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An alternative view on geotechnical parameters for tunnel design in Sydney
Bertuzzi (2014) presented an update of the geotechnical design parameters previously proposed by Bertuzzi and Pells, (2002). The updated parameters focused on tunnel design with two sets of parameters introduced to cater for different design approaches dependent on the scale of the assessments. The proposed parameters provide a useful starting point and reference for practitioners working on tunnel design. However, Bertuzzi (2014) provides no guidance on the appropriate use in a model which may, in some cases, result in discrepancies between the two different design approaches and expected range of ground behaviour. This paper provides additional comments on the differences between the two design approaches and guidance on the use of updated design parameters. The objective is a compatible behaviour independent of design approach chosen. In addition, design parameters may vary dependent on different interpretations and new sets of parameters based on the discussions in this paper are suggested as an alternative reference.
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Spoil piles – comparison of stability analysis methods
A study has been undertaken to confirm appropriate analysis methods applicable to design of granular waste materials and specifically that of “spoil piles” for open pit coal mining. The authors’ experience with spoil pile stability is that the critical failure mechanism is a two wedge mechanism in accord with that highlighted in the literature. Analysis requires careful consideration of the combination of spoil shear strengths and the angle of the “rear” scarp adopted. Moreover, the literature indicates analyses need to take account of the inclined “inter-slice” within the mechanism. To highlight these considerations an example case was assessed utilising five limit equilibrium stability packages, two “numerical based” packages and a spreadsheet developed by the author. Several analyses are based on vertical slices to pose the question: what is the magnitude of error not using an inclined “inter-slice”? The results are presented as overall spoil pile angle indicated by the use of different software, stability method and assumptions on inclination of “rear” scarp and inter-slice. The example case highlights that, provided a rigorous stability method is utilised, for a given rear scarp assumption, the results of the analyses are insensitive to the software utilised. Therefore with appropriate judgement assessment of spoil pile stability should not be predicated by use of one particular software or stability method.
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A Holistic Design Approach Using Modern Geogrids in Soft Ground Improvement
- Introduction
- Principles of Geosynthetic Reinforcement
- Independent Research findings from Large-Scale Laboratory Tests and Field trials
- Guiding Factors for Reinforcement Benefit
- Case Study: Utah Point Berth Project (UPBP), Port Hedland
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Comparison Of Mohr-Coulomb And Hardening Soil Constitutive Models For Simulation Of Settlements In The Karkheh Earth Dam
This paper presents the settlement behaviour of Karkheh earth dam during its construction and operation stages. Karkheh is one of the largest earth dams in the world in terms of its reservoir capacity and body volume. The settlement of such a large body of soil can affect the performance of the dam elements and endanger downstream areas; should a breach or failure occur in the dam, more than two million people will be affected. It is crucial to know the settlement behaviour of this structure and use the existing results to predict its future settlements and calibrate the existing stress- strain models. For anticipation of dam settlement the measured displacement from the portable probe anchor magnets installed in the dam body are compared to the results of numerical simulations. The available data cover a period of 12 years including construction, and two material impounding andoperation periods of the dam. The numerical analysis is performed in 2D plane-strain conditions and two material models are used, including Mohr- Coulomb (MC) and Hardening Soil (HS) models. The comparison between the calculation results and the measured vertical deformations in the dam site reveals that the accuracy of model for the deformations in the middle levels of dam is better than those of the crest for both applied material models in construction and impounding stages. The maximum settlement differences between computed and observed values are 0.05 m for MC model and 0.01 m for HS model. For the operation stage, the error of calculated settlements for the MC model is smaller; hence the results of this model might be more reliable for prediction of future dam settlements. The similar trends, obtained from both material models, exhibit the suitability of the model parameters used in the simulations.
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Evaluation of the Tasmanian North-West Coast landslide map series against the AGS 2007 Landslide Zoning Guideline
This paper documents an assessment of the recently produced Tasmanian Landslide Map Series in Tasmania’s North-west Coast region against the AGS 2007 Landslide Zoning Guideline and Commentary. In addition, comments are made on the Guideline and Commentary from the perspective of a practitioner using these documents.
