Changes In Factor Of Safety Research Articles

Hydrological factors affecting rainfall-induced shallow landslides: From the field monitoring to a simplified slope stability analysis

Rainfall-induced shallow landslides can seriously affect cultivations and infrastructures and cause human losses. A continuous monitoring of unsaturated soil hydrological properties is needed to understand the effects of pore water pressure and water content on shallow landslides triggering and slope safety factor. In this work, the impact of water content, pore water pressure and hydrological hysteresis on safety factor reconstruction is analyzed by applying two different models (Lu and Godt's and SLIP models) to a monitored slope located in Oltrepò Pavese (Northern Italy). A shallow landslide event in the studied slope during the monitoring allowed for identifying the triggering mechanism and modeling the safety factor changes. The conditions of instability mechanism develop in periods with frequent rainfalls: the uprising of a perched water table in the soil profile gets nil or positive pore water pressure, which, linked with the nil effective soil cohesion of some soil horizons, allows for the shallow landslide triggering. The safety factor trends correctly predicted unstable conditions (safety factor≤1.0) in correspondence of the shallow landslide triggering time on the basis of both water content and pore water pressure. A better prediction gets when the safety factor is modeled considering the hysteresis effects. Modeling the safety factor on the basis of water content can make a good assessment of shallow failures triggering conditions only for failures in unsaturated soils and for completely saturated soils. Instead, considering pore water pressure, it is possible estimating safety factor changes linked with the development of positive pore water pressures, which can be linked in some cases to shallow landslide development.

Numerical analysis on influence of principal parameters of topography on hillslope instability in a small catchment

This study was conducted to identify the influence of three principal parameters constituting topography (slope inclination, soil depth, and slope length) on hillslope instability in a small catchment, known as Higashifukubegawa of Shikoku Island, western Japan. The typhoon rainfall of 19–20 October 2004 was significant in causing a total of seven slope failures in the catchment, though other rainfall events of various intensities in the same year did not cause failure. To understand the influence of the three principal parameters, numerical modeling of seepage and slope stability was performed in slope profiles constructed by varying the three parameters across their permissible range prepared from the seven slope failures of Higashifukubegawa in GeoStudio (GeoStudio Tutorials includes student edition lessons, Geo-Slope International Ltd., Calgary, 2005 v.4). The change in porewater pressure and slope mass weight due to variation in values of principal parameters was used to interpret the change in factor of safety or instability. The results showed that (1) instability increases with increase in the values of all three selected parameters across their range in Higashifukubegawa with remarkable decreasing trend in factors of safety, (2) slope inclination and soil depth were observed to affect instability through change in both unsaturated zone moisture content and mobilizing force of slope mass, (3) but with slope length, the unsaturated zone moisture content was not found to change considerably which implies that the instability due to slope length is mainly governed by change in slope mass weight. Overall, this study has dealt in great detail with how hillslope instability changes with principal parameters of topography under the same simulating conditions of hydrological and geo-mechanical parameters.

3-D Elasto-Plastic SEM Approach for Pseudo-Static Seismic Slope Stability Charts for Natural Slopes

This paper briefly discusses on spectral element method, one of the recent method applied to evaluate the stability of natural slope. The parametric study is made to prepare stability charts in 3-D for the different seismic coefficient in case of dry and fully saturated soil slope conditions. This paper utilizes common soil engineering properties under the unified soil classification system (USCS). Stability charts of some representative soil slopes along with USCS material models can be useful to analyze the stability of slopes for general purposes. This paper analyzes the stability of dry and fully saturated soil slopes with horizontal seismic pseudo-static coefficient, K h , of 0.1, and 0.2 to prepare the stability charts. It is believed that dry and fully saturated soil slope conditions are the best and worst scenario of the stability and seismic coefficient, K h , of 0.1, and 0.2 are sufficiently addressed the seismic slope instability of severe damages. The results show that the stability condition of fully saturated soil slope is equivalent to dry seismic slope instability of pseudo-static coefficient, K h , 0.2 in most of the cases. This paper employs two roots related factors: (1) root cohesion, C r , of 0–20 kN/m2; (2) root zone h r , of 2.0 m to represent the vegetated soil slope stability. A significant change in safety factor is observed as per the soil slope models as well as soil material models including root related parameters. A linear relationship of factor of safety is found with root-cohesion up to, C r , of 20 kN/m2. It may not be true for lower and higher values of root-cohesions. This paper also presents a typical numerical problem and solution to illustrate the application of design charts.

