Positive Pore Water Pressure Research Articles

The Impact of Hydrological Parameters on Modelling Slope Safety Factor Towards Shallow Landslides: A Case Study from Oltrepò Pavese

Hydrological monitoring of slope susceptible to shallow landslides allows for identifying the triggering conditions of shallow failures and implementing slope stability analysis at site-specific scale. In this work, a case study of a long-term hydrological monitoring in a slope susceptible to shallow landslides of Oltrepo Pavese (Northern Italy) is presented. The triggering mechanism develops in wet seasons (winter and spring) due to the uprise of a perched water table at about 1 m from ground surface, in consequence of the most intense rainfalls (about > 60 mm in 48 h). Unstable conditions (safety factor < 1.0) are correctly modeled on the basis of both water content and pore water pressure, with a better prediction considering hysteresis effects. Safety factor on the basis of water content can correctly assess the triggering conditions for unsaturated and completely saturated soils. It is possible estimating shallow landslides triggering caused by positive pore water pressures only considering this parameter.

Stability and failure mass of unsaturated heterogeneous slopes

Rainfall infiltration in an unsaturated soil slope induces loss of suction (and even positive pore-water pressures), which can eventually lead to failure. This paper investigates the probability and the size of failure of an unsaturated slope with spatially variable void ratio, subjected to a constant intensity rainfall. The random finite element method is employed in conjunction with a Monte Carlo simulation to stochastically evaluate the factor of safety and the size of the sliding mass. The results indicate that the mean value and the variability of these two quantities depend on both correlation length and coefficient of variation of the void ratio field. This dependency is more prominent during the transient regime than at steady states. Notably, the factor of safety in some cases can be low but the corresponding sliding mass is relatively small, while in other instances, the factor of safety might remain large though the associated sliding mass is very sizeable. The correlation between the factor of safety and the size of the sliding mass shifts from positive to negative as the rainfall progresses. A simple quadrant plot is suggested to assess the risk associated with slope failure, taking into account both the factor of safety and the size of failure, rather than the factor of safety alone as it is usually the case. The study also demonstrates an application of a numerical approach to assess stability of geostructures composed of complex multiphase materials such as unsaturated soils or frozen soils.

Impact of Fiber Reinforcement on Deformation Characteristics of Cemented Sand-Gravel Mixtures

This paper describes triaxial compression tests conducted to determine the effect of fiber inclusion on stiffness and deformation characteristics of sand-gravel mixtures. The soil was a mixture of Babolsar sand from the shores of the Caspian Sea and Karaj River gravel in two different contents of 30% and 50%. The relative density of samples was 70% and 3% Portland cement was used as the cementing agent. Fibers of 12mm in length and 0.023mm in diameter added at 0%, 0.5% and 1.0% (wt. of dry soil) to the sand-gravel mixtures. The axial stress versus axial strain and volumetric strain or pore pressure changes have been assessed with the changes in stiffness during loading. Based on the obtained results, fiber addition caused to increase soil shear strength. Under drained conditions, fiber addition leads to increase initial compressive strains and ultimate dilation. It also increases initial positive pore water pressure and final suction in undrained condition. It has also been observed that fibers decrease initial tangent stiffness of the cemented sand-gravel mixture.

The effect of soil type on matric suction and stability of unsaturated slope under uniform rainfall

It is well understood that rainfall infiltration decreases the unsaturated shear strength of soil slope due to reduction in matric suction as the wetting front moves through it. However, how much the matric suction is reduced that may affect slope stability results is still unknown. The paper presents the numerical results of wetting front depth, matric suction and stability results in term of reliability index for a single rainfall intensity. The analyses are for five soils using the modified Mohr-Coulomb failure criterion in conjunction with a finite element saturated-unsaturated seepage model. The numerical results show that the matric suction results are differently affected for soils with rainfall infiltration. The change in reliability index result is insignificant as long the soil remains unsaturated as the wetting front moves. The reliability index results are however become noticeable if the infiltrated rainfall produces saturation and positive pore-water pressure. The difference in results may be attributed to the wetting front depth and its saturation that depends on saturated-unsaturated hydraulic conductivity and matric suction head.

Measuring the Soil Water–Retention Curve Under Positive and Negative Matric Suction Regimes

Abstract The soil water–retention curve (SWRC) is commonly theorized and measured in the positive matric suction or negative pore water pressure domain. However, all soils do not reach full saturation when matric suction decreases to zero. To date, few methods or theories have been developed to understand the SWRC in the negative matric suction domain, which could play important roles in field mechanical stability conditions, such as slopes under heavy rainfall or levees under rapidly rising water table conditions. A method employing both the transient water release and imbibition method (TRIM) and a constant flow method (CFM) is devised to measure a soil’s complete loop of the SWRC under both wetting and drying, and positive and negative matric suction conditions. Although the TRIM is used to measure both drying and wetting paths of the SWRC in the positive matric suction domain, the CFM is used to quantify the soil water–retention behavior in the negative matric suction domain. The TRIM method has been previously validated and extensively tested. The novel feature is the cyclic application of the CFM in the negative matric suction domain. The head loss in the high air entry ceramic stone because of the application of the CFM is calibrated in the range of the applied flow rates. Similar sets of flow rates are used to validate the repeatability of the measured SWRC behavior in the negative matric suction domain. Three different soils, sandy, silty, and clayey soils, are used to demonstrate the applicability of the methodology for various soil types. It is shown that, for the sandy soil, a few kPa of negative matric suction are needed to fully saturate the specimen, whereas for the clayey soil, over 10 kPa of negative matric suction are needed to fully saturate the specimen.

