Australian Geomechanics Society Newcastle, New South Wales – 2004 Meetings

• February 19, 2004

  Poulos Lecture: Tunnels and Excavations: Ground movements and the construction process

  Prof. R Neil Taylor, Geotechnical Engineering Research Centre, City University, London

  Infrastructure development invariably involves construction of excavations in the ground. These may be long and horizontal – tunnels – or short, often wide and vertical – basement excavations or cut-and-cover tunnels. The excavations need to be formed safely, so stability can be an issue, and increasingly there are major concerns about the assessment and control of deformations caused by their construction.

  The central theme of research within the Geotechnical Engineering Research Centre at City University is the investigation of ground movements associated with tunnels and deep excavations during the construction process and a major part of the work has been the application of geotechnical centrifuge modelling of these problems. An important feature of the research has been the use of digital image processing techniques that allows overall patterns of ground movements to be determined, enabling centrifuge modelling to progress from a tool for investigating stability mechanisms to one that can give valuable and detailed insight into pre-failure deformations.

  The lecture presented examples of research on tunnels and deep excavations focussing on the overall patterns of ground movements caused by these constructions, the interaction of ground movements with nearby structures and how various structural elements effectively control the magnitude and spread of ground movements.

• March 18, 2004

  Rock Stability Analysis and its Application in Large Dam Construction

  Professor Zhou, Wei Yuan, Tsinghua University, Beijing, China

  In recent years in China a lot of dams, especially large dams have been constructed and are under construction. Rock engineering research has been faced with rock stability problems in dam construction. Below are several important problems that were encountered:

  • Uncertainty in which stability models to use in dam evaluation
  • Difficulty in conducting rupture analysis of rock engineering, such as high slopes and dam foundations
  • Difficulty in conducting fracture analysis of dams

  Elasto-plastic fracture analysis with Damage Mechanics Model and Multipotential Surfaces Failure Model, and Localisation and Meshless Analysis were conducted. The most famous dams such as the Three Gorges Projects and Ertan Arch Dam were analysed by these methods. The results from the analyses as well as physical model tests were presented. These results coincided very well with prototype investigations

• March 30, 2004

  Jamuna – Bridge Foundations

  Mr Steve Kay, Senior Engineer – Fugro Engineers BV

  The Jamuna Multi-purpose Bridge, Bangladesh consists of a 5.0 km long combined road, rail and service crossing of the Jamuna [Brahmaputra] River at Sirajganj some 100 km north of the capital, Dhaka. It is the largest project ever undertaken in Bangladesh at contract value of US$ 700 million. The bridge was Bangladesh's first major river crossing. Engineer for the project was a Joint Venture of Rendel Palmer & Tritton, NEDECO and Bangladesh Consultants Ltd. The Bridge Constructor was Hyundai Engineering & Construction Co. and the Main Contractor for the massive River Training Works a Benelux consortium of HAM/VOA.

  Work commenced in 1995. The Bridge consists of seven separate six, and seven span modules of 100 m. The bridge deck is prestressed balanced cantilever units bearing on concrete piers cast on hollow precast reinforced concrete pile caps. Each pier is founded on 1:6 rake 2 and 3 tubular steel pile groups. Pile diameters are 2.5 and 3.15 m and pile penetrate to around 80 m below existing ground level. The bridge site is in a seismic area, and the piers were designed to withstand earthquakes, ship impact and massive scour. No previous attempts had been made to bridge a braided river of the strength and complexity of the Jamuna, which flows at a rate of 65,000 m3/s during the monsoon flood. The Bridge was opened for traffic in mid 1998.

  The presentation included discussion on the Bridge structure, with particular emphasis on the site investigation, soil conditions, pile load tests, pile design and driving. A principal factor affecting foundation design was that the Jamuna river bed consists of approximately 70 m of loose to medium dense highly micaceous sands.

• August 17, 2004

  Numerical Analysis: A Virtual Dream or practical Reality?

