Potential Failure Surface Research Articles

Numerical simulation of rainfall-induced deformations of embankments considering the coupled hydro-mechanical behavior of unsaturated soils

This paper presents numerical simulations of the deformation response of unsaturated embankments subjected to rainfall infiltration using a coupled hydro-mechanical constitutive model for unsaturated soils. The constitutive model accounts for the influence of degree of saturation on the stress–strain behavior and the influence of void ratio on the water retention behavior. The constitutive model was implemented in the finite difference program FLAC, calibrated using triaxial test data, and validated using measurements of wetting-induced deformations of embankment models from centrifuge tests. The validation demonstrates that the constitutive model can capture the coupled hydro-mechanical behavior of unsaturated soils under wetting conditions. Simulations of the hydro-mechanical response of unsaturated embankments subjected to rainfall infiltration indicate that the differential settlement across the top surface of the embankment between the centerline and shoulder increases significantly during rainfall infiltration, which could result in severe damage to overlying transportation infrastructure. As the rainfall infiltration increases, shear strains accumulate and form a potential failure surface at a shallow depth of approximately 2 m from the slope surface. Insights into the hydro-mechanical response of unsaturated embankments subjected to rainfall infiltration gained will be useful for considering climate change effects in design and construction of compacted embankments.

Ultimate Capacity and Load Transfer Mechanism of Ground Anchors in Granular Soils: State-of-the-Art

The grouted ground anchor is the most used geotechnical element to transfer the tension load from the superstructure or the soil mass in front of the potential failure surface (active zone) to a deeper, efficient soil layer for its reliability and feasibility. It was originally constructed by inserting the tendon, usually a steel strand, into an already drilled borehole filled with cement grout. It was successfully used in the Cheurfas dam in Algeria in (1934). According to the load transfer mechanism, there are two types of ground anchors: tension-type and compression-type. The names of these two types of anchors refer to the developed stresses in the grouted body. The present work presents a literature survey about the load transfer mechanism for both types of anchors: tension and compression embedded in granular soil. Many factors affecting the load transfer mechanism have been reviewed. The main factor affecting the load transfer mechanism is the confining pressure of the borehole. The recently developed methods of estimating the ultimate pullout capacity of the anchor have been reviewed and discussed.

Review of Quantifying Slope Stability and Assessing Landslide Susceptibility in Sabah, Malaysia

Landslides affect 3.7 million square kilometers of land and affect around 5% of the global population. In Malaysia, 21,000 locations are prone to landslides, with 16,000 in Peninsular Malaysia, 3,000 in Sabah, and 2,000 in Sarawak. Factors contributing to landslides include geological factors, weather conditions, slope steepness, vegetation, and human activities. Rainfall and geological conditions are significant external and internal causes. Landslides can be mitigated using various construction techniques, such as contagious bored pile, guniting, and rock anchoring. Rainfall and landslides are closely related, with rainfall affecting soil stability and causing slope failure. Prolonged rainfall can decrease slope stability due to increased water infiltration, leading to landslides. Analysis of slope stability is crucial in geotechnical engineering, assessing the stability of natural and man-made slopes. It involves assessing soil properties and identifying potential failure surfaces. Limit Equilibrium (LEM) and finite element (FE) methods are commonly used for slope stability assessments. LEM divides failure mass into slices, while FEM maintains global equilibrium until failure occurs. Both methods offer advantages, such as revealing working stress deformations with realistic soil compressibility data and tracking progressive failure to shear failure.

Analysis of the mechanical effects and influencing factors of cut-fill interface within loess subgrade

