Seismo-acoustic investigations of consolidated phenomena
The understanding of the settling behaviour and subsequent consolidation of sediments deposited under water has been hampered by the difficulty of recovering high quality samples. There is a need therefore to correlate in situ seismo-acoustic properties of these soils to the geotechnical properties required by engineers. To this end a laboratory study was made during which an instrumented sedimentation column was developed. This apparatus allowed the successful monitoring of compressional wave velocity, shear wave velocity, electrical resistivity, pore pressure and bulk density in a number of natural marine sediments consolidating under self weight introduced into the column as a homogeneous slurry. The seismo-acoustic responses of the sediment samples were observed during primary consolidation, a period of over a thousand hours in some cases. Shear waves were found to be small amplitude waves travelling at velocities of less than Ilm/s. Shear wave velocity was subject to time dependent increase, or creep, thought to be a result of continued clay particle floc interaction during secondary consolidation. The velocity increased with decreasing porosity and increasing overburden but the effect of creep meant that no unique velocity-porosity relationship exists. Empirical relationships were established in order to determine the degree of consolidation and frame compressibility via shear wave velocity measurement. The Biot theory was used to predict the variation of compressional wave velocity as a function of porosity, bulk density and permeability. Frame and shear modulus measurements indicated that the model is justified in assuming weak soil frame parameters and a mass coupl ing factor equal to unity. The continually changing velocity relationship with porosity and bulk density can be satisfactorily modelled using the Geertsma and smit model. The full Biot model was found to predict permeability with reasonable accuracy in high porosity sediments but failed when applied to lower porosity soils. Results suggested that the soils studied exhibited anisotropy with respect to permeability.