Transient Pore Water Pressure Research Articles

EVALUATING CUT SLOPE STABILITY DURING RAINFALL EVENTS IN HA GIANG CITY, VIET NAM

The main objective of the present study is to investigate the mechanism of triggering slope failures during extreme rainfall events under the climatic and geological conditions of Ha Giang city, Viet Nam. For this purpose, the specific research objectives of this paper were based on the analytic results of the geotechnical properties of the soil samples. The pore water pressure distribution on a slope was changed by the influence of rainfall. In order to analyze the stability of the cut slopes, the two modules SEEP/W and SLOPE/W in the GEOSTUDIO 2012 software were employed and integrated. First, the initial steady seepage condition was used to assess the effect of extreme rainfall for a transient seepage. Second, the soil sample properties and some features were assigned in the slope stability model, such as unit weight, cohesion, internal friction angle, and hydraulic functions, and then the achieved transient pore water pressure regime was used as the input parameter to calculate the factor of safety (FS) on cut slopes. Finally, the relationship between the changing of the FS over time and the hourly rainfall intensity that resulted in slope failure were evaluated. In summary, the slopes in Ha Giang city can be stable under normal conditions, but their stability is always a dilemma for local authorities due to the influence of intense rainfall.

A Novel Radial Basis Function Approach for Infiltration-Induced Landslides in Unsaturated Soils

In this article, the modeling of infiltration--induced landslides, in unsaturated soils using the radial basis function (RBF) method, is presented. A novel approach based on the RBF method is proposed to deal with the nonlinear hydrological process in the unsaturated zone. The RBF is first adopted for curve fitting to build the representation of the soil water characteristic curve (SWCC) that corresponds to the best estimate of the relationship between volumetric water content and matric suction. The meshless method with the RBF is then applied to solve the nonlinear Richards equation with the infiltration boundary conditions. Additionally, the fictitious time integration method is adopted in the meshless method with the RBF for tackling the nonlinearity. To model the stability of the landslide, the stability analysis of infinite slope coupled with the nonlinear Richards equation considering the fluctuation of transient pore water pressure is developed. The validation of the proposed approach is accomplished by comparing with exact solutions. The comparative analysis of the factor of safety using the Gardner model, the van Genuchten model and the proposed RBF model is provided. Results illustrate that the RBF is advantageous for reconstructing the SWCC with better estimation of the relationship than conventional parametric Gardner and van Genuchten models. We also found that the computed safety factors significantly depend on the representation of the SWCC. Finally, the stability of landslides is highly affected by matric potential in unsaturated soils during the infiltration process.

Physical Modeling for Large-Scale Landslide with Chair-Shaped Bedrock Surfaces under Precipitation and Reservoir Water Fluctuation Conditions

The deformation and failure mechanisms of historical landslides, characterized with different types of bedrock surface shapes which are known to have been induced by rainfall and reservoir water fluctuations, is an important issue currently being addressed by many researchers. The Zhaoshuling Landslide of the Three Gorges Reservoir Region, which was characterized with a chair-shaped bedrock surface under rainfall and reservoir water fluctuation conditions, was selected as an example in this study’s physical modeling process. The results of different parameters, including the displacements, pore water pressure, and total soil pressure during the landslide event, revealed that the Zhaoshuling Landslide with a chair-shaped bedrock surface had been extremely sensitive to heavy rainfall coupled with the rapid lowering of the water levels. Then, based on the data analysis results of the monitoring of the rainfall and groundwater levels, as well as the reservoir water levels, a conceptual model was put forward to explain the failure mechanisms. It was believed that the chair-shaped bedrock at the toe of the slope had been subjected to a localized zone of high transient pore water pressure, which had significantly adverse effects on the mechanisms of the slope stability.

