Potential Failure Surface Research Articles

Comparison of different probabilistic methods for predicting stability of a slope in spatially variable c– [formula omitted] soil

Three probabilistic methods of different complexity for slope stability calculations are in the paper evaluated with respect to a well-documented case study of slope failure in Lodalen, Norway. A finite element method considering spatial random fields of uncorrelated parameters c (cohesion) and φ (friction angle) is taken as a reference for comparison with two simpler methods based on Taylor series expansion, known as first-order-second-moment (FOSM) methods. It is shown that the FOSM method enhanced by a reduction of variance of input parameters due to spatial averaging along the potential failure surface (extended FOSM method) leads to a significant improvement in predictions as compared to the basic FOSM method. This method is computationally inexpensive and can be used in combination with any existing finite element code, it is thus a useful approximate probabilistic method for geotechnical practice. Several limitations of the extended FOSM method for calculating probability of a slope failure are identified.

Experimental Investigation of Mechanical Damage in Geogrids

This paper presents the results of a comprehensive experimental program for investigating the influence of mechanical damage on the load-displacement behavior of geogrids. Unconfined tension tests, pullout and direct shear tests were carried out on intact and damaged specimens. Natural or artificial damages were produced either by imposing heavy compaction procedures in the laboratory or by simply cutting one or more geogrid elements. It is concluded that natural damage in the geogrid may be more pronounced when aggressive compaction methods are used with coarse grained soils. Fine grained soils did not show a significant strength reduction even when subjected to heavy compaction in the laboratory. Under pullout loading, artificial damage was also noted to be of little significance for fine soil (silty clay). Rupture of the geogrid’s transverse elements led to a significant pullout strength reduction. These transverse elements are responsible for anchoring the geogrid within the soil mass. However, under unconfined tensile load, these transverse elements are responsible only for the grid’s geometrical configuration and their rupture did not induce a significant strength loss. In direct shear, the position of the geogrid relative to the potential failure surface was shown to be an important factor.

Model test on sand retaining wall reinforced with denti-strip inclusions

In conventional reinforced soil structures, the reinforcements are often laid horizontally in the soil. In this paper, a new concept of soil reinforced with denti-strip inclusions was proposed and a series of laboratory model tests were carried out on sand retaining wall reinforced with denti-strip inclusions. Besides the horizontal displacements of the facing, the lateral earth pressures acting on vertical elements were measured. A microscopic measurement was performed to investigate the deformation and progressive failure of the sand within model retaining wall. Based on the image analytical technique, the bearing capability and interaction mechanism of reinforced sand retaining wall were analyzed. The model of the initial shear failure and potential failure surface were also put forward. From the experimental results, it is shown that denti-strip inclusions can increase the bearing capability of retaining wall significantly and restrict the facing displacements efficiently, as compared with conventional horizontal reinforcement.

Rock-slides on road cuttings in the Dinaric karst of Croatia: processes and factors

The northern Adriatic coast, which forms part of the Dinaric karst system, is bordered by high mountains. Here, the occurrence of rock-slides and rock-falls in road sections with high cuttings excavated in recent times and more than one century ago has been documented. Some of these old slopes were stable for a long time before rock-slides started to occur. We propose that these mass movements are favoured by karstification processes acting along joints in the exposed rock mass. The stress changes induced by overburden removal (excavation) cause dilation in the mass joint systems reducing the rock mass strength and facilitating the circulation of aggressive water. Changes in water pressure and temperature, wetting and drying cycles, and corrosion processes along discontinuity planes entail a reduction through time of the shear strength of potential failure surfaces.

Shear transfer between ribbed slab and edge column

The results of tests on six micro-concrete wide beam ribbed slabs at edge column locations are presented. It was found that punching shear failure of a wide beam ribbed slab can occur at an edge column. Although a punching shear failure of a wide beam ribbed slab is very similar to that of a solid slab, the shear capacity is reduced relatively, because some of the potential shear failure surface is lost when it enters the ribbed section. A plastic theory for punching shear was developed, which gave good agreement with the test failure loads.

