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A parametric study of lateral load behaviour of single piles socketed into joint rock mass
Rock mass properties, joint characteristics, pile stiffness and geometry play important roles in determining the pile head load-deflection behaviour of laterally loaded rock socketed piles. In this study, a comprehensive numerical modelling was undertaken using the three-dimensional distinct element code, 3DEC. The numerical model was initially calibrated against laboratory model pile load test results. The calibrated model was then extended to investigate the effect of rock properties such as modulus, cohesion and friction angle on the pile head-load deflection behaviour. In addition, this study examined the effect of pile socket length and joint characteristics such as dip angle and spacing. This study found that rock modulus, joint characteristics and pile geometry have significant effect on the lateral load capacity of single piles socketed into jointed rock mass.
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2018 Sydney Symposium
Advances in Site Investigations, Monitoring and Instrumentation
Peter K. Robertson and Michael Nicholson
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A Novel Multiple-Liner Design For Preventing Desiccation Of Geosynthetic Clay Liners
Geosynthetic clay liners (GCLs) covered by geomembranes (GMB) often constitute a major component in barrier systems. They are used in waste containments systems such as landfills, brine ponds and solar ponds. In many of these cases, high temperatures can develop as a result of exothermic biodegradation or direct solar radiation and can cause significant desiccation of the bentonite in the GCLs. In addition, the self-healing ability of bentonite may be compromised by exposure to chemically aggressive permeants that are commonly found in such applications.
A new multiple-liner design is proposed in this paper, with two GMB-GCL composite liners sandwiching one layer of geocomposite (GC). The new design is able to actively hydrate top and bottom GCLs through the middle GC layer with clean water. A set of column model experiments simulating a typical bottom profile under a brine pond were conducted to investigate GCL hydration before and after continuous heating at 78 ± 1 o C for 14 days. The results were compared to the more conventional GCL-GMB designs. The findings revealed that the new multiple-liner system speeds up hydration of bentonite in the GCL by a factor of more than 3, increases its water content at the end of the hydration stage by up to 50%, and prevents its desiccation when exposed to high temperatures.
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AGS WA Symposium 2022
Engineering Geology and Geotechnics of Western Australia
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Variability in selecting design values in Queensland: A calibration of professional opinions
Selection of design values is often subjective although statistical approaches may also be adopted. The design value is derived from the characteristic value which is a cautious estimate of the parameter. However many practitioners rely on their judgment in arriving at a characteristic value. The use of statistics provides transparency, especially when a third party is required to review the parameter selection, as often personal judgment lacks accountability. However, to be useful, a sufficient number of test results should be obtained for statistics to be used.
This paper describes a survey carried out to assess this personal judgment, and examines how the background of the engineer influences what design value is selected. The case study is for a local road with soaked California Bearing Ratio (CBR) values provided. This survey also compares the background of participants in terms of work type as well as number of years of experience to assess how we select design values. When the participants are informed of the relative cost of their decision then a comparison of the survey results (before and after) was then used to see this effect on the recommended design value.
These “professional opinion” values were compared with statistically derived results to assess our risk appetite in Queensland (as compared to elsewhere).
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Pipe Jacking Through A Rail Embankment
Pipe jacking through rail embankments inevitably causes ground movements during excavation and construction. Excessive ground movements can lead to embankment instability and rail track distortions which could result in train derailment. Therefore, ground and track movements must be closely monitored during construction in accordance with the respective rail authority standards and geotechnical assessment. This paper presents the project overview, construction method, prediction of ground movements associated with pipe jacking, track deformation design criteria and construction performance for the Gerringong Bypass Belinda Street Culvert where seven 1500mm diameter reinforced concrete pipes needed to be constructed beneath the existing South Coast Line railway embankment. Extensive instrumentation and real time monitoring were undertaken during construction to ensure safety of the rail operation. The measured ground movements were back analysed and the assumptions were calibrated for the prediction of future movements due to subsequent pipe jacking. A contingency plan was implemented to ensure that the settlement criteria and operational safety were met.
