Predicting long term performance of offshore wind turbines using cyclic simple shear apparatus

Offshore wind turbine (OWT) foundations are subjected to a combination of cyclic and dynamic loading arising from wind, wave, 1P (rotor frequency) and 2P/3P (blade passing frequency) loads. Under cyclic/dynamic loading, most soils change their characteristics. Cyclic behaviour (in terms of change of shear modulus change and accumulation of strain) of a typical silica sand (RedHill 110) was investigated by a series of cyclic simple shear tests. The effects of application of 50,000 cycles of shear loading having different shear strain amplitude, cyclic stress ratio (ratio of shear to vertical stress), and vertical stress were investigated. Test results were reported in terms of change in shear modulus against the number of loading cycles. The results correlated quite well with the observations from scaled model tests of different types of offshore wind turbine foundations and limited field observations. Specifically, the test results showed that; (a) Vertical and permanent strain (accumulated strain) is proportional to shear strain amplitude but inversely proportional to the vertical stress and relative density; (b) Shear modulus increases rapidly in the initial cycles of loading and then the rate of increase diminishes and the shear modulus remains below an asymptote. Discussion is carried out on the use of these results for long term performance prediction of OWT foundations

Soil water retention curve of silty sand – experimental investigation using different laboratory methods

Most of the infrastructure investments and earthworks not requiring deep foundations are usually designed in the vadose zone to avoid excessive groundwater-structure interactions. The mechanical behaviour of soil material, under partially saturated conditions, is greatly influenced by pore-water tension, known as soil suction, and the characteristics of the Soil Water Retention Curve (SWRC). In the present paper, the SWRC of a silty sand was determined using two different experimental methods. In the first method, a modified pore water pressure transducer was used for suction monitoring, while the specimen was allowed to change its moisture content by natural evaporation. For the second method, a modified consolidation cell fitted with a high air entry value ceramic disc on the base pedestal was used. Suction was applied using the axis translation technique by utilising pore air and pore water pressure controllers, while moisture was monitored using a volumetric measurement system. Through the determination of the SWRC for the silty sand, this paper intends to compare the abovementioned testing methods basedon the produced SWRCs and to reveal advantages and limitations