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Low H.E.,Advanced Geomechanics | Low H.E.,Benthic Geotech Pty. Ltd. | Randolph M.F.,University of Western Australia
Journal of Geotechnical and Geoenvironmental Engineering | Year: 2010

A manually operated penetrometer (DMS) fitted with cylindrical (T-bar) and ball penetrometer tips was developed for measuring the profiles of undisturbed and remolded undrained shear strength within box-core samples. This paper summarizes the findings of a series of miniature penetrometer tests and vane shear tests that were carried out on reconstituted clay from a local site in Western Australia. The aim of the tests was to evaluate the potential of the DMS in characterizing the shear strength of seabed surficial sediments. It was found that the DMS gave essentially identical T-bar and ball penetration resistances but these were up to 17% lower than the net cone resistance. From the comparison between the T-bar and ball penetration resistance and the shear strengths measured from vane shear tests, average N factors of 11 and 14 were obtained for intact and fully remolded conditions, respectively. The test results suggest that the DMS is a reliable and efficient means of obtaining intact and remolded shear strength profiles. © 2010 ASCE.

Kong V.,Advanced Geomechanics | Kong V.,University of Western Australia | Cassidy M.J.,University of Western Australia | Gaudin C.,University of Western Australia
Canadian Geotechnical Journal | Year: 2013

Mobile jack-up drilling rigs often need to return to a site where a previous installation has left footprints in the seabed. Reinstallation near these depressions is a problematic operation because the jack-up's circular spudcan footings become subjected to eccentric and (or) inclined loading conditions. This can lead to structural failures within the jack-up legs and (or) excessive leg tilt and hull displacement. This paper reports a comprehensive set of geotechnical centrifuge experiments that investigated the effect of footprint geometry on the reinstallation response. Artificial conical shaped footprints were manually cut in the centrifuge sample, ensuring consistent shapes and minimizing any variation of undrained shear strength due to the process of initially installing and retrieving a spudcan. The effect of footprint geometry was thereby isolated. The vertical, horizontal, and moment loads induced on a model footing when penetrated at varying offsets are presented and these provide evidence on the effect of different footprint depths and angles on installation. The footprint geometry governed the horizontal force and moment observed during reinstallation between the level of the touchdown and the footprint toe. Further experimentation has shown that an equivalent skirted footing induced significantly higher horizontal forces (although it can be assumed to be significantly stiffer).

O'Loughlin C.D.,University of Western Australia | Richardson M.D.,Advanced Geomechanics | Randolph M.F.,University of Western Australia | Gaudin C.,University of Western Australia
Geotechnique | Year: 2013

This paper utilises centrifuge data to explore the penetration response of dynamically installed anchors in normally consolidated clay. The data indicate that for anchors with no flukes, expected anchor tip embedment depths are two to three times the anchor length for impact velocities approaching 30 m/s, with a strong dependence on the net density of the anchor and smaller dependence on the impact velocity. Total energy, defined as the sum of the kinetic energy of the anchor at the mudline and the potential energy released as it penetrates the seabed, is shown to be a useful quantity for comparing the embedment depth of anchors with markedly different geometries and mass, impacting the soil at different velocities. The centrifuge data were used to calibrate an analytical embedment model, based on strain-rate-dependent shearing resistance and fluid mechanics drag resistance. The merit of the anchor embedment model has been demonstrated by predicting the final embedment depths for a series of offshore field trials to within 4% of the measurements.

Sahdi F.,University Malaysia Sarawak | Gaudin C.,University of Western Australia | White D.J.,University of Western Australia | Boylan N.,Advanced Geomechanics | Randolph M.F.,University of Western Australia
Geotechnique | Year: 2014

Submarine slides are a significant hazard to the safe operation of pipelines in the proximity of continental slopes. This paper describes the results of a centrifuge testing programme aimed at studying the impact forces exerted by a submarine slide on an offshore pipeline. This was achieved by dragging a model pipe at varying velocities through fine-grained soil at various degrees of consolidation, hence exhibiting properties spanning from the fluid to the geotechnical domains, relevant to the state of submarine slide material. To simulate the high strain rates experienced by the soil while flowing around a pipe in the path of a submarine slide, tests were conducted at pipe-soil velocities of up to 4.2 m/s. The changing density and shear strength of the samples were back-calculated from T-bar penetrometer test results. A hybrid approach combining geotechnical and fluid-mechanics-based components of horizontal drag resistance was developed. This approach provides an improved method to link the density and strength of the slide material to the force applied on the pipe. Besides fitting the present observations, the method provides an improved reinterpretation of similar data from the literature.

