In situ rheometry of cohesive sediments under water wave pressure

The problems and challenges involved in in situ measurement of dynamic mechanical properties of non-rigid mud bed under water wave pressure have been investigated. A detailed preliminary investigation on a natural mud was carried out in a flume-generating monochromatic water waves at a fixed frequency in which dynamic bulk density profile, water turbidity and viscoelastic parameters from the bed response to plane shear waves, were measured with a specially designed measuring device. The device, which is based on the analysis of the characteristics of shear wave field generated within the mud bed, was used in situ at a frequency of 619 Hz. The effect of water wave pressure on the bed was picked up at a fixed location in the form of shear wave pattern for analysis using Fast Fourier Signal Processing technique. The shear wave velocity through the mud and the phase difference between shear stress and shear strain at the wave-generating surface of the device were simultaneously measured. A phase angle of about 18° was obtained for the mud with evidence of bed weakening by up to 57% within the surface wave amplitude that showed no mud surface erosion. The problems inherent in carrying out this type of investigation are also discussed alongside the result obtained that demonstrated the capability of this technique in this type of application.