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Effective remediation of groundwater in acid sulphate soil terrain
Acidic groundwater, generated from acid sulphate soil (ASS), is a major geo-environmental problem in Australia. Manipulation of groundwater through the use of weirs and gates in the nearby creeks and drains of ASS, which is being practised right now for preventing pyrite oxidation, is not effective in low land floodplains due to the risk of flooding. The application of a permeable reactive barrier (PRB) can be an alternative for remediation of acidic groundwater in such floodplains. Laboratory column experiments were carried out prior to installation of the PRB for examining the efficiency of the material. Results of these experiments have shown that recycled concrete could effectively neutralise the acidic water for longer periods with complete removal of aluminium (Al) and iron (Fe). Despite the reduction of the efficiency of the recycled concrete due to armouring by accumulated precipitates of Al and Fe, excellent performance was observed for an extended period under controlled laboratory condition. Following these results, a pilot PRB was installed in the Broughton Creek flood plains in southeast NSW to observe its performance under varying natural conditions of the field. The PRB has been maintaining near neutral pH with complete removal of Al and Fe from the groundwater of ASS matching with the results of column test. The promising performance of the pilot PRB for the last three years shows that PRB can be used as one of the cost effective and environmental friendly alternative to other recently utilised techniques in ASS.
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The hidden cost of geotechnical investigations
This paper presents the results of an analysis of 201 geotechnical reports prepared by most of the geotechnical consultants in Sydney. The analysis considers the recommendations for several important design parameters in the most common soils and rocks in the Sydney Basin and compares them against a level of Geotechnical Investigation Risk. The results of the analysis are used to demonstrate that investigations with a small scope of works result in conservative design and expensive construction – the hidden cost of low levels of investigation.
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Does emerging evapotranspiration (ET) cover technology offer a suitable alternative for landfill covers in the Hunter Region?
The NSW Environment Protection Authority (EPA) Environmental Guidelines: Solid Waste Landfills state that “the site capping (of a landfill) should ensure that the final surface provides a barrier to the migration of water into the waste, controls emissions to water and atmosphere, promotes sound land management and conservation, and prevents hazards and protects amenity”. The Environmental Guideline identifies benchmark techniques to achieve these goals. The EPA recommends that the final capping should have five parts including a seal bearing layer, a gas drainage layer, a sealing layer, an infiltration drainage layer and a revegetation layer. This traditional approach to capping of the landfill aims to seal the surface so that rainfall cannot infiltrate the waste and landfill gas cannot escape to the atmosphere. This is a highly engineered and costly means of achieving the desired environmental goals. Experience in the Hunter Region is that final covers created with a compacted clay sealing layer as recommended by the NSW EPA do not always achieve these required environmental goals. Problems relating to conventional covers include the availability of suitable materials, particularly for the low permeability sealing layer, the potential for cracking of the sealing layer particularly during extended dry conditions, the response to differential settlement within the waste and problems with revegetation. Thus opportunities exist to explore alternative options for the final cover of landfills.
One type of alternative cover that is of increasing popularity in the United States is the evapotranspiration (ET) cover. In the United States, ET covers are in place or on trial at a variety of different types of landfill including those used for hazardous and municipal waste. Evidence suggests that these can perform well in a variety of climatic conditions.
The principle behind ET covers is the effective management of water balance in the cover medium. Precipitation is balanced by evapotranspiration, water storage in the cover medium and influx to the underlying waste. In areas where the potential for evapotranspiration approaches or exceeds rainfall, the influx of water through the ET cover can be very low or neutral and can effectively control migration of water into the waste.
This paper considers the potential for the use of ET covers in the Hunter Region.
There are two key considerations:
- Climatic factors – is evaporation greater than rainfall? Are there any critical climatic factors such as prolonged periods of high rainfall and low evaporation? Sample water balances are presented to demonstrate the applicability of ET covers in the Hunter Region.
- Cover media factors – the water holding capacity of the cover medium, availability of materials and ability of the media to support vegetation. A variety of materials available in the Hunter, including naturally occurring soils and waste materials from power generation, coal mining and the waste management industries are assessed for their potential for use in ET covers.
