There is some evidence that South Asian women may have an increased risk of osteoporosis compared with Caucasian women, although whether South Asians are at increased risk of fracture is not clear. It is unknown whether older South Asian women differ from Caucasian women in bone geometry. This is the first study, to the authors' knowledge, to use peripheral Quantitative Computed Tomography (pQCT) to measure radial and tibial bone geometry in postmenopausal South Asian women. In comparison to Caucasian women, Asian women had smaller bone size at the 4% (-. 18% p. <. 0.001) and 66% radius (-. 15% p. = 0.04) as well as increased total density at the 4% (+. 13% p. = 0.01) radius. For the tibia, they had a smaller bone size at the 4% (-. 16% p. = 0.005) and 14% (-. 38% p. = 0.002) sites. Also, Asians had increased cortical thickness (-. 17% p. = 0.04) at the 38% tibia, (in proportion to bone size (-. 30% p. = 0.003)). Furthermore, at the 4% and 14% tibia there were increased total densities (+. 12% to +. 29% p. <. 0.01) and at the 14% tibia there was increased cortical density (+. 5% p. = 0.005) in Asians. These differences at the 14% and 38% (but not 4%) remained statistically significant after adjustment for Body Mass Index (BMI). These adaptations are similar to those seen previously in Chinese women. Asian women had reduced strength at the radius and tibia, evidenced by the 20-40% reduction in both polar Strength Strain Index (SSIp) and fracture load (under bending). Overall, the smaller bone size in South Asians is likely to be detrimental to bone strength, despite some adaptations in tibial cortical thickness and tibial and radial density which may partially compensate for this. © 2013 Elsevier Inc.
Cui L, Bhattacharya S (2014) Dynamic Soil-Structure Interaction around a monopile supporting a wind turbine, Geomechanics from Micro to Macro CRC Press
Kiernan S, Cui L, Gilchrist MD (2008) A Numerical Investigation of the Dynamic Behavior of Functionally Graded Foams, 19pp. 15-24 Springer
Jiang M, Zhang N, Cui L, Jin S (2015) A size-dependent bond failure criterion for cemented granules based on experimental studies, COMPUTERS AND GEOTECHNICS69pp. 182-198 ELSEVIER SCI LTD
Cui L, Mitoulis S (2014) DEM analysis of green rubberised backfills towards future smart bridges, Geomechanics from Micro to Macro CRC Press
As a result of deposition process and particle characteristics, granular
materials can be inherently anisotropic. Many researchers have strongly suggested that the inherent anisotropy is the main reason for the deformation non-coaxiality of granular materials. However, their relationships are not unanimous due to the limited understanding of the non-coaxial micro-mechanism. In this study, we investigated the
influence of inherent anisotropy on the non-coaxial angle using the discrete element method (DEM). Firstly, we developed a new DEM approach using rough elliptic particles, and proposed a novel method to produce anisotropic specimens. Secondly, the effects of initial specimen density and particle characteristics, such as particle
aspect ratio Am, rolling resistance coefficient ² and bedding plane orientation ´, were examined by a series of biaxial tests and rotational principal axes tests (RPAM). Findings from the numerical simulations are summarized as: (1) The peak internal
friction angle Õp and the non-coaxial angle i both increase with the initial density, Am and ², and they both increase initially and then decrease with Õ in the range of 0 - 90°; (2) Among the particle characteristics, the influence of Am is the most significant; (3) For anisotropic specimens, the non-coaxial angle can be calculated using the double slip and rotation rate model (DSR2 model). Then, an empirical formula was proposed based on the simulation results to depict the relationship between the non-coaxial angle and the particle characteristics. Finally, the particle-scale mechanism of non-coaxiality for granular materials was discussed from the perspective of energy dissipation.
