C.E. Augarde

Stability of an undrained plane strain heading revisited

The stability of an idealised heading in undrained soil conditions is investigated in this paper. The heading is rigidly supported along its length, while the face, which may be pressurised, is free to move. The problem approximates any flat wall in an underground excavation. Failure of the heading is initiated by a surface surcharge, acting with the self-weight of the soil. Finite element limit analysis methods, based on classical plasticity theory, are used to derive rigorous bounds on load parameters, for a wide range of heading configurations and ground conditions. Solutions for undrained soils with constant strength, and increasing strength with depth are presented. Recent improvements to finite element limit analysis methods, developed at the University of Newcastle, have allowed close bounds to be drawn in most cases. Previous research in this area has often been presented in terms of a stability ratio, N that combines load and self-weight into a single parameter. The use of a stability ratio for this problem is shown not to be rigorous, a finding that may be applicable to other stability problems in underground geomechanics.

Soil suction monitoring for landslides and slopes

Abstract Rainfall is the most frequent triggering factor for landslides and the development of early warning systems has to take account of this. It is suggested that direct measurement of pore pressure gives the most reliable prediction of failure of a slope. The amount of rainfall can be only a crude indicator of failure as the processes that occur between rain falling on a slope and the resulting pore water pressure change are complex, highly non-linear and hysteretic. The paper describes high-capacity tensiometers developed within the EU-funded MUSE Research Training Network that have been used for measuring suctions in slopes. High-capacity tensiometers are capable of direct measurement of pore water pressure down to −2 MPa and are also able to record positive pore water pressures. Two methods of field installation are discussed; one developed by ENPC in France uses a single tensiometer per hole, and the second technique, developed by Durham University in the UK, allows multiple tensiometers to be used at different depths within a single borehole. Continuous monitoring of pore water pressure has been carried out over several months and shows the responses to climatic events.

Chronological Description of the Spatial Development of Rammed Earth Techniques

Rammed earth has been used by man for thousands of years and is currently experiencing a revival in some parts of the world as a result of its inherent sustainability. Historic rammed earth structures are scattered around the world, and much information can be derived from these structures to inform the development of modern rammed earth. This article provides a chronological study of rammed earth distribution through observation of monumental buildings to aid the study of this building technique. It is shown that the rammed earth technique is likely to have originated independently in China and around the Mediterranean, and spread through the movement of people and ideas to many other parts of the world. Through observation of historic rammed earth sites, geographic and climatic limits can be placed on the extent of rammed earth. The different ways rammed earth has been used over time are explored, culminating in its current incarnation as a sustainable building material.

A new procedure for the determination of soil-water retention curves by continuous drying using high-suction tensiometers

Soil-water retention curves (SWRCs) can be determined using high-suction tensiometers (HSTs) following two different procedures that involve either continuous or discrete measurement of suction. In the former case, suction measurements are taken while the sample is permanently exposed to the atmosphere and the soil is continuously drying. In the latter case, the drying or wetting process is halted at different stages to ensure equalization within the sample before measuring suction. Continuous drying has the advantage of being faster; however, it has the disadvantage that the accuracy of mass measurements (necessary for the determination of water content) is affected by the weight and stiffness of the cable connecting the HST to the logger. To overcome this problem, an alternative continuous drying procedure is presented in this paper in which two separate but nominally identical samples are used to obtain a single SWRC; one sample is used for the mass measurements, while a second sample is used for suction measurements. It is demonstrated that the new continuous drying procedure gives SWRCs that are similar to those obtained by discrete drying.

