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Design and Construction of Plastic Geocellular Rain Water Harvesting/Stormwater Detention Tanks
Plastic voided modular structures (known as geocellular units) were first used in the mid-1980s in Europe below pavements to store stormwater. Its use has since spread to rainwater harvesting and on-site stormwater detention for residential, commercial and industrial developments. It is an environmentally friendly and sustainable solution. However, there are engineering pitfalls associated with the design and construction of plastic geocellular structures. The main pitfalls are associated with creep rupture of plastic structures, potential construction damage and the lack of care in wrapping the cells with filter fabric and backfilling procedure. As the scale and complexity of geocellular structures have significantly increased in recent years, guidance on appropriate design and construction methods has become more essential for these structures to be adopted as safe, yet economic and sustainable solutions.
In this paper, the author will describe his design and construction experience based on research associated with a court case on the damages associated with a major geocellular on-site stormwater detention project (approx. 8.5 Mega litres), and recent conversion of his backyard swimming pool to a 40,000 Litre rainwater harvesting tank. References on design and construction guidance will be described together with the authorโs personal opinion on the use of partial factors in the economic design of geocellular structures.
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Influence of trees on expansive soils in Melbourne
In Australia, distortions of residential buildings caused by tree drying settlement have been widely reported, particularly in areas of expansive or reactive soils. The paper presents the results of two major projects dealing with trees and expansive clay soils in the Melbourne Metropolitan area. The first project is the long-term field study of the effects of trees on the performance of a building, soil moisture patterns and ground movement in basaltic clay. The second project is a case study of a residential house damaged by expansive soil movement due to tree root drying. In this field study the sap flow rate of a tree was seen to be closely correlated with solar radiation. It was also observed that the soil moisture content profile near the tree was significantly lower than that away from the tree. The findings of the second study clearly indicated that trees, growing in close proximity to a house could cause more severe damage to the buildings than the expected moisture changes due to seasonal effects and re-distribution of soil moisture arising from construction on the site.
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Connecting Geotechnical Investigations with Project Risk
It is often stated by geotechnical engineers that a project pays for a site investigation one way or another as a means of justifying a scope of work aimed at managing a projectโs risk. This proposition is critically assessed through reviewing three case histories covering site investigations performed for design only, design and construct, public private partnership (PPP) and alliance contract models. An extensive site investigation was performed for the Ballina Bypass Alliance project. Most of the project risks were identified and managed. However, some risks were realised relating to detecting palaeochannels, variable subsurface topography, quantifying material parameters and coping with corestones. An extensive site investigation has been performed for the Snowy 2.0 project, but its scope has been limited by time and access constraints. Risk is managed through adoption of a geotechnical baseline report. Site investigations for Inland Rail for design only, design and construct and PPP contract models have been scoped and partially delivered. In addition, Inland Rail has developed an earthworks materials specification that can be varied to suit site characteristics. Integration of site investigations with the specification and design is shown to be key to controlling the major materials risk. Some thoughts about scoping an investigation to inform geotechnical risk when procuring a PPP are presented. Overall it is concluded that the geotechnical industry generally scopes investigations to adequately manage risk. Quality is shown to be as or more important than quantity. The critical importance of engineering geology for identification of potential risks is demonstrated thus allowing a focussed drilling and geophysics scope to be delivered. Challenges remain when communicating residual risk to stakeholders.
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Liquefaction and settlement assessment for the Mortimer Road Bridge, Perth, Western Australia
This paper presents the results of an assessment of liquefaction and settlement for a freeway interchange bridge founded on siliceous sand in an area of shallow water table at a site located south of Perth in Western Australia. The effect on liquefaction of factors such as the depth to groundwater, earthquake magnitude, ground acceleration; load applied to the soil and the depth and thickness of zones of loose sand is discussed. The study confirmed that the liquefaction potential was very low.
Predicted soil settlement was relatively large and was addressed by surcharge preloading of the site. Settlement of the preload embankment was monitored leading to an improved understanding of the non-linearity of settlement versus load response, and the development of a relationship between modulus and electric friction cone penetrometer resistance. The results from this site were compared to results from a calcareous sand in Perth, for which a relationship following a similar approach has previously been derived.