Cui L Exploring the controlling parameters affecting specimens generated in a pluviator using DEM, Discrete Element Modelling of Particulate Mediapp. 196-202 Royal Society of Chemistry
Kiernan S, Cui L, Gilchrist MD (2007) Energy absorption of a novel functionally graded polymeric foam,
Cui L, O'Sullivan C (2006) Exploring the macro- and micro-scale response of an idealised granular material in the direct shear apparatus, GEOTECHNIQUE56(7)pp. 455-468 THOMAS TELFORD PUBLISHING
Two Finite Element models approximating the dynamic behaviour of functionally graded foam materials (FGFMs) have been developed under free weight drop impact and Kolsky wave propagation conditions. The FGFM is modeled by discretising the material into a large number of layers through the foam thickness. Each layer is described by a unique constitutive cellular response, which is derived from the initial relative density, Á*, unique to that layer. Large strain uniaxial compressive tests at strain rates of 0.001, 0.01 and 0.1/s were performed on expanded polystyrene (EPS) and ALPORAS® Aluminium (Al) foam and their Ã-µ response was used as input to a modified constitutive model from the literature. Simulations were then performed on both uniform and graded specimens. For both impact and wave propagation conditions it is found that under certain conditions an FGFM can outperform a uniform foam of equivalent density in terms of reducing peak accelerations imparted from an impact, or mitigating stress wave magnitudes through increased plastic deformation. These properties provide significant insight into the hypothesised behaviour of FGFMs and elucidate the potential for the future use in the design of next generation cushioning structures. © 2010 Springer Science+Business Media B.V.
Cui L, O'Sullivan C (2003) Analysis of a triangulation based approach for specimen generation for discrete element simulations, GRANULAR MATTER5(3)pp. 135-145 SPRINGER-VERLAG
Geometrically, axi-symmetric systems are
frequently encountered in soil mechanics and geotechnical
engineering. This paper proposes a mixed boundary environment for such axi-symmetrical discrete element
analyses. In the proposed approach, only one quarter of the system is considered. Two vertical (circumferential)
periodic boundaries are used to enforce the conditions for axi-symmetry in the model. The proposed
algorithm was implemented in a three-dimensional discrete element code to model axi-symmetric triaxial tests.
To facilitate these triaxial test simulations, a cylindrical stress controlled membrane was developed. Simulations
of triaxial compression tests on specimens of spheres with regular packing configurations are used to validate
the proposed analysis approach.
Jockey head injuries, especially concussions, are common in horse racing. Current helmets do help to reduce the severity and incidences of head injury, but the high concussion incidence rates suggest that there may be scope to improve the performance of equestrian helmets. Finite element simulations in ABAQUS/Explicit were used to model a realistic helmet model during standard helmeted rigid headform impacts and helmeted head model University College Dublin Brain Trauma Model (UCDBTM) impacts. Current helmet standards for impact determine helmet performance based solely on linear acceleration. Brain injury-related values (stress and strain) from the UCDBTM showed that a performance improvement based on linear acceleration does not imply the same improvement in head injury-related brain tissue loads. It is recommended that angular kinematics be considered in future equestrian helmet standards, as angular acceleration was seen to correlate with stress and strain in the brain.
Lopez-Querol S, Cui L, Bhattacharya S (2017) Numerical Methods for SSI Analysis of Offshore Wind Turbine Foundations, In: Wind Energy Engineering: A Handbook for Onshore and Offshore Wind Turbines14pp. 275-297 Academic Press
The aim of this chapter is to provide a summary of the numerical methods
available to carry out long-term prediction analysis of offshore wind turbine
foundations. Different available methods of analysis are discussed.
As a consequence of its particulate nature, soil exhibits a highly complex response to applied loads and deformations. Traditionally, geotechnical engineers have coupled continuum numerical analysis tools (such as the finite element method) with complex constitutive models to analyze soil response. This approach does not explicitly consider the particle-scale interactions underlying the macro-scale response observed in the laboratory and field. With increasing computational speeds, particle-based discrete element methods are becoming popular amongst geotechnical engineers in both research and practice. On a practical level discrete element methods are particularly useful for studying finite deformation problems, while from a more theoretical perspective they can be used to create virtual laboratories where the micro-mechanics of soil response can be analyzed in detail. This paper describes a series of validation studies that were performed to confirm that, despite their inherent simplifications, discrete element methods can accurately capture the macro-scale response of granular materials. It is shown that, once validated, these methods can provide useful information to explain the complex response exhibited by granular materials in conventional laboratory tests.
Using discrete element simulations, one can monitor the micro-mechanisms driving the macroresponse
of granular materials and quantify the evolution of local stress and strain values. However, it is
important to couple the se simulations with carefully controlled physical tests for validation and insight.
Only then can findings about the micro- mechanics of the material response be made with confidence. Moreover,
the sensitivity of the observed response to the test boundary conditions can be analyzed in some detail.