A tying scheme for imposing displacement constraints in finite element analysis

This paper describes the formulation of elements to connect finite element meshes of differing dimensionality. The formulation employs minimization using Lagrange multipliers. While this technique is already described in many texts, this paper demonstrates how particular types of connection may be implemented as independent ‘tie’ elements. Ties for connecting 2D and 3D nodes and for 2D to 2D nodes are formulated in this paper and examples are given showing their application. Copyright

GENERATION OF SHAPE FUNCTIONS FOR STRAIGHT BEAM ELEMENTS

Abstract Straight beam finite elements with greater than two nodes are used for edge stiffening in plane stress analyses and elsewhere. It is often necessary to match the number of nodes on the edge stiffener to the number on a whole plane stress element side. Beam elements employ shape functions which are recognised to be level one Hermitian polynomials. An alternative to the commonly adopted method for determining these shape functions is given in this note, using a formula widely reported in mathematical texts which has hitherto not been applied to this task in the finite element literature. The procedure derives shape functions for beams entirely from the set of Lagrangian interpolating polynomials. Examples are given for the derivation of functions for a three and four-noded beam element.

Some Experiences of Modelling Tunnelling in Soft Ground Using Three-Dimensional Finite Elements

A three-dimensional finite element model has been developed at Oxford University to study the effects of subsidence from soft ground tunnelling on adjacent surface structures. Simulation of excavation and the ground loss associated with tunnelling are incorporated in the model. Surface buildings are also included, as groups of interconnected two-dimensional facades composed of an elastic no tension material, to model masonry. This paper describes the development, implementation and performance of procedures to model the tunnelling processes. A description is also given of the methods used to generate the finite element meshes and to post-process the data.

Parallel Hybrid Particle Swarm Optimization and Applications in Geotechnical Engineering

A novel parallel hybrid particle swarm optimization algorithm named hmPSO is presented. The new algorithm combines particle swarm optimization (PSO) with a local search method which aims to accelerate the rate of convergence. The PSO provides initial guesses to the local search method and the local search accelerates PSO with its solutions. The hybrid global optimization algorithm adjusts its searching space through the local search results. Parallelization is based on the client-server model, which is ideal for asynchronous distributed computations. The server, the center of data exchange, manages requests and coordinates the time-consuming objective function computations undertaken by individual clients which locate in separate processors. A case study in geotechnical engineering demonstrates the effectiveness and efficiency of the proposed algorithm.

The Effect of Relative Humidity and Temperature on the Unconfined Compressive Strength of Rammed Earth

Rammed earth (RE) is an ancient construction technique now being considered for construction in a wide range of climatic conditions. As such, a new range of scientific investigations treating it as a highly unsaturated soil are being undertaken by several institutions in order to fully understand its properties and behaviour. This paper introduces laboratory work determining the effect of changing climatic conditions on the unconfined compressive strength of RE and offers comments on preliminary results; full results and their interpretations shall be discussed in a forthcoming paper.

Numerical Simulation of Fracking in Shale Rocks: Current State and Future Approaches

Extracting gas from shale rocks is one of the current engineering challenges but offers the prospect of cheap gas. Part of the development of an effective engineering solution for shale gas extraction in the future will be the availability of reliable and efficient methods of modelling the development of a fracture system, and the use of these models to guide operators in locating, drilling and pressurising wells. Numerous research papers have been dedicated to this problem, but the information is still incomplete, since a number of simplifications have been adopted such as the assumption of shale as an isotropic material. Recent works on shale characterisation have proved this assumption to be wrong. The anisotropy of shale depends significantly on the scale at which the problem is tackled (nano, micro or macroscale), suggesting that a multiscale model would be appropriate. Moreover, propagation of hydraulic fractures in such a complex medium can be difficult to model with current numerical discretisation methods. The crack propagation may not be unique, and crack branching can occur during the fracture extension. A number of natural fractures could exist in a shale deposit, so we are dealing with several cracks propagating at once over a considerable range of length scales. For all these reasons, the modelling of the fracking problem deserves considerable attention. The objective of this work is to present an overview of the hydraulic fracture of shale, introducing the most recent investigations concerning the anisotropy of shale rocks, then presenting some of the possible numerical methods that could be used to model the real fracking problem.