Cut-fill interfaces within the loess subgrade tend to form potential failure surfaces, controlling the mechanical properties of cut-fill engineering. Focusing on the cut-fill interface, extensive laboratory tests in this research show the performance of interfacial mechanical properties under various test conditions, revealing interface effects and exploring the impact of different factors on these effects. The results indicate that the interface strengthens the friction angle of soil but weakens cohesion, especially impacting the cohesion, thereby reducing the shear strength of soil. The increase in dry density diminishes the enhancement effect on friction angle caused by the interface while amplifying the degradation effect on cohesion. Elevated water content has a weak influence on the enhancement effect of friction angle but diminishes the degradation effect of cohesion. Load doesn't change the impact of dry density on the interface effect but amplifies the impact of water content. The impact of various factors on interfacial shear strength manifests as follows: load > average dry density > water content. The interaction among these factors demonstrates average dry density + load > water content + load > average dry density + water content. The study indicates that the construction of loess subgrades based on the standard of maximum dry density and optimum water content may not align with the conditions required for achieving optimal stability at the cut-fill interface. These research findings reveal the crucial role of various factors in comprehensive impact of interfacial effect, offering essential support for ensuring the stability of cut-fill interfaces.

Experimental study on the load bearing behavior of shored mechanically stabilized earth wall

Shored mechanically stabilized earth (SMSE) walls have been increasingly applied in the projects of widening existing roads or new construction of roads on slopes because of their good carbon emission reduction and mechanical performance. In this paper, a scaled-down model test of SMSE wall with shoring wall batter of 1H:2V was conducted to study the load-bearing behavior of SMSE wall under this terrain condition, and the results including wall deformations, earth pressures, reinforcement strains, and potential failure surface were analyzed. The results show that although the backfill near the shoring wall was not directly compressed by the load, it still slipped along the backfill-shoring wall interface. The loading created a tensile pressure zone in the upper part of the backfill-shoring wall interface, which may lead to tensile cracking. The uppermost layer of geogrid was more prone to sliding, while the tensile deformation was smaller, and its strain value was overall smaller than the strain value of the geogrid below it. As the load increases, the potential failure surface changed from a Rankine failure surface to a bilinear potential failure surface. The potential failure surface did not pass through the heel of the SMSE wall under large loads.

Deformation Responses of a Geosynthetic Reinforced Soil-Integrated Bridge System Under Static Loading and Sensitivity Analysis of Influential Factors Based on the Improved Grey Relational Method

A numerical parametric study is conducted to evaluate the effects of various parameters on the deformation responses of a geosynthetic reinforced soil-integrated bridge system (GRS-IBS) under static load. The investigated parameters include the abutment height, reinforcement spacing, reinforcement length, reinforcement stiffness, bridge load, and backfill internal friction angle. The improved grey relational method is used to investigate the sensitivity of influential factors on the deformation responses of the GRS-IBS. The simulation results indicate that the abutment height, reinforcement spacing, and bridge load have a significant impact on the performance of the GRS-IBS with respect to lateral displacement and settlement, while the effects of reinforcement length, reinforcement stiffness, and backfill internal friction angle on the deformation behavior of the abutment are negligible. Differential settlements between the girder and approach are minimal under all conditions. The potential failure envelope of the GRS-IBS exhibits a “L” shaped configuration where the potential failure surface starts beneath the inner edge of the strip footing, extending vertically downward to half of the wall height and further bifurcating toward the strip footing and the toe of the wall. The results of the improved grey relational analysis show that reinforcement spacing is the most sensitive to the lateral displacement of the GRS-IBS. The abutment height and bridge load are more sensitive to abutment deformation than other influential factors. These three parameters are crucial to the deformation behavior and should be given primary consideration when designing a GRS-IBS.

Ultimate bearing capacity of geosynthetic reinforced soil abutment centrifuge model tests

Five centrifuge models of geosynthetic reinforced soil (GRS) abutments with segmental block facing were loaded in the acceleration field under plane strain condition. Influencing factors including reinforcement tensile strength, setback and bearing area width were considered and analysed. Results show that the abutment with stronger reinforcement remained stable under the maximum loading capacity. However, abutments with weaker reinforcement showed excessive vertical strain, local deformation or even collapse. The ultimate bearing capacity increased with the lengthening of setback but decreased due to a larger bearing area. The rupture of reinforcements was observed and considered as the cause of the failures. Compared with the measured ultimate bearing capacity, the values calculated by the semi-empirical formula of design guidelines were significantly conservative. The failure surface of failed abutment developed from the rear edge of the bearing area to the middle height of the abutment at an angle of nearly 45° + φ/2. The setback and the bearing area width affected the form and position of the failure surface. The difference between the potential failure surface predicted by available methods and the measured failure surfaces has been discussed, and suggestions for the design and ultimate bearing capacity prediction of GRS abutments with segmental block facing are provided.