Westgate Z.J.,Advanced Geomechanics | White D.J.,University of Western Australia | Randolph M.F.,University of Western Australia
Canadian Geotechnical Journal | Year: 2013

Subsea pipelines are becoming an increasingly significant element of offshore hydrocarbon developments as exploration moves into deep-water environments further from shore. During the lay process, pipelines are subject to small amplitude vertical and horizontal oscillations, driven by the sea state and lay vessel motions. Centrifuge model tests have been used to simulate these small-amplitude lay effects, with varying degrees of idealization relative to the real lay process. In the soft soils found in deep water, pipe embedment can exceed a diameter or more, thus significantly affecting the lateral pipe-soil interaction, axial resistance, and thermal insulation. In this paper, results from centrifuge model tests are used to calibrate a model for calculating the dynamic embedment of a subsea pipeline. The model uses elements of plasticity theory to capture the effects of combined vertical and horizontal loading, and incorporates the softening of the surrounding soil as it is remoulded due to the pipeline motions. Influences from the lay rate, lay geometry, and sea state are included in the calculation process. The model is compared with observed as-laid pipeline embedment data from field surveys at three different offshore sites. Using site-specific soil parameters obtained from in situ testing and idealized pipe loads and motions to represent the load and displacement patterns during offshore pipe-laying, respectively, the model is shown to capture well the final as-laid embedment measured in the field surveys.

Zhou H.,Advanced Geomechanics | Randolph M.F.,University of Western Australia
Geotechnique | Year: 2011

The influence of the shaft on the resistance of a ball penetrometer was examined using a large deformation finite-element approach. A range of ball-shaft diameter ratios between infinitely large (no shaft) to 1 were examined in terms of resistance during monotonic penetration and extraction. It was found that the steady-state resistance decreased with decreasing ball-shaft diameter ratio, and that greater displacement (as a proportion of the shaft diameter) was required to reach a steady state. The finite-element results were matched closely using an upper bound approach, treating the shaft and outer part of the ball separately. It was also found that the rigidity index of the soil affected the penetration resistance of the shafted ball, but the influence was much less than for a cone penetrometer, being essentially proportional to the shaft-ball area ratio. Finally cyclic penetration and extraction tests were modelled comparing results for the no-shaft ball and a shafted ball with ball-shaft diameter ratios of 2 and 3. The presence of the shaft was found to have minimal influence on the resistance and cyclic degradation curves, although at the later stage of cycling the penetration resistance becomes greater than the extraction resistance. The corresponding resistance ratio of extraction to penetration for each cycle reduces with decreasing ball-shaft diameter ratio and increasing soil sensitivity. In terms of the change in mean total stress, however, the presence of the shaft introduces a pronounced gradient with respect to displacement during cyclic penetration and extraction, while increasing the changes in mean stress significantly during initial penetration.

O'Loughlin C.D.,University of Western Australia | Blake A.P.,University of Western Australia | Richardson M.D.,Advanced Geomechanics | Randolph M.F.,University of Western Australia | Gaudin C.,University of Western Australia
Ocean Engineering | Year: 2014

A dynamically embedded plate anchor (DEPLA) is a rocket or dart shaped anchor that comprises a removable central shaft and a set of four flukes. Similar to other dynamically installed anchors, the DEPLA penetrates to a target depth in soft seabed sediments by the kinetic energy obtained through free-fall in water and the self-weight of the anchor. In this paper DEPLA performance was assessed through a series of beam centrifuge tests conducted at 200 times earth's gravity. The results show that the DEPLA exhibits similar behaviour to other dynamically installed anchors during installation, with tip embedments of 1.6-2.8 times the anchor length. After anchor installation the central shaft of the DEPLA, termed a follower, is retrieved and reused for the next installation, leaving the DEPLA flukes vertically embedded in the soil. The load-displacement response during follower retrieval is of interest, with mobilisation of frictional and bearing resistance occurring at different rates. The load required to extract the DEPLA follower is typically less than three times its dry weight. The vertically embedded DEPLA flukes constitute the load bearing element as a circular or square plate. The keying and pullout response of this anchor plate is similar to other vertically embedded plate anchors, with an initial stiff response as the anchor begins to rotate, followed by a softer response as the rotation angle increases, and a final stiff response as the effective eccentricity of the padeye reduces and anchor capacity is fully mobilised. For the padeye eccentricity ratios considered (0.38-0.63 times the plate breadth or diameter), the loss in plate anchor embedment is between 0.50 and 0.66 times the corresponding plate breadth or diameter. Finally, the bearing capacity factors determined experimentally are typically in the range 14.2-15.8 and are higher than numerical solutions for flat circular and square plates. This is considered to be due to the cruciform fluke arrangement which ensures that the failure surface extends to the edge of the orthogonal flukes and mobilises more soil in the failure mechanism. © 2014 Elsevier Ltd.