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Case Study Comparing Embodied Carbon Emissions in Two Road-Over-Rail Bridge Foundation Designs
This paper examines and compares the embodied carbon emissions in the earthworks and foundation design for two road- over-rail integral bridges as part of a project in Western Australia. The first bridge is supported on gravity footings and constructed using bottom-up methods, the second on load-bearing contiguous piles and constructed using top-down methods. The abutments support a cut profile of sand and limestone up to 10 m high.
A Life Cycle Assessment (LCA) for embodied carbon emissions was carried out for each bridge using the framework of PAS 2080. Construction-stage design information was used in the assessment, representing a bottom-up LCA approach to retrospectively identify carbon hotspots to inform future designs.
The assessment incorporated geotechnical site investigations; temporary works; bulk excavation for gravity footings and other minor excavations; and the raw materials for construction of the two foundation types. Transportation of materials to site, construction processes and final deconstruction and disposal of the structures were also considered. The bridge superstructure was outside the scope of the assessment.
The results are presented in total tonne CO2e per bridge and tonne CO2e per bridge deck area to allow direct comparison of the embodied carbon emissions of the two bridge foundation systems. The carbon hotspots in each design are identified, and the authors discuss how the results can be communicated to clients and contractors to be weighed alongside the various other drivers that influence construction method and design.
The paper closes with the authors’ assessment of opportunities across the design process where geotechnical designers have most influence on embodied carbon over the design life of these bridge types.
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Geotechnical investigation of the abandoned mine workings under Newcastle
There are often differences in ‘expert opinions’ of what constitutes appropriate design parameters for determining the level of stability of the existing pillars of old coal mine workings. For example, there is seldom 100% certainty about the value of any design parameter. Instead there are varying degrees of confidence (or belief) for each possible design value. This uncertainty in design parameters can be represented by probability distributions. The inclusion of such probabilistic information into a probabilistic risk analysis will enable the probability of failure to be estimated. To illustrate the utility of risk analysis for decisions taken with uncertainty, a probabilistic risk analysis has been conducted to assess the uncertainty of design parameters on the level of stability of existing pillars within a disused coal seam beneath a proposed surface development in the Newcastle area. The case study considers dimensional and level of inundation uncertainties. This case study provides a preliminary framework for a risk-based approach to decisionmaking for a geotechnical system subject to high uncertainties. The outcomes of the risk analysis are probability of failure and annual economic risks (expected losses per year). The paper will describe the steps taken in the risk assessment, risk acceptance criteria and how results from a risk analysis may be interpreted by a decision-making development consent authority.
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Use of Geopolymer for Stabilising Crushed Rock Base in Road Pavement
In recent years, the utilisation of fly ash-based geopolymer has been progressively escalating as it has proven to be a potent alternative to conventional cement. The acceptance of geopolymer is primarily credited to its ability to improve soil strength and stiffness, coupled with the advantage of reducing harmful pollution and energy usage. This study is primarily designed to investigate the mechanical performance of Geopolymer-Treated Crushed Rock Base (GTCRB), with a particular focus on uniaxial compressive strength response and the structural durability quantified by the resilient modulus and permanent deformation. This is achieved through an experimental program incorporating Unconfined Compressive Strength (UCS) and Repeated Loading Tri-axial (RLT) tests. The outcomes of the tests suggest that the use of geopolymer tends to fortify the uniaxial compressive strength and resilient modulus of the GTCRB-tested samples, indicating an overall enhancement in the mechanical properties. Of particular note, a design mix containing 6% geopolymer produced an average UCS of 1.2 MPa, aligning with the standard UCS range for cemented lightly-bound base course material, as specified by the Austroads. In terms of the resilient modulus, while treated mixtures satisfied the requirements for permanent deformation, none of the tested samples achieved the stipulated resilience modulus range.
