Intact Slope Research Articles

The field-enriched finite element method with gravity effects for simulating the cracking behaviors of large-scale engineering rock masses

The field-enriched finite element method uses a scalar field defined as a field variable to describe cracks and characterize their impact on the displacement field and stress field of the solution model. It is capable of avoiding remeshing and employing level set functions to describe cracks when simulating the propagation of cracks. In this work, a field-enriched finite element model with gravity effects is proposed to simulate the large-scale failure process of engineering rock masses, and several numerical cases of geotechnical engineering are successfully analyzed. First, by introducing the unified tensile fracture criterion into the numerical model, the large-scale failure process of the intact slope is simulated. Second, the sliding process of rock slopes containing en echelon joints is numerically investigated. Third, the cracking process of the concrete dam is analyzed. Finally, the effects of joint and bedding plane inclination angles on the stability of tunnel chamber in transversely isotropic rock mass are studied. The numerical results indicate that the numerical method proposed in this work can accurately solve the large-scale failure process of rock masses.

Effects of extreme drought–rainfall on slope failure mechanisms: centrifuge modelling

The extreme drought–rainfall seesaw is projected to occur with an increasing frequency. However, there still lacks a thorough understanding of its impacts on slope behaviour, in which desiccation crack plays a key role. To address this issue, a centrifuge test was conducted to investigate the effects of drought-induced desiccation crack on slope instability under extreme rainfall. During the test, the non-cracked slope was firstly subjected to extreme rainfall with 100-year return period. Subsequently, a long-term drying was applied to induce desiccation crack, and hence forming a cracked slope. The cracked slope is then subjected to an identical extreme rainfall. The non-cracked slope only exhibits swelling deformation, whereas for the cracked slope, a slip surface (2 m in depth) is clearly observed to initiate from one deep crack at crest. The global sliding failure of the cracked slope is mainly related to preferential flow, which could result in soil shear strength reduction.

Stability analysis of unsaturated slopes under elevated temperatures

Several natural and man-made phenomena (e.g., droughts, wildfires, shallow geothermal technologies) can subject unsaturated slopes to elevated temperatures. However, the impact of temperature on the stability of unsaturated slopes is not well understood. This study aims to examine the stability of unsaturated intact slopes under elevated temperatures. An analytical framework is developed to calculate the factor of safety (FOS) of unsaturated slopes under steady fluid flow conditions at various temperatures. The theoretical basis of the framework is built upon the concept that, within the temperatures range investigated, the influence of temperature on the soil's shear strength is primarily attributed to thermal induced changes in apparent cohesion stemming from matric suction. Temperature dependency of apparent cohesion is quantified by employing temperature-dependent soil-water retention curve (SWRC) and effective stress models, which are then integrated into an infinite slope stability analysis of unsaturated soils under steady flow. A saturation-dependent thermal conductivity is used to obtain the temperature profile versus depth. The formulations are used to determine FOS for intact slopes with different hypothetical soils including sand, silt, and clay at surface temperatures of 25, 40, and 55 °C under different steady flow rates. Results show that the effective stress, shear strength, and FOS increase by increasing temperature from 25 °C to 55 °C. Results reveal that the effect of temperature on the stability of clayey and silty slopes can be considerable. The proposed models are validated against two sets of laboratory-measured data from the literature. The presented model provides a simple and effective method for site-specific and regional-scale stability analyses of unsaturated slopes under different thermal and seepage conditions.

Q-slope and SSAM applied to excavated coal mine slopes

The Q-slope classification system is used to assess the stability of excavated rock slopes and provide an indication of long-term stable, reinforcement-free slope angles. Q-slope is based on over 500 rock slope case studies from mines, road and rail cuttings hosted in igneous, sedimentary and metamorphic rocks around the world. Q-slope can be applied for slopes ranging from less than 5 m to more than 250 m in height in both civil and mining environments.This paper describes the application of Q-slope classification system to 38 failed and intact slopes from Australian open cut coal mines. It further describes the relationship between Q-slope and Slope Stability Assessment Methodology (SSAM) ratings for stable slopes based on the available case studies.

