Rainfall-induced Slope Failures Research Articles

LABORATORY INVESTIGATION OF DRAINAGE CELL AS TRANSPORT LAYER IN RESIDUAL SOILS

Transport layer or unsaturated drainage layer is an alternative measure often employed to improve the performance of a capillary barrier system. A capillary barrier is an earthen cover used to prevent rainfall-induced slope failure. In this study, potential of using drainage cell system as transport layer is exploited using laboratory experimental methods. The drainage cell was sandwiched between grade V and grade VI soils in a two-dimensional laboratory slope model to act as transport layer. Coarse particles of gravel were compacted within the drainage cell to facilitate capillary break development at the interface. Each of grade V and grade VI soil was mixed with water content identical to residual water content determined from the soil water characteristic curves (SWCC) of each soil to enable simulation of their initial condition. Both soils are then compacted in the slope model to their dry densities. The whole set up was subjected to three rainfall intensities of 1.0586 x 10-5 m/s, 1.2014 x 10-6 m/s and 3.7337 x 10-7 m/s for 2 hour, 24 hour and 7 day, respectively. These rainfall intensities were determined from Intensity-Duration-Frequency (IDF) curve and were applied through a rainfall simulator which is part of the laboratory set up. The results shows that the transport layer formed with drainage cell was capable of producing capillary break and impedes percolation of the infiltrating water into the lower grade V soil layer. The accumulated water was later drained laterally above the interface of grade VI soil and drainage cell transport layer towards the toe of the slope model. In an event that the infiltrating water percolates the drainage cell transport layer due to longer rainfall duration, the drainage cell provides a definite direction through which the infiltrating water flow and diverted laterally. It was found that the modified capillary barrier with drainage cell transport layer performed much better than the conventional capillary barrier system.

A Study on Rainfall Induced Slope Failures: Implications for Various Steep Slope Inclinations

A rainfall induced slope failure is a common natural hazard in mountainous areas worldwide. Sudden and rapid failures which have a high possibility of occurrence in a steep slope are always the most dangerous due to their suddenness and high velocities. Based on a series of experiments this study aimed to determine a critical angle which could be considered as an approximate threshold for a sudden failure. The experiments were performed using 0.42 mm mean grain size sand in a 200 cm long, 60 cm wide and 50 cm deep rectangular flume. A numerical model was created by integrating a 2D seepage flow model and a 2D slope stability analysis model to predict the failure surface and the time of occurrence. The results showed that, the failure mode for the entire material will be sudden for slopes greater than 67°; in contrast the failure mode becomes retrogressive. There is no clear link between the degree of saturation and the mode of failure. The simulation results in considering matric suction showed good matching with the results obtained from experiment. A subsequent discarding of the matric suction effect in calculating safety factors will result in a deeper predicted failure surface and an incorrect predicted time of occurrence.

Behavior analysis by model slope experiment of artificial rainfall

Abstract. In this study, we performed a model slope experiment with rainfall seepage, and the results were compared and verified with the unsaturated slope stability analysis method. In the model slope experiment, we measured the changes in water content and matric suction due to rainfall seepage, and determined the time at which the slope failure occurred and the shape of the failure. In addition, we compared and verified the changes in the factor of safety and the shape of the failure surface, which was calculated from the unsaturated slope stability analysis with the model experiment. From the results of experiment and analysis, it is concluded that the unsaturated slope stability analysis can be used to accurately analyze and predict rainfall-induced slope failure. It is also concluded that in seepage analysis, setting the initial conditions and boundary conditions is very important. If engineers will use the measured porewater pressure or matric suction, the accuracy of analysis can be enhanced. The real-time monitoring system of porewater pressure or matric suction can be used as a warning of rainfall-induced slope failure.

불포화 토사사면 해석에 대한 강우사상과 분포의 영향

The time distribution of rainfall is one of the most important considerations for evaluating soil slope stability. In order to study the rainfall-induced slope failure, the rainfall pattern has generally been assumed as uniform rainfall intensity for rainfall duration. However, it should be noted that the time distribution of the design rainfall method has a significant effect on the soil slope instability. The study implemented Mononobe, Huff, and uniform method as three types of time distribution method of the design rainfall to estimate the factor of safety of soil slopes by rainfall duration. As a result, the difference of soil suction and unsaturated hydraulic properties in a soil by rainfall pattern was found through the application of an appropriate time distribution method to numerical simulation for rainfall-induced slope stability.

