Potential Failure Surface Research Articles

Modeling progressive failures in rock slopes with non‐persistent joints using the numerical manifold method

SUMMARYRock slope failure is a complex process that usually involves both opening/sliding along pre‐existing discontinuities as well as fracturing of intact rock bridges. Discontinuity persistence is an important factor governing rock slope instabilities. However, traditional slope failure analysis assumes persistent discontinuities, and rock slope fails along a predefined persistent continuous potential failure surface because of the limitations of the analysis tools. This paper proposes the numerical manifold method (NMM) incorporated with a Mohr–Coulomb criterion‐based fracturing algorithm to simulate the progressive failure of rock slopes with non‐persistent joints. Detailed fracturing algorithm is first presented. Then, the NMM enabling fracturing is calibrated through simulating an edge‐cracked plate and the Brazilian test. Lastly, the developed code is applied to investigate the failure process of rock slopes involving non‐persistent joints. Numerical results indicate that the proposed method can capture the opening/sliding along existing discontinuities, the fracturing in intact rock bridges and the final kinematic release. Progressive slope failure is well exhibited. Copyright © 2013 John Wiley & Sons, Ltd.

Limit Equilibrium Analyses for Internal Design of Geosynthetic Reinforced Slopes: Influence of Potential Failure Surface and Strength Distribution

The paper presents results from a computer code, based on limit equilibrium analyses, able to quantify earth pressure coefficients for the internal design of geosynthetic reinforced soil structures and identify the potential failure surfaces. Failure mechanisms assuming bilinear or logarithmic spiral failure surfaces are considered. The influence of the potential failure surface and geosynthetic strength distribution on the earth pressure coefficient is analysed. Required reinforcement tensile strengths calculated by the developed program are compared with values published in the literature. To further evaluate the capabilities of limit equilibrium analyses, the numerical modelling of a geosynthetic reinforced steep slope, designed at ultimate limit state conditions (FS = 1), is also presented. Good agreement was achieved between the potential failure surfaces predicted by limit equilibrium analyses and those obtained with numerical modelling.

Developing a new slice method for slope displacement analyses

Conventional slice methods of slope stability use a constant value of safety factor along the entire failure surface, providing no information of displacements and possible variations about safety margins for the analyzed slope. Therefore, it is of practical significance to extend the force-based slice method into a displacement-based one to provide useful information about shear displacements and associated safety margins along the potential failure surface. This study extends Janbu's Generalized Procedure of Slices (GPS) by supplementing force and moment equilibrium and the Mohr–Coulomb failure criterion, which are employed in the Janbu's GPS, with additional criteria regarding slice displacement compatibility and non-linear stress–displacement relationships represented by a hyperbolic soil model. The extended displacement-based slice method constitutes a statically determinate system which is capable of generating local stress- and displacement-based safety factors along a potential failure surface. The principles and procedures proposed here can also be applied to other slice methods, regardless of their accuracy. Results of a case study show that the observed slope displacement induced by a ground table rise during a rainstorm event can be well simulated using the proposed method. More case studies into various mechanisms of slope displacements will be performed in the future to validate the proposed method for practical engineering applications.

Assessing the Stability of a Geologically Complex Slope Where Strong Dykes Locally Act as Reinforcement

This paper presents a case study of aspects of the design of large slope cuttings in complex geological conditions in South Korea. During the original cutting of the slopes to geometries prescribed by Korean standards for rock slopes, several translational slides occurred on daylighting bedding planes. From observation it was evident that away from the areas of distress, some parts of the slopes appeared to be relatively stable, apparently because of the strengthening influence of relatively strong and massive igneous dykes and sills through the sedimentary rock. This paper describes how the geological conditions were assessed and stability analysed, section-by-section, along the road. This was achieved using a form of the method of slices to account for those sections of potential failure surfaces where sliding could occur along bedding and others where failure would necessitate shear through the intrusive igneous rock. Results were checked using UDEC models of critical sections. Following these analyses, recommendations were made for localized additional preventive measures including anchors and drainage.

