Infrastructure Engineering - Research Publications

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    Plate anchor capacity estimation through CPT tip resistance in sand
    Roy, A ; Chow, S ; Gottardi, G ; Tonni, L (Taylor and Francis group, 2022-06-15)
    Reliable estimation of plate anchor uplift capacity in sand through analytical and empirical equations is often complicated due to uncertainties in estimation of soil properties required in the equations. In order to address this uncertainty, this study proposes a correlation to estimate plate anchor vertical uplift capacity in sand based on cone tip resistance measured from cone penetrometer tests (CPT). The correlation was established using a database of reported centrifuge experiments on circular, rectangular and strip anchors in loose and dense silica sand at various embedment depths and g-levels, along with the corresponding centrifuge CPTs performed in the same testing boxes. The centrifuge cone tip resistances were also depth-corrected to remove the effect of shallow embedment. Through regression analyses, the correlation between plate anchor capacity and cone tip resistance in dimensionless form was developed, with different coefficients fitted for circular, rectangular and strip anchors respectively.
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    A non-associative macroelement model for vertical plate anchors in clay
    da Silva, AP ; Diambra, A ; Karamitros, D ; Chow, SH (CANADIAN SCIENCE PUBLISHING, 2021-11)
    This work proposes a new plastic-hardening, non-associative macroelement model to predict the behaviour of anchors in clay for floating offshore structures during keying and up to the peak load. Building on available models for anchors, a non-associated plastic potential is introduced to improve prediction of anchor trajectory and loss of embedment at peak conditions for a large range of padeye offsets and different pull-out directions. The proposed model also includes a displacement-hardening rule to simulate the force and displacement mobilisation at the early stages of the keying process. The model is challenged and validated against different sets of numerical and centrifuge data. This extensive validation process revealed that two of the four newly introduced model parameters assume a constant value for the range of simulated cases. This suggests that only two of the newly introduced parameters may need to be calibrated for the use of the proposed macroelement model in practice.
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    Macro-element modelling of plate anchor kinematics under cyclic loading in clay
    da Silva, AP ; Diambra, A ; Karamitros, D ; Chow, SH (Deep Foundations Institute, 2020)
    With the recent shift of offshore renewable energy sector towards deeper waters, the use of floating structures anchored to the seabed through mooring lines is becoming an appealing solution. Several researchers have carried out experimental and numerical analyses to assess the behaviour of plate anchors. However, being able to quickly predict the overall anchor behaviour during keying and operational loads is important for design purposes and to assess their field performance; for this purpose, macro-element modelling is a powerful and timeefficient tool which requires minimal computational effort. In that context, this paper expands a non-associative model for plate anchors to account for the effects of cyclic loading. Whereas the effects of pore-pressure generation, consolidation and eventual material de-structuration were neglected and need to be accounted for in future studies, this study shows that the anchor’s capacity decrease occurs not only due to changes in soil conditions, but also to the particular evolution of anchor’s kinematics due to anchor re-orientation during cyclic loading.
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    Consolidation effects on uplift capacity of shallow horizontal plate anchors in dilating sand
    Roy, A ; Chow, SH ; Randolph, MF ; O'Loughlin, CD (ICE PUBLISHING, 2022-11)
    This paper examines the effect of consolidation on a shallowly embedded horizontal plate anchor in medium dense and dense sand using centrifuge tests, where the loading rate was varied over four orders of magnitude. The experimental results show a 4·8 to 5·5 times increase in anchor capacity as the consolidation condition changes from drained to undrained, driven by a steadily increasing negative excess pore pressure with increasing loading rate. At the highest loading rate, under undrained conditions, the measured maximum negative excess pore pressures reach a steady limit, suggesting the occurrence of cavitation. This increase in anchor capacity with dimensionless velocity is captured using a ‘backbone curve’ interpretation framework that describes the change in capacity between the limiting values of drained and undrained anchor capacity. Calculation of drained anchor capacity is straightforward relative to the more challenging problem of calculating undrained capacity, particularly during cavitation. This was addressed separately through a numerical parametric study (pure undrained analysis) using a bounding surface soil model involving different water depths (cavitation potential), densities and embedment ratios. The numerical results are then synthesised into a simple extended analytical solution to allow estimation of undrained anchor capacity under different densities and water depths.
