Numerical modelling of transient cyclic vertical loading of suction caissons in sand

This paper presents numerical investigations of the monotonic and cyclic behaviours of suction caissons upon vertical transient loading. Both drained and partially drained conditions are investigated. Monotonic compression and traction simulations are carried out to qualitatively compare results with the literature and validate the model. They highlight the different modes of reaction of the caisson to both compression and traction loading. A sensitivity analysis points out the strong influence of some parameters on the resistance of the caisson but also on the failure mechanism. The transient behaviour of the caisson upon different kinds of cyclic load signals is analysed. Results reproduce the settlement and pore water pressure accumulations observed during experiments. The influence of the key design parameters on the settlement accumulation is also assessed. Finally a cyclic diagram is proposed to describe the evolution of the final settlement upon different magnitudes of loading

Three-node zero-thickness hydro-mechanical interface finite element for geotechnical applications

The paper presents the main features of a hydro-mechanical coupled finite element of interface. The mechanical problem accounts for the detection of contact, the development of contact pressure, shearing and relative sliding between two solids. A three-node discretisation of hydraulic problem allows the representation of fluid flow across and in the plane of the interface. The method involves a drop of pressure between each side of the interface and the inner medium. Hydro-mechanical couplings result from 1) the definition of the total pressure acting on each side of the interface according to the Terzaghi’s principle; 2) the dependence of permeability on the gap opening; 3) the variation of the fluid mass stored within the gap

DEM study of particle scale and penetration rate on the installation mechanisms of screw piles in sand

Screw piles are efficient anchors to sustain large uplift loads and can be installed with low noise or vibration. Screw piles dimensions are currently increasing, renewing research interest to reduce the installation requirements (torque and crowd or vertical force). The Discrete Element Method (DEM) is an ideal technique to investigate the complex soil behaviour during screw pile installation. Different techniques such as particle upscaling or increase of pile penetration rate have been used to reduce the CPU time to more acceptable durations (e.g. few days or weeks). This paper investigates how such techniques can affect the accuracy of the results and change the installation mechanisms. Results show that maintaining a low particle scaling factor is essential to reproduce the correct mechanism at low pile advancement ratio (AR, defined as the vertical displacement per rotation divided by the helix pitch). The pile overflighting (AR≤1) creates an upwards movement of particles, which in turn creates some tension in the pile. Smaller advancement ratios require smaller particles to accurately capture this effect. Results also show that the pile penetration rate must be maintained relatively low to avoid spurious inertial effects

Hydromechanical modelling of shaft sealing for CO2 storage

The geological sequestration of CO2 in abandoned coal mines is a promising option to mitigate climate changes while providing sustainable use of the underground cavities. In order to certify the efficiency of the storage, it is essential to understand the behaviour of the shaft sealing system. The paper presents a numerical analysis of CO2 transfer mechanisms through a mine shaft and its sealing system. Different mechanisms for CO2 leakage are considered, namely multiphase flow through the different materials and flow along the interfaces between the lining and the host rock. The study focuses on the abandoned coal mine of Anderlues, Belgium, which was used for seasonal storage of natural gas. A two-dimensional hydromechanical modelling of the storage site is performed and CO2 injection into the coal mine is simulated. Model predictions for a period of 500 years are presented and discussed with attention. The role and influence of the interface between the host rock and the concrete lining are examined. In addition the impact of some uncertain model parameters on the overall performance of the sealing system is analysed through a sensitivity analysis

Formulation of a 1D finite element of heat exchanger for accurate modelling of the grouting behaviour: Application to cyclic thermal loading

This paper presents a comprehensive formulation of a finite element for the modelling of borehole heat exchangers. This work focuses on the accurate modelling of the grouting and the field of temperature near a single borehole. Therefore the grouting of the BHE is explicitly modelled. The purpose of this work is to provide tools necessary to the further modelling of thermo-mechanical couplings. The finite element discretises the classical governing equation of advection-diffusion of heat within a 1D pipe connected to ground nodes. Petrov-Galerkin weighting functions are used to avoid numerical disturbances. The formulation is able to capture highly transient and steady-state phenomena. The proposed finite element is validated with respect to analytical solutions. An example consisting of a 100 m depth U-pipe is finally simulated. A first continuous heating simulation highlights the nonsymmetric distribution of temperature inside and near the borehole. An estimation of the error on the results as a function of the resolution parameters is also carried out. Finally simulations of cyclic thermal loading exhibit the need to take into account all daily variations if the grouting behaviour must be modelled. This is true especially in case of freeze-thaw damaging risk.Geotherwa