Progressive failure analysis of slope with strain-softening behaviour based on strength reduction method

Based on the strength reduction method and strain-softening model, a method for progressive failure analysis of strain-softening slopes was presented in this paper. The mutation is more pronounced in strain-softening analysis, and the mutation of displacement at slope crest was taken as critical failure criterion. An engineering example was provided to demonstrate the validity of the present method. This method was applied to a cut slope in an industry site. The results are as follows: (1) The factor of safety and the critical slip surface obtained by the present method are between those by peak and residual strength. The analysis with peak strength would lead to non-conservative results, but that with residual strength tends to be overly conservative. (2) The thickness of the shear zone considering strain-softening behaviour is narrower than that with non-softening analysis. (3) The failure of slope is the process of the initiation, propagation and connection of potential failure surface. The strength parameters are mobilized to a non-uniform degree while progressive failure occurs in the slope. (4) The factor of safety increases with the increase of residual shear strain threshold and elastic modulus. The failure mode of slope changes from shallow slip to deep slip. Poisson’s ratio and dilation angle have little effect on the results.

Analysis of rainfall‐induced slope instability using a field of local factor of safety

Slope‐stability analyses are mostly conducted by identifying or assuming a potential failure surface and assessing the factor of safety (FS) of that surface. This approach of assigning a single FS to a potentially unstable slope provides little insight on where the failure initiates or the ultimate geometry and location of a landslide rupture surface. We describe a method to quantify a scalar field of FS based on the concept of the Coulomb stress and the shift in the state of stress toward failure that results from rainfall infiltration. The FS at each point within a hillslope is called the local factor of safety (LFS) and is defined as the ratio of the Coulomb stress at the current state of stress to the Coulomb stress of the potential failure state under the Mohr‐Coulomb criterion. Comparative assessment with limit‐equilibrium and hybrid finite element limit‐equilibrium methods show that the proposed LFS is consistent with these approaches and yields additional insight into the geometry and location of the potential failure surface and how instability may initiate and evolve with changes in pore water conditions. Quantitative assessments applying the new LFS field method to slopes under infiltration conditions demonstrate that the LFS has the potential to overcome several major limitations in the classical FS methodologies such as the shape of the failure surface and the inherent underestimation of slope instability. Comparison with infinite‐slope methods, including a recent extension to variably saturated conditions, shows further enhancement in assessing shallow landslide occurrence using the LFS methodology. Although we use only a linear elastic solution for the state of stress with no post‐failure analysis that require more sophisticated elastoplastic or other theories, the LFS provides a new means to quantify the potential instability zones in hillslopes under variably saturated conditions using stress‐field based methods.

Erosional power in the Swiss Alps: characterization of slope failure in the Illgraben

ABSTRACTLandslides and rockfalls are key geomorphic processes in mountain basins. Their quantification and characterization are critical for understanding the processes of slope failure and their contributions to erosion and landscape evolution. We used digital photogrammetry to produce a multi‐temporal record of erosion (1963–2005) of a rock slope at the head of the Illgraben, a very active catchment prone to debris flows in Switzerland. Slope failures affect 70% of the study slope and erode the slope at an average rate of 0.39 ± 0.03 m yr¯¹.The analysis of individual slope failures yielded an inventory of ~2500 failures ranging over 6 orders of magnitude in volume, despite the small slope area and short study period. The slope failures form a characteristic magnitude–frequency distribution with a rollover and a power‐law tail between ~200 m³ and 1.6 × 106m³ with an exponent of 1.65. Slope failure volume scales with area as a power law with an exponent of 1.1. Both values are low for studies of bedrock landslides and rockfall and result from the highly fractured and weathered state of the quartzitic bedrock.Our data suggest that the magnitude–frequency distribution is the result of two separate slope failure processes. Type (1) failures are frequent, small slides and slumps within the weathered layer of highly fractured rock and loose sediment, and make up the rollover. Type (2) failures are less frequent and larger rockslides and rockfalls within the internal bedded and fractured slope along pre‐determined potential failure surfaces, and make up the power‐law tail.Rockslides and rockfalls of high magnitude and relatively low frequency make up 99% of the total failure volume and are thus responsible for the high erosion rate. They are also significant in the context of landscape evolution as they occur on slopes above 45° and limit the relief of the slope. Copyright © 2012 John Wiley & Sons, Ltd.