A Non-Linear, Time-Variant Approach to Simulate the Rainfall-Induced Slope Failure of an Unsaturated Soil Slope: A Case Study in Sapa, Vietnam

In this study, the Transient Rainfall Infiltration and Grid-Based Regional Slope-Stability Analysis (TRIGRS), v2.1 program, and module SLOPE/W in the Geostudio package were adopted for assessing rainfall-induced slope failure. TRIGRS was developed by the United States Geological Survey to determine the time-varying groundwater table at the regional scale under rainfall infiltration. The program employs partial differential equations represented by one-dimensional vertical flow in homogeneous materials for unsaturated conditions. With the application of a simple runoff routing scheme combined with the mass balance between rainfall, infiltration, and runoff over the study area, the distribution of the transient pore-water pressures within the entire landscape was simulated considering both the surface and subsurface flow. Additionally, compared to the traditional two-dimensional approach, the topographical conditions were also considered during the groundwater simulation. For conducting the slope stability analysis, a typical cross-section was constructed based on the site description. The predicted water-tables at the observed time of failure of the typical section were extracted and used in SLOPE/W to conduct the time-dependent modelling of rainfall-induced slope failures. In this study, the non-linear method was employed for simulating unsaturated soil shear strength, and the stability of the slope was evaluated using Bishop’s simplified method. We applied the approach to the landslide event that occurred on August 5, 2019, in Sapa district, Lao Cai province, Vietnam. The event resulted in severe damage and blocked the road for days. The predicted results on the stability of the slope as the factor of safety were compared with the actual slope failure during the event. The results showed that, by inputting accurate data, the applied approach could provide valuable evidence about the time of the slope failure.

Assessment of Water Retention Test by Continuous Pressurization Method

Abstract The water retention test involving the stepwise pressurization method (SPM) and a pressure plate extractor has been mainly used to obtain the soil–water characteristic curve (SWCC) of soils. The disadvantage of this test is that the measurement of the drying and wetting paths of the SWCC is time consuming. Thus, a water retention test involving a continuous pressurization method (CPM) in this paper was proposed, and the experiments for three kinds of soil samples were performed. Unlike the SPM, the pore air pressure in the CPM is continuously given to the specimen at a constant pressurization rate. In particular, for the measurement of the transient pore water pressure inside specimen according to the applied pore air pressure, a microtensiometer (height: 25 mm, diameter: 3 mm) was placed vertically at the center of the specimen. The suction for the specimen was defined as the difference between the applied pore air pressure and the measured pore water pressure. It was confirmed that the testing time for the drying and wetting paths of the SWCC in the test involving the CPM was remarkably shortened. The results of the CPM for its verification were compared with those of the SPM under the same initial condition of the specimen. The two test results showed good agreement regardless of the soil samples. Furthermore, the effect of the pressurization rate of the pore air pressure for the CPM was examined. Finally, the proposed water retention test involving the CPM was found to be highly efficient and accurately measure the drying and wetting paths of the SWCC in a short time.

Slope Stability Analysis of Levee

The stability of levees can be affected during fluctuations in the water table. It is also possible for the flood caused by the typhoon to have an influence on the position of the water table in an earth levee. This paper compares the traditional methods methodology with a combination of transient seepage and slope stability analysis for analyzing rapid drawdown scenarios. Firstly the stability of a levee was calculated by conventional methods, then considering the effect of water level changes in the seepage area, the simulated transient pore-water pressure response from a finite element SEEP / W analysis was used for levee stability simulation. The Silt Solidified Piles have a positive effect on the stability of the levee. The result s provide design basis for the projects.

Response mechanism of post-earthquake slopes under heavy rainfall

This paper uses the catastrophic landslide that occurred in Zhongxing Town, Dujiangyan City, as an example to study the formation mechanism of landslides induced by heavy rainfall in the post-Wenchuan earthquake area. The deformation characteristics of a slope under seismic loading were investigated via a shaking table test. The results show that a large number of cracks formed in the slope due to the tensile and shear forces of the vibrations, and most of the cracks had angles of approximately 45° with respect to the horizontal. A series of flume tests were performed to show how the duration and intensity of rainfall influence the responses of the shaken and non-shaken slopes. Wetting fronts were recorded under different rainfall intensities, and the depth of rainfall infiltration was greater in the shaken slope than in the non-shaken slope because the former experienced a greater extreme rainfall intensity under the same early rainfall and rainfall duration conditions. At the beginning of the rainfall infiltration experiment, the pore water pressure in the slope was negative, and settling occurred at the top of the slope. With increasing rainfall, the pore water pressure changed from negative to positive, and cracks were observed on the back surface of the slope and the shear outlet of the landslide on the front of the slope. The shaken slope was more susceptible to crack formation than the non-shaken slope under the same rainfall conditions. A comparison of the responses of the shaken and non-shaken slopes under heavy rainfall revealed that cracks formed by earthquakes provided channels for infiltration. Soil particles in the cracks of slopes were washed away, and the pore water pressure increased rapidly, especially the transient pore water pressure in the slope caused by short-term concentrated rainfall which decreased rock strength and slope stability.