The results of three-dimensional discrete element simulations of direct shear tests and as well as complementary
physical tests on specimens of steel balls are presented in this paper. Previous discrete element analyses
of the direct shear test have been restricted to two-dimensional simulations. For the simulations presented
here, an analysis of the internal stresses and contact forces illustrates the three-dimensional nature of the material
response. The distribution of contact forces in the specimen at larger strain values, however, was
found to be qualitatively similar to the two-dimensional results of Zhang and Thornton (2002). Similarities
were also observed between the distrib ution of local strain values and the distribution of strains obtained by
Potts et al (1987) in a finite element analysis of the direct shear test. The simulation results indicated that
the material response is the stress dependent. However, the response observed in the simulations was found to
be significantly stiffer than that observed in the physical tests. The angle of internal friction for the simulations
was also about 3o lower than that measured in the laboratory tests. Further laboratory tests and simulations
are required to establish the source of the observed discrepancies.
ExoMars is the European Space Agency (ESA) mission to Mars planned for launch in 2018, focusing on exobiology with the primary
objective of searching for any traces of extant or extinct carbon-based micro-organisms. The on-surface mission is performed
by a near-autonomous mobile robotic vehicle (also referred to as the rover) with a mission design life of 180 sols Patel et al. (2010).
In order to obtain useful data on the tractive performance of the ExoMars rover before flight, it is necessary to perform mobility
tests on representative soil simulant materials producing a Martian terrain analogue under terrestrial laboratory conditions. Three
individual types of regolith shown to be found extensively on the Martian surface were identified for replication using commercially
available terrestrial materials Patel (2011), sourced from UK sites in order to ensure easy supply and reduce lead times for delivery.
These materials (also referred to as the Engineering Soil Simulants (ES-x) are: a fine dust analogue (ES-1); a fine aeolian sand
analogue (ES-2); and a coarse sand analogue (ES-3). Following a detailed analysis, three fine sand regolith types were identified
from commercially available products. Each material was used in its o -the-shelf state, except for ES-2, where further processing
methods were used to reduce the particle size range. These materials were tested to determine their physical characteristics, including
the particle size distribution, dry bulk density, particle shape (including angularity / sphericity) and moisture content. The
results are analysed to allow comparative analysis with existing soil simulants and the published results regarding in-situ analysis
of Martian soil on previous NASA missions. The findings have shown that in some cases material properties vary significantly
from the specifications provided by material suppliers. It has confirmed that laboratory testing is necessary to determine the actual
parameters and that standard geotechnical processes are suitable for doing so. The outcomes have allowed the confirmation of each
simulant material as suitable for replicating their respective regolith types.
The density of foam used as energy absorbing liner material in safety helmets was optimised in this paper using Finite Element Modelling (FEM). FEM simulations of impact tests from certification standards were carried out to obtain the best performing configurations of helmet liner. For each test condition, two best liner configurations were identified as minimising peak impact accelerations: one was composed of layers of uniform foam and the other of functionally graded foam (FGF). It was found that the observed decreases in the peak accelerations for the best performing helmets in various test conditions are directly related to the contact area, the distribution of internal stresses, and the dissipated plastic energy density (DPED). Application of the methods described in this study could help increase energy absorption for current and future equestrian helmet designs.
Cui L, Mitoulis S (2015) DEM analysis of green rubberised backfills towards future smart Integral Abutment Bridges (IABs), Geomechanics from Micro to Macro, Vols I and IIpp. 583-588 CRC PRESS-TAYLOR & FRANCIS GROUP
Cui L, O'Sullivan C (2004) Exploring the Use of Triangulation to Generate Specimens for Two-Dimensional Discrete Element Simulations, pp. 71-80
When discrete element method (DEM) simulations are carefully coupled with equivalent physical
experiments, conclusions about the micro-mechanics of underlying the observed material response can be
made with confidence. A novel approach to simulating triaxial tests with DEM using circumferential periodic
boundaries has been developed by the authors. In an earlier study, this approach was validated experimentally
by considering a series of laboratory monotonic triaxial tests on specimens of uniform and non-uniform steel
spheres. The current paper extends this previous research by simulating the response of specimens of about
15,000 steel spheres subject to unload/reload cycles in quasi-static triaxial tests. In general, good agreement
was attained between the physical tests and the DEM simulations. The paper also discusses use of the DEM
simulation results to explore the particle-scale mechanics during the load reversals.
