Dr Liang Cui

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.

Fixed-bottom foundations of offshore structures, mainly monopiles, are subject to extreme events and other critical cyclic nature loads. Since offshore wind turbine structures are slender, the manufacturers of offshore wind turbines give a range of frequencies for safe operation during a structure’s life cycle. Highly reliable measurements and accurate determination of shear moduli and damping ratios are crucial to ensure the stability of these structures, for example, to avoid the resonance of the structures. Because foundation–soil properties change over a period of time due to various environmental factors, this should be taken into consideration for designs. In the current investigation, behaviours of dry sand under dynamic loads were explored. Cyclic loads of strain amplitudes of 0.05%, 0.1%, 0.25% and 0.5% were carried out in a cyclic simple shear apparatus to explore the evolution trend of the stiffness and damping ratio of the soil. Attempts were made to simulate varying weather conditions by conducting cyclic tests with different strain amplitudes representing normal weather conditions and extreme weather conditions. It was found that soil dynamic properties vary remarkably at first and then tend to stabilise under cyclic loading with the same strain amplitude. However, with varying strain amplitude, property variations continue further. From numerical analyses using the discrete element method, it was found that this is due to the disturbance of soil, causing further particle rearrangements and soil compactions, following a sudden change of strain amplitude, which leads to further property variations.

Offshore wind farms are currently being constructed worldwide, and most of the Wind Turbine Generator (WTG) structures are supported on single large-diameter steel piles, commonly known as monopile. One of the challenging design aspects is predicting the long-term deformation of the foundation and, in particular, the accumulation of rotation which is a complex Soil-Structure Interaction (SSI) problem. Accumulation of rotation requires the estimation of Load Utilisation (LU) ratio (i.e., ratio of the load-carrying capacity of the foundation to the applied loads from wind and wave). Estimation of LU for monopile is not trivial due to the simultaneous action of lateral load and moments and needs the introduction of interaction diagram concepts. This paper proposes methodologies to obtain LU for monopiles using three types of methods: (a) Simplified method, which is based on closed-form solution (where the load effects are uncoupled) and can be carried out using spreadsheets or pocket calculators; (b) Standard method based on non-linear Winkler spring (also known as p-y method) where the load effects are also uncoupled; (c) Advanced method, which uses Finite Element Method (where the load effects are coupled). Examples of monopiles are taken from European Wind Farms covering different ground profiles: Gunfleet Sands (clay profile), Walney-I (sandy profile), London Array-I (layered profile) and Barrow-II (layered profile) sites are analysed using all three methods. It is hoped that the methodology will be helpful in the design optimisation stage.

Groundwater heat pump (GWHP) systems are acknowledged as renewable and sustainable energy sources that can effectively fulfill the heating and cooling requirements of buildings on a district level. These systems harness geothermal sources available at shallow depths. To ensure the long-term sustainability of the system, the thermally used water is generally reinjected into the aquifer, creating a thermal plume starting from the injection well. Over time, this thermal plume may reach the abstraction well in the long term, potentially leading to a reduction in system efficiency. The operation types have a significant impact on this matter, and their effects have not been extensively studied in the existing literature. Therefore, this study aims to determine the optimal operating configurations for the Northern Gateway Heat Network, a GWHP system established in Colchester, UK. In this study, four distinct operation types are considered: (1) continuous heating (actual system), (2) heating and recovery, (3) heating and cooling, and (4) aquifer thermal energy storage (ATES). The results indicate that ATES operation yields the highest thermal energy output due to its ability to benefit from stored energy from the previous operation. However, implementing the ATES system may encounter challenges due to factors such as well development, hydraulic conductivity, and hydraulic gradient. On the other hand, implementing heating and cooling operations does not require additional considerations and offers not only free cooling to buildings but also a delay in thermal feedback time.

