This paper describes and benchmarks a new implementation of image-based deformation measurement for geotechnical applications. The updated approach combines a range of advances in image analysis ...algorithms and techniques best suited to geotechnical applications. Performance benchmarking of the new approach has used a series of artificial images subjected to prescribed spatially varying displacement fields. An improvement by at least a factor of 10 in measurement precision is achieved relative to the most commonly used particle image velocimetry (PIV) approach for all deformation modes, including rigid-body displacements, rotations, and strains (compressive and shear). Lastly, an example analysis of a centrifuge model test is used to demonstrate the capabilities of the new approach. The strain field generated by penetration of a flat footing and an entrapped sand plug into an underlying clay layer is computed and compared for both the current and updated algorithms. This analysis demonstrates that the enhanced measurement precision improves the clarity of the interpretation.
Shallow penetrometers are devices that penetrate into and measure the properties of surficial offshore sediments via multi-phase tests involving penetration, dissipation, and rotation stages. In ...fine-grained soils such as silts and clays, these testing stages yield undrained strength, consolidation, and friction properties relevant to subsea pipeline and shallow foundation design. This paper describes toroid and hemiball devices of the scale for use in box-core samples and associated interpretation methods for the penetration and dissipation stages. The aim of the paper is to provide all tools needed to design and interpret these tests. New large-deformation finite element (LDFE) dissipation solutions are presented, which can be used for back-analysis of the dissipation stage. Results of an extensive laboratory proof testing exercise in kaolin clay, for both the hemiball and toroid penetrometers, are also reported. These results highlight the potential of the two devices to quickly and economically assess strength and consolidation characteristics of fine-grained sediments in box-core samples recovered to the deck of a site investigation vessel.
AbstractAccurately predicting peak penetration resistance qpeak during spudcan installation into sand overlying clay is crucial to an offshore mobile jack-up industry still suffering regular ...punch-through failures. This paper describes a series of spudcan penetration tests performed on medium-loose sand overlying clay and compares the response to existing centrifuge data from tests performed on dense sand overlying clay. Together these data demonstrate that punch-through is a potential problem for both dense and loose sand overlying clay soil stratigraphies. Using this experimental database, a failure-stress-dependent model has been modified to account for the embedment depth, and the depth of occurrence of qpeak is shown to be a function of the sand thickness Hs. The model then was recalibrated, taking these findings into account, for a larger range of material properties and ratios of sand thickness to spudcan diameter (Hs/D). Finally, the performance of the modified and recalibrated model is verified by comparing its predictions with those calculated using current guidelines. The comparisons show that the modified model yields more accurate predictions of qpeak over the range of Hs/D ratios of practical interest, which when used in practice will potentially mitigate the risk of unexpected punch-through on sand overlying clay stratigraphies.
Shallow penetrometers are a new type of device that measures the properties of surficial offshore sediments via multi-phase tests involving penetration, dissipation, and rotation stages. In ...fine-grained soils such as silts and clays, these testing stages yield properties relevant to subsea pipeline and shallow foundation design; namely, undrained strength, consolidation, and interface friction. This paper describes the fundamentals of the rotation stage, including models required for data interpretation, encompassing both a total and an effective stress framework. Additionally, new relationships to evaluate the pore pressure scaling factor, which is a key interpretation parameter required to convert discrete measurements of pore pressure on the penetrometers to an average pore pressure over the contact area, are developed based on large-deformation finite element simulations. Results from an experimental campaign using kaolin clay samples are presented, illustrating the potential of the devices to rapidly and repeatably measure interface friction properties of fine-grained sediments offshore. The results compare well with comparative measures obtained from shear box tests conducted at similarly low effective stress levels. Recommendations regarding future in situ applications are given at end of the paper.
Climate models incorporate photosynthesis-climate feedbacks, yet we lack robust tools for large-scale assessments of these processes. Recent work suggests that carbonyl sulfide (COS), a trace gas ...consumed by plants, could provide a valuable constraint on photosynthesis. Here we analyze airborne observations of COS and carbon dioxide concentrations during the growing season over North America with a three-dimensional atmospheric transport model. We successfully modeled the persistent vertical drawdown of atmospheric COS using the quantitative relation between COS and photosynthesis that has been measured in plant chamber experiments. Furthermore, this drawdown is driven by plant uptake rather than other continental and oceanic fluxes in the model. These results provide quantitative evidence that COS gradients in the continental growing season may have broad use as a measurement-based photosynthesis tracer.
Spudcan foundations of mobile jack-up rigs are penetrated into the seabed under seawater ballast preload, which is shed prior to rig operations commencing. During pauses in the installation process ...and during operation, soil beneath the spudcan foundations stiffens and strengthens due to consolidation. On the application of further loading or during spudcan extraction, this causes increased resistance, which in extremis can result in punch-though type failure. This note reports results from a series of experiments with particle image velocimetry measurements that were performed in a drum centrifuge to facilitate observation of the effects of a load-hold period on the soil movements around a model spudcan during subsequent further loading. The results show that the dimensionless load-hold period dominates the enhancement in the penetration resistance, due to significantly more soil being mobilised following a long load-hold period. These observations might be useful to (a) predict the enhancement in bearing capacity factor due to a load-hold period during installation or operation and (b) predict the footing extraction resistance during jack-up re-deployment.
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