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Mark Randolph - One of the best experts on this subject based on the ideXlab platform.

  • keying of rectangular plate anchors in normally Consolidated Clays
    2011
    Co-Authors: Dong Wang, Mark Randolph
    Abstract:

    The loss in anchor embedment during keying, as it rotates to become normal to the cable load, reduces the uplift capacity of anchors in normally Consolidated Clay. The keying behavior of plate anchors has been studied previously by using centrifuge and field model tests. In this paper, a large deformation finite-element approach incorporating frequent mesh regeneration and allowing for evolution of the anchor-chain profile, was developed to simulate the keying process of rectangular and strip plate anchors. A parametric study was undertaken to quantify the loss in anchor embedment during keying in terms of the anchor geometry, soil properties, loading eccentricity, and inclination. The embedment loss decreased dramatically with increasing loading eccentricity and decreasing chain angle at the mudline to the horizontal. The loss in anchor embedment during keying increased as the local soil strength increased relative to the weight of the anchor, up to a limit determined by the eccentricity of loading. In c...

  • setup following installation of dynamic anchors in normally Consolidated Clay
    2009
    Co-Authors: M D Richardson, Mark Randolph, Conleth Oloughlin, Christophe Gaudin
    Abstract:

    This paper describes a series of centrifuge model tests designed to assess the increase in capacity of dynamic anchors due to setup in normally Consolidated Clay. The tests involved measurement of the vertical capacity of 1:200 reduced scale model anchors following various periods of postinstallation consolidation. The short-term capacity was shown to be dependent on the anchor impact velocity. Cavity expansion solutions for consolidation around a solid driven pile were found to provide agreement with the experimental results. A simplified capacity calculation technique predicted higher friction ratio values than is typically observed for driven piles; however, these calculations were complicated by the unusual dynamic anchor load — displacement response and uncertainty regarding the true sample shear strength. Dynamic anchor consolidation proceeds at a slower rate than for suction caissons and open-ended piles of similar equivalent diameter. However, the results indicate that depending on the site conditions, dynamically installed anchors remain a viable alternative to conventional deep-water mooring techniques.

  • revealing the bearing capacity mechanisms of a penetrating spudcan through sand overlying Clay
    2008
    Co-Authors: Mark Cassidy, C F Leung, Y K Chow, Mark Randolph, Chin Kau Quah
    Abstract:

    The safe and economic use of mobile jack-up structures is still hindered by limited understanding of the installation of their large conical ‘spudcan’ footings in layered sand over Clay sites. This paper addresses this by detailing experimentally observed bearing failure modes induced during the penetration of a spudcan through a layer of sand into underlying normally Consolidated Clay. Digital images were captured continuously by installing a half-spudcan against a transparent window, and analysed using a particle image velocimetry technique coupled with close-range photogrammetry correction. As the experiments were performed in a geotechnical centrifuge the observed mechanisms occurred in stress conditions of similar magnitude to the offshore case. The experimental evidence provides: (a) failure modes at different spudcan penetration depths; (b) the transitional failure mechanism at the event of peak bearing resistance; and (c) the changes in overall failure mechanism due to varying geometric and streng...

  • effect of recent load history on laterally loaded piles in normally Consolidated Clay
    2007
    Co-Authors: Nina H Levy, Itai Einav, Mark Randolph
    Abstract:

    The response of piles subjected to lateral loading (along a single line of action) has been substantially investigated in the past. However, little attention has been devoted to studying how a change in the lateral load direction (say, from east–west to north–south) may affect the overall response. This paper describes a Winkler type of soil–pile interaction model that allows coupling between the two perpendicular directions in a systematic way, by adopting local yield surfaces along the pile. A parametric study is presented for a range of pile lengths and cross sections, applied loads, and changes in direction from previous loadings. An analogy is shown to exist between the response of the pile to changes in the direction of lateral loads and the well-known elementary constitutive phenomenon of the recent stress history. As is commonly assumed in practice the model shows that the direction of the resultant pile response does not have a large impact on the load carrying capacity of the system. However, ch...

  • large deformation analysis of suction caisson installation in Clay
    2006
    Co-Authors: Hongjie Zhou, Mark Randolph
    Abstract:

    Large deformation finite element (LDFE) analyses were performed to study the installation of caissons by suction and jacking in normally Consolidated Clay. The penetration of the caisson wall was modelled between depths of one and four diameters using an axisymmetric LDFE approach, which falls in the category of arbitrary Lagrangian–Eulerian (ALE) methods. The results allowed quantification of differences in the behaviour of caissons installed entirely by jacking compared with a combination of self-weight and suction as is used in the field. For jacked installation, over the penetration range of one to four diameters, the proportion of caisson wall accommodated by inward soil flow reduced from around 45% at the start to zero at about four diameters embedment; by contrast, the proportion for suction installation stayed essentially constant, oscillating around 65% through the depth of penetration. This difference was also evident in the local incremental displacements of the soil beneath the caisson tip. Du...