Numerical analysis of reinforced soil-retaining wall structures with cohesive and granular backfills

The failure mechanisms of reinforced soil segmental walls with extensible reinforcements were studied by performing a numerical analysis using the finite element method. The numerical approach was first verified against the results of three instrumented full-scale structures reported in the literature. Finite element models with different combinations of reinforcement spacing, reinforcement length and backfill soil were analysed. The ϕ–c reduction method, which is a special shear strength parameter reduction technique, was applied to simulate the failure conditions. The results of ϕ–c reduction analysis were used to evaluate assumptions used in current design procedures for geosynthetic-reinforced soil walls. In particular, shear strains were used to identify failure surfaces. Interpretation of the results indicated that, for both granular and cohesive backfills, the potential failure surface gradually shifts to a direct sliding mode as the system approaches failure. As a result, under working loads the potential failure surface used in current design analysis is correct, but the failure plane of a geosynthetic-reinforced soil-retaining wall at failure approaches a direct sliding type or a bilinear plane, which starts from the toe of the wall with a very shallow slope.

Stability analysis of Hawaiian Island flanks using insight gained from strength testing of the HSDP core

Hawaiian Island flank failures are recognized as the largest landslide events on Earth, reaching volumes of several thousand cubic kilometers and lengths of over 200 km and occurring on an average of once every 100 000 years. The 3.1 km deep Hawaii Scientific Drilling Project (HSDP) enabled an investigation of the rock mass strength variations on the island of Hawaii [Schiffman, P., Watters, R.J., Thompson, N., Walton, A.W., 2006. Hyaloclastites and the slope stability of Hawaiian volcanoes: insights from the Hawaiian Scientific Drilling Project's 3-km drill core. Journal of Volcanology and Geothermal Research, 151 (1–3): 217–228]. This study builds on that of Schiffman et al. [Schiffman, P., Watters, R.J., Thompson, N., Walton, A.W., 2006. Hyaloclastites and the slope stability of Hawaiian volcanoes: Insights from the Hawaiian Scientific Drilling Project's 3-km drill core. Journal of Volcanology and Geothermal Research, 151 (1–3): 217–228] by considering more in-depth rock mass classification and strength testing methods of the HSDP core. Geotechnical core logging techniques combined with laboratory strength testing methods show that rock strength differences exist within the edifice. Comparing the rock strength parameters obtained from the various volcano lithologies identified weak zones, suggesting the possible location of future slip surfaces for large flank failures. Relatively weak rock layers were recognized within poorly consolidated hyaloclastite zones, with increases in strength based on degree of alteration. Subaerial and submarine basalt flows are found to be significantly stronger. With the aid of digital elevation models, cross-sections have been developed of key flank areas on the island of Hawaii. Limit equilibrium slope stability analyses are performed on each cross-section using various failure criteria for the rock mass strength calculations. Based on the stability analyses the majority of the slopes analyzed are considered stable. In cases where instability (i.e. failure) is predicted, decreased rock mass quality (strength) of the altered and highly poorly consolidated lithologies is found to have a significant influence. These lithologies are present throughout the Hawaiian Islands, representing potential failure surfaces for large flank collapses. Failure criterion input parameters are considered in sensitivity analyses as are the influences of certain external stability factors such as sea level variation and seismic loading.

Effect of Nonlinear Shear Strength Characteristic on the Slope Stability of Super High Rockfill Dam

Linear and nonlinear shear strength models of rockfill, and the safety factor criterion of nonlinear shear strength were discussed in this paper. The slope stability of 261.5 m high Nuozhadu rockfill dam was analyzed with three different shear strength models. The safety factors are almost equal and the positions of potential failure surface are similar. It is found that the allowable safety factor base on linear criterion in the existing code can be used without any change when applying nonlinear shear strength rules.

Shear transfer between ribbed slab and internal column

The results are presented of tests on 20 micro-concrete wide beam ribbed slabs at internal column locations. It was found that shear failure of a wide beam ribbed slab can occur at an internal column either by punching shear or wide beam shear. Although a punching shear failure of a wide beam ribbed slab is very similar to that of a solid slab, the shear capacity is reduced relatively, because some of the potential shear failure surface is lost when it enters the ribbed section. Plastic theories for punching shear and wide beam shear were developed, which gave good agreement with the test failure loads.