Low H.E.,Advanced Geomechanics | Maynard M.L.,University of Maine, United States | Randolph M.F.,University of Western Australia | Degroot D.J.,University of Massachusetts Amherst
Geotechnique | Year: 2011

The Burswood clay is a lightly overconsolidated and sensitive silty clay of high to extremely high plasticity. In two extensive characterisation studies, a wide range of in situ tests were carried out at the Burswood site, including full-flow penetrometer tests (T-bar, ball and plate). In addition, thin-wall tube samples and high-quality Sherbrooke block samples were collected for laboratory testing. The stress-strain-strength, consolidation and compressibility characteristics of Burswood clay were investigated, and the performance of various penetrometers in characterising the soft Burswood clay was assessed. The paper compares values of stiffness and shear strength measured by different in situ and laboratory tests, quantifying the degree of non-linearity of the stress-strain response, the effect of sample disturbance on the measured mechanical properties, the degree of strength anisotropy, and the influence of strain rate on the measured shear strength. In addition, factors relating the net penetration resistance measured by cone, T-bar and ball penetrometers to yield stress, small-strain shear modulus and different measures of undrained shear strength are also provided.

Lam S.Y.,Advanced Geomechanics | Lam S.Y.,Hong Kong University of Science and Technology | Ng C.W.W.,Hong Kong University of Science and Technology | Poulos H.G.,Coffey Geotechnics
Journal of Geotechnical and Geoenvironmental Engineering | Year: 2013

Negative skin friction (NSF) can induce an increased compressive force on piles, called dragload, and additional pile settlement caused by the downward pull of the soil, called downdrag. To investigate the efficiency of shielding effects on dragload and downdrag of piles, centrifuge tests have been carried out to study the shielding mechanisms created by installing sheet piles sleeves around an existing pile in consolidating ground. The effects of various shielded lengths have also been investigated. Comparisons between centrifuge test results and finiteelement (FE) analyses are made and discussed. Based on centrifuge tests, it is clear that the shielding effect on the dragload decreases only gently with a decrease in the shielded length, whereas the shielding effect on the downdrag decreases exponentially with a decrease in the shielded length. Numerical simulations of the centrifuge model tests on the sleeved center piles reveal that the observed shielding effects on the center pile are attributed to the stress transfer from the consolidating soft soil to the sheet pile sleeve. As consolidation proceeds, the relatively stiff sheet piles sleeve hangs up the soil, leading to a significant reduction in the vertical and horizontal effective stresses in the soil and in the NSF on the center piles.The deeper the depth, the greater the hang-up effects. Thus, the shielding effect increases with the shielded length of the center pile. The reduction in the NSF on the center piles protected by the sheet pile sleeve is more significant than the reduction in the NSF from the sacrificial piles with the same shielded length. © 2013 American Society of Civil Engineers.

Lee K.K.,Advanced Geomechanics | Lee K.K.,University of Western Australia | Cassidy M.J.,University of Western Australia | Randolph M.F.,University of Western Australia
Geotechnique | Year: 2013

When a jack-up spudcan foundation is installed on seabeds consisting of a sand layer overlying soft clay, the potential for 'punch-through' failure exists. This happens as a result of an abrupt reduction in bearing resistance when the foundation punches a block of sand into the underlying soft clay in an uncontrolled manner. This paper details an extensive series of 30 tests of flat circular and spudcan foundations continuously penetrated through samples of sand overlying clay, and performed under relevant stress conditions using a drum centrifuge. The large testing area of the drum centrifuge was used advantageously to produce test results that could be compared directly with tests covering a sand thickness over foundation diameter of 0.21 to 1.12. Results from retrospective finite-element analysis of the experiments are also described, with back-calculated values of the stress-level-dependent friction and dilation angles in the sand during peak penetration resistance shown to fit correlations in the literature. The back-analysis showed that larger values of peak resistance gave lower friction and dilation angles, which is consistent with gradual suppression of dilatancy under high confining stress. When compared with published results from visualisation experiments, the finite-element analysis showed a similar failure mechanism during peak resistance, with a frustum of sand forced into the underlying clay at an angle reflecting the dilation in the sand.

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