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Welcome To Geotechnical Engineering In South-East Queensland “Soft Clay One Day, Hard Rock The Next”
South-East Queensland is loosely defined as the area from the NSW border in the south to Gympie in the north and inland to the Great Dividing Range. The main population centres include Brisbane, Ipswich, and the Gold and Sunshine Coasts. It is a geologically complex area and consequently presents the geotechnical newcomer with a wide range of geological environments and their associated challenges.
In South-East Queensland we are fortunate to be well supplied with detailed geological maps, notes and texts from the Queensland Division of the Geological Survey of Australia, and this is the best place to start in order to understand the local geotechnical conditions. The publications available not only describe the detailed superficial geology of the area, but also, through a series of notes and texts, provide a clear outline of the geological development, rock types and stratigraphy. A simple text, which provides a very thorough geological introduction for the newcomer, is “Rocks and Landscapes of Brisbane and Ipswich” by W Willmott and N Stevens (Geological Society of Australia, 1992). Similar texts are available for other areas through Southern and Eastern Queensland. The other essential geological information available for the area is the1:100,000 and 1:250,000 geological maps. A very good series of maps, now no longer available, but still held and jealously guarded by many practitioners, is the 1965 1:31,680 series. Maps, memoirs and books can be obtained from the Queensland Division of the Geological Society by contacting them on [email protected].
To complement the geological maps, there are also soils maps available. The Department of Natural Resources and Mining, Qld (DNR) published two Acid Sulfate Soils Maps at 1:100 000 scale, which show the distribution of potentially acid sulfate soils along the coast between Tweed Heads and Noosa. The DNR also publishes several soils, vegetation and terrain maps which can be of use. These can be obtained via their website at www.dnr.qld.gov.au.
In hard rock engineering jobs, such as deep excavations in rock, rock slope stability or road cuttings etc, the rock type generally dictates the investigation and design approach. The strong, massive and widely jointed Brisbane Tuffs lend themselves to deep excavations and steep slopes, with dominant jointing patterns dictating failure modes. Support measures such as bolting and anchoring are typically adopted when necessary. The highly foliated metasediments of the Neranleigh-Fernvale beds and the related Bunya Phyllite pose different failure mechanisms and support approaches. Foliation directions and the extent of weathering are critical, and minor block and wedge failures are much more prevalent. Rockfall mesh, flatter slope angles and surface protection are typical support approaches, in addition to bolts and anchors. The Tuff–Phyllite interface is of particular importance, and clay seams, carbonaceous material and paleo- soils have all been encountered at these locations. Further afield, the Mesozoic sedimentary beds, and also the younger Tertiary sedimentary and volcanic rocks cover a large area of the south-east. These require a different geotechnical approach, with generally deeper weathering profiles and larger variations in jointing, bedding and strength.
The stability of natural slopes throughout the south-east is of concern in many areas. Consequently, engineering works routinely require special attention. The steep slopes of the Main Range from the Gold Coast Hinterland, through to Toowoomba and the D’Aguilar ranges, and behind the Sunshine Coast, have always been prone to stability problems. The south-east’s sub-tropical rainfall, characterised by intense summer storms, contribute to instability in these areas. Other areas of inherent instability occur, typically but not always due to localised steep terrain and concentrated storm runoff. In the Brisbane suburb of Oxley for example, the weathering of the underlying rocks of the Oxley Group has resulted in an area that has experienced numerous large landslips, although the natural slopes are no steeper than many other stable areas within Brisbane.
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Water content and its effect on a slope failure in Bangladesh
Most of Bangladesh is flat. However small areas of the country including Sylhet, Chittagong, Bandarban, Rangamati, Khagrachari and Cox’s Bazar are hilly. Slope failures are common in Bangladesh during the monsoon season. As a consequence, every year both property and lives are lost due to slope failure on hilly areas. In this study, the causes of slope failure and the remedial measures required are discussed with a case study of slope failure at Himsory of Cox’s Bazar in Bangladesh. Stability analyses of the failed slope have been investigated by adding water in different proportions to get the critical condition of this slope. From the test results, it is found that shear strength parameters of the soil decrease with an increase of water content. As a result the slope failure occurs when the slope becomes saturated.
