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Structure Development In Surficial Heavy Clay Soils: A Synthesis Of Mechanisms
This paper presents a synthesis of mechanisms related to structure development of surficial heavy clay soils. These clay soils develop specific structural features due to wet/dry cycles and desiccation cracking they undergo during soil “ripening”. There is substantial field and laboratory evidence to indicate that clay soils generally develop stable structures with stable material properties when they ripen under repeated wet/dry cycles of climatic change. This development occurs as a result of the re-arrangement of the soil particles to minimise the potential or free energy. Available evidence indicates that under field climatic conditions, swelling/shrinkage of clay soils occur predominantly due to water loss from interparticle and interaggregation pores. Vertisols or heavy clay surficial soils can develop special geomorphological features such as gilgai. Mechanisms of gilgai formation are also analysed, and their origin is related to the initial pattern of soil desiccation cracking. The process of shrinkage cracking and associated volume change in soils is explained on the basis of unsaturated soil mechanics theory. Crack patterns are divided into orthogonal and non-orthogonal patterns, and the conditions that lead to the development of these crack patterns are highlighted. Finally, a conceptual approach for modelling of the desiccation cracking process is presented.
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Compaction Density Of Residual Soils And Weathered Rock
Residual soils, weathered rock and overburden quarry fill represent a significant amount of fill used in road embankments. These have a high stone content which affects both the bulk density and the accuracy of the quality tests associated with compaction control. Density is the parameter that one is most likely to have a good estimate. It is used as an input in settlement and slope stability analysis of fills. The assumed density value may become critical for high fills or when embankments are placed on soft and compressible soils. Density testing is widely used in earthworks quality control (QC) and is arguably the parameter with the largest database. Construction records from compacted embankment fills from QC for major construction projects are used to check this most basic and simplest of design assumptions. Observation of commonly used density value in geotechnical investigation and design reports compared with construction records show that industry has a persistent error in analyses for an input that is arguably the easiest and simplest input parameter to determine. Three cases studies are used to illustrate the influence of an assumed density. Two of these cases led to contractual and legal issues as this simple parameter affects quantities placed, settlement and stability analysis.
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North Strathfield Rail Underpass A One Pass Synthetic Fibre Reinforced Shotcrete Lining For A Very Shallow Cover Tunnel
The single track North Strathfield Rail Underpass (NSRU) consists of two dive structures either end of a 148m long driven tunnel. The underpass is for diesel hauled freight trains up to 1.5km in length. The tunnel excavation dimensions are 7 to 8m in height and 9m in width with has a horseshoe shaped profile. The permanent tunnel structural lining consists of synthetic fibre reinforced shotcrete. The ground cover over the crown of the tunnel varies between 2.5m and 3.5m. The ground profile is predominately Ashfield Shale with graded weathering from the surface to fresh shale. Finite Element analysis, calculated the stresses in the shotcrete lining, was also used to assist in predicting surface settlements. Apart for the initial steel canopy tubes no other steel support is installed in the driven tunnel (a first for civil transport tunnel in Australia). A repeated grid pattern of thirty-five grouted 12m long fibreglass dowels ensured tunnel face stability. The tunnel face was mapped daily. The 12m long canopy tube array installations are staggered relative to the 12m long face dowels by 4.5m. The excavation/shotcrete support cycle advanced in increments and the next cycle cannot commence until the initial 150mm thickness of shotcrete has reached an early strength of 6MPa. Early strength measurements of the shotcrete are a vital part of the construction. At the tunnel face, to support the train live loads, there are three levels of redundancy, the canopy tubes, the shear capacity of the ground slot to the surface and the structural/deflection capacity of the rails. The synthetic fibres in the shotcrete provide shrinkage crack control, residual strength if cracking occurs due to deformation and enhanced durability of the tunnel lining compared any alternative using steel such as steel lattice girders(and with no electrical stray current issues). Both the macro and micro synthetic fibres (the latter in the final 100mm fire protection layer placed over a spray-on waterproofing membrane) will reduce potential fire event related shotcrete spalling. Surface settlement minimisation relies on the construction methodology with the shotcrete over the arch always being very close to the tunnel face not allowing the ground to relax. Real time surface settlement monitoring was carried out using robotic scanning theodolites aimed at reflective prisms. In tunnel monitoring included convergence taping and optical targets. Excavation of the driven tunnel commenced in February 2014 was completed in late August.
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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 a steep slope using site-won material that is not in full compliance with Roads and Maritime Services (Roads and Maritime) QA specification R57. Firstly, the local geology and the landslips that occurred along the project corridor are briefly described. The key design and material requirements for RSW are reviewed with respect to the use of site-won material. A detailed design strategy is then presented on how to mitigate the potential risks of using the non-compliant site-won fill material. Comprehensive laboratory testing of the site-won material and large-scaled pullout tests of RSW reinforcement were undertaken. With the test results and engineering judgement, the design proceeded with the following key assumptions/factors: 1) An 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 required by R57. These are to cater for long term corrosion on steel reinforcement and to ensure longevity and integrity of the RSW. Heavy rainfall in February 2010 resulted in two significant landslips within the project corridor, with the larger one being immediately down slope of the proposed reinforced soil wall (RW01). As a result an additional geotechnical investigation was carried out and landslip remedial works implemented to ensure the long term global stability of RW01. A three dimensional effect was considered in the assessment of the slope instability during the construction stage review.