Jiang MJ, Fu C, Cui L, Zhu FY, Shen ZF (2014) DEM simulations of methane hydrate dissociation by thermal recovery, Geomechanics from Micro to Macro CRC Press
Cui L, Bhattacharya S (2015) Choice of aggregates for permeable pavements based on laboratory tests and DEM simulations, International Journal of Pavement Engineering18(2)pp. 162-170
This paper presents an investigation on mechanism of the inclined cone penetration test using the numerical discrete element method (DEM). A series of penetration tests with the penetrometer inclined at different angles (i.e., 0?,15?,30?,45? and 60? ) were numerically performed under ¼=0.0 and ¼=0.5 , where ¼ is the frictional coefficient between the penetrometer and the soil. The deformation patterns, displacements of soil particles adjacent to the cone tip, velocity fields, rotations of the principal stresses and the averaged pure rotation rate were analyzed. Special focus was placed on the effect of friction. The DEM results showed that soils around the cone tip experienced complex displacement paths at different positions as the inclined penetration proceeded, and the friction only had significant effects on the soils adjacent to the penetrometer side and tip. Soils exhibited characteristic velocity fields corresponding to three different failure mechanisms and the right side was easier to be disturbed by friction. Friction started to play its role when the tip approached the observation points, while it had little influence on rotation rate. The normalized tip resistance (qc=f/Ãv0) increased with friction as well as inclination angle. The relationship between qc and relative depth (y/R) can be described as qc=a×(y/R)?b , with parameters a and b dependent on penetration direction. The normalized resistance perpendicular to the penetrometer axis qp increases with the inclination angle, thus the inclination angle should be carefully selected to ensure the penetrometer not to deviate from its original direction or even be broken in real tests.
A finite element, functionally graded foam model (FGFM) is proposed, which is shown to provide more effective energy absorption management, compared to homogenous foams, under low energy impact conditions. The FGFM is modelled by discretising a virtual foam into a large number of element layers through the foam thickness. Each layer is described by a unique constitutive cellular response, which is derived from the initial foam density, Á, unique to that layer. Large strain unixial compressive tests at a strain rate of 0.001 s-1 are performed on expanded polystyrene (EPS), and their Ã ?µ response is used as input to a modified constitutive model from the literature. It is found that under low energy impacts an FGFM can outperform a uniform foam of equivalent density terms of reducing peak accelerations, while performing almost as effectively as uniform foams under high energy conditions. These novel materials, properly manufactured, could find use as next generation helmet liners in answer to recent, more rigorous equestrian helmet standards, e.g. BS EN 14572:2005.
Darling AL, Hakim OA, Horton K, Gibbs MA, Cui L, Berry JL, Lanham-New SA, Hart KH (2012) Associations between vitamin D status and radial bone geometry in older South Asian and Caucasian women, PROCEEDINGS OF THE NUTRITION SOCIETY71(OCE3)pp. E230-E230 CAMBRIDGE UNIV PRESS
O'Sullivan C, Cui L, O'Neill SC (2008) DISCRETE ELEMENT ANALYSIS OF THE RESPONSE OF GRANULAR MATERIALS DURING CYCLIC LOADING, SOILS AND FOUNDATIONS48(4)pp. 511-530
Offshore wind turbine (OWT) foundations are subjected to a combination of cyclic and dynamic
loading arising from wind, wave, rotor and blade shadowing. Under cyclic loading, most soils change their
characteristics including stiffness, which may cause the system natural frequency to approach the loading
frequency and lead to unplanned resonance and system damage or even collapse. To investigate such
changes and the underlying micromechanics, a series of cyclic simple shear tests were performed on the
RedHill 110 sand with different shear strain amplitudes, vertical stresses and initial relative densities of soil.
The test results showed that: (a) Vertical accumulated strain is proportional to the shear strain amplitude but
inversely proportional to relative density of soil; (b) Shear modulus increases rapidly in the initial loading
cycles and then the rate of increase diminishes and the shear modulus remains below an asymptote; (c)
Shear modulus increases with increasing vertical stress and relative density, but decreasing with increasing
strain amplitude. Coupled DEM simulations were performed using PFC2D to analyse the micromechanics
underlying the cyclic behaviour of soils. Micromechanical parameters (e.g. fabric tensor, coordination
number) were examined to explore the reasons for the various cyclic responses to different shear strain
amplitudes or vertical stresses. Both coordination number and magnitude of fabric anisotropy contribute to
the increasing shear modulus.
