Suzanne Lacasse

Determination of the Transitional Fines Content of Mixtures of Sand and Non-Plastic Fines

Sand-silt mixtures show different behavior with different fines content. The transitional fines content (TFC) is a key parameter which can indicate if the soil behaves as a sand-dominated or as a fines-dominated material. The determination of the TFC for a sand-silt mixture and the possible ways of obtaining the TFC from index data and the triaxial test results are discussed in this study.

Landslide Risk Assessment and Mitigation Strategy

Each year, natural disasters cause countless deaths and formidable damage to infrastructure and the environment. In 2004–2005, more than 200 000 people lost their lives in natural disasters. Material damage was estimated at USD 300 billion. Many lives could have been saved if more had been known about forecasting and mitigation. The need to improve the ability to deal with the hazards and risks was accentuated by increased sliding and flooding in many regions around the world in recent years, concern for their disastrous consequences on mankind, infrastructure and material property and the catastrophic Indian Ocean tsunami in December 2004.

Evolution of Mass Movements near Epicentre of Wenchuan Earthquake, the First Eight Years

It is increasingly clear that landslides represent a major cause of economic costs and deaths in earthquakes in mountains. In the Wenchuan earthquake case, post-seismic cascading landslides continue to represent a major problem eight years on. Failure to anticipate the impact of cascading landslides could lead to unexpected losses of human lives and properties. Previous studies tended to focus on separate landslide processes, with little attention paid to the quantification of long-term evolution of multiple processes or the evolution of mass movements. The very active mass movements near the epicentre of the Wenchuan earthquake provided us a unique opportunity to understand the complex processes of the evolving cascading landslides after a strong earthquake. This study budgets the mass movements on the hillslopes and in the channels in the first eight years since the Wenchuan earthquake and verify a conservation in mass movements. A system illustrating the evolution and interactions of mass movement after a strong earthquake is proposed.

Runout of Landslides in Sensitive Clays

An essential part of landslide hazard and risk assessment is the estimate of the runout distance of the landslide masses. There is, however, little guidance available today on the estimation of the landslide runout in sensitive clays and no suitable model exists for predicting runout in sensitive clays. A new empirical model for the runout estimation is presented in this paper. The new model is based on empirical data, and is recommended for use in Norway until further research on analytical models becomes available. The recommended empirical procedure is based on the historical landslides in sensitive clays in Norway. The paper discusses the implementation of the proposed empirical models in a calculation tool called GeoSuite Toolbox as a part an ongoing R&D project GeoFuture II.

Hazard and Risk Assessment of Landslides

The evaluation of the safety of natural or man-made slopes involves uncertainties. With a deterministic safety factor, the geotechnical engineer tries to deal with the uncertainties by choosing reasonably conservative parameters for stability evaluation. The deterministic approach fails to deal consistently with uncertainties. As a complement to the deterministic approach, a probabilistic approach that allows for hazard and risk assessment of slopes is recommended. The paper proposes a framework integrating existing methods to exploit the benefits of the two approaches. Societal aspects, including allowable and tolerable risk, are briefly considered. The need for input on the social dimension of landslides, and the role of risk assessment in easing communication among the different experts are also discussed.

Probabilistic equivalent linear soil spring stiffness analysis for gravity platforms: Conceptual model

Abstract Horizontal and rotational soil spring stiffnesses of the foundation are needed (a) to perform the dynamic analysis of gravity platforms and (b) to obtain characteristic wave loads to be used in the geotechnical design analysis of platforms. Dynamic analyses are especially important in the design of deepwater platforms. A probabilistic procedure in the context of the first two moments is suggested to estimate the equivalent linear soil spring stiffnesses of the foundation for gravity platforms on clay loaded under undrained conditions. In calculating the first two moments of the soil spring stiffnesses, consideration is given to the influence of the uncertainties in the loads and soil properties, and the model errors encountered in applying the deterministic procedure. Limitations in the available data, the current analytical procedures, and in the probabilistic analysis are discussed. The companion paper [1] provides an illustrative example on the same topic.

Reliability of Slopes in Sensitive Clays

Risk and probabilistic analyses have now had enough applications that make them effective to use in practice. The approach provides more insight than deterministic analyses alone. They help reduce uncertainty and focus on safety and cost-effectiveness. The paper illustrates the use of reliability methods for the analysis of slopes in sensitive clays with examples of the calculation of probability of failure and run-out for the Finneidfjord and Rissa landslides in Norway. The input, model and results of the probabilistic slope analysis are described, including the uncertainties in the parameters, triggers and calculation model, as well as a brief review of the principles of the reliability approach. Reliability approaches do not remove uncertainty nor do they alleviate the need for judgment. They provide a way to quantify the uncertainties and to handle them consistently. Site investigations, laboratory test programs, limit equilibrium and deformation analyses, instrumentation, monitoring and engineering judgment are necessary inputs to the reliability approach. Landslide events, often unwittingly, are triggered or aggravated by human activity, such as change in topography (e.g. excavation or surcharge) and change in drainage conditions. Climate change can increase the frequency of landslide. The paper proposes that a probabilistic model in an event tree format should be included to ensure that all failure modes and the uncertainties have been covered and that slope failure mitigation measures are quickly available.

Reliability of API and ISO Guidelines for Bearing Capacity of Offshore Shallow Foundations

The safe bearing capacity for offshore shallow foundations has been traditionally assessed using working stress design (WSD) methods (e.g. the API RP 2GEO guideline). Other codes of practice such as the ISO standard strive to provide designs achieving a desired target reliability level in the form of the Load and Resistance Factor Design (LRFD) approach. This study compares the levels of safety achieved for offshore shallow foundations. Calculations are made for one foundation on soft clay and one on medium dense sand, using the API RP 2GEO, API RP 2GEO-LRFD and ISO 19901-4 design guidelines. Three probabilistic models were used, the first-order, second moment (FOSM) approximation, the first order reliability method (FORM) and the Monte Carlo simulation (MC) approach, to do the reliability assessment. The results showed that the reliability level achieved with current practice varies with the design methods. The FORM and MC models yielded consistent results, while the FOSM model yielded inconsistent results when the performance function was non-linear.

Design Parameters for Offshore Sands: Use of In Situ Tests

Traditionally, North Sea sands have been tested by laboratory tests on reconstituted samples and by in situ cone penetration tests. A number of other in situ test devices are already available for offshore use or will most likely be available in the near future. These include the piezocone, the pressure-meter, the dilatometer, nuclear density and electrical resistivity probes, the seismic cone and the screw plate.

Stress Testing Framework for Managing Landslide Risks Under Extreme Storms

Extreme storms in the changing climate may trigger a large number of landslides and debris flows. The consequences can be catastrophic but their occurrence probability is rather low. Stress testing is recognized as an efficient tool for managing the risks of low-frequency high-consequence hazards. This paper presents a stress-testing framework for coping with the landslide risks under extreme rainstorms. Stress testing is a targeted reassessment of safety margins of a system under extreme events. It involves testing beyond normal operational capacity, often to a breaking point. The proposed stress testing framework include (1) identifying future critical rainstorm scenarios under the changing climate; (2) evaluating the response of the slope safety system to the critical rainstorm scenarios; (3) assessing the risks posed by the multi-hazard processes; (4) evaluating the bottlenecks of the slope safety system and (5) proposing strategies for improving system performance. A preliminary stress test was performed to evaluate the response of Hong Kong Island to four storms corresponding to 29, 44, 65 and 85% of the 24-h probable maximum precipitation. The testing shows that extreme storms can pose high risks, which require suitable policies and strategies for multi-hazard risk management.