A simple method to analyze infiltration into unsaturated soil slopes
- Authors: Gavin, Kenneth , Xue, Jianfeng
- Date: 2008
- Type: Text , Journal article
- Relation: Computers and Geotechnics Vol. 35, no. 2 (2008), p. 223-230
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- Description: Assessment of the stability of embankment and cut slopes over the life of a project are critical issues for railway and motorway infrastructure projects. Experience has shown that many slope failures occur during or shortly after rainfall. Analyses show that failure is initiated by the reduction in near surface suction over some critical depth in the slope. A simple method is proposed in this paper to estimate the time needed for a wetting front to develop. The method which is a modification of the traditional Green–Ampt infiltration model assumes that ponding of water cannot occur on soil slopes and as a consequence soil in the wetted zone remains partially saturated at the point of slope failure. It differentiates between cases where the initial suction in the slope is high and the rate of infiltration is controlled by the rainfall intensity (supply controlled) and, situations where the suction is low, and the rate of infiltration is controlled by the infiltration capacity of the soil (demand controlled). When applied to a case history where field measurements of infiltration into a slope were available the new method provided a reasonable approximation of the measured infiltration time.
Effect of rainfall intensity on infiltration into partly saturated slopes
- Authors: Xue, Jianfeng , Gavin, Kenneth
- Date: 2008
- Type: Text , Journal article
- Relation: Geotechnical and Geological Engineering Vol. 26, no. 2 (2008), p. 199-209
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- Description: This paper describes the development of a model to analyse the rate of infiltration and run-off experienced by a partly saturated soil slope during rainfall. The paper first reviews some of the most popular infiltration models used in geotechnical analysis, and highlights some of the problems associated with their application. One particular model, the Horton Equation is extended to include rainfall intensity directly in its formulation. The new model is shown to predict infiltration responses, which agree with field measurements. In the final section the influence of the rainfall intensity and pattern of rainfall (variation of rainfall intensity) on the infiltration response of a soil is investigated using the new model.
Use of a genetic algorithm to perform reliability analysis of unsaturated soil slopes
- Authors: Gavin, Kenneth , Xue, Jianfeng
- Date: 2009
- Type: Text , Journal article
- Relation: Geotechnique Vol. 59, no. 6 (2009), p. 545-549
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- Description: This paper describes the development of a probabilistic design approach to analyse the stability of unsaturated soil slopes. Instability of unsaturated soil slopes is caused by the downwards progression of a wetting front, which reduces the near-surface suctions, increases the weight of soil in the failure zone and reduces the soil strength. Assuming the slip surface forms parallel to the slope surface (at a depth equal to the wetting front depth) the probability of failure is determined in this paper through the reliability index determined using a computationally efficient genetic algorithm method.
Deterministic and probabilistic multi-modal analysis of slope stability
- Authors: Reale, Cormac , Xue, Jianfeng , Pan, Zhangming , Gavin, Kenneth
- Date: 2015
- Type: Text , Journal article
- Relation: Computers and Geotechnics Vol. 66, no. (2015), p. 172-179
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- Description: Traditional slope stability analysis involves predicting the location of the critical slip surface for a given slope and computing a safety factor at that location. However, for some slopes with complicated stratigraphy several distinct critical slip surfaces can exist. Furthermore, the global minimum safety factor in some cases can be less important than potential failure zones when rehabilitating or reinforcing a slope. Existing search techniques used in slope stability analysis cannot find all areas of concern, but instead converge exclusively on the critical slip surface. This paper therefore proposes the use of a holistic multi modal optimisation technique which is able to locate and converge to multiple failure modes simultaneously. The search technique has been demonstrated on a number of benchmark examples using both deterministic and probabilistic analysis to find all possible failure mechanisms, and their respective factors of safety and reliability indices. The results from both the deterministic and probabilistic models show that the search technique is effective in locating the known critical slip surface while also establishing the locations of any other distinct critical slip surfaces within the slope. The approach is of particular relevance for investigating the stability of large slopes with complicated stratigraphy, as these slopes are likely to contain multiple failure mechanisms. © 2015 Elsevier Ltd.
Design charts for the stability analysis of unsaturated soil slopes
- Authors: Gavin, Kenneth , Xue, Jianfeng
- Date: 2010
- Type: Text , Journal article
- Relation: Geotechnical and Geological Engineering Vol. 28, no. 1 (2010), p. 79-90
- Full Text: false
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- Description: Simple limit equilibrium analyses can be performed to determine the Factor of Safety (FOS) against slope failure of unsaturated soil slopes. However, many of the input parameters needed for these analyses are highly variable, and the FOS value obtained is critically dependent on assumptions made by the designer. This paper describes a suite of reliability analyses on unsaturated soil slopes performed using an invariant reliability model. The results are presented in design charts from which a designer can choose the FOS value required to ensure a given target reliability index for a slope. The approach ensures that despite the variability of input parameters the slope will have a probability of failure of 2.23% or less.
