Actual Failure Surface Research Articles

High fill slope collapse: Stability evaluation based on finite element limit analyses

The stability of high fill slopes, which are typically encountered when building airports in mountainous areas, is influenced by complex geotechnical profiles, the fill strength, reinforcement, and rainfall conditions. On 3 October 2009, a catastrophic landslide occurred at Panzhihua Airport in Sichuan Province, China. Investigations indicated that this event was attributable to the presence of carbonaceous mudstone. However, the overall stability of the high fill slope in different construction phases has not been explored. In this study, finite element limit analyses were performed to examine the overall stability of a critical section of the high fill slope in 11 cases that differed in terms of their strength parameters, reinforcement, and rainfall conditions. The results indicate that the original slope stability determined using parameters based on the laboratory shear tests of fill materials was overestimated. Furthermore, the original slope reinforcement with three rows of stabilising piles was ineffective. The safety factor of the pile-reinforced slope was computed to be 0.95 when rainfall infiltration was considered, and the predicted slip surface coincided with the actual failure surface. Random field limit analyses of this slope indicated that the failure probability increased under the assumed rainfall infiltration conditions. Remediation plans could ensure stability against landslides. Overall, reliable parameters and methods must be used to calculate the stability of high fill slopes.

Limit Equilibrium Analysis of Landfill Instability Based on Actual Failure Surface

Slope stability is one of the key engineering problems in the whole lifetime of landfills. In this paper, combined with the wedge limit equilibrium analysis, a landfill stability analysis method based on the actual failure surface is proposed, and the model is verified according to the data of centrifuge model tests. It is found that this method can more accurately calculate the factor of safety (FS) of the slope of the landfill and evaluate the stability of the slope. Finally, sensitivity analysis was conducted to evaluate the effects of the actual failure surface angle, water level, soil parameters, and the presence or absence of a dam on the factor of safety.

Reconstruction of a wetting-induced shallow landslide in Shillong, India

The hill slopes of the north eastern region of India often encounters shallow landslide during the rainy season. Forensic study of a failed hill-slope in such areas is useful in improving the understanding of the failure mechanism. An incessant rainfall triggered a shallow landslide in the Umshing Mawkynroh locality of Shillong in North-east India during the rainy season of 2019. In this work, a forensic investigation was carried out by performing a coupled unsteady-state seepage and stability modelling to reconstruct the actual failure surface. The recorded rainfall data, slope geology, and soil hydraulic properties of the compacted laterite soil were used to back-estimate accurately the in situ failure surface. The underlying mechanism responsible for the shallow landslide during the rainfall infiltration was identified from the numerical modelling. The combined effect of wetting front advancement in the initial partial saturation zone and the rise of the perched groundwater table was found to be the mechanism responsible for the failure.

Ground thermal and geomechanical conditions in a permafrost-affected high-latitude rock avalanche site (Polvartinden, northern Norway)

Abstract. On 26 June 2008, a rock avalanche detached in the northeast facing slope of Polvartinden, a high-alpine mountain in Signaldalen, northern Norway. Here, we report on the observed and modelled past and present near-surface temperature regime close to the failure zone, as well as on a subsequent simulation of the subsurface temperature regime, and on initial geomechanical mapping based on laser scanning. The volume of the rock avalanche was estimated to be approximately 500 000 m3. The depth to the actual failure surface was found to range from 40 m at the back of the failure zone to 0 m at its toe. Visible in situ ice was observed in the failure zone just after the rock avalanche. Between September 2009 and August 2013, ground surface temperatures were measured with miniature temperature data loggers at 14 different localities, close to the original failure zone along the northern ridge of Polvartinden and on the valley floor. The results from these measurements and from a basic three-dimensional heat conduction model suggest that the lower altitudinal limit of permafrost at present is at 600–650 m a.s.l., which corresponds to the upper limit of the failure zone. A coupling of our in situ data with regional climate data since 1958 suggests a general gradual warming and that the period with highest mean near surface temperatures on record ended four months before the Signaldalen rock avalanche detached. A comparison with a transient permafrost model run at 10 m depth, representative for areas where snow accumulates, strengthen these findings, which are also in congruence with measurements in nearby permafrost boreholes. It is likely that permafrost in and near the failure zone is presently subject to degradation. This degradation, in combination with the extreme warm year antecedent to the rock failure, is seen to have played an important role in the detaching of the Signaldalen rock avalanche.

