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Unsaturated free-standing mainline railway embankments – Part 1: Can you handle the awkward truth?
The presence of negative pore pressures within cuttings and embankments, and their benefit to the reduction of the likelihood of instability, have long been recognised by the Geotechnical profession. Negative pore pressures are usually a consequence of natural environmental influences upon soils, clay in particular, and are frequently (and perhaps misleadingly) termed “soil suction”.
The recognition of pore suctions in the assessment of potential instability of embankments by way of conventional stability analyses is not common. Tools are becoming available to conduct such analyses once the boundary conditions and parameters involved are understood. Measurement of suction in-situ is beginning to assist in the identification of reliable suction values suitable for use in such analyses.
In this paper, the authors develop a philosophy to include suction and present a defensible model for the analysis of free- standing embankments. The impetus for the work was the need to demonstrate via analysis that Main Line Railway embankment infrastructure within NSW was not in jeopardy, as was indicated by “the usual” saturated soil mechanics approach. Discussion includes the challenges posed to realistically implement and consider the benefits resulting from recognition of the “known” suctions.
The paper is intended to alert the profession to an assessment technique that includes the benefit of these suctions.
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Managing Ground Movement Impacts For Australian Urban Road Tunnels
Road tunnels in Australia involve underground construction with the potential for substantial impact to both surface and underground third-party assets. This paper looks at lessons learnt from recent and current Australian road tunnels in terms of stakeholder consultation and best practice for this process.
Road tunnels in Australia are generally delivered under a model where it is the design and construction contractor’s responsibility for undertaking the ground movement impact assessment. As a minimum this requires consultation with affected third party businesses, property owners, utility owners and road/rail authorities.
Contractual obligations may require contractors to consult with affected parties and receive formal approval prior to construction commencing. Affected parties will often not have experience or resources to engage in these processes and, therefore, the requirement for consultation and/or approval of the ground movement impact assessment and potential mitigation measures can put the construction programme at risk.
This paper will investigate the following in more detail:
- Ground movement design assessment approach
- Unknown conditions of third-party assets
- Parties requirements for assessment
- Monitoring of third-party assets and trigger levels
- Contractual obligations of the contractor
- Recommendations for future Australian road tunnelling projects
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Flow category landslide susceptibility modelling of the Sydney Basin
The University of Wollongong Landslide Research Team has completed a GIS-based Landslide Susceptibility model for the entire Sydney Basin region. According to the Australian Bureau of Statistics and the 2011 Census data, the population within the Sydney Basin Study area is approximately one quarter of the population of Australia. This model has been developed with the aid of a large scale Landslide Inventory for NSW, which contains 1823 landslides to date. A composite geology dataset has also been developed using commercially available geology datasets including those from NSW Department of Primary Industries and elsewhere. The model employs a 10m pixel Digital Elevation Model (DEM) across the entire study area derived from either Local Government sourced Airborne Laser Scan data or the 30 m pixel year 2000 Shuttle Radar Topography Mission (SRTM) data. Using techniques developed over the last decade and refined ArcGIS tools developed over the last three years, Data Mining methods and ESRI ArcGIS capabilities have enabled the modelling to produce a very useful zoning outcome over the entire Sydney Basin area. The major advantage of this new tool is that it applies the See5 logic derived from rule sets over a large datasets, and produces a visually interpretable outcome. The authors expect the susceptibility zoning to be suitable for use at Regional to Local Advisory level Local Government Planning Development Control Plans.
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Ground movement and soil structure interaction – recent findings
Recently, unsatisfactory performance of some domestic structures founded on reactive soils has been reported. In Victoria, many instances of significant damage particularly in the western suburb are associated with houses built during the millennium drought. Abnormal moisture conditions (e.g., extreme climate conditions or pipe leak) lead to changes in soil moisture content under the footing system causing differential movement of slabs and supported superstructure. This paper presents a summary of research undertaken by Swinburne University of Technology with support from a number of organisations investigating the main causes of damage and some of the shortcomings in current design and construction practice.
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Methods And Approaches For Mitigating The Risk Of Ground Movement Impacts From Tunnelling
Ground movements from excavation works can present project risks with stakeholder engagement, construction approvals, 3rd party claims for damage, time delays and additional cost for mitigation works. The theory of the prediction of ground movements and phased assessment of impacts on structures and infrastructure using simplified approaches has been documented in papers by Rankin (1988), [1] Burland (1995) [2] and several others.
In reality there are numerous complexities which come into the assessment including historical movements, modelling assumptions, quality of asset data, existing condition and individual structural layouts and sensitivities. Many of these variables cannot be modelled, or it is not practical to model. To manage these risks, sets of acceptance criteria have been developed, and where the assessment is within the criteria, the risk of more onerous cracking, deformation or loss of service is considered reasonably low. Similarly, where an assessed impact is higher than a given damage category or acceptance criteria, it indicates the risk is unacceptably high, but the event/failure may not actually occur. On this basis ground movement impact assessments need to be considered as a risk assessment as opposed to an absolute prediction. Often it is the ability to repair any damage which is more important than the magnitude of any cracking itself.
This paper discusses some of the challenges, considerations and approaches that the author has used on major infrastructure projects, along with some of the opportunities and new technologies available for assessment, instrumentation and monitoring and mitigation of the risk of ground movement impacts.