Stress distribution and development within geosynthetic-reinforced soil slopes

ABSTRACT: Numerical methods combined with centrifuge tests are used to investigate the distribution and development of soil stresses and reinforcement tensile loads in geosynthetic-reinforced soil (GRS) structures. In this study, system stability indicated by the factor of safety (FS) of GRS slopes is calculated by limit equilibrium analysis. Stress information under various stress states is evaluated using finite element analysis. Advanced models and an integration algorithm are implemented in finite element code to enhance the simulation results. The proposed numerical models are validated by centrifuge tests of two GRS slopes with different backfill densities. Numerical results indicate that soil stress mobilisation can be described by the soil stress level S, which is defined as the ratio of the current stress status to peak failure criteria. For both slope models, as loading increases, backfill stresses develop and propagate along the potential failure surface. Mobilisation of soil stress was non-uniform along the failure surface. Immediately after the stress level reaches peak (S = 1), strength softening initiates at the top and toe of the slope at approximately FS = 1.2. The slope settlement rate and reinforcement tensile load increase significantly when soil softening begins. The softening occurs randomly and irregularly along the failure surface, and the formation of the soil-softening band completes at approximately FS = 1.1. The failure surface corresponds to the locus of intense soil strains and the maximum tensile loads at each reinforcement layer. Additionally, the numerical results show that the initiation of soil softening and the failure of the slope occurred earlier in the slope model with low backfill density. The numerical results support the view that peak shear strength, not residual shear strength, governs system stability. Last, the distribution of maximum reinforcement tensile loads with depth was highly uniform at low g-level and became trapezoidal at high g-level. The peak value was located at approximately mid-height of the reinforced slopes. This observation contradicted the triangular distribution with depth assumed in current design methodologies for geosynthetic structures.

Experimental studies of groundwater pipe flow network characteristics in gravelly soil slopes

Piping flow networks have often been identified in hydrogeological field studies of gravelly soil slopes in the southern part of China. The present experimental studies have shown that under long-term groundwater seepage, piping flow networks gradually develop in the slope. Factors affecting the development of flow pipe seepage network included the grain size distribution, the degree of soil compaction, and soil depth. Piping seepage networks favorably form if the content of the gravel was high, the soil cohesion was low, the degree of the soil compaction was low, or the soil depth was shallow. Due to the enhanced permeability associated with the presence of flow pipe seepage network in gravelly soil slopes, groundwater can be effectively drained away. This can beneficially prevent the rise of groundwater level in the slope during raining seasons, hence reducing pore water pressure along the potential failure surface and increasing slope stability. Once the flow pipe seepage network was disturbed or damaged, the water level in the upper portion of the slope experienced a great rise, hence reducing the slope stability. Therefore, slope toe excavation and excessive loading at the slope crest should be avoided for slopes with well-developed flow pipe seepage network in order to preserve it.

Stability analysis of infinite clayey slopes subjected to pore pressure changes

A simplified method is proposed for the stability analysis of clayey slopes subjected to pore pressure changes. The method is based on the infinite slope model and takes advantage of an analytical solution to evaluate the changes in pore pressure at the potential (or pre-existing) failure surface directly from the pore pressure measurements at a piezometer which is installed above this surface. A case study documented in the literature is analysed as an application.

Three-dimensional Analysis of Rainfall-induced Slope Failure

Rainfall leads to perched water table development, increasing the main groundwater level and soil erosion due to concentrated water flow. This results in an increase in pore water pressure causing decrease in effective stress in the soil and thus reduces the soil shear strength on the potential failure surface to a point where equilibrium can no longer be sustained in the slope, eventually failure takes place. Numerical and experimental study has been performed to investigate the mechanism of rainfall induced slope failure. The slope stability analysis has been carried out using the pore water pressure and the moisture content calculated by seepage flow model combined with 2D coupled surface water flow and erosion/deposition model. Extended Spencer method has been incorporated into an effective minimization procedure based on dynamic programming by which the minimal factor of safety and the corresponding critical non circular slip surface are determined simultaneously. Rectangular flume of length 300cm, width 80cm and depth 70cm set at 24° longitudinal slope was used in the experiment. The model slope was prepared on the rigid bed of flume by placing silica sand S6 on the flume. Three Profile probes (PRs) consisting four sensors (SRs) were used to measure the temporal variation of moisture content and three pressure transducers (PTs) were used to measure the temporal variation of air pressure at different locations inside the body of the model slope. Red colored sediment strips and red colored cotton threads were placed respectively at the side wall faces and inside the body, normal to the flume bed, so as to measure the failure surface after sliding. Experimental moisture profiles at the position of different SRs are shown in Fig.1 and experimental air pressure head profiles at the position of different PTs are shown in Fig.2.

