data. Some researchers have studied the effect of fines content on the Critical State Line (CSL) and cyclic liquefaction potential. They concluded that, as the content of fines increases, the CSL in the e-lnp space rotates clockwise around a pivot point. However, the results were based on non-plastic sand and silt mixtures with fines content less than about 30%.

The aim of the project is therefore to characterise silty soils in order to physically understand silt. The procedure for understanding silty soil behaviour is to carry out several advanced field and laboratory investigations. In order to understand the silty soil drainage conditions, a various number of field Cone Penetration Tests (CPT) must be conducted at a test site that contains a large amount of silty soil and a high water table. By taking soil samples at the test site, and conducting index- and triaxial test, knowledge of the strength-strain behaviour of silty soils will be possible.

Effective start/end date01/10/201030/09/2013