Multimode observations and 3D magnetic control of the boundary of a tokamak plasma

We present high-resolution detection and control of the 3D magnetic boundary in the High Beta Tokamak-Extended Pulse (HBT-EP) device. Measurements of non-axisymmetric radial and poloidal fields are made using 216 magnetic sensors positioned near the plasma surface. Control of 3D fields is accomplished using 40 independent saddle coils attached to the passive stabilizing wall. The control coils are energized with high-power solid-state amplifiers, and massively parallel, high-throughput feedback control experiments are performed using low-latency connections between PCI Express analogue input and output modules and a graphics processing unit. The time evolution of unstable and saturated wall-stabilized external kink modes are studied with and without applying magnetic perturbations using the control coils. The 3D dynamic structure of the magnetic field surrounding the plasma is determined through biorthogonal decomposition using the full set of magnetic sensors without the need to fit either a Fourier or a model-based basis. Naturally occurring external kinks are composed of multiple independent helical modes. Smooth transitions between dominant poloidal mode numbers are observed for simultaneous n = 1 and n = 2 modes as the edge safety factor changes. Relative amplitudes of coexistent m/n = 3/1 and 6/2 modes depend on the plasma's major radius and edge safety factor. When stationary 3/1 magnetic perturbations are applied, the resonant response can be linear, saturated, or disruptive, depending upon the perturbation amplitude and the edge safety factor; increased plasma–wall interactions from the perturbed plasma are proposed as a saturation mechanism. Initial feedback experiments have used 40 sensors and 40 control coils, producing mode amplification or suppression, and acceleration or deceleration depending on the feedback phase angle.

Research on Deformation Stability of Soft Rock Slope under Excavation Based on FLAC3D

In order to study deformation and stability of soft rock slope in excavation process, the deformation, plastic area distribution, and the change of safety factor of example slope based on FLAC3D software were studied in this paper. Results show that: (1) the vertical and horizontal displacement of slope gradually increase with excavation; (2) in the process of slope excavation, tension plastic zone occurs in slope surface, and shear plastic zone with connected properties gradually forms internally; (3) based on the strength reduction method and limit equilibrium method calculation, the safety factor of slope gradually reduces with excavation, and adopting the construction form of excavation with supporting meantime is very necessary.

Rapid assessment of regional superficial landslide under heavy rainfall

In order to establish a rapid method for regional slope stability analysis under rainfall, matric suction and seepage force were taken into account after obtaining explicit solution of infiltration depth. Moreover, simplified analysis model under 3D condition was put forward based on identification and division of slope units, as well as modification of sliding direction of each column. The result shows that explicit solution of infiltration depth is of good precision; for the given model, safety factors without taking seepage force into account are 1.82–2.94 times higher; the stagnation point of slope angle is located approximately in the range of (45°, 50°); the safety factor changes insignificantly when wetting front is deeper than 2 m; when matric suction changes in the specified range, the maximum variations of safety factor are less than 0.5, which proves that matric suction plays an insignificant role in maintaining slope stability compared to the slope angle and infiltration depth. Incorporated with geographic information system, a practical application of regional slope stability assessment verifies the applicability of the proposed method.

Stability analyses of reinforced walls subjected to simulated toe scouring

ABSTRACT: Limit-equilibrium-based internal and external stability analyses were performed for two carefully designed and monitored reinforced model walls reported in a companion paper. Failure mechanisms and associated safety factor changes against toe scouring were investigated. Safety factors against various failure modes – reinforcement over-stressing, reinforcement pullout, lateral sliding, overturning and bearing capacity – were derived, based on well-known material properties and theories. It was shown that the ultimate collapse state of the wall with a low reinforcement–facing connection strength was controlled by the pullout of the reinforcement from the facing. A low reinforcement–facing connection impeded the full mobilisation of reinforcement force in the lowermost layer of the reinforcement, which in turn reduced the ductility of the structure and accelerated the failure process. For the wall with high connection strength, the ultimate collapse of the wall was controlled by the bearing capacity failure near the slope toe. The analytical bearing capacity failure state was supported by the experimental observation of the failure wedge below the facing and the reinforced zone. Experimental results also suggested that a transitional wall displacement of Dh max/Ht = 0.2–1.0%, signalling rapid wall displacement for further inclined toe cutting (or further toe scouring), is associated with higher internal and external stability safety factors than those suggested in current design guidelines that do not take into account the effects of toe scouring.

Influence of Anisotropic Internal Friction Angle on the Stability of Uniform Soil Slopes

The effect of anisotropy of friction angle in natural deposited soil on the stability of soil slopes was studied in this paper. Stability analysis was performed on a uniform soil slope with anisotropic friction angle. Spencer’s method was used, and the variation of friction angle was assumed to be linear to the change of direction of the slip surface. It was shown that 7-10 percent of change in safety factor might achieve within a 10m-highed anisotropic soil slope. It was also found from the analysis that that frictional anisotropy had no obvious effect on the location of critical slip surface.

