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Guideline for landslide susceptibility, hazard and risk zoning for land use management
There are a number of natural hazards which are relevant to urban, residential, rural and undeveloped property throughout Australia. These include flooding, bush fire, coastal processes and landslides. This guideline addresses landslide susceptibility, hazard and risk zoning for land use planning.
In 1998, following the Thredbo landslide in which 18 persons were killed, the Institution of Engineers Australia and the Australian Geomechanics Society (AGS) formed a Taskforce on the Review of Landslides and Hillside Construction Standards. The Taskforce after reviewing the Australian Standards and relevant codes on landslides and hillside construction concluded that they were inadequate and recommended the production of four guidelines:
- Landslide hazard zoning for urban areas, roads and railways
- Slope management
- Site investigations, design, construction and maintenance
- Landslide risk assessment
The Australian Geomechanics Society “Landslide Risk Management Concepts and Guidelines”, already under preparation at the time of the Thredbo landslide, was published in 2000 (AGS 2000, 2002). This document touched on all four areas but mainly addressed the fourth. It is used extensively throughout Australia.
In 2005 the Australian Geomechanics Society in collaboration with the Sydney Coastal Councils Group, was successful in obtaining funding under the Australian governments’ National Disaster Mitigation Program (NDMP) to further the development of the guidelines which had been recommended by the Taskforce. Work to prepare these guidelines has progressed in 2005 and 2006 and has involved extensive consultation with those involved in landslide mapping for land
use planning and the application of such mapping for planning in local government.This Guideline for Landslide Susceptibility, Hazard and Risk Zoning for Land Use Planning provides:
- Definitions and terminology.
- Description of the types and levels of landslide zoning.
- Guidance on where landslide zoning and land use planning is necessary to account for landslides.
- Definitions of levels of zoning and suggested scales for zoning maps taking into account the needs and objectives of land-use planners and regulators and the purpose of the zoning.
- Guidance on the information required for different levels of zoning taking account the types of landslides.
- Guidance on the reliability, validity and limitations of the investigation methods.
- Advice on the required qualifications of the persons carrying out landslide zoning and advice on the preparation of a brief for consultants to conduct landslide zoning for land use planning.
The guideline considers landslides occurring in natural slopes and from failure of constructed slopes including cuts, fills and retaining walls and the impact of the landslides on the area to be zoned. It is intended for use by local, state and national government officials, geotechnical professionals, land use planners and project managers.
This guideline has been developed at the same time as similar guidelines prepared by the JTC-1 The Joint International Committee on Landslides and Engineered Slopes and there has been an interchange of concepts and detailed inputs between the two guidelines.
Through the NDMP, Australian governments (at Commonwealth, State and Local Government levels) are also funding the development of a Practice Note Guideline (AGS 2007c) to supersede the Landslide Risk Management Guideline (AGS 2000, AGS 2002), and a series of GeoGuides on Slope Management and Maintenance (AGS 2007e).
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Impact From Symbiotic Collaboration Between Industry And Academia In Offshore Geotechnics
Offshore geotechnical engineering is often considered to be ‘industry leading’ with respect to evolving new scientifically based design approaches for foundations and other infrastructure. A high proportion of such advances have originated from collaboration between the offshore industry and academia and, indeed, academic staff in geotechnics at the University of Western Australia have had a particularly significant impact on offshore geotechnical design practice, both locally and internationally, extending over the last 30 years. The nature of interactions with industry and the type of research methodology has varied considerably, ranging from classical doctoral research leading to a major new design approach for a generic problem, to project-specific studies initiated by industry to provide a design basis for particular seabed infrastructure. An example of the former is CPT-based estimation of axial pile capacity in sand, where the UWA approach was incorporated, initially as an alternative to traditional practice but recently as the primary approach, into international guidelines. As a contrast, project-specific studies have often involved physical model tests using the National Geotechnical Centrifuge Facility or the closed O-tube apparatus, to generate data from which to formulate or validate design approaches for a current offshore development. The paper provides examples of these different types of collaboration and their impact on practice, but also discusses the mutual benefits of working with industry, both from a professional perspective for individual academic staff and at the more fundamental level of building and sustaining an economically viable research group.
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Resilient Geotechnics – Past Failures And Future Success
The concept of resilience applied to engineering systems has gained importance in recent years. Geotechnically, all infrastructure assets interact with the ground and so resilient geotechnical solutions must meet a range of plausible conditions including not only stability and serviceability, but increasingly, repairability, growing demands, climate change and impacts from surrounding works.
