Renato Macciotta

Probabilistic estimation of rockfall height and kinetic energy based on a three-dimensional trajectory model and Monte Carlo simulation

Railways across the Canadian Cordillera have long histories of losses associated with ground hazards. The hazards most frequently reported are rockfalls, which are ubiquitous along the steep rock cuts required to accommodate the railway alignment. Several hazard control measures can be adopted in rockfall areas. However, when rockfall frequencies cannot be controlled, protective structures may be necessary to decrease rockfall-related risks to tolerable levels. Designs of protective structures require knowing rockfall trajectory heights and kinetic energies. This information is difficult to obtain even at locations where comprehensive rockfall records are kept. We present a method to calculate rockfall trajectory heights and velocities based on three-dimensional, lumped mass, rockfall simulations. Rockfall source location, model parameters and model calibration are also discussed. In this regard, the model should be calibrated against observed values of rockfall heights and velocities, and the design parameters should be validated before proceeding with the design of rockfall mitigation measures. The method is illustrated with the analysis of a section of a railway along the Canadian Cordillera. Furthermore, a probabilistic approach is adopted to calculate rockfall trajectory heights and velocities when intersecting the railway alignment. This is consistent with the natural variability of rockfall trajectories and falling block volumes. We illustrate the use of probability distributions of rockfall velocities and volumes to calculate the distribution of kinetic energy at three locations along the study section. The calculated rockfall trajectory heights are also presented in probabilistic terms and discussed. The rockfall kinetic energy distributions are used to assess the type of protective structures that could be required for further reduction of risk levels.

Quantitative risk assessment of slope hazards along a section of railway in the Canadian Cordillera—a methodology considering the uncertainty in the results

Railway alignments through the Canadian Cordillera are constantly exposed to slope instabilities. Proactive mitigation strategies have been in place for a few decades now, and instability record keeping has been recognized as an important aspect of them. Such a proactive strategy has enhanced the industry’s capacity to manage slope risks, and some sections have been recognized as critical due to the frequency of instabilities. At these locations, quantification of the risks becomes necessary. Risk analysis requires knowledge of some variables for which statistical data are scarce or not available, and elicitation of subjective probabilities is needed. A limitation of such approaches lies in the uncertainty associated to those elicited probabilities. In this paper, a quantitative risk analysis is presented for a section of railway across the Canadian Cordillera. The analysis focused on the risk to life of the freight train crews working along this section. Upper and lower bounds were elicited to cope with the uncertainties associated with this approach. A Monte Carlo simulation technique was then applied to obtain the probability distribution of the estimated risks. The risk probability distribution suggests that the risk to life of the crews is below previously published evaluation criteria and within acceptable levels. The risk assessment approach proposed focuses on providing a measure of the uncertainty associated with the estimated risk and is capable of handling distributions that cover more than two orders of magnitude.

Rock fall hazard control along a section of railway based on quantified risk

ABSTRACT Rock falls represent a large percentage of landslide-related hazards reported by Canadian railways in mountainous terrain. A 54.7 km-long section of railway through the Canadian Cordillera is examined that experiences, on average, 18 rock falls each year. An approach for rock fall hazard management is developed based on quantified risk. The approach focuses on defining railway operation procedures (freight train operations and track maintenance) that comply with quantified risks. Weather-based criteria that define periods when rock falls are more likely to occur along the study area are examined. These criteria are used herein to reduce exposure to rock falls and reduce their consequences. Several freight train operation strategies are proposed that comply with a tolerable level of risk adopted in this study for illustrative purposes. The approach provides a simple, flexible and practical strategy for railway operations that can be regularly adopted by the operators, and that is based on a more comprehensive assessment of quantified risk.

Quantifying weather conditions for rock fall hazard management

Relationships between weather conditions and rock fall occurrences have been acknowledged in the past, but seldom have such relationships been quantified and published. Rock falls are frequent hazards along transportation corridors through mountainous terrain, and predicting hazardous rock fall periods based on weather conditions can enhance mitigation approaches. We investigate the relationship between weather conditions and rock fall occurrences along a railway section through the Canadian Cordillera. Monthly weather-rock fall trends suggest that the seasonal variation in rock fall frequency is associated with cycles of freezing and thawing during the winter months. The intensity of precipitation and freeze–thaw cycles for different time-windows was then compared against recorded rock falls on a case-by-case approach. We found that periods when 90% of rock falls occurred could be predicted by the 3-day antecedent precipitation and freeze–thaw cycles. Some rock falls not predicted by this 3-day antecedent approach occurred during the first two weeks of spring thaw. These findings are used to propose a rock fall hazard chart, based on readily available weather data, to aid railway operators in their decision-making regarding safe operations.

Combining Track Quality and Performance Measures to Assess Track Maintenance Requirements

The serviceability of a section of railway highly depends on track stiffness and roughness. Railway operators regularly measure parameters associated with track stiffness and roughness to evaluate the track conditions. These measures are used in combination with performance observations to assess maintenance requirements. Although these assessments are mostly qualitative, railway operations have benefited from them. Railway operators keep comprehensive records of different types of track defects along their lines. These records are a measure of track performance and present an opportunity to quantify the relationship between track quality and performance. This brings the possibility of developing a performance-based approach for assessing the maintenance requirement along a railway track.In this paper, a database of track geometry defects along Canadian National Railway’s Lac la Biche subdivision (Alberta) has been compared against measured parameters associated with track roughness and stiffness. The analyses confirm the relationship between track stiffness and roughness, and the occurrence of track defects. This relationship is further used to define threshold values of track roughness and stiffness, and a hazard chart for maintenance requirements along the Lac la Biche subdivision is proposed.Copyright