Study on Dynamic Stability of Large-Scale Underground Cavern Group during Construction Period of Hydropower Station

Taking excavation and supporting of surge tank large-scale underground cavern group of Nuozhadu hydropower station as example, the stability of surrounding rock is analyzed systematically, and the best lining scheme is obtained. Through primary supporting calculation, the displacement and the small main stress increases continuously with excavation of cavern; the rock-point safety factor of decreases continuously with excavation of cavern; the displacement, the small main stress and the rock-point safety factor soon changed when the surrounding rock unloaded suddenly, therefore, the surrounding rock must be supported timely. After excavation, the maximum total displacement is 10.57mm, the maximum tensile stress is far less than the tensile strength of surrounding rock, and the minimum safety factor of key point does not satisfy the requirement of standard; the surrounding rock surface will be broken but not large-area damage basis of excavation. The key point’s safety factor changes and the safety factor satisfies the standard requirement only when the elevation of the second lining larger than this key point. According to lining scheme comparison, the scheme that concrete of surge tank pouring to elevation 625.5m is the most economic and reasonable lining scheme for this engineering.

Identifying the effects of parameter uncertainty on the reliability of modeling the stability of overhanging, multi-layered, river banks

Composite river banks consist of a basal layer of non-cohesive material overlain by a cohesive layer of fine-grained material. In such banks, fluvial erosion of the lower, non-cohesive, layer typically occurs at a much higher rate than erosion of the upper part of the bank. Consequently, such banks normally develop a cantilevered bank profile, with bank retreat of the upper part of the bank taking place predominantly by the failure of these cantilevers. To predict the undesirable impacts of this type of bank retreat, a number of bank stability models have been presented in the literature. These models typically express bank stability by defining a factor of safety as the ratio of resisting and driving forces acting on the incipient failure block. These forces are affected by a range of controlling factors that include such aspects as the overhanging block geometry, and the geotechnical properties of the bank materials. In this paper, we introduce a new bank stability relation (for shear-type cantilever failures) that considers the hydrological status of cantilevered riverbanks, while beam-type failures are analyzed using a previously proposed relation. We employ these stability models to evaluate the effects of parameter uncertainty on the reliability of riverbank stability modeling of overhanging banks. This is achieved by employing a simple model of overhanging failure with respect to shear and beam failure mechanisms in a series of sensitivity tests and Monte Carlo analyses to identify, for each model parameter, the range of values that induce significant changes in the simulated factor of safety. The results show that care is required in parameterising (i) the geometrical shape of the overhanging-block and (ii) the bank material cohesion and unit weight, as predictions of bank stability are sensitive to variations of these factors.

Effects of Groundwater Table Position and Soil Properties on Stability of Slope during Rainfall

Rainfall, hydrological condition, and geological formation of slope are important contributing factors to slope failures. Parametric studies were carried out to study the effect of groundwater table position, rainfall intensities, and soil properties in affecting slope stability. Three different groundwater table positions corresponding to the wettest, typical, and driest periods in Singapore and four different rainfall intensities (9, 22, 36, and 80 mm/h) were used in the numerical analyses. Typical soil properties of two main residual soils from the Bukit Timah Granite and the sedimentary Jurong Formation in Singapore were incorporated into the numerical analyses. The changes in factor of safety during rainfall were not affected significantly by the groundwater table near the ground surface due to the relatively small changes in matric suction during rainfall. A delay in response of the minimum factor of safety due to rainfall and a slower recovery rate after rainfall were observed in slopes from the sedimentary Jurong Formation as compared to those slopes from the Bukit Timah Granite. Numerical analyses of an actual residual soil slope from the Bukit Timah Granite at Marsiling Road and a residual soil slope from the sedimentary Jurong Formation at Jalan Kukoh show good agreement with the trends observed in the parametric studies.

Identifying the effects of parameter uncertainty on the reliability of riverbank stability modelling

Bank retreat is a key process in fluvial dynamics affecting a wide range of physical, ecological and socioeconomic issues in the fluvial environment. To predict the undesirable effects of bank retreat and to inform effective measures to prevent it, a wide range of bank stability models have been presented in the literature. These models typically express bank stability by defining a factor of safety as the ratio of driving and resisting forces acting on the incipient failure block. These forces are affected by a range of controlling factors that include such aspects as the bank profile (bank height and angle), the geotechnical properties of the bank materials, as well as the hydrological status of the riverbanks. In this paper we evaluate the extent to which uncertainties in the parameterization of these controlling factors feed through to influence the reliability of the resulting bank stability estimate. This is achieved by employing a simple model of riverbank stability with respect to planar failure (which is the most common type of bank stability model) in a series of sensitivity tests and Monte Carlo analyses to identify, for each model parameter, the range of values that induce significant changes in the simulated factor of safety. These identified parameter value ranges are compared to empirically derived parameter uncertainties to determine whether they are likely to confound the reliability of the resulting bank stability calculations. Our results show that parameter uncertainties are typically high enough that the likelihood of generating unreliable predictions is typically very high (> ∼ 80% for predictions requiring a precision of < ± 15%). Because parameter uncertainties are derived primarily from the natural variability of the parameters, rather than measurement errors, much more careful attention should be paid to field sampling strategies, such that the parameter uncertainties and consequent prediction unreliabilities can be quantified more robustly.