Failure mode analysis of post-seismic rockfall in shattered mountains exemplified by detailed investigation and numerical modelling

A high-position destructive rockfall, significantly influenced by post-seismic effect of Wenchuan earthquake in 2008 and associated aftershocks, was reported in Miansi Town of Wenchuan County, Sichuan Province, China. About 3,000 m3 of rock at high position detached from a dipping rocky slope, which manifested as a translational motion. Detailed field investigation, geological mapping, kinematic analysis, and numerical modelling are presented to comprehensively analyze the complex failure mode of the post-seismic rockfall. The results demonstrated that the sliding surfaces are mainly controlled by the internal shear discontinuity sets. The backscarps are progressively evolved from the tensile cracks at rear slopes caused by the Wenchuan earthquake. Because of the combination effect of heavily shocks of previous earthquake and local geomorphology of the rockfall with beneficial for facilitating the magnification effect on peak ground acceleration (PGA), the rock mass of slopes is dramatically damaged and fractured. The original intact slopes finally developed to the shattered slopes as an optimum birthplace for the post-seismic rockfalls. It should be noticed that intensive precipitation plays a triggering factor that generates the hydrostatic water pressure in the rear cracks and the uplift pressure at the bottom sliding surface within a short time. The evolution processes are divided into three stages: slope relaxation stage, rock shattering and disaggregation stage, and high-speed falling stage. Velocities, energy, and bounce heights of falling rock blocks are computed by the RocPro3D software for understanding the corresponding kinematics. The paper can provide an insight into post-seismic rockfalls in the tectonic areas associated with the combination of seismic activities and intense rainfall.

Static and seismic stability assessment of 3D slopes with cracks

The majority of extant analyses on slope stability assumed a two-dimensional plane-strain condition and/or an intact slope. However, slope failures typically exhibit three-dimensional (3D) characteristics and cracks are commonly present in cohesive slopes. This study develops a framework based on the kinematic limit-analysis method for static and seismic stability evaluation of 3D slopes with cracks. A pseudo-static approach is adopted to represent seismic effects. Two types of cracks are considered: cracks prior to slope failure (open cracks) and cracks forming contemporaneously with slope failure (formation cracks). A 3D horn-like collapse mechanism including a vertical crack is used to represent the slope collapse. A comparison with previous studies is conducted to validate the proposed approach. A series of stability charts is presented for the ease of direct use. Results show that consideration of seismic effects and existence of cracks lead to a significant reduction in slope stability, whereas the inclusion of 3D effects yields a greater safety factor. The influence of cracks becomes more significant for slopes with greater inclination angles and higher levels of soil cohesion. This study considers a more realistic situation, the results of which can serve as a reference tool in the seismic design of slopes.

Stability of intact slopes with tensile strength cut-off

Stability of intact slopes with tensile strength cut-off

Effect of the characteristics of surface cracks on the transient saturated zones in colluvial soil slopes during rainfall

The presence of surface cracks has a great influence on the formation of transient saturated zones, which are one of the main factors affecting the stability of colluvial soil slopes during rainfall. This paper aims to examine how the distribution characteristics, such as depth (h), angle (θ), and location of surface cracks, influence the development of transient saturated zones. For this reason, a series of numerical simulations involving colluvial soil slopes is performed. The results show that the development of transient saturated zones in a slope with a crack and that in an intact slope are quite different. In a cracked slope, rainwater infiltrates into the slope along the crack and forms a transient saturated zone at the crack tip. It is found that the smaller the crack depth is, the easier it is for the transient saturated zone at the crack tip to connect with that in the shallow layer, leading to an increase in the area of the transient saturated zone in the shallow layer. Moreover, the larger the crack angle is, the more easily the transient saturated zone at the crack tip connects with both the transient saturated zone in the shallow layer and the groundwater, forming a transient saturated zone that penetrates the entire colluvium. In addition, the lower the location of the crack along the slope surface is, the shorter the time is for the formation of a transient saturated zone at the crack tip, and the larger the area of the transient saturated zone.

