Progressive failure in overconsolidated brittle clay. lessons from the failure of a tailings dam - 2016 spring CROSS CANADA LECTURE
by Dr. Antonio Gens of the Technical University of Catalonia (Barcelona), Thursday, May 5
Progressive failure of brittle materials exhibit two very undesirable characteristics: i) they usually occur without warning, and ii) consequences are often catastrophic. Brittle low-permeability materials also involve significant challenges for design such as the selection of the operational strength and the slow dissipation of pore pressures. All those issues are examined in the context of the failure of a tailings dam that caused an important environmental disaster affecting a very large area. The lecture describes in detail the features of the case, the failure event and the subsequent investigation. The forensic investigation led to the unambiguous identification of the mechanism of failure that is consistent with the observations before, during and immediately after the failure.
It was conclusively shown that the main causes underlying the failure were the occurrence of progressive failure in the brittle foundation clay and the presence of very high pore water pressures in the foundation. The mechanism of the failure also explains the large post-failure movements (more than 50 m) that were responsible for the large spill of tailings. The study of the failure was supported by numerical analyses of increasing degree of complexity that provided additional insights into the problem.
The lecture concludes with some general lessons that can be extracted from the case and applied to other problems involving the potential failure of brittle stiff clays.
19 July 1985 Stava tailings dam failure: what can we learn from it?
by Luca Zorzi, Wednesday, April 13
On 19 July 1985 the failure of two tailings dams located just upstream from the village of Stava in the municipality of Tesero (Eastern Alps, Italy) triggering a vast mudflow that flowed down-channel through Stava, a small village of 20 buildings. The mudflow rapidly traveled over 4.2 km along the Stava Valley and passed through Tesero, before flowing into the Avisio River. The flowing mass had an initial volume of 180,000 m3 of mud and water which spilled out of the tailings dams plus nearly 50,000 m3 of additional material resulting from soil erosion, destruction of buildings, and uprooting of trees. The total area affected by the mudflow was about 435,000 m2 along the total stretch of 4.2 km. Along it’s run out path, the flowing mass destroyed 53 houses, 3 hotels, 6 industrial buildings and 8 bridges; 9 buildings seriously damaged; and caused erosional processes over an area of 27,000 m2. 268 people lost their lives, including 59 boys and girls less than 18 years old, 89 men, and 120 women.
In recent years, the Stava 1985 Onlus Foundation has been established aiming to keep the memory of the disaster alive. The duty of the Foundation is to keep alive the historical memory of the Stava Valley and strengthen the culture of prevention, correct territorial management, and safety since shortcomings in these fields were the cause of these and other man-induced disasters. The Foundation is principally active in the field of education and information directed at high school and university students, graduates, technicians and administrators who are in charge of the management of tailings dams and geotechnical structures throughout the explanation of genesis, causes, and responsibilities of this catastrophe.
In this presentation, an overview of the principal causes and conditions that led to the disaster, along with the responsibilities will be presented aiming to highlight what has been learnt and what we still have to learn from this tragedy.
DEBRIS FLOW HAZARD AND RISK ASSESSMENT & LESSONS FOR LIFE AS YOU HEAD INTO PRACTICE: A PERSPECTIVE ON GROUNDWATER MODELS - THE GEOLOGICAL ENGINEERING 2016 DISTINGUISHED LECTURES
by Dr. Oldrich Hungr and Dr. Leslie Smith, Tuesday, March 22, 2016
Dr. Oldrich Hungr: Debris Flow Hazard and Risk Assessment.
The Topic: A brief review will be given of a practical approach towards characterizing debris flow hazards in a watershed, estimating risks and selecting remedial measures. The presentation will focus on debris flow hazards typical of coastal B.C. and will be illustrated by some examples.
The presentation slides for Dr. Hungr are available here, and Dr. Smith here.
Dr. Leslie Smith: Lessons for Life as You Head into Practice: A Perspective on Groundwater Models
The Topic: Over the past decade there has been a substantial increase in the level of detail built into groundwater models submitted in support of environmental assessments and permitting for mining projects. A synthesis of recent case histories suggests: (i) poor choices can still be encountered in defining conceptual models; (ii) powerful modeling tools can create the illusion of knowledge; (iii) models are viewed by some as defining a decision rather than a decision support tool. This presentation suggests some simple guiding principles to consider when developing groundwater models in an environment of rising expectations.
DEVELOPMENT OF ROCK STRENGTH PARAMETERS FOR NUMERICAL MODELLING
by Dariusz Wanatowski, Wednesday, February 3, 2016
European collieries suffer from severe floor and side deformation due to depth, tectonic stress and the soft strata within and beneath the seams which are also vulnerable to degradation over time, particularly when wet. This seminar will summarise findings from the recently completed research project ‘GEOSOFT’ funded by European Commission’s Research Fund for Coal and Steel. The project developed and applied improved means of measuring, representing and analysing this behaviour and its interaction with mine support, both in the laboratory and in the field. Its objectives were to improve our understanding of the phenomena and develop enhanced design and construction solutions utilising stress control, shotcrete, improved reinforcement and/or closed support structural sections. The research partners have come from the UK, Spain and Poland and included industry representatives, consultants and research bodies.