Using reliability theory to assess the stability and prolong the design life of existing engineered slopes
- Authors: Reale, Cormac , Xue, Jianfeng , Gavin, Kenneth
- Date: 2017
- Type: Text , Conference paper
- Relation: Geotechnical Safety and Reliability: Honoring Wilson H. Tang; Dever, United States; 4th-7th June 2017; published in Geotechnical Special Publications Journal p. 61-81
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- Description: Modern engineered slopes are designed to exceed certain safety targets set out in design codes. This is in stark contrast to earthen infrastructure inherited from the 18th century which typically was constructed in a haphazard manner without design. This infrastructure seldom meets modern deterministic guidelines yet clearly exhibits some degree of safety, as a failure has not occurred in the intervening years. This paper highlights the use of reliability theory for evaluating the stability of existing engineered slopes. A comprehensive review of geotechnical uncertainty and existing reliability based techniques are outlined. Furthermore, the paper highlights the issue of finding the critical slip surface and gives a brief summary of the current state of the art. Finally a case study of an Irish railway embankment is presented and both a deterministic and reliability analysis is performed on it highlighting the benefits of probabilistic methods over traditional techniques.
- Description: Modern engineered slopes are designed to exceed certain safety targets
System reliability of slopes using multimodal optimisation
- Authors: Reale, Cormac , Xue, Jianfeng , Gavin, Kenneth
- Date: 2016
- Type: Text , Journal article
- Relation: Geotechnique Vol. 66, no. 5 (2016), p. 413-423
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- Description: Many engineered and natural slopes have complex geometries and are multi-layered. For these slopes traditional stability analyses will tend to predict critical failure surfaces in layers with the lowest mean strength. A move toward probabilistic analyses allows a designer to account for uncertainties with respect to input parameters that allow for a more complete understanding of risk. Railway slopes, which in some cases were built more than 150 years ago, form important assets on the European rail network. Many of these structures were built at slope angles significantly higher than those allowed in modern design codes. Depending on the local geotechnical conditions these slopes may be susceptible to deepseated failure; however, a significant number of failures each year occur as shallow translational slips that develop during periods of high rainfall. Thus, for a given slope, two potential failure mechanisms might exist with very similar probabilities of failure. In this paper a novel multimodal optimisation algorithm (‘Slips’) that is capable of detecting all feasible probabilistic slip surfaces simultaneously is presented. The system reliability analysis is applied using polar co-ordinates, as this approach has been shown to be less sensitive to local numerical instabilities, which can develop due to discontinuities on the limit state surface. The approach is applied to two example slopes where the complexity in terms of stratification and slope geometry is varied. In addition the methodology is validated using a real-life case study involving failure of a complex slope. © 2016 ICE Publishing. All rights reserved.
Multi-modal reliability analysis of slope stability
- Authors: Reale, Cormac , Gavin, Kenneth , Prendergast, Luke , Xue, Jianfeng
- Date: 2016
- Type: Text , Conference proceedings
- Relation: 6th Transport Research Arena; Warsaw, Poland; 18th-21st April 2016; published inTransportation Research Procedia Vol. 14, p. 2468-2476
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- Description: Probabilistic slope stability analysis typically requires an optimisation technique to locate the most probable slip surface. However, for many slopes particularly those containing many different soil layers or benches several distinct critical slip surfaces may exist. Furthermore, in large slopes these critical slip surfaces may be located at significant distances from each other. In such circumstances, finding and rehabilitating the most probable failure surface is of little merit, as rehabilitating that surface does not improve the safety of the slope as a whole. Unfortunately, existing slip surface search techniques were developed to converge on one global minimum. Therefore, to implement such methods to evaluate the stability of a slope with multiple failure mechanisms requires the user to define probable slip locations prior to calculation. This requires extensive engineering experience and places undue responsibility on the engineer in question. This paper proposes the use of a locally informed particle swarm optimisation method which is able to simultaneously converge to multiple critical slip surfaces. This optimisation model when combined with a reliability analysis is able to define all areas of concern within a slope. A case study of a railway slope is presented which highlights the benefits of the model over single objective optimisation models. The approach is of particular benefit when evaluating the stability of large existing slopes with complicated stratigraphy as these slopes are likely to contain multiple viable slip surfaces. © 2016 The Authors.
Geotechnical Challenges for the European TEN-T Network – SMARTRail and Beyond
- Authors: Gavin, Kenneth , Reale, Cormac , Xue, Jianfeng
- Date: 2014
- Type: Text , Conference paper
- Relation: 3rd International Conference on Road and Rail Infrastructure – CETRA 2014 p. 21-34
- Full Text: false
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- Description: Climate change is leading to increased incidence of slope instability across the globe. As well as causing a risk to human life, slope failures can cause severe disruption to transport networks. Infrastructure managers who control road and rail networks need to manage risks associated with earth slopes, many of which were built before the advent of modern design and construction standards. This paper discusses two key steps in the slope management process: (i) locating potential failures and (ii) rational analysis of slopes. Recent work completed in the EU FP7 SMARTRAIL project is described and a case study is presented demonstrating how the potential damaging effects of rainfall can be considered in a rational framework.