Under cyclic loading, most soils change their characteristics. Cyclic behaviour (change of shear modulus and accumulated strain) of the RedHill 110 sand was investigated by a series of cyclic simple shear tests. The effects of application of 50,000 cycles of shear loading with different shear strain amplitudes and vertical stresses were investigated. The results correlated quite well with the observations from scaled model tests of different types of offshore wind turbine foundations and limited field observations. Specifically, the test results showed that shear modulus increases rapidly in the initial loading cycles and then the rate of increase diminishes; the rate of increase depends on strain amplitude, initial relative density and vertical pressure. Complementary DEM simulations were performed using PFC2D to analyse the micromechanics underlying the cyclic behaviour of soils. It shows that the change of soil behaviour strongly related to the rotation of principle axes of fabric and degree of fabric anisotropy.
This paper introduced a testbed developed from a perspective of soil mechanics which not only focused on wheel design and optimization but also considered the elimination of the boundary effect caused by soil bin. Using this testbed, a series of experimental investigations were performed by changing the wheel rotational velocity, vertical load and towed load. Tracks were generated at a regular spacing as the wheel lugs enter and exit the soil periodically. It has been found that there is a relationship between the track length and wheel slip ratio regardless of different mechanical properties of soil. The wheel rotational velocity has little effect on the driving torque and sinkage. The towed load affects more on the driving torque than on the sinkage. However, the vertical load effects on the driving torque and sinkage are similar. The current models used for parameter estimations may not be appropriate for TJ-1 lunar soil simulant which has a relatively high internal friction angle according to the experimental results. But the internal friction angle and cohesion can still be estimated with proper selection of shear deformation modulus using the model proposed by Li et al(2011).
Many offshore wind turbines are supported by large diameter piles (known as monopiles) and are subjected to large number of cyclic and dynamic loads. There are evidences suggesting that foundation stiffness are changing with cycles of loading and this may lead to changes in the natural frequency of the system with the potential for unplanned system resonances. There are other consequences such as excessive tilt leading to expensive repair or even complete shutdown. Therefore, it is vital to understand the long-term response of wind turbine foundation so that a method to predict the change in frequency and tong term tilt could be established. This paper aims to present the experimental work of small scale physical modelling and Discrete Element Modelling (DEM) of the interaction between a monopile and the surrounding soil. Changes in soil stiffness under cyclic loading of various strain amplitudes were examined for both physical modelling and DEM. Micro-mechanics of soils underlying the soil stiffness change was investigated using DEM. Variation of force distribution along the mono-pile under cyclic loading was analysed to show the influence of monopile stability.
Using discrete element simulations, one can monitor the micro-mechanisms driving the macroresponse of granular materials and quantify the evolution of local stress and strain values. However, it is important to couple the se simulations with carefully controlled physical tests for validation and insight. Only then can findings about the micro- mechanics of the material response be made with confidence. Moreover, the sensitivity of the observed response to the test boundary conditions can be analyzed in some detail. The results of three-dimensional discrete element simulations of direct shear tests and as well as complementary physical tests on specimens of steel balls are presented in this paper. Previous discrete element analyses of the direct shear test have been restricted to two-dimensional simulations. For the simulations presented here, an analysis of the internal stresses and contact forces illustrates the three-dimensional nature of the material response. The distribution of contact forces in the specimen at larger strain values, however, was found to be qualitatively similar to the two-dimensional results of Zhang and Thornton (2002). Similarities were also observed between the distrib ution of local strain values and the distribution of strains obtained by Potts et al (1987) in a finite element analysis of the direct shear test. The simulation results indicated that the material response is the stress dependent. However, the response observed in the simulations was found to be significantly stiffer than that observed in the physical tests. The angle of internal friction for the simulations was also about 3o lower than that measured in the laboratory tests. Further laboratory tests and simulations are required to establish the source of the observed discrepancies.