Saturated and unsaturated stability analysis of slope subjected to rainfall infiltration

This paper presents results of saturated and unsaturated stability analysis of typical residual slopes subjected to rainfall infiltration corresponds to 50 years rainfall return period. The slope angles considered were 45° and 70°. The saturated stability analyses were carried out for original and critical ground water level commonly considered by practicing engineer. The analyses were conducted using limit equilibrium method. Unsaturated stability analyses used combination of coupled stress–pore-water pressure analysis to evaluate the effect of rainfall infiltration on the deformation and transient pore-water pressure on slope stability. Slope stability analyses were performed at some times during and after rainfall infiltration. Results show that the critical condition for slope made by sandy material was at the end of rainfall while for clayey material was at some specified times after the rainfall ceased. Unsaturated stability analysis on sandy soil gives higher factor of safety because the soil never reached saturation. Transient analysis using unsaturated soil concept could predict more critical condition of delayed failure of slopes made up of clayey soil.

Electrically induced displacement transport of immiscible oil in saline sediments

Electrically assisted mitigation of coastal sediment oil pollution was simulated in floor-scale laboratory experiments using light crude oil and saline water at approximately 1/10 oil/water (O/W) mass ratio in pore fluid. The mass transport of the immiscible liquid phases was induced under constant direct current density of 2A/m2, without water flooding. The transient pore water pressures (PWP) and the voltage differences (V) at and in between consecutive ports lined along the test specimen cell were measured over 90days. The oil phase transport occurred towards the anode half of the test specimen where the O/W volume ratio increased by 50% over its initial value within that half-length of the specimen. In contrast, the O/W ratio decreased within the cathode side half of the specimen. During this time, the PWP decreased systematically at the anode side with oil bank accumulation. PWP increased at the cathode side of the specimen, signaling increased concentration of water there as it replaced oil in the pore space. Electrically induced transport of the non-polar, non-conductive oil was accomplished in the opposing direction of flow by displacement in absence of viscous coupling of oil-water phases.

Design tools for thermoactive geotechnical systems

AbstractThis paper presents a review of current design tools used for thermoactive geotechnical systems, along with validation efforts. The capabilities of available analytical methods used for the thermal and thermomechanical design of these systems are evaluated and shortcomings of the existing methods are identified. Although the analytical methods permit accurate prediction of the thermal stress and strain response of thermoactive piles from readily available soil and concrete properties, current shortcomings consist of the ability of the methods to simulate cyclic heating and cooling effects, transient pore water pressure generation and dissipation, and the effects of radial stress changes. Recommendations are provided on how to properly address the current design requirements and the efforts to overcome shortcomings with the development of constitutive relationships from further full scale and laboratory scale experimental studies on thermoactive piles. Furthermore, the need for the development of b...

Space–time prediction of rainfall-induced shallow landslides through a combined probabilistic/deterministic approach, optimized for initial water table conditions

In landslide-prone areas the magnitude of events is related to recurring rainfall intensity. In a large sector of the Sannio Apennines (Southern Italy), predictive mapping of recurrent shallow landslides was undertaken by combining deterministic and probabilistic predictive approaches. This, with the aim to minimize the negative influence of the uniform distribution of the initial water table depth in steady condition that usually influence the theoretical instability resulting from the application of methods for large-scale estimation. The deterministic approach was performed by means of the Transient Rainfall Infiltration and Grid-based Regional Slope-stability model to obtain triggering maps in multi-temporal transient pore-water pressures. The optimized physical modeling was validated by back-analysis on large-magnitude landslide events which occurred in 2003 by means of the introduction of two cross-mapping correlation indexes. Subsequently, different predictive scenarios were proposed for different probabilistic return periods of the rainstorm events. The output data permitted the definition of a linear log regression curve to estimate the theoretical instability of the study area. This curve is defined as a function of cumulative precipitation, duration and return periods of the possible rainfall events.