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Design Challenges Of Road Widening In Soft Grounds: Characterisation To Numerical Analysis
This paper presents challenges involved in robust geotechnical design for upgrading an existing motorway, which is founded on soft soil. A critical review is presented on current approaches adopted for soft soil characterisation using results of laboratory and field tests commonly applied in Australian practice. Particular attention is given to assessment of undrained shear strength, overconsolidation ratio (OCR), primary and secondary compression (creep) indices and coefficient of consolidation. Some of the limitations of finite element analysis using 2D PLAXIS software, as a commonly used tool, and its ‘soft-soil creep model’ are also highlighted. Analytical approaches are presented which were used in a case study to overcome these limitations and help with deformation analysis of soft soils undergoing creep, as well as the design of rigid inclusions taking into account 3D effects.
The case study project upgrade works involved widening of the motorway embankment and in turn extension of existing culverts, which were located in a river floodplain. Preload and wick drains were considered for soft soil improvement to meet residual and differential settlement criteria over the design life of the new pavement. Rigid inclusions were also designed for the new culvert extension to reduce potential differential settlement between the proposed extension and existing culverts.
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In ground concrete durability
Durability analysis is often a difficult part of the design for structures subject to severe exposures. Project specifications often quote national standards as a minimum requirement and require a specified design life be achieved. This paper shows how national standards can be conflicting and may be inadequate with regard to design for concrete in contact with ground. This places the responsibility on the contractor to ensure that state of the art durability designs will provide the serviceability required over the design life.
The paper specifically considers in-ground deterioration applicable to loaders, linings, railways, slabs and ports. Deterioration mechanisms considered include cracking, corrosion, and chemical attack (including acid sulphate soil issues). The rate of decay is dependent on the access and penetration of contaminants and their subsequent reaction with the concrete and steel. The paper reviews these processes to give a logical approach to design. The importance of cement systems to provide chemical resistance, mix design to provide impermeable paste and aggregate selection to avoid integral problems are discussed.
Even so, the best design will fail if construction is not adequate. The paper also outlines specific concrete properties (e.g. bleed, setting time, slump loss, heat of hydration, shrinkage) that the Contractor needs to understand and use in planning concrete pours so that construction defects do not compromise durability objectives and lead to expensive repairs.
Authors’ note: Throughout this paper, we employ the use of capitalised text to denote the various roles and elements that contribute to a major project’s success, in particular for durability management.
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Characterizing mine tailings for geotechnical design
Mine tailings are ground up rock and generally consist of sand and silt size particles, without clay minerals. Tailings “storage” (actually disposal) facilities are some of the largest constructed works, with seismic design an integral component. High value mines are frequently in earthquake prone areas and tailings liquefaction is an ever-present concern. A screening level liquefaction assessment based on the CPTu is a needed first step, but cannot be accurate because the fines content of tailings can be high, and engineering behaviour is only loosely related to fines content. An engineering mechanics based procedure is laid out in this paper which is applicable regardless of the fines content. It is anchored to the eighty year old principles of critical state soil mechanics originated by the Corp of Engineers. The state parameter provides the practical engineer an entry to this framework, as well as insight into a simple stability principle that sets the strategy for a tailings characterization project aimed at the analyses that a tailings engineer needs. The approach requires systematic in situ and laboratory testing to determine the soil mechanics properties of the tailings.
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Overcoming the Current Density Testing Impediment to Alternative Quality Testing in Earthworks
Density testing has been applied widely in earthworks quality assurance (QA), yet because of its widespread usage, this now acts as an impediment to the development of alternative methods of testing. Many incorrect inferences are made from density testing. Modern geotechnical and pavement designs are based on modulus and strength values. However, when such measurements are correlated back to density testing, a poor correlation often results. Therefore, while alternative in situ testing to density provides significant benefits to the industry, the poor correlation is often questioned, and then site personnel default to the usual density testing for quality testing of earthworks. The reason for the poor correlations includes, the depth of influence being different, with the quality and compaction being combined into one parameter (say modulus). Another rationale for the poor correlation, is the density lot measurements are normally distributed due to its low coefficient of variation, while other measurements are not normally distributed and have a large variation. A new method on matching Probability density functions (PDFs) for quality assurance has been successfully used on a large earthworks project to overcome the correlation inconsistency and is introduced. Data from several test sites using a range of alternative testing equipment are compared. One must also distinguish between a test accuracy and its precision. Traditional testing density has focused on its key benefit of precision without appreciating the poor accuracy associated with this measurement. Case studies are used to illustrate this dichotomy between traditional and non- traditional testing for QA assessment of earthworks.
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Ground truth, control and design of driven piles: implementing old ways with a new twist
Piling design and verification is a fraught and risky business. The spread of pile capacity estimates submitted to conference predictions exercises is often staggering and sobering. This underlines why design of driven piles does not stop at the design engineer’s desk but continues through construction, and relies on the valuable information provided by the installation process. Each installation blow is a test – a test of the ground response to hammer input delivered into the pile. Traditionally, pile capacity has been interpreted from this input-response relationship through various and many pile driving formulae. Five decades ago, measurement systems were first used to measure and interpret the stress waves in piles generated from the hammer inputs and reflected from the ground response to infer capacity in a more sophisticated and reliable way using wave mechanics principles. Today, PDA testing and wave matching are routinely accepted practice. However, each PDA test has direct relevance only to the individual pile which is tested. This paper will argue that our fundamental task as designers and supervisors is to establish ground truth, by synthesizing the results of PDA tests into a locally-evidenced and locally-targeted dynamic formula. Therefore, only dynamic formulae, properly modified and correlated,mustbethevehiclefordeliveringlocalgroundtruthandultimatelybeingthebasisforsign-off. On a foundation-wide basis, the role of PDA tests is critical but subservient, and principally to provide the evidence on which a correlated dynamic formula is developed. Consequent implications for the foundation sign-off process, and for a proposed new approach to establishing capacity reduction factors for driven piles will also be discussed.