3D zero-thickness coupled interface finite element:Formulation and application

In many fields of geotechnical engineering, the modelling of interfaces requires special numerical tools. This paper presents the formulation of a 3D fully coupled hydro-mechanical finite element of interface. The element belongs to the zero-thickness family and the contact constraint is enforced by the penalty method. Fluid flow is discretised through a three-node scheme, discretising the inner flow by additional nodes. The element is able to reproduce the contact/loss of contact between two solids as well as shearing/sliding of the interface. Fluid flow through and across the interface can be modelled. Opening of a gap within the interface influences the longitudinal transmissivity as well as the storage of water inside the interface. Moreover the computation of an effective pressure within the interface, according to the Terzaghi’s principle creates an additional hydro-mechanical coupling. The uplifting simulation of a suction caisson embedded in a soil layer illustrates the main features of the element. Friction is progressively mobilised along the shaft of the caisson and sliding finally takes place. A gap is created below the top of the caisson and filled with water. It illustrates the storage capacity within the interface and the transversal flow. Longitudinal fluid flow is highlighted between the shaft of the caisson and the soil. The fluid flow depends on the opening of the gap and is related to the cubic law

On the modelling of coupled hydro-mechanical behaviour of interfaces for offshore foundations

Complexity and cost of offshore foundations are increasing. Small scale and large scale modelling alone, although essential, are not sufficient to elaborate accurate design methods. As a complement to physical modelling, numerical methods provide more systematic and numerous results. Sensitivity analyses and many configurations may be tested for a lower cost. Therefore numerical modelling, such as finite element method, has become an essential tool for engineers in the last decades. The derivation of the ultimate bearing capacity of the foundations is not sufficient anymore and serviceability becomes an important concern. The corollary effect is the need of modelling transient behaviour, especially displacement and rotation evolution, of the foundation system. If the investigated loading duration is sufficiently long, pore water pressure may be generated within the soil but also partly dissipated. Therefore the modelling of purely drained or undrained conditions are not relevant. The partially drained behaviour must be reproduced and coupled hydromechanical simulations must be carried out. Capturing the behaviour of interfaces is essential in order to well reproduce the complex behaviour of the soil foundation interaction. Indeed, shearing along the shaft of piles and suction caissons is the main mechanism of resistance upon traction. Losses of contact are frequent upon traction for suction caissons or upon lateral loading for piles in clay, reducing the surface for shear mobilisation. A zero-thickness hydro-mechanically coupled finite element is proposed to satisfy the need of interface modelling in partially drained conditions. The element couples a large displacement formulation of the mechanical contact with a transversal three-node formulation for fluid flow. The field of water pressure is discretised on each side of the interface but also between them. Two transversal and one longitudinal fluid flows are modelled. The hydromechanical coupling arises from the definition of an effective stress, the filling of an opening gap and the variation of the longitudinal permeability with the gap opening. This chapter provides the basic formulation of a coupled finite element of interface. It is followed by two examples of applications related to the vertical (uplift) or lateral loading of a suction caisson. In both cases, the role of interfaces is highlighted by purely and partially drained simulations. Coupled phenomena and their implications on practical results such as the elaboration of p-y curves or the ultimate bearing capacity computation are highlighted

Axisymmetric transient modelling of a wind turbine foundation in cohesionless soil using the Prevost’s model

Suction caissons are more and more used for offshore foundations. This paper deals with the cyclic modelling of suction caissons using the Prevost’s model. The case study is a 8m large diameter caisson embedded in dense No. 0 Lundsand. Parameters for the model are calibrated using drained triaxial tests. A parametric study concerning the influence of the constitutive law, the skirt length and permeability is carried out