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Landslide failure perspectives in practice in South East Queensland
Quantitative Risk Analysis (QRA) is one of the tools used in Landslide Risk management. Ideally its matrix approach should provide similar conclusions when used by different geotechnical professionals. However in practice some differences in interpretation and application occur. This assessment tool is examined for various case studies in south east Queensland.
By examining landslide events that have occurred with the benefit of hindsight in order to calibrate the approach, one finds that the interpretation of consequences has scope for variation in the application of this QRA tool if it is applied as is. The perception of risk governs landslide risk management in practice in Queensland. Other consequential variables also have a significant effect and may govern the result of the risk assessment process. The QRA is a decision tool with risk analysis at its core, but users must also not overemphasise the analysis in a risk assessment approach.
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Swelling characteristics of bentonite and bentonite-sand mixtures for nuclear waste disposal
The density of bentonite and of bentonite-sand mixture is the prime criterion in the evaluation of the swelling pressure and deformation of buffer material which must be taken into consideration in the design of any type of waste disposal facilities. A series of laboratory swelling pressure and deformation tests using variable dry density of the specimens has been carried out to investigate the characteristics of buffer material for radioactive waste disposal. Initial dry density and loading pressure on the specimens has a noticeable influence on maximum swelling rate. Temperature is also an important factor in the control of the swelling rate of compacted bentonite. The void ratio increased in high initial dry density material and decreased for low level initial dry density when compared with the initial state at the end of swelling due to static load. The swelling pressure fluctuated with elapsed time in respect to temperature. The maximum swelling pressure is dependent on the initial dry density and the content of bentonite in bentonite-sand mixture.
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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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A new lightweight dynamic cone penetrometer for laboratory and field applications
Developed by Scala (1959), the dynamic cone penetrometer (DCP) has been extensively used in recent decades for the design of flexible pavements due to its inexpensive and easy-to-operate features. This paper presents some of the results of a study concerning the effect of vertical confinement from the CBR mould on the dynamic cone penetrometer index, and the development of a lightweight DCP that can be used in the laboratory as well as in field conditions with similar results. The results show that the effect of vertical confinement is very significant, especially with a hammer mass larger than 4.6 kg. The results also indicate that the influence of the vertical confinement on the penetration index is not significant when the hammer mass is less than 2 kg. Based on these results, a new lightweight DCP is proposed with a hammer mass of 2.25 kg, which can be used in the laboratory in the CBR mould and also in field conditions with similar results for a similar soil.
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Class A prediction versus performance of O-cell pile load tests in Sydney Sandstone
As part of the Barangaroo South project, pile load tests were carried out on two prototype rock socketed piles (SC-01 and SC-02) in Sydney Sandstone to validate the design. The pile load tests were carried out using the bi-directional OCell method on 750 mm diameter piles having rock socket lengths of 7.85 m (SC-01) and 6.38 m (SC-02). The target O-Cell test load was 17 MN, giving a total potential maximum test load of 34MN (i.e. shaft plus base) on each pile. Once the testing reached the required target test load, the O-Cell was loaded to its maximum capacity of about 26 MN. Assuming the O-Cell load recorded beyond the calibrated range is valid, the maximum test load reached is equivalent to a total capacity of 52MN on each pile. Prior to the O-Cell testing of the first test pile (SC-02), a Class A prediction was carried out. In this paper, actual test pile load-deformation results are compared to the Class A prediction, together with a back-analysis of the O-Cell testing using numerical methods for calibration of rock stiffness and assessment of topload pile response.
The Class A prediction of SC-02 using the method given in Fleming (1992) and adopted foundation design parameters gave good match with the actual test results. SC-02 was back-analysed using numerical methods (program FLAC3D) to assess the equivalent secant modulus of the rock. Analyses of the top load pile response were then carried out using FLAC3D and program PIES (the software used for the foundation design of the project). By comparing the top-load analysis results with those inferred from the O-Cell test, the ultimate resistance parameters were confirmed but it was assessed that the design stiffness parameters adopted may be optimistic. Back-analysis of the test results also indicate that the top-load stiffness inferred from the O-Cell test may be over-estimated due to opposing displacement interaction effects. The conclusion is that actual pile stiffness at the serviceability load may be about 55% of the design estimate. However, pile top settlement is relatively small (i.e. ~ 2% of pile dia.) and the design estimate is considered reasonable in terms of accuracy for geotechnical design purposes.