Discrete element analysis on impact failure mechanism of “Zhengzhou 7.20” catastrophic flood for coal gangue railway subgrade

Limited research has been conducted on the failure mechanism of coal gangue railway subgrade structures during catastrophic flood disasters, including the impact of varying flood impact heights and load sizes. This study investigates the process and mechanism of coal gangue railway subgrade failure in the coal mining subsidence area, in conjunction with the “7.20” catastrophic flood in Zhengzhou. First, various factors such as impact forms, impact height, flow velocity, water depth, and wave height are considered, and a wave-water flow coupling calculation method is proposed to determine the catastrophic flood load acting on inclined slopes. The maximum impact load calculated is then applied to simulate the impact process of the catastrophic flood using Particle Flow Code 2D. As the flood-related parameters increase, the maximum impact load on the subgrade slope also increases, leading to the breakdown of the subgrade and causing fatal damage. Finally, the failure process of the coal gangue railway subgrade under the impact and scour of a catastrophic flood is divided into three stages: erosion weakening, potential failure surface formation, and overall failure. The main failure mechanisms of each stage are revealed. These research result findings offer valuable insights for the design of railway subgrade reinforcement under catastrophic flood conditions.

Predictive models for assessment of buried pipeline response under seismic landslides in Iran

In seismic regions both in Iran and around the world, subterranean gas pipelines inevitably extend through high-risk areas prone to seismic landslides. The seismic landslide-pipe failure mechanism constitutes a continuum geomechanical challenge influenced by factors such as sliding mass configuration, pipe positioning relative to potential slope failure surfaces, and seismic input characteristics. In this study, response of steel pipeline buried in sand under seismic landslide action is analyzed by finite difference models using an advanced soil constitutive model. The numerical model is first validated based on the shaking table test results and then several dynamic analyses are performed using the selected records of the Iranian ground motions database. The outcomes of the dynamic analysis demonstrate that Arias Intensity (Ia) can be identified as an optimal intensity measure (IM) for predicting the seismic response of a slope-pipe system in terms of maximum pipe deflection, axial strain, and shear stress. Predictive models are then developed based on the optimal IM for estimating the pipe deflection, axial strain, and shear stress subjected to a seismic landslide. These proposed predictive models offer valuable insights for assessing the response of buried pipelines to seismic landslides in Iran within the framework of performance-based earthquake engineering.

Forensic Analysis of a Distressed Railroad Embankment

Land subsidence is becoming a prevalent worldwide occurrence, perhaps with a devastating impact on civilian lives. Moreover, natural and man-made slopes formed are susceptible to damage unless there is adequate stability. The bulk of the slope failures is due to intensive rains. Examination of slope stability due to massive rains is an important issue to inspect in the area with high precipitation. This study is aimed at exploring the effects of severe flooding on the stability of the slope. A 30 m long railroad embankment failure that happened on May 25, 2011, in the city of Ann Arbor, Michigan, is introduced. The collapse happened due to the rainy season that brought about ponding water against the embankment and sufficiently high-water pressures that brought about the instability of the railroad embankment. The back investigation is a much more dependable and predictable strategy to evaluate the shear parameters in situ. The back investigation of a slope is solved using the Resistance Envelope method wherein the embankment is divided through a certain number of slices. Forces on each slice are estimated and the potential failure surface is identified. The shear parameters are extracted employing the resistance envelope method for the actual failure plane and are verified with the initial condition. This study aims to find the impact of precipitation on slope stability using Geostudio 2012 software.