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    Inclined loading of horizontal plate anchors in sand
    Roy, A ; O'Loughlin, CD ; Chow, SH ; Randolph, MF (ICE PUBLISHING, 2022-12)
    The performance of plate anchors in sand, relative to clay, is not well understood, particularly for the more realistic case of an inclined load. This paper investigates the effect of load inclination on horizontal plate anchors in sand through centrifuge tests and numerical finite-element simulations. The centrifuge tests were performed on rectangular plate anchors in loose and dense sand, at shallow embedment depths with four different load inclinations. The experiments showed that the anchor capacity of horizontal plates increased progressively as the load inclination became progressively more horizontal, with anchor capacity under pure horizontal loading being approximately 1·8 times higher than that under pure vertical loading. These experimental observations were also replicated in finite-element simulations using a bounding surface plasticity model. Investigation of the underlying failure mechanisms and stress paths showed that the slip planes become longer and the mobilised lateral stresses increase as the load inclination becomes increasingly horizontal, which leads to higher anchor capacities. Finally, the anchor resistance factors from the numerical analyses were decoupled into vertical and horizontal components and represented as interaction diagrams, providing a basis for performing hand calculations of anchor capacity for a given embedment depth, load inclination and relative density.
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    Cyclic capacity of plate anchors in loose sand
    Chow, SH ; Diambra, A ; Roy, A ; O'Loughlin, C ; Gaudin, C (Deep Foundations Institute, 2020)
    This paper describes a centrifuge investigation on drained monotonic and cyclic capacity of a plate anchor in dry loose sand. The model rectangular plate anchor was pre-embedded vertically at an embedment depth of 5 times the plate width, and was loaded horizontally at the mudline with monotonic or irregular cyclic loading to failure. The anchor was also instrumented with an accelerometer to investigate the plate rotation or ‘keying’ behaviour. The monotonically and cyclically loaded anchors are found to share similar plate rotation behaviour. Depending on the applied cyclic load magnitude, the anchor produces post-cyclic ultimate capacity that is up to 7% higher than the reference ultimate monotonic capacity. When compared to existing studies using similar plate anchor in dense sand, the plate anchor in loose sand exhibits a lower gain in post-cyclic ultimate capacity relative to the ultimate monotonic capacity, a softer load-displacement response and a higher plate anchor rotation at ultimate capacity.
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    Rapid penetration of spudcans in sand
    Chow, SH ; Bienen, B ; Randolph, M (Deep Foundation Institute, 2020)
    The growing pressure for jack-up rigs to relocate during more challenging metocean conditions for offshore wind turbine installation and maintenance has increased the risk of dynamic leg loading during the set down of the spudcan footings into the seabed. To better understand the rapid sand-spudcan interaction, this study presents experimental results of a model spudcan jacked at a range of penetration rates into dry and saturated sand at both 1g and 50g gravitational acceleration. In order to achieve a wide range of consolidation conditions, the sand was saturated using both water and a viscous pore fluid (methocel cellulose ethers) with kinematic viscosity 680 times higher than water in the 50g centrifuge tests. The results indicated up to 120% increase in spudcan penetration resistance due to the dilatancy-induced suction when the consolidation response changes from drained to undrained. This change in spudcan penetration resistance is quantified using a proposed backbone curve framework.