Mineral Resources Tasmania (MRT) welcomes the introduction of the Guideline, and its Commentary, and have striven to adopt them as far as practically possible within the constraints of our project. The Guideline and Commentary provide a conceptual architecture and standard terminology for the practice which has been largely accepted by MRT.
Using the Guideline’s criteria we have classified the MRT maps as suitable for local and regional development because of its medium scale. Our landslide inventory is at the intermediate zoning level and our susceptibility mapping at the preliminary zoning level, which confirms the maps general intention and suitability for regional zoning purposes. MRT has used a combination of largely heuristic and deterministic transforms to create its susceptibility maps. The combined approach is more complex than the conceptual descriptions contained in the Guideline but this is in keeping with other published examples.
The experience gained by the production of regional scale maps at MRT enables the authors to comment on the usefulness of the Guideline and Commentary. Landslide zoning is a challenging exercise that requires considerable effort, skills and resources to put into practice. From the perspective of practitioners, it is the observation of the authors that some aims, realistically, may not be attainable within the constraints of most regional projects. Furthermore, the science underpinning the practice of landslide zoning is rapidly evolving and this requires the practitioner not to rely on the Guideline alone but to keep abreast of developments that are typically only published in the international literature.
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Suction-monitored direct shear apparatus: A simple device for unsaturated soil testing
Two very common devices used in unsaturated shear strength tests are suction-controlled direct shear apparatus and triaxial devices. During the tests, matric suction is applied to the specimen by controlling pore air and pore water pressure. Compared to using triaxial device, the suction-controlled direct shear apparatus is considered to be simpler to use due to the shorter drainage path of the specimen. A relatively new and a simpler technique for unsaturated shear strength is the suction-monitored direct shear test. The device is made by modifying the conventional direct shear apparatus with the attachment of low capacity tensiometer on its top cap; this is connected to an electronic readout monitor which assesses negative pore pressure during the test. The aim of the study is to find out the capability of the suction-monitored direct shear apparatus with regard to use in unsaturated soil. Conventional, as well as suction-monitored direct shear tests have been conducted on various compacted brown sand-kaolin clay mixtures of differing proportions. The first one was performed to obtain the effective shear strength parameters of the saturated specimen, with subsequent testing carried out on unsaturated specimens. The results indicates that in general, shear strength with respect to matric suction, exhibits a bilinear envelope with an initial value of φb higher than the effective internal
friction φʹ. This phenomenon was due to the effect of dilation on the strength development of sand. Matric suction of the specimen was generated indirectly by adjusting the specimen’s water content to the desired value, and for this purpose the Soil-Water Characteristic Curve (SWCC) can be a very useful tool for predicting the required water content. Despite the suction capacity of the tensiometer being relatively low, the suction-monitored direct shear apparatus was effective for soil with a low high entry value, such as sand or sand with relatively small portion of fine grained material. -
Seasonal variations in the performance of driven piles in sand
This paper explores the performance of model pipe piles driven into a silica sand deposit at the Water Authority Research Station in Perth, Western Australia. The principal aim of the research was to explore pile response over a range of conditions from fully closed-ended to fully open-ended. There has been some considerable debate of the relative capacity of closed and open-ended piles driven into similar deposits of sand. In many design codes, it is customary to assume lower radial stresses (and hence shaft friction) for open-ended piles, than for solid or closed-ended piles. The base capacity is often taken as the same for both pile types, provided it can be demonstrated that the pile will ‘plug’ under static loading conditions. Potentially lower shaft capacity of open-ended piles is compensated for by the ease of installation since, in principle, the base resistance only has to be overcome around the steel annulus, as a plug of soil moves up the inside of the pile. However, in some circumstances open-ended piles may plug during driving, which leads to increased driving resistance and possible refusal before the design penetration has been achieved. This tendency may be alleviated by the use of an appropriate internal driving shoe, which provides stress relief for the soil plug.
