Abstract
This thesis introduces a new in situ site investigation tool aimed at providing an accurate and rapid assessment of the undrained shear strength of near-surface offshore soil. The research describes a new spherical free-falling penetrometer (FFP) which represents a new design approach for FFPs which are almost invariably slender, full-shafted projectiles. The custom-made free-fall sphere is 250 mm in diameter and consists of two hemispheres that are bolted together with an internal vertically orientated cylindrical void to accommodate instrumentation and a motion logger. The free-fall sphere is designed to be released from a hanging position above the seabed and penetrate the seabed by the kinetic energy obtained through free-fall in water.
This thesis carried out an experimental study involving field tests and centrifuge experiments. The field tests comprise dynamic embedment data for 87 different tests undertaken in two soft soil sites: (i) an inland lake, Lower Lough Erne in the Northwest of Ireland and (ii) an offshore site in the Firth of Clyde which is located off the coast of Scotland in the Irish Sea. The centrifuge experiments comprise dynamic tests in three different soils: (i) Laminaria soil, recovered from the Timor Sea, (ii) West Africa clay, recovered from the Gulf of Angola and (iii) kaolin clay. In both sets of experiments, the sphere contained instrumentation that accurately measured the motion history in soil. These data led to the development of a newly proposed theoretical framework for assessing the dynamic resistance forces acting on the free-fall sphere. The framework was cast in terms of both fluid mechanics drag resistance and geotechnical shear resistance, but formulated in terms of a single capacity factor. In each soil a power law function was adopted in order to account for the strain rate dependency. The appropriateness of the strain rate parameter was demonstrated by varying β within the typical range reported from variable rate penetrometer tests (β = 0.05 to 0.09). In the field and centrifuge experiments, the best-fit rate parameter was calculated using β = 0.07.
To improve the strength characterisation of near surface seabeds with a shallowly-embedded ball penetrometer or free-fall sphere, the thesis describes centrifuge experiments designed to quantify the shallow penetration effects. The experiments were carried out with an 11.3 mm diameter ball penetrometer penetrating kaolin clay under undrained conditions over a range of normalised strength ratios, su/γ'D. The tests captured the influence of two important mechanisms: (i) the varying soil buoyancy with penetration depth and (ii) the reduced bearing factor, Nb-shallow, arising from the shallow failure mechanism. The centrifuge results were combined with reinterpreted data from LDFE analyses to form a unique relationship between the transition depth and the normalised strength ratio over the range su/γ'D ≈ 0.07 to 40. This led to the development of a shallow penetration framework to determine more accurately the undrained shear strength of near surface soil.
Both frameworks described in the thesis represent significant advances in the understanding of FFPs and when combined provide accurate estimation of the undrained shear strength with the IFFS compared to a conventional ball penetrometer.
This thesis carried out an experimental study involving field tests and centrifuge experiments. The field tests comprise dynamic embedment data for 87 different tests undertaken in two soft soil sites: (i) an inland lake, Lower Lough Erne in the Northwest of Ireland and (ii) an offshore site in the Firth of Clyde which is located off the coast of Scotland in the Irish Sea. The centrifuge experiments comprise dynamic tests in three different soils: (i) Laminaria soil, recovered from the Timor Sea, (ii) West Africa clay, recovered from the Gulf of Angola and (iii) kaolin clay. In both sets of experiments, the sphere contained instrumentation that accurately measured the motion history in soil. These data led to the development of a newly proposed theoretical framework for assessing the dynamic resistance forces acting on the free-fall sphere. The framework was cast in terms of both fluid mechanics drag resistance and geotechnical shear resistance, but formulated in terms of a single capacity factor. In each soil a power law function was adopted in order to account for the strain rate dependency. The appropriateness of the strain rate parameter was demonstrated by varying β within the typical range reported from variable rate penetrometer tests (β = 0.05 to 0.09). In the field and centrifuge experiments, the best-fit rate parameter was calculated using β = 0.07.
To improve the strength characterisation of near surface seabeds with a shallowly-embedded ball penetrometer or free-fall sphere, the thesis describes centrifuge experiments designed to quantify the shallow penetration effects. The experiments were carried out with an 11.3 mm diameter ball penetrometer penetrating kaolin clay under undrained conditions over a range of normalised strength ratios, su/γ'D. The tests captured the influence of two important mechanisms: (i) the varying soil buoyancy with penetration depth and (ii) the reduced bearing factor, Nb-shallow, arising from the shallow failure mechanism. The centrifuge results were combined with reinterpreted data from LDFE analyses to form a unique relationship between the transition depth and the normalised strength ratio over the range su/γ'D ≈ 0.07 to 40. This led to the development of a shallow penetration framework to determine more accurately the undrained shear strength of near surface soil.
Both frameworks described in the thesis represent significant advances in the understanding of FFPs and when combined provide accurate estimation of the undrained shear strength with the IFFS compared to a conventional ball penetrometer.
| Original language | English |
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| Qualification | Doctor of Philosophy |
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| Publication status | Unpublished - 2015 |