Stability of intact slopes with tensile strength cut-off

The Mohr–Coulomb yield criterion, typically used in slope stability analyses, predicts the uniaxial as well as the isotropic (triaxial) tensile strength. In this study, the tensile strength was reduced or eliminated from the yield condition in order to determine whether it has any consequences on the outcome of the stability analyses of slopes. It was discovered that the consequences on the slope stability factor are very significant. Slopes that are most affected by the tension cut-off are steep slopes, particularly in the presence of seepage. The difference in stability factor for steep slopes calculated with the classical Mohr–Coulomb strength envelope and a criterion with tensile strength cut-off can exceed 50% for steep slopes subjected to seepage (70% for vertical slopes). Tension cut-off used in the analysis permits deformation modes with the dilatancy angle exceeding the internal friction angle, and this is the likely cause of producing more critical mechanisms of slope collapse. The novelty of this study is in presenting a new slope stability analysis that accounts for tension cut-off, and in demonstrating the significant impact of reduced tensile strength on the outcome of the stability assessment.

Study on Early Recognition of Loess Landslides Based on Field Investigation

For the purpose of agriculture irrigation, river water has been introduced into Heifangtai region from the Yellow River several times a year since 1968, which ensued a steady rise in underground water level by a yearly rate of 0.18 m with a cumulative total rise of up to 20 m over Heifangtai catchment. This ponding effect of water injection triggered landslides (3~5 events per year) at the edge of Heifangtai tableland. Failures of loess slopes have caused high causalities, and inhibited the local economy. This study aims at investigating the distribution patterns of loess landslides and their formation conditions in Heifangtai region, so as to establish approaches to early recognition and prediction of underlying landslides. Landslides in Heifangtai were delineated through visual interpretation of high resolution images obtained by unmanned aerial vehicle photogrammetry. Digital Elevation Model with high resolution 10 cm was acquired by 3D laser scanning technology and close-range photogrammetry. With field investigation, landslide distribution patterns and characteristics, as well as the local geological structure, were examined and two early recognition criteria for landslides are established. The results indicate that an intact slope section between two old landslide sections is the most possible locations for new landslides and areas with rising groundwater level are prone to landslides. For example, in 2015 a landslide occurred at the gaps of a 2014 landslide body. The rising of local groundwater level was induced either by blockage of underground water outlets by landslide deposits and freeze in winter or by heavy rainfall. Early recognition of loess landslides would protect local communities and land resources from landslide hazards.

Earthquake-induced displacement of cohesive-frictional slopes subject to cracks

The upper bound theorem of limit analysis together with Newmark's method are employed to evaluate the displacement of soil slopes subject to cracks. The pseudo static approach has been routinely used in the literature to estimate the seismic displacement of soil slopes. However, the effect of cracks on the slope displacement has yet to be tackled. In this paper, a new technique is proposed to estimate the horizontal displacement at the slope toe due to a given earthquake postulating rough estimation of real time crack formation. Rotational failure mechanisms for intact slopes exhibiting the formation of cracks as part of the failure process and the case of cracks which are pre-existing in the slope were considered. On the basis of Newmark's method, the seismic-induced displacement is calculated by incorporating a stepwise yield acceleration corresponding to the cracks occurring in the slope. Results of the proposed technique can reasonably bridge the gap between the conservatism of assuming the slopes subject to the most detrimental cracks, and the overestimation of slope stability resulted from the neglect of crack formation. An example illustrating the procedure for a given earthquake is presented. Also, charts providing the values needed to calculate the stepwise yield accelerations are proposed.