A conceptual three-dimensional frictional model to predict the effect of the intermediate principal stress based on the Mohr-Coulomb and Hoek-Brown failure criteria

The intermediate effective principal stress has a considerable influence on rock strength as shown by true-triaxial data on intact rock. Therefore, a failure criterion that incorporates all three principal stresses may provide an improved prediction of failure than one that considers only the major and minor effective principal stresses. This study introduces a generalisation method, which converts two-dimensional failure criteria (i.e., Mohr-Coulomb and Hoek-Brown failure criteria) to three-dimensional failure criteria, for the prediction of this true-triaxial data. This method is based on a physical mechanism, and it assumes that the potential failure surface of a material has roughness, which provides additional frictional resistance when the intermediate effective principal stress is greater than the minor effective principal stress. This roughness is introduced theoretically by assuming a repeating diamond-shaped element over the failure surface. These roughness elements have the same dip as predicted by the Mohr-Coulomb and Hoek-Brown failure criteria but are offset by the dip direction angle which is assumed similar to that calculated traditionally from the major effective principal stress direction to the failure surface. With this roughness, it is shown that the major effective principal stress at failure can be predicted using an explicit solution that uses the intermediate and minor effective principal stresses and the Mohr-Coulomb or Hoek-Brown parameters as inputs. This generalisation predicts the same strength as the Mohr-Coulomb or Hoek-Brown failure criteria under conventional triaxial loading conditions, when the intermediate and minor effective principal stresses are equal. Under general stress conditions, this generalisation produces results that are aligned closely with the true-triaxial data as well as other previous theoretical predictions. This method importantly gives a possible physical mechanism for the strengthening influence of the intermediate effective principal stress. This may lead to a more accurate prediction of the rock strength under general stress conditions.

Force distribution on polymeric geostrip reinforcements in MSE walls under seismic excitation

In this experimental study, the values and distributions of dynamic tensile force along the polymeric geostrip reinforcements were investigated under stepped increasing sinusoidal excitations with a series of shaking table tests on 1500 mm high or 1/3 scale physical mechanically stabilized earth wall (MSE) model. The effects of the peak ground acceleration ([Formula: see text]), slope angle of the cohesionless backfill material ([Formula: see text]), and stiffness of the polymeric geostrip reinforcement were assessed on both the potential failure surface geometry in the backfill soil and dynamic tensile forces along the lengths of polymeric geostrips. Three potential failure surfaces from the results of shaking table tests in the backfill cohesionless soil were detected as for states of quasi-elastic, plastic, and failure. However, neither the current design codes for MSE walls nor the theoretical recommendations truly reflect potential failure surface geometry in the backfill soil. Additionally, the values and distribution geometry of maximum incremental dynamic tensile forces ([Formula: see text]) on the reinforcements along the height of MSE wall recommended by design codes are not compatible with test results; however, the normalized location of total maximum incremental dynamic tensile force ([Formula: see text]) acclaimed by design codes is consistent with the test results. The traditional pseudo-static limit equilibrium methods result in over conservative horizontal dynamic earth pressure coefficient ([Formula: see text]) values generally as compared with the test results. The slope angle of the backfill material and stiffness of reinforcement have a negligible effect on the potential failure surface geometry; however, incremental dynamic tensile forces, total maximum incremental tensile forces, and horizontal dynamic earth pressure coefficient values increase with an increase in the stiffness of reinforcement and inclination angle of the backfill and peak ground acceleration value.

Modified minimum principal stress estimation formula based on Hoek–Brown criterion and equivalent Mohr–Coulomb strength parameters

The most critical parameter for determining equivalent values for the Mohr–Coulomb friction angle and cohesion from the nonlinear Hoek–Brown criterion is the upper limit of confining stress. For rock slopes, this value is the maximum value of the minimum principal stress (sigma_{3,max }^{prime }) on the potential failure surface. The existing problems in the existing research are analyzed and summarized. Using the finite element method (FEM), the location of potential failure surfaces for a wide range of slope geometries and rock mass properties are calculated using the strength reduction method, and a corresponding finite element elastic stress analysis was carried in order to determine sigma_{3,max }^{prime } of the failure surface. Through a systematic analysis of 425 different slopes, it is found that slope angle (β) and geological strength index (GSI) have the most significant influence on sigma_{3,max }^{prime } while the influence of intact rock strength and the material constant m_{i} are relatively small. According to the variation of sigma_{3,max }^{prime } with different factors, two new formulas for estimating sigma_{3,max }^{prime } are proposed. Finally, the proposed two equations were applied to 31 real case studies to illustrate the applicability and validity.