Field monitoring evaluation of geotextile-reinforced soil-retaining walls

The use of geotextiles as reinforcement in soil-retaining walls may offer benefits over the use of other types of reinforcement, such as metallic strips or geogrids. These include ease of construction, expediency, and significant cost reduction. However, the lack of field monitoring results documenting the behavior of geotextile-reinforced structures, mainly regarding deformations, has precluded widespread utilization of these systems. A better understanding of the behavior of geotextile-reinforced soil structures could be gained by systematic monitoring of full-scale structures, combined with a thorough material characterization program. Accordingly, eight prototype geotextile-reinforced soil structures were built as part of a comprehensive study aimed at analyzing their behavior. This paper presents the characteristics of the prototype wall used as baseline for this series of structures, including construction details, instrumentation layout, and the results of laboratory tests conducted to characterize the soils and geosynthetics involved in this structure. The lateral displacements at the face of the wall were small, with increasing lateral displacements observed over time, particularly in association with precipitation events during the summer season in Brazil. The largest horizontal strains occurred towards the face, at mid-height of the prototype wall. The use of sigmoid curves to fit displacement results within the geotextiles was found to be very useful to define the value and the location of the peak strain in each reinforcement layer. The location of maximum reinforcement strains within each layer was found to be consistent with the development of a potential failure surface starting at the toe of the wall and propagating into the soil mass. The maximum reinforcement tension predicted using the K-stiffness method was found to compare well with reinforcement tension values obtained from monitoring results.

Spatio-temporal estimation of shallow landslide hazard triggered by rainfall using a three-dimensional model

A shallow landslide triggered by rainfall can be forecast in real-time by modeling the relationship between rainfall infiltration and decrease of slope stability. This paper describes a promising approach that combines an improved three-dimensional slope stability model with an approximate method based on the Green and Ampt model, to estimate the time–space distribution of shallow landslide hazards. Once a forecast of rainfall intensity and slope stability-related data, e.g., terrain and geology data, are acquired, this approach is shown to have the ability to estimate the variation of slope stability of a wide natural area during rainfall and to identify the location of potential failure surfaces. The effectiveness of the estimation procedures described has been tested by comparison with a one-dimensional method and by application to a landslide-prone area in Japan.

Failure mechanisms and triggering factors in calcareous cliffs of the Subalpine Ranges (French Alps)

In order to enhance the detection of prospective rock falls in calcareous cliffs, 25 rock falls have been described in a more detailed way than for an inventory. They are representative of middle size rock falls (10 to 100,000 m 3) occurring in the French Subalpine Ranges, at an elevation between 200 m and 2000 m. Structural conditions of the rock masses, morphology of the initial cliff surface and the scar, possible failure mechanisms and processes have been studied. Typical failure configurations have been identified, based on the attitude of the failure surface, in relation to the bedding planes and the cliff surface. Irregular cliff morphology appears to be another important susceptibility factor. In most cases, the classical comparison of the average planes of the main joint sets with the average plane of the slope could not define the potentially unstable masses. Rather, those ones are due to joint planes that deviate from their mean set plane or to irregularities of the cliff surface. The proposed investigation method to detect prospective rock falls mainly consists in observing stereoscopic aerial photographs in order to look for critical configurations. Once a critical mass has been detected, its failure probability for a period of the order of one century must be evaluated (or its life expectancy). The main factor to consider for this purpose appears to be the proportion of rock bridges in the potential failure surface. The triggering factors of rock falls in our study area have been investigated, by analysing an inventory of 46 rock falls. Statistical tests have been carried out to study the relation between rock falls and daily rainfall, freeze–thaw cycles or earthquakes. A good correlation has been obtained with freeze–thaw cycles, a slight correlation with rainfall and no correlation with earthquakes. This suggests that ice jacking could the main physical process leading to failure by causing microcrack propagation.

Upper-bound solutions of three-dimensional passive earth pressures

This paper presents a novel approach to the determination of passive soil pressures: using the upper-bound method within the framework of limit analysis theory. It is based on a three-dimensional, kinematically admissible, rotational, hyperbolical failure mechanism. The failure mechanism is composed of the central and two lateral bodies, which are connected by a common velocity field. This approach is similar to two-dimensional stability analyses, where the log spiral potential failure surface is considered. The front surface of the central body interacts with the retaining wall; the upper surface can be loaded by surcharge loading; and the log spiral segment defines the curved failure surface of the central part. The cross sections of the lateral bodies are in agreement with the cross section of the central body. On the outer side, they are laterally bounded by a curved and kinematically admissible hyperbolic surface, which is defined by enveloping the hyperbolical half cones and part of the case surface of the leading half cone. The results give values for the passive soil pressure coefficients that are for most cases lower than the values determined by the upper-bound method of limit analysis for a translational failure mechanism, as published in the literature.Key words: limit analysis, earth pressure, passive pressure, failure surface, soil–structure interaction.