Design of brick-faced retaining walls reinforced with geotextiles

Since the 1960s, retaining structures with reinforced backfill have undergone a remarkable development. Although important progress has been made regarding understanding of their behaviour, much remains to be done, especially on their design. The work described herein aims to contribute to the development of a design method for brick-faced reinforced retaining walls that should be very simple to use. A study was carried out by using some known methods of analysis, and by comparing the results obtained with the behaviour already known for this type of wall from previous laboratory and numerical studies. Both Rankine's and Coulomb's modified approach were used, and also three other methods allowing potential failure surfaces that were planar, circular and bilinear in shape. The comparative studies performed showed good predictions when a circular or a bilinear failure surface was considered, although the latter method allows a more versatile definition of the potential failure surface.

A simplified method for predicting ground settlement profiles induced by excavation in soft clay

A series of parametric studies was performed to examine the influence factors affecting the settlement influence zone induced by excavation in soft clay. It was found that the excavation depth, width, the soft clay bottom depth and the rock-like soil depth are all related to the settlement influence zone. The potential failure surface, as deduced from the failure mechanism, covering the above-mentioned parameters, is consistent with the settlement influence zone. Thus, a simple method based on the analysis results is proposed to predict the settlement influence zone. Ten case histories and statistical data for the settlements in the Shanghai area were used to verify the proposed method.

C3 Soil slope stabilization

This section focuses on techniques of soil slope stability analysis and stabilization and provides selected case studies to illustrate some of the approaches commonly adopted. The first distinction to make is whether the materials forming the slope under consideration behave primarily as soil or rock. A soil slope will fail through its granular mass, whereas a rock slope (Section C4) usually fails principally along discontinuities (bedding, foliation or other joints). Relict joints may be present in soil weathering profiles, associated with the original rock structure (Section A3.1). These normally have a weakening effect and, if adversely orientated, may become preferred planes of failure. In practice, a clear distinction between soil slopes and rock slopes cannot always be made. As described in Section A3, rock structure, weathering profiles and taluvium/colluvium deposits on mountain slopes can be highly variable; it is not uncommon to find that a given slope will fail partly through rock and partly through soil. In in situ weathered soil profiles it is often the weathering grade boundaries, for example between weathering grade III and IV and IV and V, that tend to form the basal slip surfaces of shallow rainfall-induced landslides in the humid tropics and subtropics. However, in weathering grade I–III rock, the presence of jointing, folding and faulting often results in the development of complex weathering and strength profiles to the extent that zones of ‘rock’ with strengths not much greater than that of soil may be present at considerable depth (as illustrated in Sections B4 & B5). This will often exert significant control on the depth and configuration of existing or potential failure surfaces. ### C3.2.1 Ground model Before a slope stability assessment or analysis is made it is first necessary to establish a ground model. This is a normal progression from the field mapping and ground investigation activities …

Engineering behaviors of reinforced gabion retaining wall based on laboratory test

In order to study the engineering behaviors of reinforced gabion retaining wall, laboratory model test was carried out. Cyclic load and unload of five levels (0–50, 0–100, 0–50, 0–200 and 0–250 kPa) were imposed. Vertical earth pressure, lateral earth pressure, deformation behaviors of reinforcements, potential failure surface and deformation behaviors of wall face were studied. Results show that vertical earth pressure is less than theoretical value, the ratio of vertical earth pressure to theoretical value increases nearly linearly with increasing load, and the correlation coefficient of regression equation is 0.92 for the second layer and 0.79 for the fifth layer. The distribution of lateral earth pressure along the wall back is nonlinear and it is less than theoretical value especially when the load imposed at the top of retaining wall is large. Therefore, reinforced gabion retaining wall will be in great safety when current method is adopted. The deformation behaviors of reinforcements both in the third layer and the fifth layer are single-peak distributions, and the position of the maximum strain is behind that determined by 0.3H (Here H refers to the height of retaining wall) method or Rankine theory. Lateral deformation of wall face increases with increasing load, and the largest lateral deformation occurs in the fourth layer, which lead to a bulging in the middle of wall face.