Effects of freeze–thaw on the determination and application of parameters of slope rock mass in cold regions

Freeze–thaw could significantly affect the deterioration of mechanical properties of slope rock mass, and should be taken into consideration when determining the parameters of slope rock mass in cold regions. In this paper, we first measured the freeze–thaw coefficient using the freeze–thaw cycle test on intact slope rock mass from Mengku Mine, then determined the freeze–thaw correction coefficient by taking field geological investigation and rock hardness into consideration, and established a linear relationship between Geological Strength Index (GSI) and Tianshan slope rock mass rating (TSMR) system (a rock mass classification method applicable in cold regions) by incorporating the freeze–thaw correction coefficient into the expression of rock mass effect. The results show that freeze–thaw correction coefficient varies between 0.71 and 0.93, and GSI varies between 45 and 63. We further determined the parameters of slope rock mass by employing the generalized Hoek–Brown criterion and using it established a numerical rock slope excavation model and analyzed the rock mechanic features and stability status. The results indicate that the maximum ground deformation of slop rock mass is 15mm, and the floor rebound is about 42mm. Overall, the method described in the present paper can be used to select slope rock mass in cold regions and calculate the slope rock stability.

Bioengineering Techniques Associated with Soil Nailing Applied to Slope Stabilization and Erosion Control

Slope failures in the tropics are directly related to the rainfall intensity. Refining techniques to prevent rain-induced slope failures play an important role, especially for the risky occupied areas close to big cities—the final destination from people who migrates from rural to urban areas. This paper presents the implementation of bioengineering associated with nailing techniques to control erosion and prevent rain-induced failures in a 35-m-high rocky slope (Ponteio Slope). The exposed phyllite rocks of this slope and the absence of a proper drainage system have been the identified as the trigger mechanism to the successive failures verified since 1953. In the past 15 years, berms and a concreted cover were built as attempts to stabilize this slope, but no drainage system was associated with those works. In December 2003, the implementation of bioengineering and soil nailing techniques in a total area of 1,700 m2 with a declivity, for the intact slope, of about 63° began. After the cleanup of the talus and the fissured concrete cover, the rocky mass was reinforced by connecting the superficial loose phyllites to the deeper and more resistant quartzite layer with 6-m steel reinforced bars, 130 in total. The subsequent operations carried out were seeding, rolling out the coconut fiber blankets and reinforced mat onto the slope surface, and finally, the placement of a metallic net anchored to the rods by bolts. The natural aesthetics of the whole bioengineering system adopted was improved as well as helping to improve the natural drainage system and avoiding the uplift pore pressures. The association of “soil nailing” and bioengineering techniques were cost-effective and succeeded in stabilizing the slope after several previous attempts. The slope was monitoring during the techniques implementation in 2003 and in the subsequent year, 2004. No failure occurred.

A centrifugal model study of slope instability in Ottawa area Champlain Sea clay

Problems of serious slope instability in quick Champlain Sea clay, also known as Leda clay, found in eastern Canada, have been the subject of debate regarding mechanisms of failure and relevant strength characteristics. Eight intact samples of Ottawa area Champlain Sea clay were retrieved, formed into 19 different slope configurations, and tested as reduced-scale models on the Cambridge University 4.25 m diameter 150 g geotechnical centrifuge. Of these, 14 showed some manner of slope instability, ranging from deep intact slope movement, to slope degradation, to flowsliding. Speculation is made on the importance of time-dependent horizontal stress release, on the possibility that prototype failure is not circular and rotational, and that uplift may affect subsequent retrogressions in prototypical landslides but not initial failure. Inconsistencies in the behaviours of models were attributed in part to certain possible limitations on reduced-scale modelling; these included the effects of the unscaled network of fissures and the loss of strength as a function of displacement rather than of strain. Key words: centrifuge, clays, failures, simulation models, slope stability.