Offshore Wind Turbines are a complex, dynamically sensitive structure owing to their irregular mass and stiffness distribution and complexity of the loading conditions they need to withstand. There are other challenges in particular locations such as typhoon, hurricane, earthquake, sea-bed current, tsunami etc. As offshore wind turbines have stringent Serviceability Limit State (SLS) requirements and need to be installed in variable, and often complex ground conditions, their foundation design is challenging. Foundation design must be robust due to the enormous cost of retrofitting in a challenging environment should any problem occurs during the design lifetime. Traditionally, engineers use conventional types of foundation system such shallow Gravity-Based Foundations (GBF), suction caissons or slender pile or monopile owing to prior experience with designing such foundations for the oil and gas industry. For offshore wind turbine, however, new types of foundations are being considered for which neither prior experience nor guidelines exist. One of the major challenges is to develop a method to de-risk the life cycle of offshore wind turbines in diverse met-ocean and geological conditions. The paper, therefore, has the following aims: (a) Provide an overview of the complexities and the common SLS performance requirements for offshore wind turbine; (b) Discuss the use of physical modelling for verification and validation of innovative design concepts, taking into account all possible angles to de-risk the project. (c) Provide examples on applications of scaled model tests.

Advanced therapies are commonly administered via injection even when they act within the skin tissue and this increases the chances of off-target effects. Here we report the use of a skin patch containing a hypobaric chamber that induces skin dome formation to enable needleless delivery of advanced therapies directly into porcine, rat, and mouse skin. Finite element method (FEM) modeling showed that the hypobaric chamber in the patch opened the skin appendages by 32%, thinned the skin, and compressed the appendage wall epithelia. These changes allowed direct delivery of an H1N1 vaccine antigen and a diclofenac nanotherapeutic into the skin. Fluorescence imaging and infrared mapping of the skin showed needleless delivery via the appendages. The in vivo utility of the patch was demonstrated by a superior IgG response to the vaccine antigen in mice compared to intramuscular injection and a 70% reduction in rat paw swelling in vivo over 5 h with diclofenac without skin histology changes.

In order to better understand the ice-induced collision/vibration mechanism of offshore wind turbines (OWTs) with ice-breaking cones, the Tsai-Wu yield criterion of ice for bending failure is coupled to the nonlinear collision simulation tool ANSYS/LS-DYNA in this paper, and a numerical approach is adopted to predict dynamic ice force. Through comparisons of ice force between simulations and physical tests, the accuracy of dynamic ice force simulated by the proposed approach is verified. Meanwhile, to consider the pile-soil interaction (PSI) effect, the Euler-Bernoulli Beam theory and the Timoshenko Beam theory are used, respectively, and the dynamic characteristics of the OWT are compared and analyzed. The discrepancies between the calculation results of the two methods and the influence of PSI on OWTs under combined wind-ice conditions are discovered. Further, the fully coupled interaction model between ice and flexible OWT with PSI under wind and ice loadings is established. Based on the full three-dimensional (3D) interaction model, the influence of the ice velocities, ice thicknesses, and cone angles on the dynamic ice force and structural response is investigated. The area damage rate and fatigue damage rate of the monopile foundation are proposed to further carry out damage and fatigue analyses under wind and ice loads. Higher fatigue damage is associated with smaller area damage rate. In addition, the ice thickness has a more significant effect on OWT damage and fatigue compared with the ice velocity.

Nearly 40% of Europe’s total energy consumption is dedicated to buildings and heating/cooling make a significant part of this consumption. Groundwater heat pumps (GWHP) are highly efficient, and low-carbon technology that can supply heating/cooling to buildings on small or large scales. Thus, they contribute to achieving European targets of net-zero greenhouse gas emissions by 2050. In the literature, studies on the utilisation of GWHP at a district scale, particularly in chalk aquifers, are relatively rare. The implementation of district-scale geothermal heat pump (GWHP) systems poses several challenges, including dealing with the scale and complexity of the systems, addressing geological variability, managing high initial investments, balancing energy demand and supply, ensuring proper maintenance and monitoring, and mitigating potential environmental impacts. These challenges require careful consideration and strategic planning to ensure the successful deployment and sustainable operation of these systems., This study numerically investigates a district-scale GWHP system and analyses the thermal plume development created due to the heating operation, offering insights into system performance. A good match was found between field results and simulation results for water level increase and drawdown. However, there is a difference of approximately 11% in system efficiency between field tests and simulations due to the lower abstraction temperature detected in the simulation. The simulation results show that cooler water injection into the fractured chalk aquifer creates a thermal plume radially spanning out to 50 m. The thermal plume has no effect on the abstraction temperature and system performance. This result can be attributed to the large distance between injection and abstraction wells and the low hydraulic gradient.