Model tests of geosynthetic-reinforced soil walls with marginal backfill subjected to rainfall

A series of model tests were conducted to investigate the performance of geosynthetic-reinforced soil (GRS) walls with marginal backfill subjected to rainfall infiltration. The effectiveness of improvement measures—such as decreasing reinforcement spacing and increasing sand cushion thickness—to prevent the GRS wall failure due to heavy rainfall was evaluated. The distribution and variation of the volumetric water content, porewater pressure, wall deformation, and reinforcement tensile strain were monitored during the test. The advancement of the wetting front and the drainage function of sand cushions were visually observed using the fluorescent dyeing technique. For the baseline case, the wall began to deform as rainfall proceeded, causing the potential failure surface to gradually move backward. When the potential failure surface moved beyond the reinforced zone, the pullout of the topmost reinforcement layers occurred, resulting in the collapse of the GRS wall in a compound failure mode. Decreases in reinforcement spacing and increases in sand cushion thickness effectively reduced wall deformation and enhanced wall stability. The placing of sand cushions between the reinforcement layers can also delay water infiltration and reduce the accumulation of porewater pressure inside the wall. Suggestions for designing rain-resistant GRS walls are also proposed based on the findings.

Reinforcement placement on mechanics and deformation of stepped reinforced retaining wall experimental study of characteristics

Abstract Three sets of indoor model tests of reinforced retaining walls were conducted to study the effects of reinforcing material placement on the displacement of reinforced retaining walls, wall top settlement, earth pressure distribution, and potential failure surface. The test results show that under different reinforcement laying conditions, the maximum horizontal displacement of the lower wall panel appears at the top of the lower retaining wall, and the maximum horizontal displacement of the upper wall panel appears at 0.6H. The settlement of the top of the wall decreases by about 9.1% when the reinforcement is laid in the lower layer. Under the condition of 160 kPa, the maximum horizontal and vertical earth pressures increase by about 19.2 and 12.4%, respectively, and the position of the potential fracture surface of the lower wall moves up to the back of the wall with the position of the reinforcement laying. When the reinforcement is laid in the upper layer, the fracture surface of the upper wall is furthest away from the panel.

Seismic evaluation of helical soil-nailed walls using shaking table testing

The effects of the nail arrangement and nail inclination on the dynamic performance of helical soil-nailed walls (HSNWs) were evaluated using a series of shaking table tests. The results showed that although a uniform increase in the nail length along the wall height significantly improved the seismic performance of the HSNWs, this improvement could also be achieved to some extent by increasing the length of the nails locally in the lower and upper halves of walls reinforced with horizontal and inclined nails, respectively. The use of inclined nails instead of horizontal ones was an efficient solution for reducing the lateral displacement, the acceleration amplification and increasing the fundamental frequency of HSNWs. The effectiveness of this solution was reduced with the use of shorter nails in the upper half of a wall and eventually minimized by reducing the length of the nails across the wall height. The nails located in the lower half of the wall were identified as having the greatest effect on the seismic performance of HSNWs when horizontal nails were used. The opposite occurred when inclined nails were used. A parabolic failure surface with a specific inflection point was observed to be the potential failure surface of the HSNW. The dimensions of the potential failure surface increased with an increase in the length and inclination of nails. Also, a combination of overturning and base sliding was identified as the predominant deformation mode in HSNWs, although the base sliding mode faded with an increase in the nail inclination.

The effect of surficial soil on the seismic response characteristics and failure pattern of step-like slopes

