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Soil-Structure Interaction Of Battered Minipile Groups In Sandy Soil
Battered minipile groups mimicking tree root networks have been gaining popularity as a footing solution for light structural applications in residential, commercial and infrastructure sectors, recently. Battered minipile group configurations are recently in the limelight due to advantages such as ease of installation and environmentally friendly nature. The lateral load resistance of battered minipile groups is investigated in this paper through a combination of physical and numerical modelling. Two-unconventional battered minipile groups with configurations representing the root network of trees with the capacity of engaging a larger volume of soil compared to conventional battered minipile group configurations are studied. A conventional battered minipile group is also included in the study to draw a direct comparison with the new minipile group configurations introduced in this paper. The conventional battered minipile group has two positively and two negatively 25° battered minipiles. The second type of group has one 25° perpendicularly battered minipile in the leading and trailing row each. Another unique orientation of the battered minipile group is also introduced in this study which has four diagonally outward 25° battered minipiles. The third type of minipile group with four diagonally outward battered minipiles offered the highest lateral resistance among the three groups. This better performance capability was attributed to the engagement of a larger volume of soil in resisting lateral load applied at the minipile head. Through this study, the industrial application of the unconventional minipile group configuration with better performance capability in terms of lateral load resistance can be advocated more confidently.
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Geotechnical Advances and Challenges in Urban Development
2020 AGS Sydney Golden Symposium
Prof Robert Mair, Dr Chris Haberfield, Paul Hewitt, Idy Li, Theva Muttuvel, Philippe Vincent, David Lees, Hashan Subasinghe, Helen Baxter-Crawford, Peter Sun, Antonio Ramirez Martinez and Bosco Poon & Rikito Gresswell
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The Role Of Progressive Brittle Fracture In The 1931 Landslide At Dogface Rock, Katoomba
The 1931 Dogface Rock landslide in Katoomba NSW was a complex, progressive cliff collapse with a failure volume in the order of 100,000 m3 that was triggered by the extraction of remnant coal pillars from the Katoomba Colliery, about 200 m below the top of the escarpment. Although underground coal mining is generally accepted as a cause of the rockslide, previous studies have not explicitly investigated the role of progressive brittle fracture in the collapse. This paper presents an integrated study which incorporates remotely piloted aircraft photogrammetry with a discrete element method numerical investigation of the landslide, and thereby explores the role of progressive brittle fracture, and re- examines the failure mechanism and runout motion of this multi-stage landslide.
Remotely piloted aircraft photography is used to build a georeferenced 3D model of the site with Structure-from-Motion photogrammetry software. A digital geotechnical mapping workflow is demonstrated to investigate the morphology of the landslide scar, extract statistics on discontinuity orientation, persistence, and spacing, and undertake trace mapping of newer brittle fractures that interacted with pre-existing high persistence joints as the landslide rupture surface developed. A series of discrete element method numerical laboratory tests are used to calibrate bonded block contact properties that reproduce laboratory scale intact rock index parameters including UCS and tensile strength. Upscaled rock block contact parameters are then applied to a cliff-scale model that investigates the progressive development of rock mass damage induced by mining. Following extraction of the remnant pillars, rock mass damage develops mostly by extensile strains that produce tension cracks. Brittle fractures propagate upwards from the mine level and eventually initiate toppling of massive sandstone slabs defined by high persistence pre-existing subvertical joints. The investigation illustrates how the integration of photogrammetry with discrete element numerical methods can be used to characterise progressive brittle failure and runout of large rock slope failures.
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Geotechnical Challenges For Construction Of Diaphragm Walls And Foundation Of Sydney’s Tallest Building, Crown Sydney Hotel Resort
Crown Sydney Hotel Resort is the Stage 1C component of Barangaroo South and is being developed as a single high rise mixed use tower of 72 stories (271m high), rising over a multi-level podium and a 3 level basement car park (total 75 levels). The Crown Sydney Hotel Resort basement retaining wall comprised 33 diaphragm wall (D-wall) panels and 36 barrettes for the foundation of the main tower and more than 130 bored piles (including bored compression piles, bored tension piles, bored sleeved piles and permanent plunge column piles). AECOM were engaged as designers of the foundation works by Piling Contractors Bauer Australia Joint Venture (PCBAJV) who constructed the foundation works as the D&C foundation contractor. The depth of foundation elements varied from 25 m to 50 m below ground level.