Using low-temperature (shallow) groundwater as a heat source or heat sink is a common practice to supply space heating or cooling, especially in the United States, Canada, China, and several European countries. The groundwater heat pump (GWHP) system has been extensively studied in recent decades using numerical approaches, which have some limitations in understanding the soil’s thermal behavior. Therefore, a laboratory-scale experimental study involving cooling tests was carried out to investigate the impact of GWHP on system performance and sustainability with varying groundwater flow velocities and injection and abstraction rates. The results demonstrated that groundwater flow velocity, as well as injection and abstraction rates, significantly impact thermal plume development. Higher injection and abstraction rates create a larger thermal plume, thereby decreasing abstraction temperature. However, groundwater flow prevents heat development around the well by dispersing the heat in the groundwater flow direction. Furthermore, the results indicate that the energy gain only increased by 81% and 107%, with a respective increase of 100% and 200% in injection and abstraction rates.

Large-span roof structure is widely used in important landmark buildings. This paper presents the experimental investigations of the deformation and stress distribution of standing seam metal roof system (SSMR) under the condition of a uniform temperature field. The experimental results show that with the increase of temperature, the stresses of slab rib, support, bottom and surface of the roof slab increase gradually, in which the maximum stress growth rates are 1.77, 0.87, 0.64, and 1.68, respectively. The minimum stress increase appears at the slab bottom, while the maximum stress increase with a value of 83.7% of yield strength appears at the slab rib. With the enhancement of the constraint boundary, the stress of the two specimens gradually increases. In particular, the stresses increased the most under fixed at both ends constraints. The stresses show significantly different values in different positions of the roof slab. Both the horizontal and vertical displacement of the roof slab increased with the temperature. And the maximum horizontal displacement of the roof slab is respectively 14.85 mm and 20.42 mm (free one end), in which maximum vertical displacement is 2.34 mm and 6.11 mm, respectively (fix two ends).

Resilience of bridges in seismic zones can be realised by taking the advantage of rocking isolation which aims at reducing the permanent drifts after a seismic event. The seismic forces at the base of the bridge can be reduced by allowing uplift in the foundation when subjected to ground shaking. Conventional monolithic connection of bridge pier to the foundation often leads to severe damages (or even collapse) during high magnitude earthquakes. In this context, this article proposes a novel seismically resilient pier footing which rocks on elastomeric pads and external restrainers (provided by shape memory alloy bars). Seismic performance of a typical existing overpass motorway bridge is compared with the proposed rocking isolation concept. The proposed technique shows good re-centering capability during earthquakes with negligible residual drifts. Furthermore, it is also observed that forces in the pier and size of pier footing are reduced as compared with the reference bridge considered in this study.

Continuous supply of cooling power is a critical aspect in operations of Nuclear Power Plants (NPPs) as evidenced in the major nuclear disasters. Design engineers follow rigorous standard guidelines in planning several levels of safety for power sources in NPPs. However, any unprecedented man-made or natural events may lead to the loss of coolant requirements. This study proposed a seismic resilient strategy using sustainable wind power through which the robustness of cooling power for NPPs during seismic events can be enhanced. Proposed strategy involves various steps starting from estimation of coolant power requirements for nuclear reactors, design of offshore wind turbine (OWT) and supporting system, and seismic safety assessment of proposed OWT for scenario levels at the site of interest. An existing NPP in India (Madras Atomic Power Station-MAPS, Chennai) is chosen as the case study for demonstrating the applicability of proposed strategy. Seismic safety analysis of proposed OWT is performed considering the state-of-the-art understanding on existing seismic scenario, geological and loading conditions, nonlinear soil structure interaction (SSI), and liquefaction susceptibility. Based on the analysis, it is concluded that the proposed offshore wind power is seismically resilient for the anticipated scenario at the site and could cater the coolant requirements of MAPS.

