Jim Shiau

Undrained stability of footings on slopes

Solutions for the ultimate bearing capacity of footings on purely cohesive slopes are obtained by applying finite element upper and lower bound methods. In a footing-on-slope system, the ultimate bearing capacity of the footing may be governed by either foundation failure or global slope failure. The combination of these two factors makes the problem difficult to solve using traditional methods. The importance of a dimensionless strength ratio in determining the footing capacity is broadly discussed, and design charts are presented for a wide range of parameters. In addition, the effect of footing roughness and surface surcharge are briefly quantified.

Numerical analysis of passive earth pressures with interfaces

Elasto-plastic analysis for classical lateral earth pressures is presented in this paper by applying the explicit finite difference method using FLAC. The numerical model presented here consists of a rigid elastic structure for the gravity wall, a zero thickness interface for modelling the sliding and separation, and a Mohr-Coulomb soil model for the backfill. The rigid wall was pushed into the backfill soil to induce passive failure and the ultimate load required for the failure is calculated. Numerical results are compared with other available solutions and a number of modelling issues in creating an accurate model are discussed. It is hoped that, through these experience learnt, some numerical pitfalls can be avoided by practicing engineers in their future analysis.

The use of sinkhole models in advanced geotechnical engineering teaching

This paper demonstrates an approach to integrate geotechnical research into an advanced geotechnical engineering course. It involves the use of a simple 3D small-scale sinkhole model, a 2D numerical model of an idealized sinkhole, and an advanced 3D numerical model. A three-week teaching activity that integrates this material into the course is developed, ranging from the introductory material through to experiment conduction and reporting. By incorporating some advanced topics such as this into the course, it is expected that the students will have an increased awareness and understanding of natural geotechnical phenomena. It is also hoped that the model will help foster further interest in the geotechnical discipline, for either research or practice.

Introducing advanced topics in geotechnical engineering teaching - tunnel modelling

This paper demonstrates a strategy to integrate research into teaching at a postgraduate level. It involves the use of simple small-scale physical models of a tunnel heading, the introduction of particle image velocimetry, an advanced computing technique used to further visualize the movement of soil, and a numerical model of a tunnel heading. Based on staff experience and student feedback, it is concluded that this is an effective and satisfying way to engage students with the subject matter. It is also hoped that discussing and displaying some of these advanced topics will encourage further student interest in geotechnical engineering research.

Stability charts for a tall tunnel in undrained clay

The stability of a plane strain tall rectangular tunnel in undrained clay is investigated in this paper using shear strength reduction technique. The finite difference program FLAC is used to determine the factor of safety for unsupported tall rectangular tunnels. Numerical results are compared with upper and lower bound limit solutions, and the comparison finds a very good agreement with solutions to be within 5% difference. Design charts for tall rectangular tunnels are then presented for a wide range of practical scenarios using dimensionless ratios ~ a similar approach to Taylor’s slope stability chart. A number of typical examples are presented to illustrate the potential usefulness for practicing engineers.

AN OVERVIEW ON OIL CONTAMINATED SAND AND ITS ENGINEERING APPLICATIONS

Oil leakage is considered as one of the significant environmental issue worldwide, which affects the physical and chemical properties of the surrounding sand. A range of remediation methods for oil-contaminated sand was recommended but none are considered to be cost effective especially in developing countries. In order to find an alternative and cost-effective remediation method, the use of oil contaminated sand in engineering and construction has been considered. This paper reviews the main sources of oil contamination, the existing remediation methods and critically analysed several factors that affecting the properties of oil contaminated sand. Emerging applications of oil contaminated sand in engineering and construction are also presented.

Stability charts for unsupported circular tunnels in cohesive soils

This paper investigates the stability of a plane strain circular tunnel in cohesive soils. A strength reduction technique and the finite difference program FLAC, are used to determine the factor of safety for unsupported circular tunnels. Results from the finite difference approach are presented alongside rigorous upper and lower bound limits computed using Optum G2. A thorough comparison between these two methods finds very good agreement. As the model is shown to be accurate, design charts are presented for a wide range of practical scenarios using dimensionless ratios. The potential usefulness of this approach is demonstrated using a number of examples. This is considered a simpler way of analysing the stability of an unsupported tunnel that may be as practical as similar methods used in slope stability analysis, such as the commonly used Taylor’s design charts.

Estimation of tunneling induced ground settlement using pressure relaxation method

The prediction of ground settlement caused by tunnelling is frequently estimated using a specified tunnel volume loss, and by applying a semi-empirical method involving the Gaussian equation, and relying on engineers experiences. One of the key parameters in the semi-empirical method, K, is generally estimated using basic soil classifications, which has the potential to lead to inaccurate judgement from engineers. Better estimation of this constant K has had limited attention by other studies. This research uses a force relaxation technique and the finite difference program, FLAC, to estimate the transverse settlement profile for a range of different scenarios. A number of particular cases are numerically simulated with variation in the factors that influence the tunnel transverse settlement including tunnel depth to diameter ratios (C/D), clay strength ratios (γD/S u ), Young’s Modulus (E), and volume loss (% of tunnel). Using these settlement profiles, a K parameter can be accurately fitted for each case. Results from this study compare favourably with previous empirical and analytical studies. A range of K values is proposed for any combination of soil strength, Young’s Modulus, tunnel geometry, and volume loss.

STABILITY CHART FOR UNSUPPORTED SQUARE TUNNELS IN HOMOGENEOUS UNDRAINED CLAY

This short technical note investigates the stability of a plane strain square tunnel in homogeneous undrained clay using shear strength reduction technique. The finite difference program FLAC is used to determine the factor of safety for unsupported square tunnels where an automatic generation of program script is developed using FISH programming. This developed procedure enables parametric studies to be conducted in an effective way with great efficiency. Numerical results, expressed in term of factor of safety, are compared with published classical upper and lower bound limit solutions. The comparison between these two numerical methods finds a very good agreement and the confidence level of the current FLAC model has increased significantly. With the success of model validations, a number of stability design charts for square tunnels using dimensionless ratios are presented for practical scenarios in a similar way to Taylor’s slope stability charts. Examples are illustrated to show the potential usefulness of the produced design charts for practicing engineers.

A revisit to Nicoll Highway excavation in Singapore

This paper demonstrates the use of explicit finite difference program FLAC to numerically simulate staged braced excavation involving various stages of excavation and strutting. The short term undrained condition is considered in the analyses. A FISH script is developed, with automatic mesh generation capacity, to study the behavior of wall deformation and ground settlement during the stages of construction. The numerical model is developed using the case study of Nicoll Highway in Singapore. Both the geotechnical design and the causes of failure of the Nicoll Highway excavation are re-visited through extensive comparisons with existing published data. It is considered as a successful development with the main purpose in mind to develop a numerical model for the analysis of staged excavation in the construction of underground basements. The success of the development is to be used in assisting other design projects in the future.