Resilience may be described as the ability of a system to adjust its functioning in response to changes while satisfying performance, economy and safety objectives. In the infrastructure engineering context, the notion of resilience can apply to fixed assets, but more perhaps more influentially it applies to the organisations that design, construct and operate those assets. This paper documents historic examples of geotechnical and other engineering failures where geotechnical resilience was deficient, and lessons learnt which can be used to increase resilience in future applications. Failures are reviewed in the lights of “traditional” or “linear” safety engineering concepts which include contributing factors such as people, processes and products. The evolution of safety engineering concepts is also examined by looking at improving risk management, design standards and construction processes towards Resilience Engineering (considering both assets and organisations), where risk is actively managed to achieve superior outcomes. Despite these advances over time, recent failures show that some of these lessons must be painfully re-learned; wisdom is difficult to teach.
Looking forward towards to achieving resilience in future infrastructure design, this paper considers global economic, social, and environmental factors which interact with the field of geotechnics, and how this discipline plays a role in creating robust, flexible infrastructure organisations and assets which are safe, secure, and resilient to what the future may hold. Examples of how climate change and changing societal needs may impact projects are discussed alongside future research trends and emerging geotechnical innovations such as Building Information Management (BIM) and performance based design.
Collectively, the failure examples, risk management guidance and Resilience Engineering concepts herein are provided so that geotechnical practitioners can benefit from case history learnings and can apply new tools to future geotechnical engineering challenges. By knowing what to do, what to look for, what to expect and what has happened (historically and in the project timeframe) then safe, reliable and efficient infrastructure can be created.
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GSI Adjustments for Directional Hoek-Brown Strength Calibrated by Step-Path Case Studies
Step-Path methods aim to quantify the negative rock mass shear strength impact of geological defects co-aligned with failure paths through rock slopes and the positive strength impact of intact rock ‘bridges’ between some or all of the co- aligned defects. In effect, Step-Path considers directional strength in rock masses.
Conceptually Step-Path may be reconciled with the Hoek-Brown Method by adjusting the Geological Strength Index (GSI) input to Hoek-Brown equations so that both Step-Path and Hoek-Brown yield the same shear strength outcomes. The GSI adjustment is a two-step process. GSI is first negatively adjusted for the relative portion of the failure path that is defined by geological defects co-aligned with the failure path. The GSI is then positively adjusted for the relative portion of the failure path that is defined by intact rock ‘bridges’ between co-aligned defects. For a ‘general’ rock mass, the adjusted GSI is derived via the following equations:
GSI design = GSI general rock mass – GSI defects adjustment + GSI rock bridges adjustment
Where
GSI general rock mass = GSI rating that would have been conventionally estimated for the ‘general’ rock mass
GSI defect adjustment = 0.4 x co-aligned defect occurrence (%)
GSI rock bridge adjustment = 1.2 x intact rock ‘bridge’ occurrence (%)
The above GSI adjustment factors of 0.4 and 1.2, respectively, are calibrated by reviewing 230 Step-Path case study models developed on projects over the last 20+ years. Key considerations impacting Hoek-Brown rock mass and Barton geological defect shear strengths and their influence on Step-Path strength are flagged. Challenges facing the reconciliation task are identified and discussed. In addition to GSI adjustment factors for the ‘general’ rock mass condition, adjustment factors are also suggested for rock masses partitioned by mi grouping and rock type. Limiting conditions are identified.
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Commentary on Guideline for landslide susceptibility, hazard and risk zoning for land use management
There have been examples of landslide susceptibility and hazard zoning in use since the 1970’s (e.g. Brabb et al., 1972; Nilsen, et al., 1979; Kienholz, 1978). The hazard and risk maps have usually incorporated the estimated frequency of landsliding in a qualitative sense rather than quantitatively. These examples of zoning have generally been used to manage landslide hazard in urban areas by excluding development in some higher hazard areas and requiring geotechnical engineering assessment of slope stability before development is approved in other areas. In some countries landslide susceptibility, hazard and risk maps are being introduced across the country. For example the PPR (Plans de Prevention des Riques Naturels Previsibles) in France and the Cartes de Dangers or Gefahrenkarten in Switzerland are carried out at the Canton level but with Federal funding support (Leroi et al., 2005).
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Geotechnical stability analysis: New methods for an old problem
Geotechnical stability analysis is traditionally performed by a variety of approximate methods that are based on the theory of limit equilibrium. Although they are simple and appeal to engineering intuition, these techniques suffer from a number of serious disadvantages, not the least of which is the need to presuppose an appropriate failure mechanism in advance. This feature can lead to inaccurate predictions of the true failure load, especially for realistic problems involving layered materials, complex loading, or three-dimensional deformation.