Evaluation of rainfall induced slope failure in tumulus mounds and conservation of the damaged tumuli

Evaluation of rainfall induced slope failure in tumulus mounds and conservation of the damaged tumuli

Development of the heavy rainfall induced slope failure hazard maps for the villages located in the mountainous area close to Median Tectonic Line

Development of the heavy rainfall induced slope failure hazard maps for the villages located in the mountainous area close to Median Tectonic Line

Site-specific warning system for rainfall-induced slope failure

Site-specific warning system for rainfall-induced slope failure

A physically-based multi-hazard risk assessment platform for regional rainfall-induced slope failures and debris flows

Rainfall-induced slope failures and debris flows are two major hazards in mountainous areas. A physically-based multi-hazard risk assessment platform for regional rainfall-induced slope failures and debris flows has been developed in this study. The platform enables prompt assessment of risks posed by regional rainfall-induced slope failures and debris flows across multiple catchments, which is required in landslide risk management in a large area. It considers the contribution of slope failures to debris flows and the scenario of a location impacted by multiple slope failures or debris flows or both. The contribution of slope failures to debris flows is considered by adding the increased amount of channel deposit from slope failures to the source material of debris flows. The platform is applied to a highway near the epicentre of the 2008 Wenchuan earthquake. The platform predicts the impact areas and runout distances of regional debris flows reasonably well. The risk assessment results indicate that both slope failures and debris flows pose a great danger to travellers along the road shortly after the earthquake due to the presence of a large amount of loose landslide deposits on steep terrains. The materials from the slope failures triggered during a storm substantially increase the channel deposit volume, leading to significantly increased debris flow volume and risk. A multi-hazard risk assessment approach is necessary to consider the scenario of a location impacted by multiple slope failures or debris flows or both, since assessing risks of slope failures and debris flows separately may underestimate the risk.

Laboratory and modelling investigation of root-reinforced system for slope stabilisation

Most natural slope failures are induced by seepage and/or rainfall. Soil bioengineering is an environmentally friendly method which employs vegetation to reinforce the soil in sloping terrain. The vegetation can contribute to slope stability in two ways, mechanical and hydrological. This paper demonstrates the effect of a vegetation root matrix on a soil slope and focuses on mechanical reinforcement using an example of vetiver grass. Vetiver grass (Vetiveria nemoralis A. Camus) specimens, grown for under a year, were used in this study. The investigation programme includes root observations, direct shear tests and centrifuge model tests. The growing rate of the vetiver roots and the root area ratios were observed during the tests. The cohesion and angle of internal friction of root-reinforced soils were determined from a standard direct shear apparatus and a large direct shear apparatus. A series of centrifuge tests was carried out to demonstrate the effect of vegetation on seepage- and rainfall-induced slope failures. The results indicate that the vetiver roots showed rapid growth within a year and that the shear strength of the root-reinforced soil was significantly increased by the bundle of roots. The results also reveal that the bundle of root fibres in the centrifuge model tests helped to reduce the deformation of the soil slope due to instability by increasing the shear strength of the slope.

Improved measurement of soil moisture using an ultrasonic waveguide to predict rainfall-induced slope failure

Field monitoring of soil moisture and groundwater level is important to predict shallow slope failure stemming from heavy rainfall. We previously proposed a monitoring technique using ultrasonic waveguides in which soil moisture and groundwater levels are monitored on the basis of the intensity and propagation time of ultrasonic waves reflected from the underground soil surface, respectively. In field monitoring, the reflective intensity depends on the depth of monitoring points and the inhomogeneity of grains, so it has been difficult to examine the reflective intensity change quantitatively. In the present work, we propose improvements that will allow us to measure the intensity change quantitatively. Specifically, we utilize an ultrasonic waveguide of a constant length by using casing pipes with different lengths depending on the monitoring depth and cover the surface of coarse-grained soil with a homogeneous fine soil to obtain repeatable data for wetting and drying cycles. The positive effects of these improvements were confirmed by a field monitoring test.

