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Dealing with expansive clay soils through a national standard
Australia has had a national standard for site classification and design of footings for reactive soils for nearly 30 years. From the outset, the Standard, AS2870, has implemented a simple rational model for quantifying reactive soil foundation movement. The parameters of the model have from necessity been calibrated against empirical observations. The philosophical approach adopted in the Standard envisages that footing designers, builders and homeowners all have parts to play in managing sites on reactive soils. This paper reviews the background to the Standard and its development over time. The paper presents the site classification process, tests to quantify reactive clay reactivity, design suction changes, the Thornthwaite climate index, the impact of trees on footing design and current practice. Examples are provided of ground movement calculations for normal site classification, classification adjusted for reactive soil fills and tree-drying settlement. Research areas which are needed to improve the Standard are discussed.
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A Review On Performance Of Stone Columns As A Ground Improvement Technique: Lessons Learnt From Past Experiences And Prospect For Future Development
Since the population growth is creating a strong demand for urban development, the need for construction in soft soils is dramatically increasing. Accordingly, ground improvement is an important requirement to avoid problems such as non- uniform settlements, failure due to low bearing capacity or liquefaction. Stone columns are used as one of the ground improvement techniques to stabilize the soil through increasing soil stiffness and shear resistance while decreasing the compressibility and settlement. Predicting the behaviour of a stone column needs to meet technical challenges, particularly in soft cohesive soils. Therefore, the aim of this paper is to make a broad assessment of the performance characteristics of stone columns in clayey soils as a review. In this study, the stone columns behaviour has been studied through analytical, experimental and numerical techniques, and failure modes and design of stone columns and their installation techniques are discussed. Based on previous investigations, it is gathered that in very soft soils, the dry-bottom feed vibro replacement technique is preferred to other methods and usage of geosynthetic encasement is very efficient where insufficient lateral confinement of the soil is problematic. According to past findings, the friction angle of the stone material and the diameter of the column are significant parameters for the design of the bearing capacity of the column. Furthermore, apart from ground improvement benefits, stone columns are used as vertical drains, which can decrease the pore water pressure during earthquakes and therefore mitigate the liquefaction potential. In addition, the cost-effectiveness of using low priced materials instead of aggregates without disturbing the overall performance of stone columns seems to be viable and can be explored further in future. This review can give an enhanced viewpoint to engineers and practitioners considering the use of stone columns in their projects.
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Volume 36, Number 2 — Other
Table of contents, editorial and chairman’s column for Australian Geomechanics, Volume 36, Number 2.
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Commentary on practice note guidelines for landslide risk management
In 2000 the Australian Geomechanics Society (AGS) published “Landslide Risk Management Concepts and Guidelines” (AGS 2000). In 2002 the content and application of AGS (2000) were demonstrated around Australia by “the Risky Roadshow” which was sponsored by Emergency Management Australia and AGS. Papers for the “Roadshow” were published in Australian Geomechanics Vol 37 No 2 May 2002. Since then there have been many published papers and an extensive body of discussion which has progressed the use of Landslide Risk Management (LRM) as discussed further below.
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Volume 37, Number 5 — Other
Table of contents, editorial and chairman’s column for Australian Geomechanics, Volume 37, Number 5.
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Reconnaissance Report on Geotechnical Damage Caused by Tropical Cyclone Jasper in December 2023
This paper is focussed on the Department of Transport and Main Roads (TMR) initial geotechnical response to damages caused by Tropical Cyclone Jasper (TC Jasper) to road infrastructure and batter slopes along the state-controlled road network in TMR’s Far North Queensland (FNQ) District of Queensland, Australia.
TC Jasper delivered extreme rainfalls over six days between 13 and 18 December 2023 in FNQ District. The highest accumulated total being 2,096mm in the area of Port Douglas and Mossman north of Cairns. Within areas of the TC Jasper path, rare to extremely rare rainfall occurrences resulted in severe flooding and geotechnical damage and closure of key state-controlled roads in FNQ. The roads closed included all four range crossings providing access to the Northern Tablelands. Communities were isolated, and tourism and regional transport severely impacted.
This paper provides a report on the geotechnical damage caused by TC Jasper to seven key state-controlled roads in TMR’s FNQ District, and outcomes from the event reconnaissance approach used by TMR’s Engineering and Technology Geotechnical Section in collaboration with TMR FNQ District office and RoadTek teams to re-open most of the closed roads and secure significantly damaged road sections within four days of the cyclone passing. Ongoing recovery efforts are progressively re-opening remaining roads.