Revisiting Nigawa landslide of the 1995 Kobe earthquake

A simplified pseudo-static limit equilibrium approach is used to analyse a full-scale slope failure that occurred during the 1995 Kobe earthquake. The slope is 28 m high, having a gentle angle of 26·5°, and has a factor of safety as high as 2·43 in the absence of an earthquake. In addition to a factor of safety, the critical failure surface obtained from pseudo-static analysis is compared with that of the actual failure surface. Contradictory to some design codes that use only a horizontal seismic coefficient, this study finds that a rational analysis must take into consideration both horizontal and vertical seismic accelerations. The analysis including vertical acceleration may give a shallower or deeper failure surface than cases where vertical acceleration is ignored. An analysis combining horizontal and upward accelerations results in a deeper failure surface than when only downward or no vertical acceleration is considered. The effects of soil properties on the slope stability are also investigated. The study suggests that for near-fault earthquakes, slopes may be subjected to vertical (upward) seismic coefficient as large as 90% that of horizontal, larger than that specified in existing design codes.

Discussion of “Shear Strength in Preexisting Landslides” by Timothy D. Stark and Manzoor Hussain

Discussion of “Shear Strength in Preexisting Landslides” by Timothy D. Stark and Manzoor Hussain

Closure to “Shear Strength in Preexisting Landslides” by Timothy D. Stark and Manzoor Hussain

Closure to “Shear Strength in Preexisting Landslides” by Timothy D. Stark and Manzoor Hussain

Advanced response surface method for mechanical reliability analysis

Based on the classical response surface method (RSM), a novel RSM using improved experimental points (EPs) is presented for reliability analysis. Two novel points are included in the presented method. One is the use of linear interpolation, from which the total EPs for determining the RS are selected to be closer to the actual failure surface; the other is the application of sequential linear interpolation to control the distance between the surrounding EPs and the center EP, by which the presented method can ensure that the RS fits the actual failure surface in the region of maximum likelihood as the center EPs converge to the actual most probable point (MPP). Since the fitting precision of the RS to the actual failure surface in the vicinity of the MPP, which has significant contribution to the probability of the failure surface being exceeded, is increased by the presented method, the precision of the failure probability calculated by RS is increased as well. Numerical examples illustrate the accuracy and efficiency of the presented method.

On the Active Failure Surface in the Backfilled Clay behind Rigid Retaining Wall in Slope Engineering

In the classical Coulomb’s earth pressure theory, the failure surface in the backfilled clay behind rigid retaining wall in slope engineering is assumed a plane. However, it has been proved by a number of laboratory and field tests that this failure surface is actually a curving surface. In this paper, based on the vertical differential element method and the variational principle, a new analytic solution to determine the actual failure surface in the backfilled clay is derived, and the effects of the backfilled clay’s properties as well as the effects of the retaining wall’s smoothness are discussed. The result obtained from the proposed approach is compared with Coulomb’s earth pressure theory.

Discussion of "Bearing capacity prediction of spatially random c – ϕ soils"