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Structural Reliability Of Retaining Walls Designed To AS 4678 For Permanent, Imposed And Seismic Loads
Part 1 – External Stability
AS 4678-2002 Earth retaining structures may be used to design gravity retaining walls (including reinforced soils, segmental gravity retaining walls and cantilever retaining walls). This paper compares the structural reliability of retaining walls designed to AS 4678, to those designed using other codes, British Code of Practice CP2 and the National Concrete Masonry Association (USA) method of designing reinforced soils.
Part 2 – Earthquake
AS 4678 Appendix I sets out the options for designing retaining walls for earthquake, based on a classification method consistent with AS 1170.4-1993 (which is now superseded) and simplified rules. Some combinations lead to non-conservative design, but only where “failure would result in not more than moderate damage and not more than minimal loss of access”, or “failure would result in moderate damage and loss of services” with low or medium seismicity.
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Risk Based Approach In A Spillway Upgrade
In line with worldwide best practice, major dam upgrades are currently being undertaken in New South Wales (NSW) to ensure the continued safe operation of these assets in line with the Dam Safety Management Guidelines set out by Australian National Committee on Large Dams (ANCOLD) and NSW Dams Safety Committee requirements. Burrendong Dam, which is one of the major dams owned by State Water, is located in the greater Murray Darling River catchment on the upper reaches of the Macquarie River. The dam is currently being upgraded to increase the flood handling capacity of the spillway to cater for extreme flood events.
The conventional spillway design was found to be expensive therefore State Water embarked on a risk based approach to reduce the risk below As Low as Reasonably Practicable (ALARP). Saddle Dam A, which is located approximately 1.5km east of the existing spillway, was considered to be a viable option for an auxiliary spillway. The main issue was, however, the presence of a shear zone containing brecciated rock and clay gouge, which consisted of broadly graded silty sandy gravels and sandy clays. Hence, the potential for piping and foundation erosions were identified as the major risks for the project. The conventional design approach of a traditional concrete apron slab with ground anchors was found to be very expensive. Therefore, the erosion risks for not providing an apron slab to protect the shear zone were thoroughly investigated. The geotechnical conditions were analysed in order to satisfy the nominal criteria for energy dissipation of spillway discharge flows into the natural environment with an acceptable degree of erosion risk. The outcome of this analysis led to a significant reduction in cost as well as risk.
This paper focuses on the analysis and design of the geotechnical aspects adopted in the risk based approach, which led to significant cost savings to the spillway upgrade project. This paper is intended to provide valuable information for geotechnical professionals handling similar types of projects – especially where there is a potential to reduce cost by accepting a risk based approach.
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2018 Sydney Symposium
Advances in Site Investigations, Monitoring and Instrumentation
Peter K. Robertson and Michael Nicholson
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Difficulties in assessment of liquefaction from SPT
The difficulties in assessing liquefaction potential from the results of Standard Penetration Tests (SPTs) are illustrated by a case study. The method of Youd et al. (2001) is used to predict the potential for liquefaction. The important parameters for use in a ‘design earthquake’, namely peak ground acceleration and earthquake magnitude, have been derived for the site of a large industrial complex in Port Adelaide, South Australia. An assessment of the potential for liquefaction was required, as the site contained loose saturated sands and a loss of foundation support would have considerable implications for footing and retention wall designs. Questions in the assessment arose when the factor of safety against liquefaction determined from the results of SPT differed significantly from those determined from cone penetration tests (CPTs). Based on the SPT results, the site was predicted to be unstable against liquefaction for the design earthquake event, but the CPT assessment showed that the site had an adequate factor of safety. An investigation into the differences between results on this site and for a number of neighbouring sites revealed that the SPT N values were generally lower than values expected from published correlations with CPT profiles. A discussion of possible reasons for the disparities is presented within this paper and it is concluded that assessments based on SPT results alone are over-conservative for the ground conditions considered. Assessments using CPT results will provide a better assessment of liquefaction potential, as the CPT results are more reliable. The industrial site was therefore considered to have an adequate factor of safety against liquefaction for the design earthquake, resulting in considerable construction savings.