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Modelling issues in simulation of deep excavations
This paper investigates three different modelling approaches available in ABAQUS/Standard finite element program for the simulation of short-term behaviour of deep excavations in saturated clayey soils below the water table. The first approach is based on the total stress principle with the undrained soil properties, which is the conventional way of analysing deep excavations using the finite element method with the undrained assumption. The other two approaches are based on the effective stress principle but the analysis time is very short preventing any excess pore pressure dissipation, replicating undrained behaviour. In the first approach based on the effective stress principle, a partially coupled analysis is carried out considering the excess pore pressures and effective stresses. ABAQUS does not have the ability to incorporate initial hydrostatic pore pressure distribution in the partially coupled excess pore pressure analysis. To simulate the hydrostatic pressures applied on the wall during dewatering of the excavation, body loads are applied along the wall. In the second approach, which resembles the real scenario, a fully coupled analysis was carried out considering the total pore pressures and effective stresses. In this analysis, initial hydrostatic pressures can be established before the excavation begins. Undrained and drained shear strength parameters relevant for the same clayey soil derived from triaxial tests were used to perform the total and effective stress analyses, respectively. Results obtained from a hypothetical case show that the use of the conventional way of analysis based on the total stress principle assuming undrained behaviour is not suitable to predict wall and ground deformations during excavations if the excavation is deep (more than 10 m). Partially-coupled analysis over predicted wall deformations and ground surface settlements compared to the fully coupled analysis, confirming that a fully coupled analysis is necessary to obtain the wall deformation and ground surface settlement corresponding to the short-term behaviour. Finally a case study reported in the literature for a staged deep excavation carried out in Taiwan is simulated using the two methods based on the effective stress principle. These results show that both partially and fully coupled approaches are suitable for the simulation of deep excavations, when excess pore pressure dissipation is allowed during the excavation.
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Developing engineering intuition
The limitations of our knowledge base result in occasions when “engineering judgement” is required. But what is this, and can it be developed? Engineering and geotechnical training focusses on technical and perhaps managerial aspects but pays little attention to the development of engineering instincts.
This paper explores the concept of engineering intuition and suggests directions that may be pursued by (young) engineers to encourage its growth. This is based on reflections of my own experience and is primarily explored and illustrated within a geotechnical context.
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A study of reactive soil influence on small diameter pipe failures in Melbourne
Melbourne’s water reticulation system experiences about 4000 pipe breakages or bursts in each year, causing difficulties to both water utilities and water users. The majority of these failures are in old cast iron pipes that can be up to about 100 years old. For reticulation or small diameter cast iron pipes (diameter less than 300mm), the failures occur around the pipe circumference (known as broken back failures) mainly due to pipe bending. It is well established that the seasonal ground movement in reactive soil zones in Melbourne has a notable impact on pipe bending and resultant failures. The present study examines in detail the failure process of small diameter pipes that are affected by reactive soils. Finite element models of unsaturated soil-pipe interaction are used to simulate the response of the pipe to reactive ground movements that are governed by soil moisture variations. Locations where high pipe stresses due to ground movements are likely to occur are identified as stress “hotspots” to determine the potential for pipe failure. The concept of these stress hotspots is verified or corroborated by collecting field information from recent pipe failures in Western suburbs of Melbourne where highly/extremely reactive soils are commonly present. The results of this study are presented in the form of a simplified analytical method to estimate pipe stresses based on the soil moisture changes at the pipe level. With this development, mechanistic failure models are developed that could be implemented in a GIS platform for failure prediction and visualisation.
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Analysis of rainfall-induced landslides in northern New South Wales, Australia
Although rainfall-induced landslides are common phenomena in Northern New South Wales (NSW), no systematic studies have been performed to date to better understand this natural disaster. This study seeks to determine the common characteristics of these landslides, including geology, slope geometry, rainfall distribution, and soil properties. The study area includes mountain passes such as Mt Lindesay, Mallanganee Range, Ramornie–Cangai Bluff, Gibraltar Range, Dorrigo Mountain and Mt Seaview; and riverbanks, embankments and cuttings where more than 100 landslides occurred between 2009 and 2017. Field survey of sixteen landslide sites was carried out to collect soil samples, which were tested in a shear box apparatus. This study reveals that many natural slopes affected by landslides consist of weathered sedimentary rocks (mostly sandstone) while the soil from the landslide mass contains a significant amount of coarse material. In addition, landslides tend to occur every two years on natural slopes which are inclined at about 35. Analysis of rainfall characteristics enabled the development of an indicative rainfall threshold associated with landslide occurrence in the studied area. Rainfall index studies show that landslides could occur at Mt. Lindesay and Dorrigo Mountain if daily rainfall reaches 88 mm and 136 mm respectively.