AECOM provided an initial concept design followed by a detailed design services and then, during construction, full time on-site geotechnical inspection of the basement diaphragm walls and foundations. This paper will focus on the challenges of geotechnical verification of diaphragm wall panel, barrette and pile foundation construction and how these challenges were met. During fulltime site inspection, hydraulic trench cutter penetration rates various sandstone rock classes have been measured and compared with the borehole data. Rate of penetration of the piling rig into the various sandstone rock classes, rock quality and rock apparent temperature were closely monitored and recorded as part of verification of the socket requirements. Monitoring, data collection and comparing the data with available boreholes, allowed AECOM to develop a method to reliably check the rock socket compliance with requirements across the site. Other geotechnical observations and lessons learned during the inspection of the pile, diaphragm wall and barrette construction are also presented in this paper.
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Ground characterisation and performance of a sinking motorway, waterview connection project, New Zealand
The paper presents the results and interpretations of an extensive data collected during the procurement phase of SH16 motorway upgrade. The strength and consolidation characteristics are investigated for two prevalent soil units (AH and ATcl). The AH soil is identified to manifest a response which is typical of a sensitive structured soft soil, whereas the ATcl soil is noticed to manifest an over-consolidated behaviour. The estimates based on either CSSM and SHANSEP have limitations to predict undrained shear strength profiles for the sensitive AH soil, but predicting rather well the shear strength of ATcl soil. The undrained shear ratio su/’v at OCR = 1 for AH soil appears to be a consistent indicator of shear strength development with depth. The non-linear one-dimensional compression displayed by the AH soil is proposed to model using a unique relationship between liquidity index and vertical effective stress. The predictive capability of this relationship is demonstrated by numerical simulations of settlement monitored during the construction and post-construction phase of the original SH16 motorway embankment.
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Ground surface deformation adjacent to a deep excavation in shale
This paper presents a case study of a deep basement excavation in Sydney’s Ashfield Shale. The intention of the paper is to present the high quality movement monitoring data that was gathered and analysed, and provide some accompanying interpretive commentary. These data may be of use when designing similar excavations.
Three mechanisms observed to cause ground surface deformation adjacent to the excavation are compared and contrasted:
1 – Lateral soil pressure acting on the shoring system, causing deflection of the shoring system.
2 – Excavation induced reduction in confining stress, leading to inwards movement of the rock mass.
3 – Drilling holes for ground anchor construction.
Mechanisms (1) and (2) above are those that geotechnical engineers normally consider in the design of deep excavations. Geotechnical engineers can usually predict these movements with reasonable accuracy. Lateral soil pressures and resulting deflections of shoring systems can be predicted or modelled using a range of basic and advanced techniques. These predicted deflections of the shoring system can be minimised if necessary by adjusting the design and the construction sequence. Rock mass relaxation resulting from a reduction in confining stress is more or less independent of the shoring design and as far as the authors are concerned cannot be controlled by any practical means.
Proven rules of thumb for common geological conditions in Sydney enable these deformations to be estimated with adequate accuracy for the majority of purposes.
Conversely we cannot usually predict ground deformations caused by anchor drilling using theory, modelling, or rules of thumb, with any real accuracy. Such deformations are highly dependent on the volume of soil removed by drilling, which in turn depends on the combination of geotechnical conditions, anchor hole drilling and construction methodology, and anchor design details including spacing, depth, hole diameter and declination. The volume of soil removed by drilling usually exceeds the theoretical volume, thereby meaning theoretical approaches are not usually particularly useful (at least for predictive purposes). These movements (and other movements due to shoring construction, e.g. pile excavation) are not always considered in a deep excavation design.
The case study highlights the significance of ground movements caused by shoring construction relative to total ground movements. Some conclusions are also drawn with respect to predicting and controlling anchor drilling induced deformations and appropriate types of monitoring for similar excavations.