Methane hydrate (MH, also called fiery ice) exists in forms of pore filling, cementing and load-bearing skeleton in the methane hydrate bearing sediment (MHBS) and affects its mechanical behavior greatly. To study the changes of macro-scale and micro-scale mechanical behaviors of MHBS during exploitation by thermal recovery and depressurization methods, a novel 2D thermo-hydro-mechanical bonded contact model was proposed and implemented into a platform of distinct element method (DEM), PFC2D. MHBS samples were first biaxially compressed to different deviator stress levels to model different in-situ stress conditions. With the deviator stress maintained at constant, the temperature was then raised to simulate the thermal recovery process or the pore water pressure (i.e. confining pressure for MH bond) was decreased to simulate the depressurization process. DEM simulation results showed that: during exploitation, the axial strain increased with the increase of temperature (in the thermal recovery method) or decrease of pore water pressure (in the depressurization method); sample collapsed during MH dissociation if the deviator stress applied was larger than the compression strength of a pure host sand sample; sample experienced volume contraction but its void ratio was slightly larger than the pure host sand sample at the same axial strain throughout the test. By comparison with the laboratory test results, the new model was validated to be capable of reproducing the exploitation process by thermal recovery and depressurization methods. In addition, some micro-scale parameters, such as contact distribution, bond distribution, and averaged pure rotation rate, were also analyzed to investigate their relationships with the macroscopic responses.
In this paper, the wheel-soil interaction for a future lunar exploration mission
is investigated by physical model tests and numerical simulations. Firstly, a series of
physical model tests was conducted using the TJ-1 lunar soil simulant with various
driving conditions, wheel configurations and ground void ratios. Then the
corresponding numerical simulations were performed in a terrestrial environment
using the Distinct Element Method (DEM) with a new contact model for lunar soil,
where the rolling resistance and van der Waals force were implemented. In addition,
DEM simulations in an extraterrestrial (lunar) environment were performed. The
results indicate that tractive efficiency does not depend on wheel rotational velocity,
but decreases with increasing extra vertical load on the wheel and ground void ratio.
Rover performance improves when wheels are equipped with lugs. The DEM
simulations in the terrestrial environment can qualitatively reproduce the soil
deformation pattern as observed in the physical model tests. The variations of traction
efficiency against the driving condition, wheel configuration and ground void ratio
attained in the DEM simulations match the experimental observations qualitatively.
Moreover, the wheel track is found to be less evident and the tractive efficiency is
higher in the extraterrestrial environment compared to the performance on Earth.
This paper proposed a method for predicting the failure loads of masonry wall panels subject to uniformly distributed lateral loading based on a concept of structural stress state. Firstly, the characteristics of the structural stress state of masonry wall panels subjected to uniform distributed lateral loading were investigated through experimental results. Then, a new parameter was proposed to characterize the structural stress state. Next, the relation of the failure loads between a specified base wall panels and other wall panel was established using the proposed parameter. In this way, a method (called as stress state (ST) method) based on structural stress state parameter to predict the failure load of masonry wall panel from the base wall panel was established. The following case studies validated the ST method by comparing the predicted failure load with experimental results as well as those predicted from the existing yield line theory(YLT), the FEA method and the GSED-based cellular automata (CA) method. The ST method provided an innovative way of structural analysis on the basis of structural stress state.
Cui Liang, Azizul Moqsud MD, Hyodo Masayuki, Bhattacharya Suby (2018) Methane Hydrate as a ?new energy?,In: Letcher Trevor (eds.), Managing Global Warming: An Interface of Technology and Human Issues
Elsevier: Academic Press
Methane hydrate (MH) becomes a promising new energy in some countries including China and Japan due to its huge reservation. The key mission is to find the safe and efficient exploitation method. The exploitation processes will cause stress changes, which may induce submarine landslides and failures of engineering projects. This chapter described some state-of-art exploitation methods reproduced in laboratory and in numerical modelling to understand the responses of soils during exploitation process. These studies could provide valuable guidance for real life projects.
Monopiles are currently the preferred option for supporting offshore wind turbines (OWTs) in water depths up to about 40 m. Whilst there have been significant advancements in the understanding of the behaviour of monopiles, the guidelines on the prediction of long term tilt (Serviceability Limit State, SLS) under millions of cycles of loads are still limited. Observations and analysis of scaled model tests identify two main parameters that governs the progressive tilt of monopiles: (a) Loading type (one-way or two-way) which can be quantified by the ratio of the minimum to maximum mudline bending moments (Mmin/Mmax); (b) factor of safety against overturning i.e. the ratio of the maximum applied moment (Mmax) to the moment carrying capacity of the pile or Moment of Resistance (MR) and therefore the ratio Mmax/MR. Due to the nature of the environmental loads (wind and wave) and the operating conditions of the turbine, the ratio Mmin/Mmax changes. The aim of this paper is to develop a practical method that can predict the nature of loading for the following governing load cases: Normal Operating Conditions, Extreme Wave Load scenario, and Extreme Wind Load scenario. The proposed method is applied to 15 existing wind farms in Europe where (Mmin/Mmax) and (Mmax/MR) are evaluated. The results show that the loading ratio is sensitive to the water depth and turbine size. Furthermore, under normal operating conditions, most of the wind turbine foundations in shallow waters are subjected to one-way loading and in deeper waters and under extreme conditions the loading is marginally two-way. Predictions for the nature of loading for large wind turbines (8MW and 10MW) in deeper waters are also presented. The results from this paper can be used for planning scaled model tests and element tests of the soil.