Wet winter pore pressures in railway embankments

This paper demonstrates the influence of extreme wet winter weather on pore water pressures within clay fill railway embankments, using field-monitoring data and numerical modelling. Piezometer readings taken across the London Underground Ltd network following the wet winter of 2000–2001 were examined, and showed occurrences of hydrostatic pore water pressure within embankments, but also many readings below this. A correlation was found between the maximum pore water pressures and the permeability of the embankment foundation soil, with high-permeability foundation soils (of chalk or river terrace deposits) providing underdrainage and maintaining low pore water pressures within the overlying clay embankment fill. Numerical modelling of transient water flow in response to a climate boundary condition supports this conclusion, and has been used to demonstrate the influence of clay fill and underlying foundation permeability on transient pore water pressures during extreme (c. 1 in 100 years) and intermediate (c. 1 in 10 years) wet winter rainfall. For clay-founded embankments, extreme wet winter conditions increased pore water pressures significantly compared with an intermediate winter, whereas for embankments underlain by a permeable stratum pore water pressures were less sensitive to the extreme winter rainfall.

Research of Slope Stability Analysis Considering Rainfall Infiltration

Based on the saturated-unsaturated seepage theory and considering soil-hydraulic permeability coefficient characteristic curves of rock slope, the variation of suction in unsaturated region and transient saturated zone formation of rock slope were analyzed. Combined with engineering example, the strength reduction methods were adopted to analyzing the rock slope stability influence factors considering unsaturated seepage with different rainfall intensity and duration. The results show that the flow domain owing to rainfall infiltration mainly appears surface layer region of slope. The rainfall infiltration caused the groundwater level rise, the rising of transient pore water pressure and the fall of suction in unsaturated region caused the slope stability decrease. The rainfall intensity and duration have obvious influence on slope stability, and in the same rainfall duration condition, the safety coefficient of slope decreases with the accretion of rainfall intensity. With the rainfall duration increasing, the water in soil has more deep infiltration, the water content and pore water pressure was higher in the same high position, the decreasing of suction caused the safety coefficient of slope has more reduce.

Monitored and simulated variations in matric suction during rainfall in a residual soil slope

This research combines field, laboratory and numerical investigations to estimate the development of a wetting front within a 1.2 m residual soil mantle on a steep forested slope during rainfall events. The field-monitored variations in matric suction due to rain-water infiltration during various events revealed that the maximum infiltration rate was much higher when the wetting front resided in the upper 20 cm of soil compared to the case when the wetting front advanced to depths > 20 cm. Laboratory investigations on soil hydraulic properties (i.e., soil water characteristic curve, and hydraulic conductivity) were useful to establish the parameters of a multilayer finite-element model for one-dimensional vertical infiltration. These parameters were subsequently calibrated by matching the predicted and field measured transient pore water pressure responses during actual rainstorms with irregular rainfall patterns. The calibrated simulation model was used to assess the migration of the wetting front under uniform rainfall with different intensities. Based on the numerical results, a hyperbolic equation was developed to predict the duration of uniform rainfall required for the propagation of wetting front to a certain depth for a given rainfall intensity. The proposed equation was subsequently tested against field-monitored advancements of the wetting front during real rainstorms with variable rainfall intensity.

Reliability Analysis for Effects of Reservoir Water Level Change on Embankment Dam

This paper is mainly concerned with the saturated-unsaturated seepage analysis of embankment dams based on unsaturated soil theory and the reliability analysis of embankment dam based on conception of reliability. The transient seepage due to change of the water level is calculated using the finite element method based on unifying saturated-unsaturated seepage governing equations. The transient pore water pressures are then used for stability analyses of embankment dam considering the effects of suction on shear strength of unsaturated soils. Meanwhile, combined with reliability computation, the Monte-Carlo stimulation method is used to calculate the corresponding reliability index dealing with the stochastic features of soil parameters. The reliability of different water level condition of embankment dams has been analyzed. The effects of different reservoir water level drawdown and raise speeds on reliability of embankment dam are discussed.

Centrifuge modeling of one-step outflow tests for unsaturated parameter estimations

Abstract. Centrifuge modeling of one-step outflow tests were carried out using a 2-m radius geotechnical centrifuge, and the cumulative outflow and transient pore water pressure were measured during the tests at multiple gravity levels. Based on the scaling laws of centrifuge modeling, the measurements generally showed reasonable agreement with prototype data calculated from forward simulations with input parameters determined from standard laboratory tests. The parameter optimizations were examined for three different combinations of input data sets using the test measurements. Within the gravity level examined in this study up to 40g, the optimized unsaturated parameters compared well when accurate pore water pressure measurements were included along with cumulative outflow as input data. With its capability to implement variety of instrumentations under well controlled initial and boundary conditions and to shorten testing time, the centrifuge modeling technique is attractive as an alternative experimental method that provides more freedom to set inverse problem conditions for the parameter estimation.