PREDICTING TIMING AND LOCATION OF RAINFALL TRIGGERING SHALLOW LANDSLIDES

For an assessment of how hydrologic processes influence the location and timing of landslide or to assess how landslide risk might change in response to changed landuse pattern, some sort of mathematical model is needed, which can incorporate all triggering factors explicitly. Proposed landslide model explicitly analyzes the effect of high-intensity and short-duration rainfalls on development of pore water pressure. Richards equation is used to evaluate the pore waterpressure response to the changing rainfall pattern and attendant change in Safety Factor of the stability of a location. The numerical solution of the unsteady pore waterpressure is experimentally verified in lab. The model is able to predict the timing and location of the shallow landslides. It is used to simulate the landslides occurred in 1973/10/26 at the Takora basin located upstream of Kizu river basin. The result of model encourages for development of more generalized model in future incorporating more factors that influence landslide.

Effects of pipeflow on the changes in safety factor in slopes: A numerical modeling approach

Effects of pipeflow on the changes in safety factor in slopes: A numerical modeling approach

The effect of antecedent rainfall on slope stability

A case study is presented in order to identify the effect of antecedent rainfall on slope stability for Singapore. A storm in February 1995 (during which 95 mm of rain fell in 2\(\frac{1}{2}\) h) caused more than twenty shallow landslides on the Nanyang Technological University Campus. Details of the location, size and morphology of the landslides are presented. The antecedent rainfall during the five days preceding the event was significant in causing these landslides since other rainfall events of similar magnitude (but with less antecedent rainfall) did not cause landslides. To further understand the effect of antecedent rainfall, numerical modelling of one of the slope failures is presented. The changes in pore-water pressure due to different rainfall patterns were simulated and these were used to calculate the changes in factor of safety of the slope. The results demonstrate that antecedent rainfall does play an important role in slope stability.

Change of Safety Factor of Landslide during Field Loading Test

A field loading test on a natural slope was conducted in order to study suitable slope stability evaluation technique. Safety factors by Fellenius method and simplified Janbu's method were calculated based on the results obtained from the field loading test. The differences of the analysis and the relation between the displacement of the slope and the change of safety factor in each loading step were considered. The results are summarized as follows.(1) The Fellenius method and the simplified Janbu's method gave the almost same value to the test slope.(2) When safety factor by the Fellenius method decreased about 5% from the time that monitoring equipment detected dis-placement, the rate of displacement reached a steady state or had an acceleration tendency, which suggested that the slope changed into a limit equilibrium.

Determination of field line location and safety factor in TEXT-U

Biasing a Langmuir probe to collect electrons creates a magnetic perturbation by drawing current along a field line. This perturbation propagates along the magnetic field as a shear-Alfvén mode and marks the affected field lines with a potential perturbation at the probe sweep frequency. This technique has been used in TEXT-U edge plasmas to locate a connection between two sensor probes separated by either 12 or 28 m along a field line, and this connection has been verified using magnetic field line tracing codes. The driver probe creates a shadow which spreads in the poloidal direction with increasing distance parallel to the magnetic field due to both the change of safety factor with minor radius and collisional effects. Determination of the radial and poloidal structure of the perturbation down the magnetic field therefore provides a measurement of the safety factor q as a function of minor radius in the edge. In addition, the technique for locating field lines has been used to establish a purely parallel connection between two floating probes and therefore provides a measurement of parallel wave number k∥⩽0.1 m−1 and parallel correlation length l∥⩾12 m.

An influence line approach to the stabilization of slopes by cuts and fills

The effects on stability of concentrated downward and upward influence loads acting at successive positions between the head and toe of a landslide with a fixed failure surface are expressed in terms of influence lines for the resultant changes in the factor of safety. Of particular importance are the positions where no change in factor of safety is produced. On a slope section these are referred to as "neutral points": the traces of these in plan are termed "neutral lines." These concepts provide valuable guidance on the most effective locations and distributions of corrective cuts and fills, particularly for slides on pre-existing slip surfaces.Stabilization of slides by loading the toe is shown to be generally preferable to unloading the head. In complex slides, corrective cuts and fills are best restricted to the extreme head and toe positions, intermediate cuts or fills being avoided.The conditions are defined under which the flattening of slopes by the removal of material reduces stability. In certain cases such a flattening can occur naturally by the local collapse of the steep toe of a landslide. This can lead to an immediate reduction in its factor of safety and a consequent sudden acceleration. Keywords: Slope stabilization, corrective cuts, corrective fills, influence lines, neutral points, neutral lines, slope flattening.