Monitoring of unstable slopes by MEMS tilting sensors and its application to early warning

The present paper addresses the newly developed early warning technology that can help mitigate the slope failure disasters during heavy rains. Many studies have been carried out in the recent times on early warning that is based on rainfall records. Although those rainfall criteria of slope failure tells the probability of disaster on a regional scale, it is difficult for them to judge the risk of particular slopes. This is because the rainfall intensity is spatially too variable to forecast and the early warning based on rainfall alone cannot take into account the effects of local geology, hydrology and topography that vary spatially as well. In this regard, the authors developed an alternative technology in which the slope displacement/deformation is monitored and early warning is issued when a new criterion is satisfied. The new MEMS-based sensor monitors the tilting angle of an instrument that is embedded at a very shallow depth and the record of the tilting angle corresponds to the lateral displacement at the slope surface. Thus, the rate of tilting angle that exceeds a new criterion value implies an imminent slope failure. This technology has been validated against several events of slope failures as well as against a field rainfall test. Those validations have made it possible to determine the criterion value of the rate of tilting angle to be 0.1 degree/hour. The advantage of the MEMS tilting sensor lies in its low cost. Hence, it is possible to install many low-cost sensors over a suspected slope in which the precise range of what is going to fall down during the next rainfall is unknown. In addition to the past validations, this paper also introduces a recent application to a failed slope in the Izu Oshima Island where a heavy rainfall-induced slope failure occurred in October, 2013.

Flume Experiments for Investigation of Rainfall-Induced Slope Failure

The rainfall-induced slope failure is the major geo-hazard all over the world including Malaysia. Number of studies done already to investigate the process of slope failure, however the parameters that control the initiation of flowslide type of failure is still lacking. In tropical areas different mass movements occur from erosion to flow type of failure, in order to understand the mechanism will help to mitigate the posed risk. The flowslide is also type of slope failure that mostly occur in granular type soils, when initiated attain the higher velocity and fluid like motion, and it is dangerous than other types of landslides. The flowslide type of slope failure mainly due to rainfall. Field studies are timing consuming and expensive, while numerical studies requires lot of characteristics related to geology of the materials. Therefore laboratory flume experiments used in order to understand the mechanism and behavior of slope failure by changing different parameters such as density, rainfall intensity, thickness and initial moisture condition. The model slope prepared by sandy type of the soil and failure induced by artificial rainfall by installing the sprinklers above the model flume. During the experiments the pore pressure and moisture content were measured. From the detailed experimental study it was observed density of soil slope controls the initiation of the flowslide type failure. Small density of the soil slope suffers from flowslide type of the failure at smaller rainfall intensity. However in case of higher density even higher rainfall intensity accompanied with significant initial moisture conditions did not trigger the flowslide. The erosion gullies formed from toe to crest of the slope in the case of dense slope. The measurements of moisture content at the lower parts of the slope can be used as early warning of slope failure, however piezometers less reliable for prediction of slope failure in advance. Before large failure the settlement occur at the crest of the slope.