UNDERSTANDING SLOPE STABILITY THROUGH REMOTE SENSING
by Matthew Lato, Wednesday, January 20, 2016
An engineer’s ability to understand the conditions and mechanics of the environment they work in is critical to developing practical solutions. This ability becomes increasingly complicated when working on problems such as landslides where information is traditionally limited to sporadic (and costly) borehole mapping and instrumentation data, discrete surface measurements and observations, and interpolated geophysical data. Over the past ten years, advances in remote sensing technologies have enabled engineers to observe, interpret, and understand the physical environment and its evolution through time, at previously unimaginable levels of detail. This talk will cover the development of remote sensing tools, case studies, and state-of-the-art research and development. Three case studies from BC and Alberta will be presented:
i. understanding the movement of Fountain Slide between 2006 and 2015 through airborne lidar data and how this information is assisting in developing stability options for the site;
ii. identification of pre-failure rock block deformation from a natural slope along the CN railway line in the Fraser Canyon using high resolution ground based lidar data;
iii. mapping potential rock fall source zones and assessing preventative measures for a 150 metre high vertical rock face along a railway line in the Robson Valley using helicopter photogrammetry.
The presentation slides are available here.
VGS-TAC HOLIDAY PARTY
Presentation by Tim Smith, Tuesday December 8, 2015 @ Steamworks (Gastown)
The presentation will draw on Tim's personal experience of surviving the April 2015, Gorka Earthquake in Nepal. It is a unique perspective of what it is like for someone who normally deals with post natural disaster issues, when they are thrown in the middle of a natural disaster. Tim will discuss the earthquake events from a seismic perspective, what it actually feels like from a personal perspective to be in the middle of such a large disaster and the lessons we could learn from this disaster at a personal level as well as at a larger-scale as a community in a seismically vulnerable zone.
Doors at 1730, presentation at 1830. $10 for VGS/TAC members, $15 for non-members. Cover includes one drink and appetizers.
CANADA'S 5TH GENERATION SEISMIC HAZARD MODEL FOR THE 2015 NATIONAL BUILDING CODE
by Trevor Allen, Wednesday, November 25, 2015
Canada’s 5th Generation seismic hazard model has been developed to generate seismic design values for the 2015 National Building Code of Canada (NBCC2015). The model updates the earthquake catalogue, consistently expresses earthquake magnitudes in terms of moment magnitude, revises earthquake source zones, and includes probabilistic treatment of Cascadia and other fault sources, so as to estimate mean ground shaking at the 2%/50-year probability level. Hence it takes advantage of newer knowledge and replaces the 4th Generation 'robust' combination of alternative models used for NBCC2010 by a fully probabilistic model. The ground-motion models (Atkinson and Adams, 2013, Can. J.Civil Eng.) use a “reference suite” approach and represent a major advance over those used for the 4th Generation model. Seismic design values (mean-hazard on Soil Class C at VS30=450 m/s) for PGA, PGV and for Sa(T) for T = 0.2, 0.5, 1.0, 2.0, 5.0, and 10.0 s will be used in NBCC2015. The NBCC2015 specifies new period-dependent soil factors F(T) (replacing amplification factors Fa and Fv) related to a region-dependent PGA on reference rock.
The presentation slides are available here.
RUSKIN DAM UPGRADE
by Nathan Sweeny and Li Yan, Wednesday October 21, 2015
Ruskin Dam is located near Mission, British Columbia and is one of three BC Hydro facilities in the Alouette Stave Ruskin Hydroelectric System. The dam was constructed between 1929 and 1930 and is a concrete gravity structure founded primarily on bedrock. At the right abutment, which consists mainly of glacially deposited sands and silts, the dam connects to a cut-off system consisting of sloping concrete slabs, founded on retaining walls and sheet piles, which extend upstream of the dam. Significant seepage and piping issues occurred at the right abutment after first filling of the reservoir in 1930, and a number of remedial actions were carried out in an attempt to address the problems. Through extensive investigations it was determined that the reoccurring abutment seepage and piping issues, as well as the low seismic withstand of the concrete slab cut-off, posed significant dam safety risks to the facility. A seepage control upgrade project was initiated to address the deficiencies, which resulted in the construction of a new seepage cut-off wall with a special tie-in to the concrete dam, and a reverse filter blanket and drainage system on the downstream slope to collect and measure seepage. Analyses were carried out to evaluate the performance of the proposed upgrade during the design earthquake and to model the seepage regime.
The presentation slides are available here.
Reliability-Based Geotechnical Design: Link between Theory and Practice - 2015 Fall Cross Canada Lecture and annual general meeting
by Dr. Gordon Fenton of Dalhousie University, Thursday, October 1, 2015
Geotechnical design codes are increasingly migrating towards reliability-based design concepts. What this means is that geotechnical designs are starting to be specifically targeted at a failure probability that is societally acceptable and that depends on the severity of failure consequences. For example, the foundation of a hydro-electric dam, whose failure may result in significant downstream damage and potential life-loss, must be designed to have a lower failure probability than the foundation of a storage shed.
In order to properly employ reliability-based design concepts, a basic understanding of the probability concepts, as well as the link between site and model understanding and failure probability, is required. This lecture explains the basic ideas of probability theory and how they are used in modern geotechnical design concepts. Questions, such as "How are standard site investigation results used to estimate the probability of failure of a designed geotechnical system?” are addressed and illustrated using a number of examples.
The presentation slides are available here.