Analysis of time-varying rainfall infiltration induced landslide

A case study of rainstorm-induced landslide is modeled following the hourly rainfall time series from the stage of infiltration caused by induced slope movement and soil saturated to excess pore pressures—Transient Rainfall Infiltration and Grid-based Regional Slope-Stability Analysis (TRIGRS). The grid-based landslide stability analysis was conducted to model the increased pore pressures and runoff in the study area under the specified rainfall conditions. The generated time variances of pore pressures help determine landslide characteristics and mechanisms under rainfall conditions. Inputs of soil properties and permeability parameters for landslide stability analysis in the study area were prepared by TRIGRS adopted for transient infiltration analysis. Results of the analyses show that under heavy rainfall conditions, the infiltrated slope is unstable and the time of debris masses movement initiated is correlated to the recorded time. In the initiated landslide, characteristics and effects are considered and reflected in the numerical modeling under combinations of topography, land use, climatic and geological conditions. Results reveal that there is a plane failure surface and a potential circular failure surface at the study site besides the rock topple failures in the crest. A grid-based slope-stability analysis incorporated with the GIS spatial functions is more advantageous than the traditional two-dimensional analysis for specified slope profiles to determine the whole behavior of a slope.

Fluctuant bearing capacity of shallow foundations during earthquakes

A limit equilibrium method to compute both dynamic bearing capacity and earthquake-induced settlements of foundations on soils using a log spiral potential failure surface is presented in this paper. The method is limited to soil materials that conform to the Mohr–Coulomb criterion. Time-varying inertia forces can be applied directly to the building. The procedure is capable of estimating vertical displacements and foundation tilting by integrating the differential equation developed from equilibrium of acting and resisting moments. The reliability of the method is assessed by comparing calculated and observed permanent movements of foundations for the September 1985 Earthquake in the soft ground area of Mexico City. The position of the failure surface is obtained by a minimization process at every time increment during the shaking. Results show that when an earthquake occurs the potential failure surface, obtained from static load, moves upwards and its length shortens as the seismic accelerations increase, reducing the bearing capacity and thus increasing its seismic risk. In this paper, the influence of earthquake acceleration on the time-varying positions of the potential failure is shown graphically.

Failure Analysis of Modular-Block Reinforced-Soil Walls during Earthquakes

Several modular-block reinforced-soil retaining walls failed during the 1999 Ji-Ji (Chi-chi) earthquake of Taiwan. Similar walls showed distress during the 1994 Northridge, Calif., earthquake. The instability or failure of these walls offered an opportunity to validate the simplistic pseudostatic limit-equilibrium procedures. In this study, the Ta Kung Wall of the Ji-Ji earthquake is analyzed, and the Gould and Valencia Walls of the Northridge earthquake are revisited with an improved estimation of local site acceleration. The local acceleration was estimated by using simple attenuation relationships obtained through the earthquake records. The results of analysis indicate that these three walls had adequate internal stability under estimated site acceleration. The geosynthetic length was inadequate to resist compound modes of failure where the potential failure surface extends beyond the reinforced zone. The external stability was most critical in the presence of horizontal and vertical accelerations.

The effect of internal gas pressurization on volcanic edifice stability: evolution towards a critical state

AbstractResults from simple physical and numerical models investigating the effects of increased internal pore‐fluid pressures of a Mohr–Coulomb volcanic edifice are presented. Physical experiments make use of a heap built from angular sand on top of a stiff substrate of variable angle, with the provision for injection of internal fluid (gas) pressures into the base. The resulting failure geometries arising from internal pressurization of the model appear similar to some natural examples of sector collapse. Two‐dimensional limit equilibrium models analysing 42 500 possible failure surfaces were run with internal pressures (P0) in the range 5–35 MPa, and show that the potential critical failure surface migrates to increasingly deeper levels with increasing internal pressure. Although internal pressurization alone is unlikely to reduce the factor of safety (FS) below unity, the edifice is driven towards a state of criticality that will render in susceptible to any internal or external perturbations.

Effect of unbalanced topography and overloading on Coulomb wedge kinematics: Insights from sandbox modeling

This study addresses the effect of variable unbalanced topography and overload on the kinematics of a fold and thrust belt developed within a collisional belt that underwent a subduction polarity reversal event. This was done by physical modeling of doubly vergent Coulomb wedges, using sand as an analogue material. During the experimental procedure a preexisting topography was generated by a first phase of subduction in one direction. A second phase of subduction was then initiated in the opposite direction (simulating a subduction flip). An anomalous, strong frontal growth for the wedge during the second phase was experimentally shown to be dependent upon surface slope breaks and the critical asymmetrical architecture achieved by the wedge at mature stages of deformation. The latter is suggested to be the rule for doubly vergent orogens at steady state, even after a subduction flip. During the experiments, surface processes, like syntectonic erosion and sedimentation, markedly altered mass transfer within the wedge. In particular, lowering the surface slope by syntectonic erosion favored cycling between accretion and underthrusting modes. By contrast, a sudden syntectonic sediment load in the prowedge region promoted prolonged phases of underthrusting, retarding accretion of new imbricates at the prowedge toe. However, at later stages of deformation the prowedge was forced to regain its characteristic minimum critical taper as predicted by theory and did so by the sudden nucleation of long, flat thrust units, rapidly rebalancing the asymmetry between prowedge and retrowedge regions. The experiments suggest that given a fixed critical taper related to time‐invariant frictional properties along the wedge bounding surfaces, deformation does vary within the wedge according to time‐varying location of normal stress surpluses and unbalanced topography acting on potential failure surfaces. These, in turn, alter the equilibrium between prowedge and retrowedge basal detachments that is here considered to be a major factor controlling the self‐regulating dynamics of collisional orogens.

