The use of wind power energy has gradually increased as we pay attention to mitigate the CO2 emission and climate change. South Korea government intends that the electricity production by wind power energy would be 20% in 2030, which may lead more offshore wind farms. For economical design of offshore wind farm, we need credible geotechnical parameters. Site investigation and lab test should provide appropriate geotechnical parameters. For laboratory tests, undisturbed samples are collected for geotechnical tests. We should determine whether the undisturbed sample is applicable to provide credible geotechnical parameters by designated tests. Sample disturbance can be quantified by non-destructive and large-deformation methods. This study shows a potential method of elastic wave velocity measurements to validate and quantify undisturbed sample quality on shipboard.

We conducted P- and S-wave velocity measurements through reconstituted soil samples of sand and illite inside thin-walled tubes. The diameter of the sample is 72mm and the length is from 7cm to 1m based on thin-walled tube sizes and length for fine-grained samples. We used illite and sand for soil specimens. The illite, one of dominant clays in marine sediment conditions, has liquid limit of 47 and plastic limit of 29. The water content of illite was close to plastic limit. The sand is Jumunjin sand with the maximum void ratio of 0.919 and minimum void ratio of 0.625. The sand was set at dense state. We used piezo-crystal discs for P-wave velocity measurements and bender elements for S-wave velocity measurements. The source signal was 20Hz of square wave at 10 volts from peak to peak. The receiver was installed to a filter/amplifier and an oscilloscope. We determine Vp and Vs using the first arrival time of the received signal in time domain. The experimental configuration of elastic wave measurements was validated by measuring Vp with known material of the plastic cylinder.

Results indicate that the acrylic and stainless steel thin-walled tubes have higher stiffness than the soil specimens, which can cause elastic waves to be reflected and disturb elastic waves at the receiver. S-wave velocity can be measured in short wave travel distance in illite and sand. However, as the sample length increases, elastic wave reflections and refractions generate noises and that causes difficult to determine the first arrival time in time domain analyses. P- and S-wave signals become weak and it is hard to detect the first arrival of signals. When P-wave in the radial direction goes through specimens in thin-walled tube, the wave propagation may happen along the thin-walled tube, so the waves should be identified whether signals pass the specimen. It is suggested that further methodological improvements are required to enhance the reliability of these measurements.