In this paper, a series of excavation tests were conducted with a carefully designed apparatus and testbed based on soil mechanics theories to obtain reliable excavation forces in Tongji-1 lunar soil simulant at first. Then the measured data were compared with the forces predicted by six typical analytical models to verify their capability of accurately capturing the effects of cutting depth, rake angle, blade width and cutting speed. The results show that for the horizontal excavation forces, the Zeng model, the Kobayashi model, the Mckyes model and the Swick and Perumpral model can capture the effects of cutting depth, and the Lockheed-Martin/Viking model could capture the effects of the cutting depth, blade width and rake angle. For the vertical excavation forces, the Swick and Perumpral model and the Mckyes model can capture the effects of the cutting depth, blade width and rake angle. The overall assessment of excavation force predictions shows that the Lockheed-Martin/Viking model, the Zeng model, the Swick and Perumpral model and the Mckyes model are recommended for predicting the horizontal excavation force, and the Swick and Perumpral model and the Mckyes model are recommended for predicting the vertical excavation force.
The shear behaviors of granular mixtures are studied using the discrete element method. These granular materials contain real gravel-shaped coarse particles and spherical fine particles. Dense samples have been created by the isotropic compression method. The samples are then sheared under drained triaxial compression to a large strain to determine the peak and residual shear strengths. The emphasis of this study is placed on assessing the evolutions of contributions of the coarse-coarse (CC) contacts, coarse-fine (CF) contacts and fine-fine (FF) contacts to the peak and critical deviator stresses. The results are used to classify the structure of granular mixtures. Specifically, the granular mixtures are fine-dominated or coarse-dominated materials when the coarse particle content is <30%?40% or >65%?70%, respectively. A comparison with previous findings suggests that the spherical binary mixtures will become coarse-dominated materials at a relatively larger coarse particle content (i.e., 75%?80%) than this study (i.e., 65%?70%), which is attributed to the particle shape effect of coarse particles. A microscopic analysis of CC, CF and FF contacts at the peak and critical states, including normal contact forces and proportions of strong and weak contacts of each contact type to total contacts, reveals why the contributions of CC, CF and FF contacts to the peak and residual shear strengths are varied. Finally, a detailed analysis of the anisotropies indicates that the increases of peak and residual shear strengths are primarily related to the gradual increases in geometrical anisotropy ac and tangential contact force anisotropy at to compensate for the continuous decrease in normal contact force anisotropy an. Furthermore, it is interesting to note that the branch vector frame provides a better linear relationship between the stress ratio and the geometric anisotropy of the strong and nonsliding subnetwork than the contact frame for the coarse-dominated materials.
This paper presents a Discrete Element modelling of the installation responses of rigid open-ended piles with different diameters. A two-dimensional model of granular soil is generated using the Particle Flow Code (PFC). The model is generated under 100g gravity to simulate deep foundations. The simulation results show good agreement with the published experimental measurements. Particle-scale information is retrieved to reveal the micromechanics underlying these dynamic behaviours. It is found that relative particle displacement field provides clear micro-scale evidence to the development of shear zones. The instantly fully plugged mode and the partially plugged mode continuously exchange during pile installation, which is more apparent in slimmer pile. Pile base resistance (plug resistance plus annulus resistance) contributes to the majority of pile resistance. The plug resistance is only concentrated within the zone 2-3 times of pile diameter above the pile base. The unit lateral resistance at a specific depth decreases in pile installation. Pile installation causes increases in the average stresses surrounding the pile, with the major influence zone concentrated within six times of pile diameter.
