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Application of statistical techniques for geotechnical site investigation and design
Uncertainty is a universal and important aspect of geotechnical engineering and it is comprised of several different aspects. The most significant is likely that derived from spatial variability, where the properties vary from one location to another as a result of the processes that form the ground. Secondly, statistical uncertainty is a critical element of geotechnical engineering. Other important sources of uncertainty are those associated with the testing process itself, the transformation of the test results to design values, and uncertainties derived from human error. The paper discusses each of these uncertainties in some detail and provides examples and guidance on how to quantify and account for these in the geotechnical design process. The paper also presents two examples that demonstrate the power of statistical simulation in geotechnical engineering practice.
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Piling Works Tour
New Bridgewater Bridge Project
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Design methodology and input parameters applicable to foundation design for large complex towers
As buildings are progressively getting taller, traditional methods of design that generally relied on considerations of the vertical load-carrying capacity of the foundation system, assessed by empirical methods and a lumped factor of safety, have been largely replaced by serviceability-based methods of design which typically result in an optimised foundation design. Serviceability-based designs typically rely on powerful commercial software packages to enable advanced numerical analysis of foundation systems. This paper briefly discusses case studies of foundation design processes including soil-structure interaction analyses adopted for the serviceability design of tall towers. In order to obtain accurate building movement prediction from complex computer analysis, it is imperative that materials and ground stiffness properties be accurately characterised and measured. This paper presents ground stiffness properties measured from various types of tests at different strain levels (i.e., geophysical testing, pile load tests, pressuremeter and laboratory tests) that have been adopted as input parameters in the numerical analyses. The higher allowable shaft friction values from serviceability analysis compared to those from traditional methods, are further justified on the basis of bi-directional static pile testing.
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The applicability of index tests on weathered rock: Interrelationships and experience from The Pilbara, Western Australia
Index tests can provide useful correlations with the mechanical properties of rock, but must be used with caution in lower strength and weathered materials. The authors summarize the results from laboratory testing and visual examination of rock from 1300m of core taken from 69 nearshore boreholes on the coast of the Pilbara in Western Australia. Interrelationships between weathering and index properties are discussed; correlations between the parameters are proposed and the limitations of index tests are considered.
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Effects of electrokinetic treatments on the properties of a salt affected soil
This paper presents experimental results of a study undertaken to investigate the effects of electrokinetic treatments on selected chemical and physical properties of a salt contaminated (saline) soil. The study was conducted as a laboratory scale pilot project using locally available saline soil samples. The soil was subjected to an electric gradient by passing a direct current between inserted electrodes. After certain electrokinetic treatment periods, the properties of the soil were evaluated. The experimental data reveals that electrokinetic techniques could offer a low cost, rapid solution to treat saline soils. The removal efficiency of sodium ions was found to be greater than 90% within a relatively short time period of 14 to 30 days, using low current and voltage systems. After 14 to 30 days, the degree of salinity and sodicity decreased to a very low or negligible level. The unconfined compression strength of the soil increased by between 30% to 100% in 30 days of electrokinetic treatment indicating the improvements in the physical properties, especially in the stress-strain characteristics of the soil. The liquid limit (LL) and plastic limit (PL) increased at the cathode.
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Mechanical behaviour of hydrated cement treated crushed rock base (HCTCRB) under repeated cyclic loads
This paper aims to report the mechanical behaviour of hydrated cement treated crushed rock base (HCTCRB) as granular road base material subjected to repeated cyclic loads from Repeated Loads Triaxial (RLT) tests with various stress paths in order to improve more understanding of such Western Australian roads based materials on mechanistic-empirical pavement design and analysis. As known, pavement surface rutting, longitudinal and alligator cracks are normally the main cause of damage in flexible pavements. Factors contributing to such damage are the excessive irreversible and reversible deformation of a base layer including the behaviour of a mechanical response of unbound granular materials (UGMs) under traffic load is not well understood. In this study, the shakedown concept was utilized to describe and determine limited use of HCTCRB subjected to different stress conditions. The concept is the theoretical approach of the UGMs used to describe the behaviour under RLT tests. The shakedown concept utilizes macro-mechanical observations of the UGM’s response and the distribution of the vertical plastic strain in the tested material. While the shakedown limit of an UGM is known, whether the limitation of the accumulated plastic strain in an unbound granular layer causing rutting in pavements can be predictable. In this paper, compacted HCTCRB samples were subjected to the various stress condition defined by the stress ratio (the ratio of a vertical major stress, σ1 and a horizontal minor stress, σ3) in order to simulate the real condition of pavement. The study reports that HCTCRB was defined the working stress ratio of 11 in pavement structure and will be achieved stable state at the large number of load cycles. Moreover, the mechanical responses were investigated and the limit ranges of using HCTCRB in pavements were determined.
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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.
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Geotechnical investigations for the penstock plug of the Anthony Underground Power Station, Tasmania
From a reservoir on the Anthony River water flows through a headrace tunnel and a short power tunnel to the Anthony Power Station. The last section of the power tunnel into the underground power station chamber is lined with a concrete backfilled steel penstock. This backfilled steel penstock forms a plug to prevent high pressure water in the unlined power tunnel from penetrating into the power station. This paper describes the investigations that were carried out to determine the length of penstock required and to confirm the water tightness of the finally adopted design for the penstock plug.
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Experimental Investigation of coal-fouled ballast stabilised with geogrid
This paper presents the results of an experimental study of coal-fouled ballast reinforced with geogrid, at various degrees of fouling and subjected to cyclic loading. A novel Track Process Simulation Testing Apparatus was used to realistically simulate fouled rail track conditions. The laboratory results demonstrated that coal fines acted as a lubricant, causing ballast aggregates to displace and rotate and, as a result, increase the deformation of ballast. However, coal fines also reduced ballast breakage somewhat because they fill the voids between the ballast particles and coat surface of ballast aggregates which reduce the inter-particle attrition. The placement of a geogrid at the interface between the ballast and sub-ballast layers provides extra internal confinement and interlocks the grains of ballast in its apertures, which also reduces ballast deformation. Based on laboratory results, a threshold value of VCI=40% is proposed to assist practitioners in conducting track maintenance as fouling beyond this threshold significantly reduces the reinforcement effect of geogrid so that fouled ballast experiences premature dilation leading to track instability. A novel equation incorporating the Void Contaminant Index and number of load cycles is also introduced to predict the deformation of fouled ballast, improve the design of rail tracks and help make the correct decisions with regards to track maintenance.