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    A Cyclic Macro-Element Framework for Consolidation-Dependent Three-Dimensional Capacity of Plate Anchors
    da Silva, AP ; Diambra, A ; Karamitros, D ; Chow, SH (MDPI, 2021-02)
    This paper presents a new macro-element modelling framework for plate anchors which enables the effect of pore water pressure changes and the related evolution of soil strength during the process of cyclic loading and consolidation to be captured. The proposed modelling framework combines an advanced macro-element model for plate anchors, expanded to capture the cyclic loading behaviour, with a simple one-dimensional model of undrained shearing and consolidation for a soil element representative of the whole soil mass around the anchor. The representative soil element tracks the effects of changes in effective stress on the soil strength, which in turn governs the anchor capacity in the macro-element model. The two modelling components are linked through a mobilised capacity compatibility condition. It will be firstly shown that such modelling framework is able to capture the expected changes in an anchor’s capacity related to cyclic pore pressure generation and consolidation under one-dimensional cyclic loading of the anchor. Then, the model will be used to explore the plate anchor’s behaviour and failure mechanisms under loading conditions which mobilise its full three-dimensional cyclic loading capacity. The macro-element model will identify some conflicting mechanisms (i.e., the anchor’s kinematic/rotation and soil weakening/strengthening) governing the three-dimensional capacity of the anchor.
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    Penetrometer testing in a calcareous silt to explore changes in soil strength
    Chow, SH ; O'Loughlin, CD ; Zhou, Z ; White, DJ ; Randolph, MF (Thomas Telford Ltd., 2020-01-01)
    This paper describes a centrifuge study using a range of penetrometer tests (T-bar, piezocone and free-fall piezocone) to explore strength changes in a reconstituted, normally consolidated, natural calcareous silt. Various penetrometer test procedures were applied to measure the penetration resistances including monotonic, cyclic and twitch-type movements as well as pauses for pore pressure dissipation. These mobilised combinations of partial or full remoulding, strain softening, consolidation and viscous rate effects. The penetrometer resistance – representing a proxy for strength – reduced by a factor of 4·1 from drained to undrained conditions (at the lowest fully undrained penetration rate). In undrained conditions, viscous enhancement of the penetration resistance raised the tip and shaft resistance in free-fall piezocone tests by ∼2·8 and ∼3·6 times, respectively. The ‘restart’ resistance immediately after the dissipation tests was ∼2·5 times higher than the resistance prior to dissipation, giving an indication of consolidation-induced strength gain. The ‘twitch’ test (using sequential steps decreasing the velocity) captured drainage and viscous rate effects, and also gave a ‘restart’ resistance that showed even greater consolidation effects than from a dissipation test. Overall, the different penetrometer test types and procedures measured resistances in the same soil sample that varied by a factor exceeding 20 from highest to lowest, resulting from different penetration rates and history, due to strain rate, strain level (or remoulding) and consolidation. An expression for the monotonic penetration resistance combining drainage and viscous rate effects was fitted to the response of all tests, spanning >7 orders of magnitude in strain rate.
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    Consolidation effects on monotonic and cyclic capacity of plate anchors in sand
    Chow, SH ; Diambra, A ; O'Loughlin, CD ; Gaudin, C ; Randolph, MF (Thomas Telford Ltd., 2020-01-01)
    This study investigated the change in monotonic and cyclic capacity of a plate anchor across different degrees of consolidation in dense sand. To quantify the effect of consolidation on anchor capacity, a framework is introduced and validated using centrifuge model anchor test data. The centrifuge tests considered a rectangular plate loaded at varying rates in dense sand, under both monotonic and irregular cyclic conditions, at a fixed embedment depth and with a horizontal load inclination (at the seabed). In order to vary from drained to undrained conditions, the sand was saturated using both water and a viscous pore fluid with viscosity approximately 700 times higher than water. The anchor's ultimate monotonic capacity in dense sand increased by up to 173% as the consolidation response evolved from drained to undrained with generation of dilation-induced suction. This increase in capacity across the consolidation regime can be adequately quantified using the proposed framework; however, uncertainty arises in achieving the theoretical undrained capacity. Both drained and undrained irregular cyclic loading resulted in anchor capacity increases of up to 33%, attributed to soil volume changes associated with cyclic densification under drained cyclic loading and excess pore pressure dissipation under undrained cyclic loading.