Offshore wind turbine foundations are subject to 107 to 108 cycles of loadings in their designed service life. Recent research shows that under cyclic loading, most soils change their properties. Discrete Element Modelling of cyclic simple shear tests was performed using PFC2D to analyse the micromechanics underlying the cyclic behaviours of soils. The DEM simulation were first compared with previous experimental results. Then asymmetric one-way and two-way cyclic loading pattern attained from real offshore wind farms were considered in the detailed parametric study. The simulation results show that the shear modulus increases rapidly in the initial loading cycles and then the rate of increase diminishes; the rate of increase depends on the strain amplitude, initial relative density and vertical stress. It shows that the change of soil behaviour is strongly related to the variation of coordination number, rotation of principal stress direction and evolution of degree of fabric anisotropy. Loading asymmetry only affects soil behaviours in the first few hundred of cycles. In the long term, the magnitude of (³max - ³min) rather than loading asymmetry dominates the soil responses. Cyclic loading history may change the stress-strain behaviour of a soil to an extent dependent on its initial relative density.
The mechanical properties of the lunar soil, e.g. the bearing capacity and the deformation modulus, are of great importance for the design of the foundations of permanent lunar outposts. To measure these two parameters, the plate loading test (PLT) is a reliable and the most popular method on the Earth, but is not directly applicable on the lunar surface due to the difficulties in setting up the test equipment. An alternative and indirect method is the cone penetration test (CPT) mainly due to its simplicity in operation. The current study concentrates on the relationships between the two mechanical parameters obtained from PLTs and the penetration resistance obtained from CPTs. Both PLTs and CPTs were carried out on Tongji-1 lunar soil simulant (TJ-1 simulant) in a calibration chamber to establish their relationships and then on a large-scale man-made ground to validate the relationships. The test results show that: the link between the acceptable bearing capacity qa, the deformation modulus E0 and the average penetration resistance qavg was established and validated as: qa = 0.25/D0.63qavg and E0 = 7.16qavg for TJ-1 simulant ground. They can be used directly in the foundation design for lunar outposts.
Bearing capacity failures of offshore pile foundations under cyclic loads in sandy soils are usually initiated by soil strength reduction due to cyclic pile-sand interface sliding. In this paper, a large-scale constant normal stiffness (CNS) cyclic direct shear apparatus is designed to simulate this sliding mechanism, with an effective shearing area of 0.14m2. The cyclic pile-sand interface behaviour of two common types of piles (concrete and steel piles) is subsequently investigated under different confining pressures and cyclic deformation amplitudes. The experimentally obtained results, including the induced shear/normal stress-displacement behaviour, stress path behaviour and particle distribution at the pile-sand interface, are analysed. This further leads to a detailed study for the associated pile-sand interface cyclic weakening mechanism, through the investigations of cyclic attenuation of shear/normal stress amplitude, strength reduction and particle crushing, respectively. Furthermore, experimental results highlight the effect of initial confining pressure, cyclic shear deformation amplitude and pile surface roughness on the investigated pile-sand interface cyclic weakening mechanism.
Foundations are one of the most expensive items in the capital cost break down of an offshore wind farm and the design is a challenging and multidisciplinary task that requires an understanding of the aerodynamics, hydrodynamics, structural dynamics, and soil-structure interaction. Though there has been extensive research, foundation codes are still not fully developed and are heavily dependent on the principles developed for oil & gas platforms which have distinct differences with offshore wind installations. Furthermore, current offshore wind turbines are becoming larger in size and installed in deeper waters, thus jackets are becoming a more attractive option when compared to the conventional monopiles.
However, the current design methods for jackets are computationally challenging and time consuming, and often is the case, require data that is unavailable in the public domain which makes concept designs a difficult process. Due to the lack of simple integrated approaches, this thesis focuses on developing an integrated and modular design approach which can be easily implemented on spreadsheet type software and result in a conservative foundation size with adequate accuracy. The design criteria covered in this thesis are the Ultimate Limit State (ULS), Serviceability Limit State (SLS), and the natural Frequency requirements.
For the dynamic analysis of foundations, a novel approach is proposed that expresses the natural frequency of the system in terms of mechanics based approaches considering the flexibility of the jackets and supporting foundations. The analysis steps from the thesis are compiled into an integrated design approach applied to jackets and their supporting foundations named as the ?10-step method?. It is shown that following these steps will result in a similar jacket and foundation size as detailed designs. The approaches of this thesis are expected to be a very powerful tool in the concept design stage when the financial viability of a wind farm is assessed. The work of this thesis also sets ?templates? of appropriate jacket and foundation sizes in the detailed design stage. Finally, future works and enhancement to the work of this thesis are also provided.