The 10-Mile Slide and Response of a Retaining Wall to Its Continuous Deformation

Open image in new window The 10-mile Slide has a volume of about 750,000 m3 and is sliding on a through-going shear surface at velocities up to 10 mm/day. Its importance is associated with the location of a highway and a railway line within its boundaries. Risks posed to the railway were managed through monitoring and running patrols in front of trains, and a pile retaining wall was installed immediately downslope from the tracks to prevent deformations caused by loosening of materials associated with the slope deformations and delay the retrogression of the landslide. Displacement measurements of the piles have monitored the response of the wall as the landslide retrogressed upslope from the railway track. This paper presents a brief description of the 10-mile Slide geologic context, its kinematics, mechanism, and evolution followed by a presentation of measured response of the retaining wall as the landside retrogressed.

Development and application of a quantitative risk assessment to a very slow moving rock slope and potential sudden acceleration

The benefits of quantitative risk assessments for landslide management have been discussed and illustrated in several publications. However, there still are some challenges in its application for low-probability, high-magnitude events. These challenges are associated with the difficulties in populating our models for risk calculations, which largely require the input of expert opinion. This paper presents a quantitative risk assessment to a very slow moving rock slope within a dam reservoir in the Province of British Columbia, Canada. The assessment is focused on the risk to the population in the vicinity of the dam and the populated areas downstream. Expert opinions quantified the slope failure probabilities in the order of 10−3 to 10−1 per year for the smallest failure scenario considered and less than 10−6 for a failure of the entire slope. However, these estimations are associated with high levels of uncertainty. Our approach starts with the calculation and assessment of the magnitude and probability of the potential slope failure consequences, minimizing the uncertainties associated with estimated slope failure probabilities. Then, these consequences and failure probabilities are combined to obtain a measure of risk. The uncertainty associated with the slope failure probabilities is managed by the estimation of plausible ranges for these. The calculated risk levels are then presented as ranges of values and assessed against adopted evaluation criteria. The consequence and risk assessment of the rock slope suggest that the risk to the population exposed in the vicinity of the dam and populated areas downstream is under adequate control. The probability of large consequence scenarios is extremely low, in the order of 10−7 chance of an event causing more than 100 fatalities. We propose an observational technique to assess changes in risk levels and decide when to update the risk management approach or deploy emergency measures. The technique is focused on the detection of changes in the slope deformation patterns that would indicate an increase in the potential failure volumes or an imminent failure. It can be considered an extension to the current early warning system in place, easy to implement and enhanced with the strength of the comprehensive analysis required for a quantitative risk assessment.

Managing uncertainty in underground excavation design associated with rock mass characterization using core drilled for mineral exploration

Design of underground mining operations, in their preliminary stages, often requires obtaining geomechanical data from re-logged rock core initially drilled for mineral exploration. The degree of alteration of this core due to handling and storage time increases uncertainty in the geotechnical parameters assessed and in the estimated required excavation support. Uncertainty is primarily related to changes in observed joint infill characteristics during storage. This paper presents a simple approach to assess this uncertainty quantitatively, illustrated with a case study in the Peruvian Cordillera. The Q-System was adopted for such assessment and to estimate the required excavation support. From results of the case study presented in this paper, uncertainty associated with the use of stored core was not considered significant when compared to the natural variability of the rock mass. Numerical simulations of excavation geometries for different rock mass qualities were performed to validate estimated support requirements through the Q-System. These simulations required estimations of strength characteristics of the rock mass and discontinuities. These were estimated from measurements in all core available (old and fresh) and considered the uncertainty in the adopted strength envelopes quantitatively. The range of model results and sensitivities to effects of modeling validated the recommended support requirements. During excavation, support requirements would be based on an assessment of Q as the excavation proceeds (an observational approach).

Framework for developing risk to life evaluation criteria associated with landslides in Canada

The application of quantitative risk assessments is increasing for decision-making in many industries and contexts, with the evaluation of risks against some adopted criteria. In this article, we review risk criteria developed and used for landslide management, in particular criteria associated with risk to life. We show that while this natural hazard is encountered worldwide, the social and regulatory contexts under which evaluation criteria are developed can vary significantly. Thus, the applicability of developed criteria to any specific situation should be assessed before adopting them elsewhere. We describe selected considerations for developing risk evaluation criteria, propose a framework for defining these criteria in Canada, and assess the applicability of previously proposed criteria. Examples of risk criteria development and adoption for new and existing residential developments and for railway employees are presented to illustrate some of these concepts.

Quantitative relationship between weather seasonality and rock fall occurrences north of Hope, BC, Canada

The relationship between rock falls and weather conditions has been widely recognized and attempts have been made to develop weather-based approaches for rock fall hazard management. This dependency of rock fall occurrences on weather suggests that rock fall trends and their associated risks will vary following climatic changes. In this regard, tools that quantify the relationship of weather seasonality and climate with rock fall trends provide an opportunity to forward model potential variations in rock fall trends considering diverse climate scenarios. This paper illustrates the application of one such tool along a section of a transportation corridor through the Canadian Cordillera. von Mises probability distributions are fitted to monthly trends of precipitation and freeze–thaw cycles and combined to develop a probability density model of rock fall occurrences. The methodology is outlined in detail and the model shown to fit the rock fall database with a correlation coefficient of 0.97. Further, the paper discusses limitations of the approach and potential opportunities for improvement, encouraging the use of the method at other sites and building a robust case study database for further enhancement of the approach.