The test results indicated a significant effect of season on the performance of the piles. The test programme included both dynamic and static testing of each pile, allowing a comparison between dynamic and static measures of capacity in tension and compression. Those tests carried out in the summer showed greater pile capacities in both tension and compression than those tests carried out in the winter. Equally the tests carried out in the summer showed a stiffer initial response than those carried out in the winter. The static test results were consistent when examined in conjunction with blow-records and the dynamic data. The test results indicated an optimum design for the driving shoe, which maximised soil plug penetration into the pile under hard driving. Of particular interest was the finding that the pile tip condition appeared to have little effect on the shaft capacity, but did influence the rate at which the end-bearing resistance was mobilised (due to compression of the internal soil plug). Thus, where the base resistance was defined at a fixed tip displacement, open-ended piles had a lower resistance than closed-ended piles. The test results are reviewed in the light of current design methods for driven piles in sand, and recommendations made for future design.
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Guidelines On Settlement Criteria For Design Of Highway Projects
This paper examines the implication of post-construction settlement and differential settlement on highway pavements constructed on soft soils by considering a number of a factors including: type of pavement, rate of settlement, ride quality, and likelihood of pavement distress. The main purpose of this paper is to provide some guidance and clarity on how differential settlement criteria should be specified and allowance made in the design, and in the selection of pavement types. Experience is drawn from monitoring and maintenance records gathered on various sections of the Pacific Highway Upgrade and other NSW roads. Extensive research has been carried out by the Roads and Maritime Services of NSW (RMS) and we have quoted extensively from their draft RMS (2009) “Guide for design and performance of concrete pavements in areas of settlement”. Structural performance of the pavement is considered in addition to ride quality functionality, and guidance is provided for designers to select the appropriate settlement and differential settlement criteria for highway projects. The RMS document uses radius of curvature as a basis for design, which is strictly speaking correct but difficult to predict when designing embankments on soft soils due to small allowable post-construction settlements. This paper provides some guidance on the correlation between the more commonly used limits on “change in grade” specified on recent road projects and “radius of curvature” and provides warning on the potential misuse of these values in assessing the length of transition zone required behind bridge abutments.
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A review of wave transmitting boundaries used in numerical modelling
Wave transmitting boundaries are used to truncate an infinite domain when analysing soil-structure interaction problems involving wave propagation using dynamic finite element analysis procedures. This facilitates analysing a problem in an infinite domain considering a finite domain with boundaries, which has the ability to absorb the waves propagating away from the finite domain without reflecting them. Simulating proper non-reflective conditions at the boundaries during a finite element analysis is therefore vital for the accuracy of numerical solutions. Numerous approaches have been introduced to simulate non-reflecting boundary conditions in the past few decades and the accuracy or the capacity to absorb waves propagating into these boundaries has improved over the years. This paper presents a review of wave transmitting boundaries found in the literature. The performance of several wave transmitting boundaries is investigated by simulating them using the finite element program ABAQUS.
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Excavation Induced Ground Movements And Risk Management Strategies
As well as the need to maintain stability, retention systems for excavations in urban environments are required to limit ground movements to mitigate adverse impact to adjacent structures. Tolerable ground movements will depend on the circumstances of the site; in particular the proximity and type of adjacent structures and their foundation type, underground services or tunnels and their current structural condition. It is impractical to limit excavation induced movements to zero even though this may be the wish of the adjacent property and infrastructure owners. Therefore, a site specific risk management strategy is essential for the selection, design and construction of an appropriate retention system for a particular set of site circumstances.
This paper provides a brief overview of excavation performance in different types of ground including a review of measured horizontal movements in deep basement excavations in the Sydney region and a literature study of excavations in soils. The paper then discusses prediction methods including analytical, numerical and empirical techniques, impact on adjacent structures and mitigation measures. Two short case studies are then given to illustrate the adopted risk management strategies. The first case study concerns an excavation adjacent to several existing railway tunnels, for which a risk register together with a monitoring system were used to manage the rail authority’s concerns on potential damage to the tunnel linings. The second case study describes the application of damage assessment that enabled the client to be informed of the risks and the selection of the appropriate excavation and retention method to reduce risks associated with excavation adjacent to heritage listed buildings.