기반암 경계조건을 적용한 Green-Ampt 침투모델에 의한 사면의 얕은 파괴 해석

세계의 많은 지역이 강우에 의한 사면파괴에 취약하다. 우리나라는 풍화잔류토 층의 깊이가 얕고 호우가 빈발하여 포화대가 지표로부터 하강하면서 기반암에 도달하여 토층 전체가 포화상태에 이르는 침투거동이 사면파괴를 일으키는 중요한 원인 중 하나이다. 따라서 파괴면의 깊이가 얕고 풍화토층과 기반암의 경계면 근처에 형성되는 특징을 가지고 있다. 본 연구에서는 한계평형법에 바탕을 둔 사면의 얕은 파괴 해석을 위하여 경사면을 고려한 일차원 침투모델인 Green-Ampt 모델을 적용하였다. 시간에 따라 변하는 강우강도를 고려하였으며 얕은 토층을 고려하기 위하여 Green-Ampt 모델에 얕은 기반암의 경계조건을 도입하였다. 해석결과에 의하면 제안된 해석법은 사면에 대한 강우의 침투 평가 시 포화대의 기반암으로의 진행을 계산함으로써 강우의 침투거동을 사면안정에 적절하게 반영할 수 있음을 보여준다. Many regions around the world are vulnerable to rainfall-induced slope failures. In Korea, the depth of weathered residual soil is shallow and rainfall of strong intensities is frequent. Therefore, saturation of whole soil layer caused by the reaching of saturated wetting front from the slope surface to impermeable bedrock by rain infiltration is one of important causes of slope failure. This leads to the shallow depth of slope failure that passes through the interface between soil and impermeable bedrock. In this study, one-dimensional Green-Ampt model on sloping surfaces in infiltration analysis is used for the shallow slope failure analysis based on the limit equilibrium method. Time-varying intensity of rainfall was considered and shallow impermeable boundary condition was imposed in the Green-Ampt model to simulate the impermeable bedrock underlying the shallow residual soil. The results of example analysis showed that the proposed method can be used to efficiently consider the rainfall infiltration behavior caused by shallow impermeable bedrock boundary in stability assessment of infinite slope.

Modified triaxial apparatus for determination of elastic wave velocities during infiltration tests on unsaturated soils

An ordinary triaxial apparatus was modified to reproduce field stress path and the failure mechanism during rainfall induced slope failures. In addition, the system was equipped with piezoelectric sensors for the measurement of shear, and compression wave velocities during the water infiltration process. The fundamental construction of axial loading system, for the application of constant total stress is discussed herein. The axial loading system was found to be capable of maintaining constant axial stress for long duration of times with an accuracy of ±1%. A new technique of water injection through base pedestal ceramic disk, and its distribution in the specimen through a helical filter paper is presented. Distribution of injected moisture with helical filter paper was found to be reasonably uniform with maximum moisture variation in the specimen observed to be less than 2%. A miniature pore water pressure sensor, placed mid-height of the specimen was used to measure soil matric suction. Performance evaluation of miniature sensor with respect to suction measurement through base pedestal ceramic disk is presented. Effects of various types of sealing methods on the performance of miniature sensor are also discussed. Calibrations and performance of disk type piezoelectric transducers, which are capable of measuring both shear and compression wave velocities in triaxial specimen, are discussed. Effect of isotropic confining stress, on elastic wave velocity measured by disk transducers is found to be consistent with previous researches.

Application of dynamic programming to locate the critical failure surface in a rainfall induced slope failure problem

In this paper, a dynamic programming method is employed in conjunction with limit-equilibrium techniques to determine the location of the non-circular critical slip surface in the analysis of a slope failure due to a rainfall event. The Spencer method of slope stability analysis was incorporated into dynamic programming to predict the time of a slope failure and shape of the failure surface. A one-Dimensional (1D) sliding block model was used to analyze the motion of the failure mass. Furthermore, during the movement of the sliding mass, the stability of the model slope was analyzed by updating the shape of the model slope according to the new position of the sliding mass. The suction head in the soil pore provides soil shear strength to maintain the stability of the slope. The positive pore water pressure reduces the soil shear strength so that the slip surface beneath the water table will be more unstable. Numerous studies (using non-cohesive soil) have been conducted to locate the critical slip surface in slope stability problems without considering the increase in shear strength due to suction and apparent cohesion. The studies do not clearly illustrate the reasonable boundary conditions to avoid the slip surface alignment totally outside the actual slope. This paper clearly discusses the boundary conditions for such a slope stability problem reasonably. The step-by-step calculation procedure is presented through the flow diagram. Three cases of slope failure obtained from a laboratory flume experiment have been analyzed using the proposed method. The numerical simulation results and the results obtained from experiments are comparable.