A much more rigorous method for assessing the stability of geostructures became available with the advent of the limit (or bound) theorems of classical plasticity in the 1950s. These theorems can be used to give upper and lower bounds on the predicted collapse load (a most valuable property in practice), do not require assumptions to be made about the mode of failure and use only simple strength parameters that are familiar to geotechnical engineers. Although many ingenious bound results have been derived using analytical or numerical methods, practical application of the limit theorems has been restricted by the need to develop specific solution strategies for each problem. Over the last decade, the Newcastle Geotechnical Research Group has developed powerful new methods for performing stability analysis that combine the limit theorems with finite elements and optimisation. These methods are very general and can deal with layered soil profiles, anisotropic strength characteristics, complicated boundary conditions and complex loading in both two and three dimensions. Indeed, they have already been used to obtain new stability solutions for a wide range of practical problems including soil anchors, slopes, foundations under combined loading, excavations, tunnels, mine workings and sinkholes.
This paper gives an outline of the new techniques and considers a number of practical applications. Future research developments will also be highlighted.
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Soil stiffness for shallow foundation design in the Perth CBD
Foundation systems for high-rise structures in the Perth CBD include the whole range of footing types: individual spread footings, single rafts, piles, and piled rafts. Of these, raft foundations are the most common. The design of raft foundations (and indeed all foundation types) relies heavily on calculations of the anticipated total and differential settlements. For these calculations, the most crucial material parameters are the stiffnesses of the soils underlying the foundation. In the Perth CBD, the soil types consist of interbedded layers of dense to very dense sand or fine gravel, and stiff to hard clays, overlying bedrock. In the period since the 1970s, when most of the current high rise structures in the CBD were built, a number of methods of determining the soil stiffness have been used. Very little information is available regarding the actual settlement performance of these structures. However, two important publications from the 1970s provide back-analysed stiffness parameters from the measured performance of 4 moderate rise structures (up to 40 storeys high) and these are regarded as benchmark values. The paper discusses the various methods used in Perth for determining stiffness, both ‘traditional’ and ‘modern’, and the results obtained using these methods are compared to the benchmark values. Data from a number of sites, mostly at the west end of the CBD, are discussed in detail, as a number of insitu test methods for determining stiffness have been used at some of these sites, including seismic CPT, Marchetti dilatometer (DMT) and self-boring pressuremeter (SBP). Some comments are also included about stiffnesses of sands in other parts of the Perth area, compared to the CBD area.
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Towards the development of a new guideline for the design of geosynthetic-reinforced column-supported embankments
Deep cement mixed (DCM) columns with geosynthetic reinforcements are used as integrated foundation systems for construction of embankments over soft soils. Several design guidelines are available in the literature for these embankments based on the soil arching and membrane theories. This paper identifies some inconsistencies in applying these design guidelines, especially the shape of the arches formed and their evolution. A two-dimensional numerical model calibrated using a well-established case study confirmed that soil arches formed within the embankment fill are semi-circular or catenary in shape and the size changes during the construction process. Using the same numerical model and the field measurements from the case study, three different design procedures currently available for the design of geosynthetic reinforced-column supported (GRCS) embankments are investigated. All three design methods yielded uneconomical and over conservative predictions for the geosynthetic reinforcements while giving unsafe predictions for DCM columns. Thereby gaps in current design practice are identified and some future research directions are proposed for the development of better design guidelines for these embankments.
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Shear Strength Of Stockpiled Coking Coal – Insights From Stability Analysis Of Two Instrumented Stockpiles
ACARP Report C4057 (Eckersley, 2000) describes flowslides and other stability issues in stockpiles of coking (metallurgical) coal at Australian coal operations and export terminals, and summarizes 1973 to 2000 research at James Cook University (JCU). Eckersley (2022) partly updated that work with SEEP/W transient seepage modelling of a 12m high coal stockpile constructed at Hay Point in late 1991.
Eckersley (2023) summarized available laboratory strength data for saturated and unsaturated coking coal to assist in selection and critical assessment of parameters for slope stability analyses of coal stockpiles. The current paper explores application of this data to stability analyses of two instrumented experimental stockpiles constructed at Hay Point, one of which collapsed suddenly and completely by flowsliding after extensive wetting. The stability analysis results tentatively confirm that the parameters and approach proposed are reasonable where stockpiles are subject to potential liquefaction-induced collapse.
Significant questions raised by Eckersley (2023) regarding how the coking coal strength data should be applied are considered in the context of the stability analyses. The analyses tentatively confirm that effective strength parameters for saturated coal derived from peak deviator stress in isotropically consolidated, undrained (CIU), strain controlled triaxial tests are reasonable. For loose saturated coal these are at low strains and substantially less than critical state values. However, for unsaturated coal forming the bulk of a stockpile, unsaturated strength and apparent cohesion should be assessed from the effective friction angle at critical state and not the value mobilized at low strains. Use of total stress parameters derived from testing unsaturated coal may over-estimate factor of safety.
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Sydney Symposium 2022
Reliability-based Design: Advances, Innovation and Experiences