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.

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, ρ*, 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 σ-ε 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.

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.

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 β and bedding plane orientation δ, 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 ϕp and the non-coaxial angle i both increase with the initial density, Am and β, and they both increase initially and then decrease with ϕ 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.

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.

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.

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.

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.

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).

The energy absorbing foam liner used in safety helmets was optimised using finite element modelling. Computational simulations of certification standard tests were carried out to obtain the best performing configurations of helmet liner. For each test condition, the best configuration of helmet liner was identified. Two alternative designs were considered: the first was composed of three layers of different foam density, the second was a conventional liner of one single uniform density. The observed reduction in peak acceleration for the best performing helmet liners in various test conditions are directly related to the contact area, the distribution of material stresses and the dissipated plastic energy density (DPED). Peak linear accelerations are shown to be lowered by increasing the contact areas of the inner and outer surfaces of the energy absorbing liner, or by varying the foam density through the thickness of the liner to ensure that the foam absorbs energy plastically when the stress reaches the late plateau stage of the foam stress–strain curve.

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.

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 μ=0.0 and μ=0.5 , where μ 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/σv0) 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.

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.

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 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.

In order to establish a new bond failure criterion for bonded particles to be used in two-dimensional Discrete Element Modelling, the mechanical behavior of cemented granules with different cement sizes were investigated. In this paper, experimental results of five series of loading tests on cemented granules with four cement widths were presented. Then incorporated the previous experiments with different cement thicknesses, a bond failure envelope dependent on the cement size was proposed. Among a total of about 600 pairs of tested samples, three failure modes were found. Only the failure mode, in which the failure surface was regular and mechanical responses were stable and repeatable, was considered as the representative results and adopted to establish the bond failure model. The experimental results showed that: the peak normal forces in the compression and tension tests are dependent on the cement size; the peak shear forces or peak torsions in the loading tests with complex loading paths, i.e. combined compression shear test, combined compression torsion test and combined compression shear torsion test, are dependent on the normal force and cement size. The peak shear force or peak torsion increases with the normal force when the normal force is smaller than a critical value, while decreases as the normal force is larger than the critical value. The failure envelope in the nomal–shear–torsion space is in an ellipsoidal shape with a cut-off as the normal force reaches the peak compression force. It expands with the cement width and shrinks with the cement thickness.

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.

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 (γmax - γmin) 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.

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.

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.

Suction cups are widely used in applications such as in measurement of mechanical properties of skin in vivo, in drug delivery devices or in acupuncture treatment. Understanding mechanical response of skin under hypobaric pressure is of great importance for users of suction cups. The aim of this work is to predict the hypobaric pressure induced 3D stretching of the skin. Experimental skin tensile tests were carried out for mechanical property characterization. Both linear elasticity and hyperelasticity parameters were determined and implemented in Finite Element modelling. Skin suction tests were performed in both experiments and FEM simulations for model validation. 3D skin stretching is then visualized in detail in FEM simulations. The simulations showed that the skin was compressed consistently along the thickness direction, leading to reduced thickness. At the center of the dome, the radial and angular strain decreases from the top surface to the bottom surface, although always in tension. Hyperelasticity modelling showed superiority over linear elasticity modelling while predicting the strain distribution because the stretch ratio reaches values exceeding the initial linear elastic stage of the stress-strain curve for skin. Hyperelasticity modelling is an effective approach to predict the 3D strain distribution, which paves a way to accurately design safe commercial products that interface with the skin.