Anatomy of a lava dome collapse: the 20 March 2000 event at Soufrière Hills Volcano, Montserrat

A second extrusive phase of the currently ongoing 1995–2003 eruption of Soufrière Hills Volcano (SHV), Montserrat, commenced in mid-November 1999 following ∼19 months during which no fresh lava had reached the surface. By mid-March 2000, a new andesite lava dome constructed within a collapse scar girdled by remnants of the 1995–1998 dome complex had attained an estimated volume of ∼29±3 million m 3 (Mm 3). On 20 March 2000, during a period of heavy rainfall on the island, a significant collapse event ensued that removed ∼95% of the new lava dome (∼28±3 Mm 3) during ∼5 hours of activity that generated ∼40 pyroclastic flows and at least one magmatic explosion. The associated ash cloud reached an altitude of ∼9 km and deposited ash on the island of Guadeloupe to the southeast, and a number of lahars and debris flows occurred in valleys on the flanks of SHV. A large quantity of observational data, including contemporaneous field observations and continuous data from the broadband seismic network on Montserrat, allow a detailed reconstruction of this dome collapse event. In contrast to most of the large dome collapses at SHV, the 20 March 2000 event is distinguished by a lack of short-term precursory elevated seismicity at shallow depths beneath the lava dome. Broadband seismic amplitude data recorded during the event are used to infer the cumulative volume of collapsed dome as the collapse progressed. These data indicate that the high-velocity pyroclastic flows observed at the climax of the event removed by far the largest portion (∼68%) of the lava dome at peak discharge rates (estimated from the seismic record) of ∼2×10 4 m 3 s −1. Following the 20 March 2000 collapse, lava dome growth recommenced immediately and continued without significant interruption until another, larger dome collapse occurred on 29 July 2001. The 29 July 2001 event also coincided with heavy rainfall on Montserrat [Matthews et al. (2002) Geophys. Res. Lett. 29; DOI:10.1029/2002GL014863] and lacked precursory elevated seismic activity. We attribute the initiation of the 20 March 2000 collapse to a prolonged spell of heavy rainfall on the lava dome prior to and during the event. The precise causal mechanism remains controversial, though some combination of mechanical erosion and/or destabilization of a critically poised face of the lava dome, the action of pressurized steam or water on potential failure surfaces within the dome, rapid cooling of hot lava and small phreatic explosions seems likely. Anecdotal evidence exists for other rainfall-induced activity on Montserrat, and the triggering of explosive or pyroclastic flow activity by rainfall has been noted at dome-forming volcanoes elsewhere, including Merapi, Indonesia [Voight et al. (2000) J. Volcanol. Geotherm. Res. 100, 69–138], Unzen, Japan [Yamasato et al. (1997) Papers Meteorol. Geophys. 48], Santiaguito, Guatemala [Smithsonian Institution Global Volcanism Network Bull. 15 (1990)] and Mount St. Helens, USA [Mastin (1994) Geol. Soc. Am. Bull. 106, 175–185]. Hazard mitigation plans at dome-forming volcanoes would therefore benefit from the inclusion of meteorological forecasting and rain monitoring equipment, particularly in the tropics.

Earthquake-induced slope displacements

Este artículo presenta un método de análisis para evaluar los desplazamientos inducidos sísmicamente en taludes. El procedimiento considera las características no lineales de los materiales, la variación espacial de la resistencia (fricción) a lo largo de la superficie potencial de falla y el efecto inercial de la cuña deslizante, la cual es considerada como un sistema flexible de múltiples grados de libertad. Los efectos potenciales son mostrados a través de casos hipotéticos por medio de varios modelos y dos sismos diferentes. El método de análisis propuesto se evalúa por medio de comparaciones con métodos existentes.