Fast iterative solvers for geomechanics in a commercial FE code

There is a pressing need to improve the feasibility of three-dimensional finite element (FE) methods applied to many problems in civil engineering. This is particularly the case for static analyses in geotechnical engineering: ideally, models would be 3D, follow the actual geometry, use non-linear material formulations and allow simulation of construction sequences, and all of this with a reasonable degree of accuracy. One major obstacle to improvements in this regard is the difficulty in solving of the set of (linearised) algebraic equations which arises from a typical discretisation approach. Very large systems become cumbersome for direct techniques to solve economically. This paper describes the incorporation of iterative (rather than direct) solution techniques, developed through University research, into commercial FE software for geotechnics

A fully automatic polygon scaled boundary finite element method for modelling crack propagation

An automatic crack propagation remeshing procedure using the polygon scaled boundary FEM is presented. The remeshing algorithm, developed to model any arbitrary shape, is simple yet flexible because only minimal changes are made to the global mesh in each step. Fewer polygon elements are used to predict the final crack path with the algorithm as compared to previous approaches. Two simple polygon optimisation methods which enable the remeshing procedure to model crack propagation more stably are implemented. Four crack propagation benchmarks are modelled to validate the developed method and demonstrate its salient features

Three-dimensional FE-EFGM adaptive coupling with application to nonlinear adaptive analysis

Three-dimensional problems with both material and geometrical nonlinearities are of practical importance in many engineering applications, e.g. geomechanics, metal forming and biomechanics. Traditionally, these problems are simulated using an adaptive finite element method (FEM). However, the FEM faces many challenges in modeling these problems, such as mesh distortion and selection of a robust refinement algorithm. Adaptive meshless methods are a more recent technique for modeling these problems and can overcome the inherent mesh based drawbacks of the FEM but are computationally expensive. To take advantage of the good features of both methods, in the method proposed in this paper, initially the whole of the problem domain is modeled using the FEM. During an analysis those elements which violate a predefined error measure are automatically converted to a meshless zone. This zone can be further refined by adding nodes, overcoming computationally expensive FE remeshing. Therefore an appropriate coupling between the FE and the meshless zone is vital for the proposed formulation. One of the most widely used meshless methods, the element-free Galerkin method (EFGM), is used in this research. Maximum entropy shape functions are used instead of the conventional moving least squares based formulations'. These shape functions posses a weak Kronecker delta property at the boundaries of the problem domain, which allows the essential boundary conditions to be imposed directly and also helps to avoid the use of a transition region in the coupling between the FE and the EFG regions. Total Lagrangian formulation is preferred over the updated Lagrangian formulation for modeling finite deformation due to its computational efficiency. The well-established error estimation procedure of Zienkiewicz-Zhu is used in the FE region to determine the elements requiring conversion to the EFGM. The Chung and Belytschko error estimator is used in the EFG region for further adaptive refinement. Numerical examples are presented to demonstrate the performance of the current approach in thre

Earth stabilisation via carbonate precipitation by plant-derived urease for building applications

Raw (unfired) earth represents a sustainable and efficient alternative to traditional construction materials but its dissemination into building practice has been hindered by a relatively high vulnerability to water erosion. Enzyme induced carbonate precipitation (EICP) can improve the durability of earth materials without using traditional chemical binders such as cement and lime. EICP utilises the urease enzyme to catalyse the hydrolysis of urea, which produces carbonate ions that react with the calcium ions dissolved in the pore water, thus resulting in the precipitation of calcium carbonate. The calcium carbonate fills the soil voids and binds particles together, which reduces water permeability and increases material strength. The urease enzyme is a hexameric protein that is found in the tissues of many common plants. This work proposes a low-cost and simple stabilisation technology that makes use of crude urease enzyme extracted from soybeans. This technology is applied to the stabilisation of compacted earth, whose properties are then assessed via unconfined compression, moisture buffering and durability tests. The findings suggest a noticeable improvement of material strength and durability, though further investigation is necessary to increase the competitiveness of EICP stabilisation against standard techniques using cement and lime

A comparative study of the effects of particle grading and compaction effort on the strength and stiffness of earth building materials at different humidity levels

This paper presents an investigation of the mechanical properties of three different earth building materials manufactured by compacting two soils with distinct particle size distributions under two markedly different efforts. Multiple samples of each material have been equalised either inside a climatic chamber at different humidity levels or oven-dried, before being subjected to shearing inside a triaxial cell to measure the corresponding levels of strength and stiffness. Triaxial shearing has been performed under different levels of radial stress to investigate the effect of material confinement inside thick walls. Consistent with previous research, the study has indicated that strength and stiffness increase as ambient humidity reduces and degree of saturation decreases, though the actual variation of these properties strongly depends on the dry density and clay content of the material. Most importantly, particle grading has emerged as a key material parameter, whose impact on earth building has often been overlooked. Particle grading appears to influence strength and stiffness even more than compaction effort, dry density and average particle size, which are usually quoted as the most important variables for the design of earth building materials

Improving seabed cable plough performance for offshore renewable energy

Seabed ploughing plays an important part in providing protection to subsea cables that connect offshore renewables, and the process represents a significant portion of the overall installation cost. Current models for predicting the required tow forces for seabed ploughs are based on semi-empirical methods as conventional geotechnical finite element analysis is not suited to modelling this process due to the large de-formations involved. The project described in this paper aims to develop a new material point method numerical modelling software to predict seabed plough response, which will enable new designs to be optimised. This will be validated against physical modelling, a part of which is the focus of this paper. This includes both 1g and centrifuge testing as well as new techniques such as 3D soil surface scanning. The testing also provides insights into share geometry influences on plough behaviour, and will allow improvements to existing empirical models

Cone Penetration Tests (CPTs) in layered soils: a Material Point approach

Cone Penetration Tests (CPTs) can be used to determine in-situ soil properties and represent a practical choice for site investigation offshore, especially for linear infrastructure, such as offshore wind export cables. Information gained from CPTs is key for predicting soil-structure interaction behaviour, for example when predicting the tow forces involved in seabed ploughing, as the CPT provides an analogue to the process. The numerical modelling of CPTs is challenging due to the significant distortion in the soil displaced by the penetrating cone. This means that solving this sort of problem using finite elements, although not impossible, is numerically tiresome in terms of remeshing and mapping of state variables. Therefore, in this paper we adopt the Material Point Method (MPM) to develop a CPT prediction tool in layered soils. This MPM is combined with a novel non-matching mesh frictional boundary to represent the penetrometer. The developed tool will be used to understand the response of layered soils commonly found offshore as a step towards predicting the interaction of ploughs and anchors with the seabed

Tensile strengths of flocculated compacted unsaturated soils

Flocculating agents can be introduced to soils through a number of natural or anthropogenic processes. This paper investigates the effect of flocculant addition (aqueous calcium chloride (CaCl2)) on the tensile strengths of two soils of differing flocculation susceptibility. Tensile strengths were found using the Brazilian (direct splitting) test for a range of suction values. A decrease in tensile strength was found for a soil with high clay content, which was consistent with previous findings in the literature. However, the strength of the soil with lower clay content unexpectedly increased. Results were interpreted using the extended Mohr–Coulomb (EMC) yield criterion fitted to data above the residual suction value. Changes in EMC parameters were used to infer changes in material behaviour on calcium chloride addition. Results have important implications for the design of geotechnical structures, for example engineered cover systems, exposed to flocculating conditions. </jats:p