Evaluation of landslide triggering mechanisms in model fill slopes

Hong Kong is particularly susceptible to landslide risk due to the steep natural topography and prolonged periods of high intensity rainfall. Compounding the risk of slope failure is the existence of loose fill slopes which were constructed prior to the 1970’s by end-tipping. A clear understanding of the underlying triggering mechanisms of fast landslides in fill slopes is required to analyse landslide risk and to optimise slope stabilisation strategies. The work described here had the objective of evaluating two candidate triggering mechanisms—static liquefaction and the transition from slide to flow due to localised transient pore water pressures—against observations of slope behaviour obtained from highly instrumented centrifuge model tests. These results indicate that static liquefaction is unlikely to occur if the model fill is unsaturated and the depth to bedrock large, as the high compressibility and mobility of air in the unsaturated void spaces allows the model fill slope to accommodate wetting collapse without initiating undrained failure. In contrast, high-speed failures with low-angle run-outs are shown to be easily triggered in model fill slopes from initially slow moving slips driven by localised transient pore water pressures arising from constricted seepage and material layering.

Wave-induced transient porewater pressure and seabed instability in surf zone : Sakai, T; Hattori, A; Hatanaka, K Proc International Conference on Geotechnical Engineering for Coastal Development, GEO-COAST'91, Yokahama, 3–6 September 1991V1, P627–632. Publ Japan: Coastal Development Institute of Technology, 1991

Wave-induced transient porewater pressure and seabed instability in surf zone : Sakai, T; Hattori, A; Hatanaka, K Proc International Conference on Geotechnical Engineering for Coastal Development, GEO-COAST'91, Yokahama, 3–6 September 1991V1, P627–632. Publ Japan: Coastal Development Institute of Technology, 1991

Wave‐induced displacements in seafloor sands

AbstractThe stability and deformation of the seafloor in response to storm wave loading is an important consideration in the design of pipelines, anchors, and offshore structures such as gravity‐and pile‐supported platforms. Two quite different approaches have been proposed for investigating the effects of storm waves on seafloor sands. These approaches derive from quite different considerations of soil‐water interaction.In one of these approaches (exemplified by Yamamoto and Madsen) the seafloor is visualized to be a porous elastic or Coulomb‐damped porous nonlinear medium. In this approach, the response of the seafloor, i.e. the displacements, stresses and porewater pressures, are computed for a given wave. The response is transient and is, therefore, related to the largest wave in the storm wave group.On the other hand, in the approach proposed by Seed and Rahman, only the development of residual porewater pressures (for estimating liquefaction potential) due to the storm wave group are considered. The effects of transient porewater pressures and stresses on the seafloor sands are ignored. The residual porewater pressures are computed using properties based on initial effective stresses through a porewater generation‐dissipation model.The application of these techniques to problems of offshore structures such as pile‐supported platforms has led to considerable uncertainty and confusion. In this paper, both of these approaches are combined to investigate seafloor response to a storm wave group. For typical storm wave conditions that exist in the North Sea, it appears that the inclusion of damping, inertia and anisotropic permeabilities in the study is not important relative to ocean sands. Parametric studies show that, for a given wave, the thickness and the stiffness properties of the soil deposit dictate the response of the deposit. The displacements in the deposit are, therefore, affected by the generation of residual porewater pressures.

Model Studies of Wave-Seabed Interactions

Several factors such as the wave characteristics, the seabed type, the sediment size, and the porosity characteristics influence the transient porewater pressure response below the mudline due to wave-seabed interaction. Experiments were conducted in a soil-wave tank in which the pressure variation profile inside a porous bed of sand was measured for different wave types. The measured pressures were correlated with simple theories based on wave energy dissipation in permeable seabed. Tests were conducted in which the combination of wave and soil conditions caused soil liquefaction and instability. These results were extrapolated to predict the potential conditions that may cause seabed instability in the Hibernia area of the Newfoundland Grand Banks. The experimental assembly, the results and correlations, and the limitations are discussed in this paper.