Mark Cassidy - One of the best experts on this subject based on the ideXlab platform.

  • large deformation analysis of spudcan penetration into sand overlying normally Consolidated Clay
    2013
    Co-Authors: Pan Hui, Mark Cassidy, Dong Wang, Qing Yang
    Abstract:

    Inverted conical spudcan footings are the most popular foundation for jack-up platforms in the offshore industry. Before a jack-up can be located at a site, the penetration resistance during spudcan installation must be accurately predicted. This is usually performed using bearing capacity formulae from industry standards, with the use of site-specific numerical modelling not yet common place. In this paper, the Mohr-Coulomb sand model and Tresca Clay model are incorporated with a large-deformation approach called the Coupled-Eulerian Lagrangian (CEL) method within the commercial package ABAQUS. It is used to simulate the installation process of spudcans in sand overlying Clay. The robustness of this large-deformation analysis method is validated against published results of numerical analyses of spudcan penetration into Clay. This comparison is performed to show that the numerical simulations are suited to capture the behaviour of spudcan penetration into seabeds. A series of parametric studies are then conducted to investigate the potential for punch-through during spudcan penetration in sand overlying Clay. The influence of soil properties, such as the friction and dilation behaviour of sand, the normalised sand thickness and the undrained shear strength of Clay, on the bearing capacity of the spudcan, the soil failure mechanism and the depth of peak penetration resistance are discussed.

  • centrifuge model study of spudcan penetration in sand overlying Clay
    2010
    Co-Authors: C F Leung, Y K Chow, Mark Cassidy
    Abstract:

    A series of centrifuge model tests has been conducted to investigate the bearing resistance of spudcan foundations of offshore jack-up rigs in sand overlying normally Consolidated Clay. The spudcan bearing-resistance profiles measured in most of the tests recorded a peak resistance at a shallow depth within the upper sand layer, followed by an abrupt post-peak reduction in resistance causing the spudcan to plunge into the underlying soft Clay. This phenomenon is commonly termed as ‘spudcan punch-through hazard', which potentially leads to severe structural damage of jack-up rigs in the field. The centrifuge test results revealed that the ratio of upper sand layer thickness over spudcan diameter and the ratio of bearing resistance between the upper sand and underlying Clay affect the development of spudcan bearing resistance. The limitations of existing design methods, which were derived for the ultimate bearing capacity of pre-embedded shallow foundation to assess the spudcan bearing resistance-depth prof...

  • the influence of pull out load on the efficiency of jetting during spudcan extraction
    2009
    Co-Authors: Britta Bienen, Christophe Gaudin, Mark Cassidy
    Abstract:

    Abstract Mobile jack-up drilling rigs are deployed at many locations during their service life. This necessitates retrieval of the platform’s legs and spudcan footings before the rig move. In soft soils, where the spudcans embed deeply, the extraction process can be difficult, time consuming and therefore costly. Water jetting systems, devised to ease spudcan extraction, are a common feature on modern jack-up units. However, their effectiveness in reducing the pull-out load required is questioned by the offshore industry. To investigate their efficiency, centrifuge experiments of a reduced scale spudcan model with jets have been performed at the University of Western Australia. The footing was extracted from penetrations of up to 1.5 diameters in normally Consolidated Clay. Similar to spudcan extraction in the field, these were carried out under load control, applying a constant extraction force. Both influences of pull-out load magnitude and jetting flow rate were investigated. The study demonstrates that jetting is efficient in facilitating spudcan extraction, as it reduces the required uplift load. Practical guidance is provided in applying the results to field conditions.

  • revealing the bearing capacity mechanisms of a penetrating spudcan through sand overlying Clay
    2008
    Co-Authors: Mark Cassidy, C F Leung, Y K Chow, Mark Randolph, Chin Kau Quah
    Abstract:

    The safe and economic use of mobile jack-up structures is still hindered by limited understanding of the installation of their large conical ‘spudcan’ footings in layered sand over Clay sites. This paper addresses this by detailing experimentally observed bearing failure modes induced during the penetration of a spudcan through a layer of sand into underlying normally Consolidated Clay. Digital images were captured continuously by installing a half-spudcan against a transparent window, and analysed using a particle image velocimetry technique coupled with close-range photogrammetry correction. As the experiments were performed in a geotechnical centrifuge the observed mechanisms occurred in stress conditions of similar magnitude to the offshore case. The experimental evidence provides: (a) failure modes at different spudcan penetration depths; (b) the transitional failure mechanism at the event of peak bearing resistance; and (c) the changes in overall failure mechanism due to varying geometric and streng...