Popescu The writer would like to commend the authors for a particularly interesting analytical and numerical study of the effect of spatial variability of soil properties on the bearing capacity. It is generally acknowledged that the problem of accounting for the stochastic spatial variation of soil properties is a particularly complex one, both from the datacollection point of view and from the methodological point of view. Generally accepted techniques have not yet been established to accomplish these tasks. To initiate a constructive discussion on this subject, some comments are provided here on the selected range of coefficients of variation (COVs) of soil properties and on the methodology suggested by the authors to simulate the stochastic soil properties. Range of coefficients of variation Confidence in design soil properties, and therefore in predicted structural response, is affected by a series of uncertainties arising from several sources, such as intrinsic spatial variability (i.e., actual variation of soil properties from one location to another), measurement errors, and insufficient field information. Those sources of uncertainty are often considered together as one source of variability in the response. It was observed by the authors, and by other researchers, that the intrinsic spatial variability of soil properties affects not only the variability of the predicted structural response, but also the mechanical behaviour itself. As explained, for example, by Focht and Focht (2001), because of the presence of weaker zones in a soil deposit, “the actual failure surface can deviate from its theoretical position to pass through weaker material so that the average mobilized strength is less than the apparent average strength.” It appears that different types of uncertainties (insufficient information, measurement errors, etc., on the one hand, and actual spatial variability, on the other hand) affect the structural response in different manners, and they may have to be analyzed separately. It is the understanding of the writer that this was in fact the intention of the authors, namely to analyze the effects of intrinsic spatial variability alone. A number of researchers have provided information on ranges of COVs for spatial variability of different soil properties, as obtained from in situ soil investigations. In most cases, those ranges also include variability due to measurement errors. There is, however, a very comprehensive study in this respect (Phoon and Kulhawy 1999), mentioned also by the authors, in which approximate guidelines are provided for the COV of intrinsic spatial variability, separated from other sources of uncertainty. It is the writer’s opinion that, until more results become available, the guidelines provided by Phoon and Kulhawy (1999) are a good indication of the order of magnitude for the COV of soil variability. COVs as high as 5 (i.e., one order of magnitude larger than those usually reported in the literature) are assumed in the paper for the soil parameters. In justifying this range, the authors present a very interesting set of considerations related to soil variability at various scales, starting from the soil particle scale (of the order of micrometres) and going to the regional scale (of the order of kilometres). It is demonstrated that, within such limits, the range of COVs assumed in the paper is reasonable. The writer believes, however, that with respect to intrinsic soil spatial variability, the scales applicable here are bounded by the volume of soil involved in the bearing capacity failure and by the finite element mesh size. 367

Transition from maximum tension line to actual failure surface in reinforced soil model walls : Arenicz, R M Geotech Test JV13, N2, June 1990, P83–87

Transition from maximum tension line to actual failure surface in reinforced soil model walls : Arenicz, R M Geotech Test JV13, N2, June 1990, P83–87

Transition from Maximum Tension Line to Actual Failure Surface in Reinforced Soil Model Walls

Abstract This paper presents the results of laboratory tests conducted on reinforced soil model walls. The effect of model height on distribution of tensile stress in an instrumented reinforcement strip was investigated. It was found that, with increasing height of wall, a continuous redistribution of tensile stress had occurred with the point of maximum tension moving towards the actual failure surface of the model. On that basis, a correlation between the position of maximum tension in reinforcement and the overall safety factor of model walls was investigated. General and specific equations of maximum tension line in terms of wall height and safety factor were suggested.

Slope instability in a profile of weathered norite

Abstract On Christmas Day 1982, a combination of heavy rain and a broken water supply pipe resulted in significant mass movement on a hillside in the Bukit Gombak area of Singapore. Subsequent investigations revealed a sliding mass of about 50000 m 3 , approximately 100 m from crest to toe in plan with a height of 22 m. Average slope angle was about 12°. Ground conditions consisted of fill overlying residual soil derived from the underlying norite bedrock. This resulted in a situation where heavy rainfall caused rapid rises in ground water levels followed by very slow dissipation. Mass movements occurred when water levels in the slope rose above a certain value. From observations and local knowledge of the area, it was concluded that the mass movements had been taking place over many years previously and were still continuing. The geometry of the actual failure surface was not established, however analyses of a number of possible slip surfaces indicated an average ø’ of less than 18°. Two ring shear tests on samples of residual soil from the slope gave values of ø r ’ a little below 15°. Thus, although the data measured are limited, these results suggest that the stability of the slope was controlled by residual strength. Furthermore, the case history demonstrates how permeability distribution within a slope can have a major influence on its stability.