Influence of transport Layer on Transient Suction Distribution in a Two-Layered Slope

Residual soil slope failure due to rainfall infiltration is one of geotechnical hazards receiving much attention in many tropical climate countries. The infiltrating water eliminates matric suction in the residual soil slope and results in slope failure. A capillary barrier is used to prevent excessive rainfall infiltration and preserve matric suction in the residual soil slope and hence prevent rainfall-induced slope failure. A numerical study to examine the performance of a transport layer in a two-layered slope using capillary barrier principle was presented in this paper. Material properties of tropical residual soils consisting of Grade V (silty gravel) and Grade VI (sandy silt) were used and modelled a two-layered slope. These material properties were obtained from representative soil sample of Balai Cerapan slope in Universiti Teknologi Malaysia, Johor Bahru campus. A granite chips (Gravel) was also incorporated to act as a transport layer in the numerical model. The simulated slope model was then subjected to three different rainfall intensities of 9 mm/h (rainfall 1), 22 mm/h (rainfall 2) and 36 mm/h (Rainfall 3) representing short, medium and high intensity rainfalls, respectively. A total of six numerical schemes were performed by restricting the thickness of the transport layer to 0.1 m. However, to assess the effect of the transport layer thickness on suction distribution; the thickness was increased to 0.2 m. The results of the study show that inclusion of gravelly transport layer enables the top layer of fine sandy silt residual soil to retain the infiltrating water as a result of capillary break developed at the interface and also divert it above the interface towards the direction of the toe of the slope. Similarly the transport layer is found to be effective in preventing water breakthrough occurrence into the underlying coarser soil layer of the two-layered slope, especially when the thickness of the transport layer is optimum.

Spatiotemporal regional modeling of rainfall-induced slope failure in Hulu Kelang, Malaysia

This study demonstrated the use of transient rainfall infiltration and grid-based regional slope stability analysis model incorporated with spatial rainfall distribution model for regional mapping of rainfall-induced slope failures in a slope failure-prone area in Malaysia, namely Hulu Kelang. Infinite slope analysis was incorporated into the improved model to compute the pore water pressure and slope stability of the region. Digital terrain maps, spatial rainfall distributions, soil profiles, groundwater table, and soil properties were input into the model. The results showed that the slope failures in the study area can be grouped into shallow failures (<4 m) and sub-shallow failures (4–8 m). Most of the shallow slope failures (<4 m) were triggered by the northeast monsoon while the NE monsoon that characterized by more prolonged and intense rainfalls. Spatial temporal distributions of shallow and sub shallow slope failures predicted using the improved model showed reasonably good comparison with the historical slope failure regions. Monsoon events have affected slope stability by reduction in matric suction, complete loss of matric suction, and development of positive pore water pressure (PWP). Computation for the effects of rainfall infiltration on PWP response and consequent effects on slope stability is enhanced by the spatial rainfall distribution model. The pore water pressure response to monsoon rainfall is a function of rainfall duration, the rate of infiltration, groundwater depth, soil thickness, and slope inclination. The amount of the local daily rainfall is not the only factor affecting slope stability. The prolonged antecedent rainfall could have a role to play in building up the slope failure mechanism.

Effect of Surface Flux Boundary Conditions on Transient Suction Distribution in Homogeneous Slope

Rainfall-induced slope failure is a common natural disaster that occurs frequently in many parts of the world. Many studies have been conducted on the effect of advancement of wetting front on reduction or elimination of the additional shear strength provided by matric suction in unsaturated soil. However, the inclusion of boundary flux condition such as evaporation in predicting soil suction for rainfall-induced slope failure analyses is still limited. Therefore, this study investigate the combined effect of two boundary flux (infiltration and evaporation) on transient suction distribution and hence the factor of safety (FOS) of a modelled slope which consist of slope height and horizontal distance of 5 and 47 m respectively with a slope angle of 21°. One year rainfall and evaporation data of a site in Johor Bahru, Malaysia were collected for the study and two months were selected to represent the wet and dry seasons based on the two predominant wind systems which brings rainfall in Malaysia (Northeast and Southwest monsoons). Soil samples were collected from the site for laboratory testing and the relevant soil properties required in the analyses were obtained. Initial soil condition was determined from Soil Water Characteristics Curve (SWCC) of the soil sample from the study area and these initial conditions were simulated at the beginning of each analysis to enable realistic condition of the soil to be obtained. The seepage analyses were conducted with Seep/w and the pore water pressure determined from the seepage analyses were used as input parameters in the slope stability analyses carried out with slope/w. The results of the analyses show that considering flux boundary condition has a great influence in maintaining negative pore water pressure in unsaturated soil especially during wet period when there is more water infiltrating the soil. Moreover, inclusion of evaporation in seepage analyses give more realistic pore water pressure and hence better prediction of FOS of the slope. Keywords: Evaporation, Flux Boundary, Rainfall, Slope Stability, Suction Distribution