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Crossrail Tunnelling In The London Clay Formation
The Crossrail project in London (UK) spent seven years excavating and lining tunnels and shafts within the London Basin. Construction activities encountered, and dealt with, a range of geological strata, including recent deposits of Made Ground, Alluvium and River Terrace Gravels, over-consolidated London Clay, variable mixed sediments of sands gravels and clays, uniform and fine-grained sands, and the underlying sedimentary Limestone (Chalk). Approximately 42km of segmentally lined bored tunnel were completed using pressurised Tunnel Boring Machines (TBMs), and over 12km of tunnel were supported utilising Sprayed Concrete Linings (SCL) with spans of up to 17m, along with sprayed concrete, diaphragm wall and piled shafts and underground structures.
This paper provides an outline of the project, concentrating on the central tunnelled section. The broad geological/hydrogeological setting of the central tunnelled area is described, with an examination in more detail of the perception and reality of tunnelling in the over-consolidated London Clay. This material has often been described as the ideal tunnelling material, but it is variable and not risk free, and offers challenges for both design and construction. The London Clay has been compared with the similar Keswick and Hindmarsh Clays that underlie much of the Adelaide city area. Tunnel construction utilising TBM and traditional excavation coupled with the use of SCL for support through the London Clay will be discussed. The paper will consider in particular the influence of historical experience of tunnelling in London Clay on modern approaches and risk perceptions.
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Probabilistic analysis of a spatially variable c’-ϕ’ slope
In practice, inherent soil variability is not commonly considered in routine slope stability analysis. This is due mainly to the fact that the effect of soil variability is complex and difficult to quantify. Furthermore, the majority of available slope stability analysis computer programs used in practice, which adopt conventional limit equilibrium methods, are unable to consider this aspect explicitly. To predict the stability of a slope more accurately, especially the marginally stable ones, the effect of soil variability needs to be accounted for. In this paper, an advanced probabilistic analysis method called the random finite element method (RFEM), developed by Griffiths and Fenton in the 1990s, is used to investigate the effect of soil variability on the reliability of a c’-ϕ’ soil slope. The results from the probabilistic study demonstrate that soil variability has a significant effect on the reliability of a slope. It is concluded that the deterministic factor of safety (FOS) is not a reliable measure of the true safety of a slope with spatially variable soils.
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Introduction to the ‘CGSE Special Issue’ of Australian Geomechanics
The Newcastle Chapter of the Australian Geomechanics Society is pleased to dedicate its themed issue of Australian Geomechanics to the activities of the Australian Research Council Centre of Excellence for Geotechnical Science and Engineering (CGSE), which combines three of Australia’s leading geotechnical research groups: the Centre for Geotechnical and Materials Modelling at The University of Newcastle, the Centre for Offshore Foundation Systems at The University of Western Australia, and the Centre for Geotechnics and Railway Engineering at the University of Wollongong. With a forecast investment of over 250 billion dollars in Australia’s energy and transport infrastructure over the next five years, there is an unprecedented need to design and build this infrastructure as cheaply and safely as possible. In light of the size of investment involved, even small percentage savings resulting from scientific research will lead to huge returns in absolute dollar terms. Through advanced laboratory testing, physical modelling, full-scale field testing and cutting-edge computational simulations, the CGSE is providing engineers with new science-based tools for designing safer and cheaper energy and transport infrastructure such as roads, railways, port facilities, tunnels, dams, pipelines, mining operations and offshore oil and gas facilities. The CGSE has four geotechnical science themes, each of which is linked to advanced computational modelling, state-of-the-art physical modelling and laboratory testing, and engineering applications: Geomaterial Science, Multiphysics Modelling, Moving Boundary Problems and Georisk.
The collection of articles in this issue highlights the complementary skills and facilities brought together from each of the nodes and the innovative research produced by the CGSE. The first of two articles that summarise recent developments in the testing equipment and physical modelling techniques available within the CGSE is the contribution by Cassidy et al. (2014), which describes the new National Geotechnical Centrifuge Facility, a recently established mobile in situ testing laboratory, and the new national facility for the cyclic testing of high-speed rail. The second paper by White et al. (2014) describes the recirculating flumes, or O-tubes, that allow for simulation of ocean-structure seabed interactions in offshore applications and, in particular, the stability of pipelines on mobile seabeds.