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Evaluation Of The Water Retention Curves Of Cement- Bentonite Cut-off Wall Materials
Filter Paper, Dynamic Vapour Sorption and Vapour Equilibrium Methods were used to investigate cement-bentonite (CB) materials unsaturated properties. These measurement methods provide an insight into the potential for self-desiccation of CB materials and their ability to uptake and release of moisture. The current paper shows that the type and mineralogy of bentonite play a role in CB mixture performance criteria (i.e., workability/mechanical properties) and water retention behaviour. It was observed that the presence of a polymer in CB materials impaired the slurry performance criteria and water retention properties. The air entry value (AEV) was high in the CB mixture with a higher percentage of silica, and it slightly increased with time due to a pozzolanic reaction. It is believed that the chemical interaction between cement and bentonite is directly responsible for the formation of the CB internal structure matrix that affects its unsaturated behaviour.
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The Challenges Of Field Measurement Of Suction Within Free-standing Mainline Railway Embankments
For over 100 years, adequate performance has been observed for free-standing mainline railway embankments despite the typically rudimentary earthworks techniques used. Notwithstanding this performance, use of effective stress limit equilibrium analyses often do not satisfy the design criteria adopted today. Embankments on the Main Southern Railway are frequently up to 20m high and have been subjected to environmental events; including drought, intense rainfall, flooding on their upslope side and substantial earthquake loading over the last century. In addition, these same embankments can be subjected to the influence of subsidence from underground mining.
One feasible explanation of their adequate performance is the presence of suction within the body of the embankments.
The phenomenon of suction has attracted much study and is thought to be understood by the geotechnical profession. Nevertheless, recognition of suctions within engineering analysis are seldom attempted. What is known as suction can be measured under laboratory conditions and its presence is accepted in the field. However, its measurement in the field, especially at depth, is technically challenging. There is a dearth of research and reported installations of this nature, particularly for embankments and the authors are endeavouring to correct this.The hope is that this paper will stimulate discussion within the geotechnical profession by providing an update on the authors attempts to measure suction at depth in the field with commercially available instruments, and whilst illustrating the challenges faced.
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Ground Improvement And Validation For Stage 1 IMEX Earthworks
This paper presents a case study of the use of an alternative ground improvement technique to treat contaminated uncontrolled fill other than traditional โremove and replaceโ for major earthworks for the proposed container Import and Export (IMEX) Terminal at Moorebank, Sydney. First, a brief discussion of the options considered for the ground improvement including removal and replacement and the Impact Roller Compaction (IRC) method will be presented. The local geological setting and the historical form of the existing Stage 1 IMEX Terminal site will be described, with the geotechnical model and associated design engineering parameters being surmised. The key points in the development of a technical specification are presented to take account of the performance requirements, including on-site IRC trial and validation testing. The primary validation measures adopted comprise plate load testing, cone penetration profiling (CPT), insitu density testing, dilatometer (DMT) testing and proof rolling after IRC treatment. Surcharging was considered for the remediated contaminated land areas where the details of the remedial treatments were not available at the Stage 1 IMEX development stage to ensure there will be no issues resulting from long-term settlement. At the time of writing this paper the Stage 1 IMEX works have been completed and are operational. The monitoring results indicate the performance of the site is satisfactory.
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2021 AGS Victorian Symposium
Innovations in Geotechnical Design
Dr Scott Taylor, Professor Jinsong Huang, Dr Julian Seidel and Chris Lyons
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Spatial and statistical distribution models using the CBR test
The California Bearing Ratio (CBR) is the most common test used in the design of pavements. This simple test has many pitfalls in its application. This is an empirical test, and one must be aware of its many considerations if the design value is to be appropriately applied.
A design value must consider its spatial variation, level of compaction and its relationship with its surrounding layers. The design risk is used to determine the characteristic value for a given project. Characterisation using the spatial and statistical variation of a CBR is used for a project site in Queensland to illustrate the requirement to use an appropriate prediction model. The results of this curve fitting show the normal distribution is inappropriate due to negative values and the lognormal distribution is an appropriate statistical model to characterize the design CBR.