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Volume 37, Number 1 — Other
Table of contents, editorial and chairman’s column for Australian Geomechanics, Volume 37, Number 1.
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Strength And Modulus Changes Associated With Compaction Of Residual Soils And Weathered Rock
Density testing has been applied widely in earthworks quality testing. A common inference from density testing is that as the dry density ratio (DDR) increases, the strength and modulus also increase. This is a well-established principle as under compaction can result in collapse, low strength and settlement of the placed fill. However, there is no clear relationship that a specified DDR will result in a design strength of or modulus. At high levels of compaction of residual soils and weathered rock materials with heavy vibratory rollers, when strength or modulus measurements are correlated back to density testing, a poor correlation often results. This is due to factors such as the depth of influence is different, and with the quality and compaction being combined into one parameter (say modulus). Data from field trial embankments with 3 different materials using residual soils and weathered rocks were tested using a range of alternative testing equipment. Compaction shows an increase in the DDR with the number of passes, but a decrease in modulus and friction angles with 8 No. of passes as material breakdown occurs with heavy vibratory rollers. This breakdown varies with the type of roller. DDR is shown to be a non-reliable indicator of strength and modulus at high compaction, if paired data matching was used. A compaction DDR of 95% is shown to have an associated friction angle variation of over 5 degrees depending on the type of material compacted. Similarly, the modulus can vary by factor of 2 at a given DDR depending on the material type and compaction roller used. This observation at high stiffness (DDR > 98%) should not detract from the benefits of compaction as overall and at (DDR < 98%) the strength and modulus increase with number of passes.
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Managing Geotechnical Uncertainty With Simulation Models: An Introduction
In a standard deterministic analysis discrete scenarios are considered, and a moderately conservative “characteristic” value is used as a design basis. However, fixed or exact values in a real-world geotechnical site seldom occurs. Deterministic approaches may not explicitly consider the ground uncertainty. Simulations using various probabilities provides for this uncertainty as each parameter input is treated as a random variable within certain measured ranges or ability to evaluate. Monte Carlo (MC) sampling is a traditional technique for generating random numbers to sample from a probability distribution. When low probability events occur, a small number of MC iterations might not sample sufficient quantities of these outcomes for inclusion in the simulation model. Latin Hypercube (LH) sampling uses stratification of the input probability distributions, to overcome the limitations of Monte Carlo sampling. The simulation results show low probability outcomes are included in the sampling for the simulation model. At a high number of simulation iterations both provide similar outputs, but at low simulation iterations the LH is more reliable. However, both the MC and LH sampling suffer from impractical values at low or high probability events when the normal probability density function (PDF) is adopted. The normal PDF is commonly used in statistical modelling. Non-normal PDFs often represent the best fit PDF when a goodness of fit test is carried out. The errors associated with using the common normal PDF are shown with the above-mentioned simulation models. This best fit PDF applies whether simulation models as described above is used or even with simple “what if” sensitivity models in traditional analysis.
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PRB Technology Incorporating Acidic Ground Conditions and Bio-Geochemical Clogging – A Critical Review
Groundwater acidity resulting from pyrite oxidation in acid sulphate soil terrain presents a severe threat to the environment. The exposure of low-lying acidic coastal belts to the atmospheric oxygen, exacerbated by phreatic surface lowering in dry seasons and activities like infrastructure development and agriculture, leads to pyrite oxidation and sulfuric acid production in soil. This paper reviews the challenges posed by acid sulphate soils by emphasising the environmental and infrastructure damage caused by acidic soil leaching into water bodies. Permeable reactive barriers (PRBs) have emerged as a promising method of passive treatment for mitigating groundwater acidity in pyritic terrain. This review mainly focuses on the effectiveness of PRBs in low-lying floodplains by addressing the bio-geochemical clogging that diminishes the reactivity and porosity of PRBs over time. This paper also summarises the numerical methods needed to design PRBs in acidic terrains by identifying gaps in current research that could enhance the accuracy of future PRB designs. This comprehensive review contains valuable insights into the ongoing efforts of addressing the challenges associated with groundwater contamination in regions containing acid sulphate soil.