  • a comparison of the combined load behaviour of spudcan and caisson foundations on soft normally Consolidated Clay
    2004
    Co-Authors: Mark Cassidy, B W Byrne, Mark Randolph
    Abstract:

    Experimental data from loading tests of model circular footings on soft normally Consolidated Clay are presented. The experiments were carried out on a drum centrifuge at a radial acceleration level equivalent to 100 times Earth's gravity, ensuring conditions of stress similitude between the model and the prototype scale. The aim of the experiments was to compare the undrained response of different offshore foundations to the same loading conditions. Two different types of foundation were targeted for investigation: spudcans and suction caissons. The spudcan, typically an inverted shallow cone, is the traditional footing for mobile drilling units (also known as jack-up rigs). An alternative foundation concept that is being increasingly considered is that of foundations skirted about the perimeter and installed by suction. A loading arm that incorporated an internal hinge was used so that combinations of load appropriate to the foundations of jack-up units could be applied to the models. Although it was an...

Motohiro Sugiyama - One of the best experts on this subject based on the ideXlab platform.

  • undrained cyclic shear behaviour of normally Consolidated Clay subjected to initial static shear stress
    1994
    Co-Authors: Masayuki Hyodo, Yoichi Yamamoto, Motohiro Sugiyama
    Abstract:

    ABSTRACT A series of undrained cyclic triaxial compression tests has been performed on a high plasticity marine Clay. Testing was performed using various combinations of initial static and subsequent cyclic shear stress on isotropically and anisotropically Consolidated specimens. Additionally, monotonic triaxial compression and extension tests were also performed on the same Clay with the same initial condition. The cyclic shear strength was considered first, and then, the accumulated shear strain was investigated related to the effective stress ratio at the peak cyclic stress. Based on the experimental results, a semi-empirical model is proposed for evaluating the development of pore pressure and residual shear strain during cyclic loading. The model successfully explains the behaviour of Clay subjected to various magnitudes of initial static and subsequent cyclic shear stresses.

Hideki Ohta - One of the best experts on this subject based on the ideXlab platform.

  • changes in the instantaneous shear modulus of normally Consolidated Clay with shear history
    2005
    Co-Authors: Atsushi Iizuka, Shinya Tachibana, Katsuyuki Kawai, Hideki Ohta
    Abstract:

    ABSTRACT Changes in the instantaneous shear modulus associated with the non-coaxiality of NC Clay materials are presented. A series of hollow cylindrical shear tests was performed to measure the changes in the instantaneous shear modulus with shear history. After each normally Consolidated Clay specimen was compressively sheared until a prescribed shear level, it was continuously torsionally sheared under a constant volume to measure the instantaneous shear modulus. Coaxial constitutive relations, such as these of the conventional Cam-Clay type of models, predict that the instantaneous shear modulus obtained, does not change regardless of the compressive shear history. On the contrary, it is known that non-coaxial constitutive relations theoretically bring about changes in the instantaneous shear modulus in relation to the compressive shear history. The test results supported the predictions made by the non-coaxial constitutive models. Finally, the proper formulation for the non-coaxiality between the stress and the incremental plastic strain is discussed.

Knut H. Andersen - One of the best experts on this subject based on the ideXlab platform.

  • Recompression of normally Consolidated Clay after cyclic loading.
    1991
    Co-Authors: Kazuya Yasuhara, Knut H. Andersen
    Abstract:

    ABSTRACT Cyclic loading may cause settlements in addition to the settlements due to static loads. 'This paper presents a study of recompression settlements occurring in normally Consolidated Clay when pore pressure generated by undrained cyclic loading dissipates. The study was based on 18 stress-controlled cyclic direct simple shear tests and 6 conventional consolidation tests on normally Consolidated Drammen Clay. The specimens were subjected to either one or several series of undrained cyclic loading and drainage. Afterwards, most of the specimens were Consolidated further to a vertical stress three times the effective stress at start of cycling. The study showed that undrained cyclic loading and drainage makes normally Consolidated Clay more resistant to later undrained cyclic loading. The recompression index was influenced by cyclic loading, and for normally Consolidated Drammen Clay the settlements occurring when the cyclically induced pore pressure dissipates, may be calculated by a recompression index which is 1.5 times the recompression index from a conventional consolidation test. The compression index of specimens which where subjected to undrained cyclic loading and then Consolidated past the initial consolidation stresses decreases with increasing cyclically induced pore pressure and increasing cyclic shear strain at the end of cycling.