Earthquake-induced shallow landslides are widely distributed in earthquake-affected areas. These hazards mainly originate from fragmented surface rock masses or surficial loose deposits overlaid on bedrock structure slopes. The seismic response of the surficial soil-bedrock slope is the key to revealing the dynamic failure mechanism of earthquake-induced landslides. This study investigates the seismic response of a surficial soil-bedrock step-like slope to evaluate the acceleration ground motion amplification along the slope. Parametric analysis focusing on the geometry of the surficial soil, thickness of the surficial soil, surficial soil-bedrock impedance contrast and geometry of the slope is conducted using the finite difference modeling code FLAC3D. The results show that both the surficial soil characteristics and geometry of the slope have great influences on the acceleration amplification factor in the horizontal direction (AAF). A maximum AAF zone is always formed near the interface between the surficial soil and bedrock, and the AAF always abruptly amplifies near the slope surface, especially at the crest of the slope. The potential dynamic failure surfaces under earthquakes are formed near the interface between the surficial soil and bedrock, which is in accordance with the AAF amplification zone of the slope, and the depth of the dynamic failure of the surficial bedrock slope is deeper than the static failure surface. It can be concluded that the topographic effect on the seismic response of the slope is strongly dependent on the surficial soil material characteristics and geometry. The findings of this study can contribute to a better understanding of the mechanism of shallow earthquake-induced landslides and have a number of important implications for disaster prevention and mitigation.

Edge punching shear of waffle slabs subjected to moment parallel to the slab's free edge

The test results of five one-tenth scale micro-concrete waffle slabs subjected to edge punching shear in the presence of moment transfer parallel to the slab's free edge are presented. Although the observed punching failure mechanism of the waffle slabs was found to be very similar to that of a flat slab, the shear capacity was reduced because some of the potential failure surface is lost when it extends into the waffle section. Since the current codes of practice do not specifically consider the punching shear mechanism of waffle slabs, comparisons with test results revealed that the application of various codes overestimated the punching capacities of waffle slabs. An upper-bound plasticity theory model was developed to predict the punching shear failure load. The proposed model assumes concrete to be a perfectly rigid plastic material with the modified Mohr–Coulomb failure criterion. The model showed good agreement with the test results.

Experimental study on the mechanical behavior of shored mechanically stabilized earth walls for widening existing reinforced embankments

Shored mechanically stabilized earth (SMSE) walls have been increasingly applied in embankment widening projects because of their good mechanical performance, simple construction, low cost, and low site requirements. In this paper, several large-scale model tests were conducted to explore the mechanical behavior of the composite structures with different connection forms and relative densities, and the wall deformation, earth pressure, reinforcement strain, potential failure surface and the effects of the connection forms behind SMSE walls were also analyzed. The results show that the deformation of SMSE walls is mainly concentrated on the upper middle part, showing a “bulging” failure trend. The deformation of the SMSE walls can be effectively controlled by improving the relative density and adopting a “sandwich” connection behind the walls. The horizontal earth pressure against the SMSE wall facing shows a “K”-shaped distribution, and the vertical earth pressure is large in the upper part and small in the lower part. The potential failure surface originated at the junction of the old and new retaining walls, forming a “double-line” failure surface. For a “sandwich” connection, the failure surface moves forward and occurs where the primary and secondary reinforcements overlap, and this connection form is recommended in engineering practice.

A rare case of downward progressive failure in eastern Canada: the 1976 Saint-Fabien landslide

The Saint-Fabien landslide occurred in December 1976, 270 km northeast of Quebec City, Canada. The landslide took place in a valley filled with glaciomarine sensitive clay covered by peat bog deposits and presenting an upwards hydraulic gradient. Unique features of this event are the ground surface and the failure surface that are both close to the horizontal. In addition, passive and active failure zones were observed in the debris after the landslide, indicating a possible downward progressive failure mechanism, rarely observed in Eastern Canadian clays. Possible triggers of the event are: the continuous exploitation of the peat downslope and (or) the roadworks going on along Road 132 at the time of the event, which is crossing the landslide area upslope. Conventional stability analysis for the site gives a factor of safety of 1.43, indicating that limit equilibrium analysis cannot explain the formation of the failure surface observed at Saint-Fabien. The initiation and the propagation of the failure were analysed through 1D numerical modelling using the concept of progressive failure. The methodology consists of two steps: first the calculation of the initial stresses in the slope with the finite element software PLAXIS 2D; and second, the modelling of initiation and propagation of the progressive failure along the potential failure surface with the finite element code BIFURC, developed at the Norwegian Geotechnical Institute. Results from the modelling with BIFURC show that the concept of progressive failure is able to explain the extent of the landslide observed at Saint-Fabien with parameter values based on what was measured in situ.