A three-dimensional procedure for evaluating the stability of gravity dams against deep slide in the foundation

In the design of a gravity dam, any dam monolith must be checked to determine if the stability against sliding along a potential failure surface in the foundation meets the requirement specified by the design code. Till date, such verification is usually carried out based on two-dimensional limit equilibrium methods. Two-dimensional analyses are sometimes too conservative to meet the requirements. For this purpose, in the context of the limit equilibrium methods, a rigorous three-dimensional procedure is developed, which can accommodate failure surfaces of any shape and satisfy all equilibrium conditions. Meanwhile, the procedure considers the reinforcement of concrete plugs that are laid between the sliding body and the rock foundation. On the base of distribution of factors of safety of all dam monoliths, an idea called “the design of equal safety” is suggested, which says that all monoliths should have nearly equal factors of safety as far as possible.

Reinforcement load and deformation mode of geosynthetic-reinforced soil walls subject to seismic loading during service life

A Finite Element procedure was used to investigate the reinforcement load and the deformation mode for geosynthetic-reinforced soil (GRS) walls subject to seismic loading during their service life, focusing on those with marginal backfill soils. Marginal backfill soils are hereby defined as filled materials containing cohesive fines with plasticity index (PI) >6, which may exhibit substantial creep under constant static loading before subjected to earthquake. It was found that under strong seismic loading reinforced soil walls with marginal backfills exhibited a distinctive “two-wedge” deformation mode. The surface of maximum reinforcement load was the combined effect of the internal potential failure surface and the outer surface that extended into the retained earth. In the range investigated, which is believed to cover general backfill soils and geosynthetic reinforcements, the creep rates of soils and reinforcements had small influence on the reinforcement load and the “two-wedge” deformation mode, but reinforcement stiffness played a critical role on these two responses of GRS walls. It was also found that the “two-wedge” deformation mode could be restricted if sufficiently long reinforcement was used. The study shows that it is rational to investigate the reinforcement load of reinforced soil walls subject to seismic loading without considering the previous long-term creep.

Quantitative Risk Assessment of Cut-Slope Projects under Construction

In some cut-slope projects landslide is a common problem during construction due to unfavorable geomorphological and geomechanical conditions. It is necessary to do a quantitative assessment of the risk posed by landslide before determining the budget or tender price. This paper outlines a general procedure for doing this, followed by an example to demonstrate the approach in comparison to a known failure. Finite-element analyses identify the most dangerous landslide scenario among all construction steps. The slope failure probability is then estimated using reliability theory based on the most dangerous construction step. After identifying the potential failure surface and estimating the volume of the sliding mass, the runnout behavior of sliding mass is simulated to delimit the extent of likely impacted area. Then, the exposed elements at risk and their vulnerabilities are identified and analyzed. The landslide risk is assessed quantitatively for three types of consequences: casualties, economic loss, and time overrun. Compared with actual consequences, the estimation results were in acceptable agreement with the case study. The paper demonstrates that it is feasible to analyze the risk associated with landslides during construction of cut-slopes.

The Hunters Crossing weathered rock slide, Haywood County, North Carolina, USA

Abstract The Hunters Crossing landslide is a slow-moving, weathered rock slide affecting a small community of condominiums in the town of Waynesville in the Blue Ridge Mountains of Haywood County, North Carolina. In November 2005, studies were begun to assess the characteristics of this landslide and the potential for further movement and damage to structures. Work included drilling several boreholes, performing seismic velocity surveys, and surveying benchmarks among other investigations. Data indicate that the potential failure surface is located no more than 11 m below the ground surface, possibly at the contact between saprolite and partially weathered rock. However, inclinometers installed at two locations on the slope have not detected enough movement to corroborate that assessment. Studies continue at this site to determine the location of the failure surface, to identify the mechanisms that accelerate movement, and to relate these findings to a broader understanding of weathered rock slides elsewhere in the southeastern USA.

Field Behavior of a Geogrid Reinforced Soil Retaining Wall with a Wrap-Around Facing

Abstract The earth pressure and deformation characteristics of a geogrid reinforced soil retaining wall were monitored during and for a period of 1.5 years after construction. Both vertical and lateral soil stresses were recorded with vibrating-wire earth pressure cells, and the reinforcement deformations were measured using flexible displacement sensors. The maximum vertical foundation pressure along the wall’s reinforcements occurred at the center of the reinforcement, gradually decreasing towards the front and back ends. The measured lateral earth pressure within the reinforced soil wall is non-linear along the wall height, and the value is less than the theoretical active lateral earth pressure. The distribution of tensile strain along the reinforcements within the lower portion of the wall has two peak values. The potential failure surface of the wall closely follows the theoretical Coulomb failure surface of an unreinforced backfill.