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Challenges for Australian flexible airport pavements
Since the end of direct Commonwealth government management of Australian airports in the 1990s, there has been no coordinated up-keep of airport pavement technology and practice in this country. In contrast, aircraft technology has advanced significantly. To address significant risks, a number of construction contractors have introduced new products and methods. However, the risk-averse nature of the airport industry has sometimes resulted in good solutions being resisted. A number of challenges relating to flexible aircraft pavements have resulted, including aircraft with higher tyre pressure and wheel load combination than previously experienced and an associated inability to ‘prove’ the upper fine crushed rock base layers during construction. Also, a general reduction in bitumen reliability has resulted in the increased use of polymer modified bitumen for asphalt production, but these products are not readily available in regional areas. Moreover, airport asphalt was routinely rejuvenated to extend the period between resurfacing, but increased concern for the impact of surface treatments on skid resistance has resulted in some designers no longer recommending such maintenance. A reduction in the number of specialised airport pavement engineers has meant that expertise, particularly in spray seal design for airports, has become limited. The reduction in expertise has also seen the development of new methods for expedient airport pavement rehabilitation be substantially left to construction contractors. A collaborative, airport-industry-wide initiative is essential to addressing these challenges in the future.
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Advances in offshore and onshore anchoring solutions
The paper presents a snapshot of the research performed within the ARC Centre for Geotechnical Science and Engineering (CGSE) on anchoring systems. The focus is on the determination of nominal bearing factors and the performance of three different types of anchors: the suction embedded plate anchor, the dynamically embedded plate anchor and the helical anchor. The paper emphasises the wide range of analytical, numerical and physical modelling techniques developed within the CGSE to (i) develop a rigorous understanding of the soil-anchor interaction and (ii) create practical tools that can be readily used for design.
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Geotechnical Aspects Of The Mid-West Rail Upgrade Project In Western Australia
This paper presents a description of the geotechnical aspects associated with the design and construction of the MidWest Rail Upgrade project in the Mid-West region of Western Australia. It covers aspects of site investigation on existing track formation and track duplication alignments, assessment of the capacity of existing bridge foundations, methods for assessment of the suitability of existing rail formation and upgrades required, analysis of railway behaviour in salt lake areas and recommended ground improvement schemes as well as design of formation-to-bridge transition zones. The paper presents results from a constructability study carried out to assess the stability of operational existing track during construction of duplicated portions of the line. It also describes methodology and results of borrow source investigations for both main sections of the project and geotechnical related issues that arose during the construction stage.
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Geotechnical aspects of earthquake hazard assessment for Brisbane
An effective method was needed to resolve earthquake hazard at the city-scale during the development of an earthquake risk assessment for Brisbane. While an assessment of regional seismicity is a fundamental input to the process, it can only provide a broad idea of earthquake hazard. Two of the major determinants in magnifying local earthquake shaking above regional levels are (i) the presence of significant thickness of weak foundation sediments with high void ratios and, (ii) the effects of ridge-tops and steep slopes. Soft, Recent sediments, deposited in low-lying areas around the Brisbane River and Moreton Bay coastal plain will tend to magnify the effects of earthquakes. Steep, narrow ridges are a common feature in the central and western districts of the city. Recent acquisition by Brisbane City Council (BCC) of a high-resolution digital elevation model (DEM) of the city area and a broader range of sub-surface geotechnical information has enabled an enhancement of earlier hazard analyses. The geotechnical borehole dataset available to BCC, although now broader, is nevertheless limited as a much larger database remains for potential exploitation. The new DEM enabled refinement of geological boundaries to match the scale of assets at risk, and provided additional visual clues as to subsurface conditions and indications of sediment thickness, as well as providing accurate delineation of slope and slope change. The existing coverage of geological mapping is comprehensive but mapping is at a coarse scale compared to that at which city assets are described. Available microtremor recordings are available but too restricted to be sufficiently representative for a whole-of-city assessment. Taken together, however, the DEM, available geotechnical borehole data, geological mapping and microtremor recordings are able to provide a reasonable delineation of potential variations in the magnitude of earthquake shaking, within the constraints of the given regional seismic setting. While such studies provide an assessment of hazard, further combination with information on the demographic setting, vulnerability and assets at risk through risk-GIS was necessary to quantify risk. The study shows that while nowhere is entirely immune to earthquake effects, the chance of an earthquake causing significant damage in Brisbane city is unlikely. The earthquake hazard to Brisbane is low by global standards. Less than ten minor earthquakes have been felt in the city since its founding, some 180 years ago. The largest of these, the Richter magnitude ML 4.4 Mt Glorious earthquake of 1960 caused minor damage but was felt throughout Brisbane with intensities as high as Modified Mercalli intensity MM IV. A maximum Richter magnitude of ML 6.5 is theoretically possible within the region which includes Brisbane. However, concentrations of vulnerable populations, housing and infrastructure largely lie outside of the zones of highest earthquake hazard, reducing the overall risk to Brisbane considerably.
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Rock Mass Parameters For The Brisbane CBD
Rock mass parameters are presented for the typical range of rock conditions encountered in the Brisbane CBD and surrounding area. Rock mass units are classified based on lithology, weathering, intact rock strength and degree of disturbance. The rock mass parameters are based on the Author’s combined experience from Brisbane infrastructure projects including the M7 Clem Jones Tunnel, Airport Link and Cross River Rail. The parameters may be useful for design and construction of future ground engineering projects in Brisbane.