  Prof. David M Potts, Imperial College, London

  The development of numerical analysis and its application to geotechnical problems over the past 20 years have provided geotechnical engineers with an extremely powerful analysis tool. However, the use of such analysis is still not widespread, and when it is used there is all too often evidence of bad practice. Part of the reason for this is a lack of education and of guidance, especially from codes of practice, as to the appropriate use of such methods of analysis. Clearly, some form of initiative is required to promote good practice and allow the full potential of this analysis tool to be realised, both from a safety and an economy perspective. This lecture reviewed the key advantages of numerical analysis over conventional analysis tools, and debated whether or not it can replace the conventional analysis tools in the design process. Examples from engineering practice were used extensively to illustrate the arguments both for and against the use of numerical analysis.

  The power of numerical analysis to predict mechanisms of behaviour was clearly demonstrated with the implication that conventional analysis tools could soon become extinct. Attention was focussed on some of the pitfalls that commonly arise and some of the problems related to the use of numerical software. These were shown to be both significant and extremely worrying and must be resolved if numerical analysis is ever going to fulfill its enormous potential. The successful use of the analysis tool requires a user to be proficient in many areas, some of which are not covered adequately either at undergraduate or postgraduate level. To rectify these deficiencies, changes in the education of geotechnical engineers are necessary, with all the implications this brings to the profession.

• June 1, 2004

  Geotechnical Stability Analysis: New Methods for an Old Problem

  Prof. Scott W Sloan, Geotechnical Research Group, University of Newcastle

  In practice, geotechnical stability analysis is usually performed by a variety of approximate methods that are based on the notion of limit equilibrium. Although they appeal to engineering intuition, these techniques have a number of major disadvantages, not the least of which is the need to presuppose an appropriate failure mechanism in advance. This feature can lead to grossly inaccurate predictions of the true failure load, especially for realistic problems involving layered materials, complex loading, or three-dimensional deformation.

  A much more rigorous method for assessing the stability of geostructures became available with the advent of the limit (or bound) theorems of classical plasticity in the 1950s. These theorems can be used to give upper and lower bounds on the predicted collapse load (a most valuable property in practice), do not require assumptions to be made about the mode of failure, and use only simple strength parameters that are familiar to geotechnical engineers. Although many ingenious bound results have been derived using both analytical and numerical methods, the true power of the limit theorems has been greatly restricted by the need to develop specific solution strategies for each practical problem. Over the last decade, the speaker and his group at the University of Newcastle have developed powerful new methods for performing stability analysis that combine the limit theorems with finite elements and optimisation. These methods are extremely general and can deal with layered soil profiles, anisotropic strength characteristics, complicated boundary conditions, and complex loading in both two and three dimensions. Indeed, they have already been used to obtain new stability solutions for a wide range of practical problems including soil anchors, slopes, foundations under combined loads, excavations, tunnels, mine workings, and sinkholes. The Lecture gave a brief outline of the new techniques and consider a number of practical applications. Future research developments were also be highlighted.

• July 14, 2004

  Geosynthetics in Reinforced Earth Structures

  Raymond Chow, Maccaferri Australia

  The presentation provided a brief overview of the world of Geosynthetics and latest technologies, with an emphasis on reinforced soil structures and other typical civil engineering applications. Some interesting case studies, local and abroad, were briefly discussed. Innovative technologies in the field were also looked at, including instrumentation using geosynthetics and the use of poor-draining soils for reinforced soil walls.

• September 15, 2004

  A Study of Direct Shear Testing using a Large Shear Box

  Dr Stephen Fityus, The University of Newcastle

  Reinforced soil walls are finding increasing application in Australian civil engineering infrastructure. The commonly applied Q181C ‘Test Method’ of Direct Shear Testing to estimate the ‘Effective angle of internal friction at constant volume conditions for granular (coarse grained) materials’ is commonly applied to assess the suitability of backfill soils. This presentation presented the outcomes of a comparison between the results of three series of shear box tests on a typical ripped rock material that might be considered as a possible backfill material for a reinforced earth wall. One set of tests was performed using a 300mm shear box, on a soil sample containing only sub-19mm fractions. The second set of tests was also carried out using a 300mm shear box, but on a sub-4.75mm sample, derived from the sub-19mm sample by removal of the 4.75-19mm size fractions. The third set of tests was performed using a standard small 60mm shear box on the same sub-4.75mm fraction of the soil. Tests were performed at a range of shearing rates. The effect of sample preparation (by pre-testing of samples or by using different consolidation stresses) was also considered. The results showed that accurate measurements of effective friction parameters for coarse grained, granular backfill soils require:

  • the use of fresh soil specimens for each shearing test;
  • the use of a large shear box that can accommodate soils with relatively large particles;
  • careful selection of shearing rates so that shearing takes place under drained conditions.
• October 3, 2004

  Risk Assessment for Civil Engineering Systems

  Assoc. Prof Mark Stewart, The University of Newcastle

  The talk presented an overview of how measures of risk can assess and manage the safety and performance of civil engineering systems such as dams, embankments and buildings. Typical measures of risk include the probability of system failure, annual fatality risk or expected damages or losses. Techniques for quantifying uncertainty and risk were described, as well as how estimates of risk can be used to aid engineering decision-making. Decisions may involve questions of safety (how safe is safe enough?) or the cost-effectiveness of proposed projects. The differences between factor of safety and risk were also be explained.

• October 29, 2004

  Special Function

  Local AGS Members

  To celebrate the completion of the 2004 Geotechnical Design Project at the University of Newcastle, the Newcastle Chapter of the Australian Geomechanics Society invited its members to join the 4th year design students to a recovery function on 29 October 2004 (just after they submitted their major assignment work). It proved to be an excellent opportunity for our imminent graduates to meet members of the local geotechnical profession and discuss potential career opportunities. A number of students secured employment following the function. The Chapter was also fortunate to sign up additional student members as a result of the function.

• November 24, 2004

  Regional landslide hazard maps of Tasmania

  Colin Mazengarb, Mineral Resources Tasmania

  Land stability is a significant problem in parts of Tasmania but useful information outlining the spatial and temporal distribution of this hazard is limited. To address this problem a methodology has been developed to record known areas of instability and predict areas where various landslide process could occur in the future. Specifically the project models rockfall, debris flows and deep-seated landslide processes utilising GIS technology. A comparison of known landslides to the predictive models is encouraging. Currently the approach is substantially deterministic, but probabilistic components are being developed that will eventually allow a regional risk assessment to be achieved. The talk will largely outline the technical methodology but will be expanded to discuss the successful partnerships between State and Local Government that have been achieved, and the challenge of effectively communicating complex geo-scientific hazard information to end-users.

• December 10, 2004

  Energy Method for Structures Interacting with Dissipative Soils

  Itai Einav, APD Fellow, Centre for Offshore Foundation Systems, University of Western Australia

  The combination of energy methods and variational principles has provided a valuable tool to analyze the behaviour of elastic structures for quite a long time. It has also been used infrequently to study soil-pile interaction problems, considering the soil as elastic material by following the traditional method that was developed for the elastic structures. However, soil is not a non-dissipative elastic medium. In this seminar an alternative energy-based variational approach will be presented, showing how to account for the dissipative nature of soils. The applicability of the new method will be discussed for the modelling of (a) rigid shallow foundations and (b) deformable piles. (a)Rigid shallow foundations: The general formulation allows idealization of the problem using uni- or bi-continuous (Winkler) model systems, all encapsulated in two thermodynamics potential functionals. In the uni-continuous system, the global foundation behaviour can be described by defining a continuous field of reaction stress-displacement models along the interface of the problem. By also ormulating each of these reaction stress-displacement interface models to be continuous along an internal coordinate, a bi-continuous system can be modelled. The seminar presents an outline of the approach with examples for strip and circular footing under both monotonic and cyclic loading conditions.(b)Deformable piles: An interesting feature of the approach is that the possibility of internal elastic deformations within the structures may be incorporated within the two potential functionals. For that purpose, the seminar demonstrates the applications for the classical problem of lateral and axial loading of piles. The new method enhances previous energy based variational methods that were applied for this problem. Compared to those methods the current method allows accounting for the dissipative nature of the soil and thus enables to model the response of the pile when it is subjected to cyclic loading conditions.