A series of papers covers various aspects of the work being conducted in conjunction with soft soils, for which the CGSE has developed Australia’s first National Soft Soil Field Testing Facility (NFTF) in Ballina, NSW. These articles include an overview of the site and the in situ testing programme (Kelly et al., 2014), a discussion of current and future work on sampling and laboratory testing on Ballina clay (Pineda et al., 2014), an investigation of spatial variability based on field tests completed at the NFTF (Li et al., 2014), and a study aimed at investigating the effects of strain rate and softening in vane shear testing (Ansari et al., 2014). A fifth article on soft soil focuses on developments in the understanding of vacuum preloading as a means of accelerating consolidation (Indraratna et al., 2014a).
Three articles describe past, present, and future research on onshore and offshore foundations. Gaudin et al. (2014) give a comprehensive review of the analytical, numerical, and physical modelling techniques developed within the CGSE to understand and predict the performance of offshore anchoring systems. Hambleton et al. (2014a) present findings within a new focus area on modelling the installation process for helical anchors and piles. Gourvenec and Feng (2014) describe the innovative design methodologies developed to economise offshore foundations through analysis of the capacity, foundation configuration, soil characteristics, and the mode of operation.
In the area of transportation geotechnics, two contributions focus on the performance of railway foundations. The study by Tennakoon et al. (2014) considers the influence of contamination of ballast on its drainage and shear strength characteristics, and Indraratna et al. (2014b) review laboratory and field testing on shock mats which can be used to reduce ballast degradation and improve stability of railroad tracks. A third study on transportation geotechnics (Heitor et al., 2014) examines cost-effective techniques for assessing the adequacy of compaction in projects with high fills covering large areas, where conventional quality control methods can be prohibitively expensive.
Two articles illustrate the challenges associated with problems involving ultra-large deformations. O’Loughlin et al. (2014) provide an overview of the centrifuge modelling, field testing, and numerical modelling being completed to understand and predict the behaviour of free falling projectiles such as torpedo anchors and penetrometers. Hambleton et al. (2014b) highlight current and future research initiatives within the CGSE on modelling the progressive displacement of soil in ploughing and cutting processes, which lie at the heart of earthmoving operations occurring at numerous scales in various environments.
Three papers showcase advances in the numerical and analytical tools developed within the CGSE. Kardani et al. (2014) demonstrate the improved computational performance that can be achieved in the numerical analysis of coupled consolidation problems through the use of high-order elements in adaptive finite element methods. Vinod et al. (2014) present a technique based on the discrete element method (DEM) for investigating the mechanical behaviour of sand containing methane hydrate. Huang et al. (2014) discuss the means by which Bayesian statistical methods can be used for improved prediction of performance geotechnical projects, considering two examples involving load testing of piles and settlement prediction from field monitoring data.
Each of the papers in this edition was anonymously peer-reviewed, and the Newcastle Chapter extends its sincerest gratitude to each of the reviewers.
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Use of shock mats for enhanced stability of railroad track foundation
Increasing demand for High Speed Rail (HSR) and fast heavy haul poses a serious challenge for stability of tracks on problematic ground. Ballast is a key track foundation material placed underneath the sleepers which provides structural support against high cyclic and impact stresses caused by moving trains. Degradation of ballast contributes to a large percentage of track maintenance costs apart from affecting longevity and stability. In recent years, use of elastometric soft pads underneath sleepers has become increasingly popular as means of reducing track damage. The ‘shock mat’ placed under the sleeper is traditionally called Under Sleeper Pad (USP), and when it is placed under ballast, the term Under Ballast Mat (UBM) is often used. Currently there is lack of comprehensive assessment on the geotechnical behaviour of ballast using these artificial inclusions under impact and cyclic loading. In this study, a series of large-scale laboratory tests were conducted to understand the performance of these energy absorbing ‘shock mats’ in the attenuation of impact and cyclic stresses and subsequent mitigation of ballast degradation. Impact loads were simulated using a high-capacity drop-weight impact testing equipment, while the cyclic loads were simulated using a large-scale prismoidal process simulation test apparatus. This paper presents a state-of-the-art review of laboratory studies and field trials demonstrating the benefits of USPs and UBMs in rail industry.