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Innovative Design Of Reinforced Soil Wall On A Steep Slope Subject To Land Slip Risks
This paper presents a case study of an innovative reinforced soil wall (RSW) design on steep slope using site-won material that is not in full compliance with Roads and Maritime Services (RSM) QA specification R57. The steep slope was impacted by a landslip occurred in February 2010. Firstly the local geology and the landslips occurred along the project corridor are briefly described. The key design and material requirements for RSW have been reviewed with respect to the use of site-won material. A detailed strategy is presented on how to deal with the potential risks of using the non-compliance site-won fill material to the specification R57. A comprehensive laboratory testing regime of the site-won material and large-scaled pullout tests of RSW reinforcement were undertaken. With the test results and engineering judgement, the design was proceeded with the following key assumptions/factors: 1) the allowable fines (<75 microns) content of up to 25%, clay (<2.4 microns) content of up to 7%; 2) the use of reduced friction angle of 30 degrees; 3) a 10% reduction in the calculated pullout capacity of reinforcement; and 4) 1.5 times sacrificial allowance as per R57. These were to cater for long term corrosion on steel reinforcement and to ensure longevity and integrity of the RSW. A heavy rainfall in February 2010 resulted in two significant landslips within the project corridor, with the larger one being immediate at the down slope of the reinforced soil wall (RW01). This event required us to carry out an additional geotechnical investigation and landslip remedial works to ensure the long term global stability of RW01. Three dimensional effects were considered in the assessment of the slope instability in the construction stage review.
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Climatic factors for AS2870 for the metropolitan Sydney area
AS2870-1996 “Residential Slabs and Footings – Construction” correlates depths of design suction change (Hs) to regional climatic zones as delineated by the Thornthwaite Moisture Index (TMI). TMI is a measure of aridity, quantifying the cyclical wetting and drying of soils. TMI is a function of rainfall, potential evapotranspiration and soil water-holding capacity, and provides an indication of moisture balance.
Further to work done by Chan and Mostyn (2004), this paper includes the complete TMI dataset for the metropolitan Sydney area. This paper provides improved estimates of Hs that can be considered in the calculation of characteristic surface movement (ys) in metropolitan Sydney when classifying residential sites under AS2870-1996.
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Revisiting the applicability of Voussoir Beam Theory for tunnel design in Sydney
The design of semi flat-roofed tunnels, i.e. with a high arch radius to roof span ratio, in Sydney has been proven successful over time. The horizontally bedded nature of Sydney’s Hawkesbury Sandstone draws designers to the voussoir beam theory. Such analogy and the associated method of analysis can be easily implemented in computer spreadsheets, which significantly facilitates the design of semi flat-roofed tunnels in geological conditions such as Hawkesbury Sandstone. Like any other engineering simplified model or theory, the voussoir beam theory has some limitations. However, it seems that some of these limitations are not well understood and often ignored and/or misinterpreted. Such lack of understanding of the theory and its limitations often raises question about the applicability of the analytical solution in practice, misleading engineers to believe that the only reliable and comprehensive design method is through numerical analysis such as Distinct Element Method (DEM). This paper investigates the applicability of the analytical or closed-form solution of the voussoir beam theory through comparison with numerical modelling, focusing on some of the perceived limitations and their impact on the design of tunnels in Sydney. The results illustrate that the voussoir beam theory can be confidently used in practice if its limitations are well understood and good engineering judgment is applied to take the local geology into account. In addition, the results also demonstrate that some of the limitations can be on the conservative side. For example, the potentially positive effect of high horizontal stresses ignored in the voussoir beam theory may explain why some of the unfavourable conditions are less pronounced in practice.
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Bayesian Approach To Improve The Confidence Of The Estimation Of The Shear Strength Of Coarse Mine Waste Using Barton’s Empirical Criterion
The evaluation of the shear strength of waste rock is required for the verification of the stability of high waste dumps, especially those that reach hundreds of meters in height. Mine waste rock material in open pit mining contains particles of metric scale which precludes the utilisation of commercial laboratory testing equipment. To overcome testing limitations, the shear strength of waste rock is frequently estimated using the empirical criterion of Barton-Kjærnsli. This criterion takes into consideration the nonlinearity of the shear strength envelope, characterising the behaviour of very coarse granular materials submitted to high loads. In the criterion, a stress-dependent structural component of the shear strength is parametrised with the equivalent roughness (R) and equivalent strength (S) and the structural component is added to the basic friction angle (φb) of the parental rock to determine the shear strength of the waste rock material. This paper demonstrates the use of Bayesian inference to determine the best set of parameters φb, R and S that satisfied both: large-scale laboratory testing results characterising a waste rock material, and reconciliation data from observations of stability of the waste dumps. The methodology allows the estimation of project-specific model parameters that honour both, laboratory data and site performance information. This objective is achieved through the estimation of correction factors to downgrade the strength from laboratory to field scale.