On the installation effects of open ended piles in chalk

Chalk is a highly porous rock formed by cemented calcite grains. It covers areas of the UK and is widespread under the North Sea where offshore wind turbines (OWT) are currently being installed and where future offshore expansion will be sited (Figure 1(a)) [1]. Large piles are often driven in chalk to support OWT. The installation process causes the intact rock below the pile tip to crush into a putty characterised by a mechanical behaviour very different from the intact chalk. The difficulty to predict the final state of the putty and the stress around the pile after installation is the underlying reason for inadequate current design guidance for piles in chalk. Considering that for OWT, foundations account for 20-25% of the total development cost , pile design improvements in chalk, would be extremely beneficial from an economical and environmental perspective.Current guidelines for the design of piles in chalk (CIRIA C574) originate from the analysis of a limited number of pile tests [2]. These guidelines suggest crude average ultimate unit shaft friction (tsf) design values of between 20 and 120 kPa for low-medium density and high/very-high density chalk, respectively. tsf estimates are thought to be conservative hence introducing significant increases in cost and carbon footprint (Figure 1(b)). Reducing the level of conservatism (e.g. enabling more confident use of higher tsf) would reflect in significant savings of steel and consequent reduction of the cost and embodied carbon. Such design considerations are possible but require a better understanding of the long-term mechanical behaviour of the damage processes intact chalk experiences during dynamic installation in hydro-mechanical (HM) coupled conditions.In this work the coupled HM effects developing during pile installation in chalk are investigated numerically using a robust and mesh-independent implementation of an elasto-plastic constitutive model at large strains. The model, implemented into an open-source Geotechnical Particle Finite Element (G-PFEM) code [3], is shown to be able to capture the damage of the rock until the formation of a chalk putty layer around the shaft of a model piles jacked in chalk. In particular the complex flow processes occurring in the soil around both open and closed ended piles of variable shape jacked into saturated chalk are investigated. A fully coupled hydro-mechanical formulation, based on regularized, mixed low-order linear strain triangles is used [5]. To capture the relevant features of the mechanical response of chalk, a finite deformation, non-associative structured modified cam clay is used [6]. The model formulation is based on a multiplicative decomposition of the deformation gradient and on the adoption of an elastic response based on the existence of a suitable free energy function. Bonding-related internal variables, quantifying the effects of structure on the yield locus, are used to provide a macroscopic description of mechanical destructuration effects. To deal with strain localization phenomena, the model is equipped with a non-local version of the hardening laws [7]. The G-PFEM model is shown to be capable of capturing the destructuration associated with plastic deformations below and around the pile shoulder; the space and time evolution of pore water pressure as the pile advances; the effect of soil permeability on predicted excess pore water pressures, and the effect of chalk putty formation on predicted values of the load displacement curve. Installation effects are highlighted by comparing the axial performance between wished in place piles and piles which considered the full installation process

Micromechanical investigation of grouting in soils

Grouting and jet grouting are geotechnical consolidation techniques commonly employed to improve the mechanical behaviour of soils. Although these techniques are common, the micromechanical processes taking place at the local level are not yet totally understood and modelled. In this work, such a problem has been approached from a micromechanical perspective via the discrete element method by considering the local interaction among soil grains and pseudo-fluid particles. Homogeneous representative elementary volumes of a virtual analogue of silica sand have been first generated and tiny rigid frictionless particles have been subsequently injected through them, to simulate the grouting in granular materials. Various injection pressures, initial soil pressures and initial soil densities have been considered. The different diffusion patterns, the flow rate, the consequent increase in local stresses and the consequent reduction in local porosity have been discussed. To overcome the DEM computational restrictions and to speed up the injection simulations, a novel procedure based on the replication of pre-equilibrated cells has been adapted for both the initialization and injection phase. Finally, a qualitative laboratory-scale pressure grouting test has been reproduced to validate the results

Effect of Flexible Membrane in Triaxial Test on the Mechanical Behaviour of Rockfill Material using Discrete Element Method

The investigation of rockfill materials poses challenges due to their large particle size, associated high cost, and long laboratory testing duration. As a result, empirical correlations based on historical experimental studies are commonly used to design and analyse rockfill structures. However, the extensive use of rockfill in a wide range of applications and limited understanding of its mechanical behaviour emphasize the need for further research. These make it necessary to develop a robust technique capable of capturing key parameters such as particle shape and breakage, allowing for the simulation and study of large-scale assemblies with realistic boundary conditions. Given that the behaviour of rockfill is highly scale-dependent, primarily due to particle breakage, the simplified laboratory tests on the scaled-down assemblies can be misleading. Particle breakage is a fundamental phenomenon in the mechanical behaviour of rockfill and significantly affects shear strength, deformability, and porosity under different stress levels. The particle breakage is influenced by factors such as the rockfill’s maximum particle size, mineralogy, particle shape, gradation, and confining stresses. This study adopts a computationally efficient breakage method called the Modified Particle Replacement Method (MPRM) based on the Discrete Element Method. A Tile-Based Flexible Membrane (TBFM) for triaxial test modelling has been developed by employing segmental rectangular walls to create a deformable membrane. The effects of critical parameters, including particle shape, confining stress, membrane resolution, degree of flexibility, and the characteristic strength of the particles, are examined. The findings of the combined MPRM-TBFM approach demonstrate the significant influence of membrane flexibility on volumetric-related behaviour