Effect of surface flux boundary conditions on transient suction distribution in homogeneous slope

Rainfall-induced slope failure is a common natural disaster that occurs frequently in many parts of the world. Many studies have been conducted on the effect of advancement of wetting front on reduction or elimination of the additional shear strength provided by matric suction in unsaturated soil. However, the inclusion of boundary flux condition such as evaporation in predicting soil suction for rainfall-induced slope failure analyses is still limited. Therefore, this study investigate the combined effect of two boundary flux (infiltration and evaporation) on transient suction distribution and hence the factor of safety (FOS) of a modelled slope which consist of slope height and horizontal distance of 5 and 47 m respectively with a slope angle of 21°. One year rainfall and evaporation data of a site in Johor Bahru, Malaysia were collected for the study and two months were selected to represent the wet and dry seasons based on the two predominant wind systems which brings rainfall in Malaysia (Northeast and Southwest monsoons). Soil samples were collected from the site for laboratory testing and the relevant soil properties required in the analyses were obtained. Initial soil condition was determined from Soil Water Characteristics Curve (SWCC) of the soil sample from the study area and these initial conditions were simulated at the beginning of each analysis to enable realistic condition of the soil to be obtained. The seepage analyses were conducted with Seep/w and the pore water pressure determined from the seepage analyses were used as input parameters in the slope stability analyses carried out with slope/w. The results of the analyses show that considering flux boundary condition has a great influence in maintaining negative pore water pressure in unsaturated soil especially during wet period when there is more water infiltrating the soil. Moreover, inclusion of evaporation in seepage analyses give more realistic pore water pressure and hence better prediction of FOS of the slope.

확률론적 침투해석을 통한 무한사면 파괴의 특성 연구

세계의 많은 지역이 강우에 의한 사면파괴가 취약하다. 사면파괴의 발생 메커니즘을 파악하기 위해 지금까지 다양한 방법들이 제안되어져 왔으나 현재 사용되는 방법들은 비균질한 지반분포와 수리학적 거동이 강우로 인한 사면파괴에 미치는 효과를 고려하지 못한다. 본 연구에서는 강우 시 토층의 두께에 따른 사면파괴의 발생 메커니즘을 연구하기 위하여 불투수 기반암 위에 존재하는 풍화 잔류토 사면에 대한 확률론적 사면안정 해석을 수행하였다. 불균질한 투수계수의 공간적 분포로 인한 불확실성이 강우침투에 의한 불포화 사면의 파괴에 미치는 영향을 고려하기 위하여 일차원 랜덤필드에 기초한 일련의 침투해석과 사면 안정해석을 수행하였다. 해석결과에 의하면 확률론적 해석법은 사면에 대한 강우의 침투 평가 시 투수계수의 공간적인 변동에 의하여 발생하는 다양한 파괴 패턴을 효과적으로 고려할 수 있음을 보여준다. Many regions around the world are vulnerable to rainfall-induced slope failures. A variety of methods have been proposed for revealing the mechanism of slope failure initiation. Current analysis methods, however, do not consider the effects of non-homogeneous soil profiles and variable hydraulic responses on rainfall-induced slope failures. In this study, probabilistic stability analyses were conducted for weathered residual soil slopes with different soil thickness overlying impermeable bedrock to study the rainfall-induced failure mechanisms depending on the soil thickness. A series of seepage and stability analyses of an infinite slope based on one-dimensional random fields were performed to consider the effects of uncertainty due to the spatial heterogeneity of hydraulic conductivity on the failure of unsaturated slopes due to rainfall infiltration. The results showed that a probabilistic framework can be used to efficiently consider various failure patterns caused by spatial variability of hydraulic conductivity in rainfall infiltration assessment for a infinite slope.