To validate the application of three-dimensional (3D) discrete element method (DEM) on modeling the excavation process, a discrete analogue of lunar regolith simulant is excavated under terrestrial conditions using DEM and the results were compared with the previous experimental data using TJ-1 lunar soil simulant. The soil failure mechanism is first described at different scales, with detailed DEM studies of excavation force, earth pressure, soil heap, void ratio changes, APR (average micro-pure rotation rate) field, particle displacements and velocities. Following these, the effects of cutting depth, cutting angle, blade width on the excavation force and size of the affected zone are analyzed and compared with the experimental data. The results illustrate that a “real-time” affected zone can be identified from the APR field and particle velocities which fluctuate significantly in a similar tendency to the excavation force at the post-peak stage. In contract, a “cumulative” affected zone can be identified from the void ratio and particle displacements, which remain increasing gradually at the post-peak stage and invariable when the excavation displacement is large enough to allow a stable soil heap to form. In addition, the simulation results can capture the effects of cutting depth, cutting angle and blade width, which reveals the validity of the numerical modeling approach and thus further studies on the effects of lunar environments on soil excavation can be carried out using DEM.

The offshore pile foundations are subjected to lateral cyclic loadings such as winds, waves, and currents with complex force characteristics, which can cause complex structure responses. In this paper, the discrete element method (DEM) is used to carry out a numerical study on the force characteristics of open-ended pipe piles under lateral loading. The simulation results show that the pile rotates under cyclic loading, the centre of rotation is about 85% of the buried depth of the pile, and the cumulative angle of the pile is less than 0.1°. When a periodic load is applied, the soil around the pile is disturbed, resulting in plastic displacement, and the soil affected area around the pile forms a “butterfly shape”. The displacement in the “active zone” is larger than that in the “passive zone”. As the cycle time increases, the horizontal stiffness gradually decreases in the first 10 cycles, and the attenuation amplitude reaches more than 29% after 100 cycles. The reduction of the internal and external frictional resistance of the pile mainly occurred in the first 10 cycles, and the reduction was less than 15%. It can be seen from the static p-y curve of the pile foundation after cyclic loading that the larger the load amplitude, the greater the reduction in the bearing capacity of the pile foundation under cyclic loading. In addition, the reduction in the bearing capacity of the pile foundation under one-way loading is larger than that under two-way loading. •The double-wall model pipe pile numerical model was used innovatively.•It can effectively separate the inner and outer friction resistance of the open pipe pile under the horizontal cyclic load.•It studied the influence of unidirectional and bidirectional cyclic load on the bearing performance of pile foundation.•The attenuation mainly occurs in the first 10 cycles, and it accounts for more than 80% of the total attenuation.•The reduction in the bearing capacity of the pile foundation under one-way loading is larger than that under two-way loading.

Objectives To develop a simulation model to explore the interplay between mechanical stretch and diffusion of large molecules into the skin under locally applied hypobaric pressure, a novel penetration enhancement method. Methods Finite element method was used to model the skin mechanical deformation and molecular diffusion processes, with validation against in-vitro transdermal permeation experiments. Simulations and experimental data were used together to investigate the transdermal permeation of large molecules under local hypobaric pressure. Results Mechanical simulations resulted in skin stretching and thinning (20%–26% hair follicle diameter increase, and 21%–27% skin thickness reduction). Concentration of dextrans in the stratum corneum was below detection limit with and without hypobaric pressure. Concentrations in viable epidermis and dermis were not affected by hypobaric pressure (approximately 2 μg ∙ cm−2). Permeation into the receptor fluid was substantially enhanced from below the detection limit at atmospheric pressure to up to 6 μg ∙ cm−2 under hypobaric pressure. The in-silico simulations compared satisfactorily with the experimental results at atmospheric conditions. Under hypobaric pressure, satisfactory comparison was attained when the diffusion coefficients of dextrans in the skin layers were increased from ∼ 10 μm2 ∙ s−1 to between 200–500 μm2 ∙ s−1. Conclusions Application of hypobaric pressure induces skin mechanical stretching and enlarges the hair follicle. This enlargement alone cannot satisfactorily explain the increased transdermal permeation into the receptor fluid under hypobaric pressure. The results from the in-silico simulations suggest that the application of hypobaric pressure increases diffusion in the skin, which leads to improved overall transdermal permeation.

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.

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.

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.

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, ρ, 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 σ −ε 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.

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.

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.

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.

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.

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.

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.