A time-to-facture DEM model for simulating creep in rough crushable sand

A contact model able to capture creep of crushable sands within a discrete element method (DEM) framework is presented. Time dependency is established through stress-corrosion induced grain fracture. This is grafted onto a pre-existing particle-splitting model developed to simulate rough-crushable sands. The model is calibrated for Fontainebleau sand and applied to simulate soil creep at high-confining pressures. Creep simulation is advanced using the off-DEM ageing technique. The crack-velocity evolution in particle scale and laboratory oedometer creep curve were successfully captured. Moreover, the onset of creep failure during triaxial creep curve under high deviatoric stress can also be simulated using this model. The results obtained indicate significant potential of this model for micromechanically based investigation of time-dependent soil behaviour.The first author has received support from the Chinese Government Scholarship. (CSC No.202108390006) Spanish Research Agency support (AEI) through re-search project PID2020-119598RB-I00 is also acknowledged.Postprint (published version

Effect of Flexible Membrane in Triaxial Test on the Mechanical Behaviour of Rockfill Material using Discrete Element Method

The investigation of rockfill materials poses challenges due to their large particle size, associated high cost, and long laboratory testing duration. As a result, empirical correlations based on historical experimental studies are commonly used to design and analyse rockfill structures. However, the extensive use of rockfill in a wide range of applications and limited understanding of its mechanical behaviour emphasize the need for further research. These make it necessary to develop a robust technique capable of capturing key parameters such as particle shape and breakage, allowing for the simulation and study of large-scale assemblies with realistic boundary conditions. Given that the behaviour of rockfill is highly scale-dependent, primarily due to particle breakage, the simplified laboratory tests on the scaled-down assemblies can be misleading. Particle breakage is a fundamental phenomenon in the mechanical behaviour of rockfill and significantly affects shear strength, deformability, and porosity under different stress levels. The particle breakage is influenced by factors such as the rockfill’s maximum particle size, mineralogy, particle shape, gradation, and confining stresses. This study adopts a computationally efficient breakage method called the Modified Particle Replacement Method (MPRM) based on the Discrete Element Method. A Tile-Based Flexible Membrane (TBFM) for triaxial test modelling has been developed by employing segmental rectangular walls to create a deformable membrane. The effects of critical parameters, including particle shape, confining stress, membrane resolution, degree of flexibility, and the characteristic strength of the particles, are examined. The findings of the combined MPRM-TBFM approach demonstrate the significant influence of membrane flexibility on volumetric-related behaviour

Lesson learnt from static pulling tests on trees: an experimental study on toppling behaviour of complex foundations

Standard procedures for stability assessment of unstable trees are based, among other, on the interpretation of on-site, non destructive static pulling tests. To this goal, a simple phenomenological equation is usually adopted in professional agronomic practice, and an estimation of the ultimate toppling resistance is extrapolated by fitting the test data, without taking root geometrical parameters and soil mechanical properties into account. From a geotechnical point of view, however, the root plate of a tree plays the role of a ‘‘living foundation’’, and its behaviour under toppling actions (like those produced by intense wind gusts) conceptually corresponds to the mechanical response of shallow foundations under rocking loads. In the paper, several static pulling tests on real-scale trees (some of them have been run until the complete collapse, after some unloading–reloading cycles) and some tests taken from the literature are considered in order to investigate the toppling behaviour. A possible new interpretative equation is proposed and critically compared with the existing one against experimental results. The new equation allows for a mechanically meaningful description of the toppling curve of the tree and accounts for strength and deformability issues. It allows to introduce innovative ‘‘perfor mance-based’’ approaches, which are commonly neglected by practitioners and professional agronomists in this field. Nevertheless, the experimental results show that tree toppling is a complex phenomenon, and capturing its failure condition requires more advanced multi-mechanism models and second-order effects to be accounted for. From a practical point of view, the proposed equation, employed within the same standard interpretative procedure currently adopted in practice for pulling tests, seems to provide conservative estimations of ‘‘operational’’ values of the ultimate toppling resistance, and in perspective, it could be used to significantly optimize—when needed—the design of structural stabilizing interventions on potentially unstable tree

Grain roughness effect on the critical state line of crushable sands

A recently proposed DEM model for materials with rough crushable grains (Zhang et al. 2021; Ciantia et al. 2015; Otsubo et al. 2017) is here employed to examine the effect of contact roughness on the critical state line, a property of granular materials which is a) fundamental for the evaluation of lique-faction risk and liquefied responses and b) easily accessible through DEM simulation (Ciantia et al. 2019).Postprint (published version

Rupture et état critique via DEM

Discrete-element simulations are used to explore the relation between breakage-induced grading evolution and the critical state line position on the compression plane. An efficient model of particle breakage is applied to perform a large number of tests, in which grading evolution is continuously tracked using a grading index. Using both previous and new experimental results, the discrete element model is calibrated and validated to represent Fontainebleau sand. The results obtained show that, when breakage is present, the inclusion of a grading index in the description of critical states is advantageous. This can be simply done using the critical state plane concept.Postprint (published version

Effect of boulder shape on the response of compound meshes subject to dynamic impacts

Rockfall is a type of natural hazard associated with the detachment of one or several boulders in steep slopes. Passive risk mitigation strategies are based on intercepting these blocks during their movement, using rigid barriers, embankments, and flexible protection systems. In recent years, the advancement of remote sensing techniques based on discrete fracture networks allows the characterisation of the shape and size of these boulders even before their detachment [13]. However, physical and numerical modelling of the impact on flexible protection system typically considers spheres [4] and truncated cubes [3] as boulder shape. In this work, the local, i.e. bullet effect [5] and full-scale effect of the aspect ratio of the block is investigated during its impact with a full-scale barrier model. The barrier is characterised by a compound mesh, formed by interweaved double-twisted hexagons and strand ropes stretched between two fence posts. The mesh geometry is reproduced within the Discrete Element framework, using the remote contact interaction approach and the fast mesh generation technique described in [11]