A combination of soft sealant closed-cell foam tape was applied on the majority of the interface and hard rubber at the bucket tips Figures 1 c and 2 , which were adequately lubricated with petroleum jelly before each test.
This provided: i sealing of the bucket interior from the external fluid; ii sufficient resistance against peeling of the foam tape during installation improved by the presence of the hard rubber strips at the skirt tip ; and iii minimal friction generation during suction bucket installation.
The horizontal degree of freedom of the actuator allowed the pressure of the suction bucket model against the PIV window to be optimised, while minimising the contact friction. Details of the geometry of the aluminium model used in this centrifuge experimental campaign are shown in Figure 2.
Table 1 provides a summary of the properties of the very fine silica sand used in this investigation. Each sample was prepared by pluviating the silica sand into the PIV strongbox, either from an automated sand rainer for dense samples or by hand for the medium dense sample.
Source: After Chow et al. Matching the viscosity with the target centrifuge acceleration level was necessary to correctly scale the drainage properties of the soil Bienen et al. Sample saturation was achieved through the base of the PIV strongbox. Care was taken to ensure that the fluid reservoir elevation was sufficiently low to prevent piping in the sample.
Hence, the closest rigid boundary is at a distance of 16 cone diameters, greater than the minimum ten diameters recommended in Bolton et al. This provided a basis for characterising the samples and quantifying potential density variations between samples. A single suction bucket test was performed in each sample. The temperature in the centrifuge chamber was monitored throughout for any changes to the pore fluid viscosity, which was confirmed to be negligible for all tests.
This enabled comparison of fully jacked against suction-assisted installation. Each of the remaining tests involved an initial self-weight installation, achieved by penetration of the bucket in displacement control mode i. Once the target self-weight embedment was achieved, the actuator was switched to load control and the achieved vertical load, V s-w was kept constant during the following suction-assisted stage.
Before actuating the syringe pump, the valve on the bucket lid was set to hydraulically connect the suction bucket to the syringe pump.
As discussed in Bienen et al. All of the aforementioned steps were undertaken while the centrifuge was spinning at the target acceleration.
The module relies on an array of subsets to analyse the soil domain, with the subset size and spacing of the subsets defined by the user. A subset size of 50 pixels was adopted here, equally spaced every 25 pixels i. A variation of either the subset size 25 pixels, with spacing of 25 or the subset spacing 10 and 50 pixels, with subset size of 50 resulted in negligible changes to the observations, thus proving that the conclusions generated by interpreting the PIV measurements were insensitive to the PIV analysis parameters.
The automatic reference image updating scheme described in Stanier et al. In general, modern image-based deformation algorithms can tolerate relatively large deformations, so long as the deformations are not overly localised Stanier et al.
In this work occasional problems occurred in the upper part of the sand plug due to localised piping, where the minimum correlation coefficient CC ZNCC for a given image pair was less than the default criteria for GeoPIV-RG recommended by Stanier et al.
In this way the validity of the measurements is ensured at the same time as allowing completion of the analysis in the event of localised piping occurring. This section reports the interpretation of the results obtained through PIV analyses. Initial attention is dedicated to the development of the deformation mechanisms underlying both the self-weight and suction-assisted bucket installation phases.
The effects of parameters such as stress level, relative density and pumping flow rate are illustrated. This is followed by examination of the changes in the soil state, experienced during installation, underpinned by analyses of volumetric and shear strain contours. Finally, the formation and development of the internal soil plug heave is analysed. The dashed-dotted lines represent the lateral limits of the micro-camera field view, and not the rigid side of the strongbox.
Figure 4. The mechanism at the skirt tips transitions from a shallow failure mechanism that propagates to the surface both internally and externally to a quasi-deep mechanism comprised of tip resistance, external and internal skirt friction. The increase in vertical stress generated by the penetrating skirts is thought to increase the skirt tip resistance to a greater extent in the interior of the suction bucket than the exterior, leading to an asymmetric change in effective stress.
This down-drag enhancement cannot be neglected in a suction bucket installation prediction Houlsby and Byrne, b. This is in broad agreement with the work of Bolton et al. It is finally noted in the PIV analyses that the soil contained by the skirt the soil plug is largely unaffected by the self-weight installation Figure 4 d , as the minimal plug heave observed confirms negligible changes in volume. Figure 5. In terms of deformation mechanism, a response similar to that shown in Figure 5 b was observed Figure 6 , implying that the relative density of the soil has little influence on the form of mechanism generated during self-weight jacked penetration.
Figure 6. However, compared with the self-weight phase, the mechanism is asymmetric about the skirt tip, with a bias towards soil flow to the bucket interior. The induced seepage flow reduces the effective stresses and generates steep displacement field gradients around the skirt tips. Consequently, the soil is displaced predominantly inside the bucket and upwards, which further facilitates installation.
Figure 7. This links to the study of the stress level effect during self-weight penetration in Figure 5. A similar deformation mechanism is observed for the two cases, where the soil under the skirt tips is quickly displaced, following a prevalent inwards motion. This is indicative of effective stresses under the skirt tips being reduced as a result of the applied suction. Although suction-induced seepage is responsible for reducing the effective stresses around the skirt tips and thus the tip resistance to extremely low values when critical hydraulic gradients are approached, the difference in the initial effective stresses at the skirt tips is reflected in the final penetrations achieved.
Figure 8. Relative density did not appear to influence the shape of the mechanism Figure 9 , but did affect the final penetration depth achieved. Figure 9. This is a result of the larger suction generated at the lid invert and hence larger seepage around the skirt tip and through the soil plug, which encourages a larger amount of soil to be displaced upwards. Figure However, the curvature of the test result indicates more significant seepage losses when compared with the other tests with a theoretical constant suction bucket penetration rate signifying no seepage losses , brought about by increasing permeability through loosening of the soil plug.
The remainder of embedment is achieved through suction: a remote-operated vehicle ROV pumps water out of the top suction port after sealing pile top valves. Pile top and ROV instrumentation contribute to a precise installation. The pile can also be retrieved by reversing the installation process, applying an overpressure inside the caisson. For nearly three decades, suction piles have been effective foundations for pipeline initiations and terminations, as well as subsea manifolds and pump stations.
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