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Use of digital imaging for gradation and breakage of railway ballast
The foundation of ballasted railway usually consists of a graded layer of granular media of ballast placed above naturally deposited subgrade. The ballast layer is responsible for limiting the vertical stress magnitudes applied to the weaker subgrade and also prevents the vertical and lateral train-induced sleeper movements. In recent years, the progressive use of faster and heavier trains has compromised the ability of ballast to resist such movements due to the increased magnitude of ballast breakage (degradation). Therefore, accurate determination of ballast breakage is important. The amount of ballast breakage can be estimated by comparing the gradation (particle size distribution) curves of fresh and degraded ballast, using a ballast breakage index. However, the Australian railway authorities currently rely on the visual inspection for estimating ballast breakage. Despite the fact that the visual inspection is convenient, as it does not require transport of ballast samples or testing, it is subjective and can lead to uneconomical maintenance cycles. In this paper, an attempt is made to utilise the digital imaging technique for gradation analysis and breakage estimation of ballast. The technique is fast and convenient, can be applied to both the laboratory and field conditions and, hence, can be used successfully to replace the visual inspection of ballast breakage.
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Highly flexible catch fences and high performance drape mesh systems for rockfall protection in open pit operations
Rockfall hazards in open pit applications mainly occur in steep open pit walls due to aggressive pit design, in flat walls without berms while following shallow dipping ore bodies or locally on batter level. Areas with high damage potential such as decline portals or haulage ramps are especially hazardous. Dangers from falling rocks have to be reduced as much as possible. The protection systems to cope with such hazards from the manufacturer Geobrugg which are described in this paper are highly flexible and consist of high-tensile steel components. They are field tested and are thus rated with a certain energy absorption capacity. In order to get impact velocities and energies, a rockfall simulation is run, utilising actual slope characteristics. This study deals with three case studies of rockfall protection systems recently implemented in Western Australia. The first study describes a portal protection fence, the second one a ramp protection fence and the third one a high-performance drape system with impact section.
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AGM and Social Night – “Behind the Streets of Adelaide”
Dr Jeff Nicholas
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Significance of geosynthetic reinforcement in embankment construction
Geosynthetic reinforced pile-supported (GRPS) embankments emerge as a promising ground improvement technology when construction needs to be undertaken over soft soil deposits. This method has the potential to overcome many problems that arise due to undesirable characteristics of soft soil during embankment construction. There are many advantages in this method compared to conventional consolidation based ground improvement methods such as higher reliability, less time consumption and the ability to use even in very adverse soil conditions. This study concentrates on the significance of geosynthetic reinforcement in embankment construction. The effect of the geosynthetic reinforcement in a GRPS embankment is discussed in detail using three different analysis cases. Case 1 has no pile supports or geosynthetic reinforcement, Case 2 has only pile supports and Case 3 has both pile supports and geosynthetic reinforcement. An in depth analysis was carried out in order to investigate the influence of geosynthetic stiffness, interface friction coefficient of the soil-geosynthetic interface, height to the geosynthetic layer from the pile heads and the number of geosynthetic layers on the overall behaviour of GRPS embankment systems.
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Case study: Using limiting equilibrium analysis in landslide risk assessments
Many regulatory bodies (e.g Councils) relate slope stability performance criteria to a Factor of Safety (typically 1.5). Frequently the input parameters for such analyses are selected in a subjective manner following relatively expensive subsurface investigation and laboratory testing. Subsurface investigation works, logging and laboratory testing works are all undertaken following specific procedures and standards. No such standards exist when it comes to modelling parameter selection and analysis. This paper addresses this issue and provides a methodology for linking limiting equilibrium analysis results to landslide risk assessments.
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2019 Queensland AGS Symposium
Risk & Resilience in Geotechnical Engineering
Dr Tim Mote, Deryk Forster, Dr Bindumadhava Aery, Ian Shipway, David Folan, William Eom, Greg Anderson, Amir Shahkolahi, Mehdi Davari, Stephen Buttling and Jun Sugawara
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Geophysical properties of soils
Low energy perturbations used in geophysical methods provide insightful information about constant-fabric soil properties and their spatial variability. There are causal links between soil type, index properties, elastic wave velocity, electromagnetic wave parameters and thermal properties. Soil type relates to the stress-dependent S-wave velocity, thermal and electrical conductivity and permittivity. The small strain stiffness reflects the state of stress, the extent of diagenetic cementation and/or freezing. Pore fluid chemistry, fluid phase and changes in either fluid chemistry or phase manifest through electromagnetic measurements. The volumetric water content measured with electromagnetic techniques is the best predictor of porosity if the water saturation is 100%. Changes in water saturation alter the P-wave velocity when Srà100%, the S-wave velocity at intermediate saturations, and the thermal conductivity when the saturation is low Srà0%. Finally, tabulated values suffice to estimate heat capacity and latent heat for engineering design, however thermal conductivity requires measurements under proper field conditions.