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.

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.

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.

Precipitation Influence on the Distribution of Pore Pressure and Suction on a Coastal Hillside

The frequent interruption of roads in some freeways in Brazil due to slope failure has caused economic losses and potential harm to users. The paper presents data from a monitoring system installed at Serra do Mar in Santa Catarina State, Brazil. The slope monitored is called Morro do Boi and is located on BR-101 south, near the municipality of Itapema. It was stabilised using anchor and flexible metal mesh. Hence, all the pore water pressure measured is due to environmental changes. After approximately sixteen months of readings, it was possible to observe that the suction variation in the unsaturated layers presents a time delay in relation to the rainfall observed in the area. The ground water table presented a variation of about 1.5 m. The analyses of the data allowed establishing a trigger accumulated rainfall that reduces the negative pore water pressure to values below 10 kPa. So far, no significant positive pore water pressure has been observed above the ground water table.

Closing the Loop of the Soil Water Retention Curve

AbstractThe authors, to their knowledge for the first time, produced two complete principal soil water retention curves (SWRCs) under both positive and negative matric suction regimes. An innovative testing technique combining the transient water release and imbibition method (TRIM) and constant flow method (CFM) was used to identify the principal paths of SWRC in the positive pore-water pressure regime under unsaturated conditions. A negative matric suction of 9.8 kPa is needed to reach full saturation or close the loop of the SWRC for a silty soil. This work pushes the understanding of the interaction of soil and water into new territory by quantifying the boundaries of the SWRC over the entire suction domain, including both wetting and drying conditions that are relevant to field conditions such as slope wetting under heavy rainfall or rapid groundwater table rise in earthen dams or levees.

Rainfall thresholds for landslide activity in Portugal: a state of the art

Rainfall is the most important physical process responsible for the landslide triggering in Portugal. Results obtained worldwide have shown that control of rainfall on landslides differs substantially depending upon landslide depth and kinematics and the affected material. Therefore, the critical rainfall conditions for failure are not the same for different types of landslides, and may be strongly influenced by regional geologic and geomorphologic conditions. Rapid debris flows are typically triggered by very intense showers concentrated in just a few hours, and shallow translational soil slips are usually triggered by intense precipitation falling within the 1–15 days long range. On the contrary, activity of deep-seated landslides of rotational, translational and complex types is related to periods of nearly constant rainfall, lasting from several weeks to several months. The different rainfall intensity–duration conditions are associated with different hydrologic mechanisms for slope failure. The generation of surface run-off and high peak discharges in first-order mountain catchments is a critical triggering mechanism for debris flows. The intense rainfall allows the rapid growth of pore water pressure and the drop of capillarity forces that sustain the apparent cohesion of thin soils. As a consequence, shallow soil slips occur within the soil material or at the contact with the underlying less permeable bedrock. Long lasting rainfall episodes enable the steady rise of the groundwater table and the development of positive pore water pressures into the soil. Consequently, deep-seated failures occur in relation to the reduction of shear strength of affected materials. In this work, we present the state of the art concerning the proposition of empirical rainfall thresholds in Portugal for different types of landslides observed in different zones of the country: the Lisbon region, the Douro Valley and the NW Mountains, and the Povoacao Municipality in Sao Miguel Island (Azores). The empirical thresholds applied in Portugal are based on the identification of past landslide events and include (i) the computation of antecedent rainfall threshold defined by linear regression, (ii) the normalization of rainfall by the mean annual precipitation, (iii) the definition of lower limit and upper limit rainfall thresholds and (iv) the definition of combined rainfall thresholds, which integrates the rainfall event and the antecedent rainfall for different time periods.

Physical Model for Studying the Migration of Fine Particles in the Railway Substructure

Abstract To study the interlayer creation and the mud-pumping phenomena in the conventional French railway substructure, a physical model was developed with a 160-mm-thick ballast layer overlaying a 220-mm-thick artificial silt layer (mixture of crushed sand and kaolin), both layers being compacted in a transparent cylinder of a 550-mm-inner diameter. One positive pore water pressure sensor, three tensiometers, and three time domain reflectometer (TDR) sensors were installed around the ballast/silt interface allowing the evolution of pore water pressure (negative or positive) and volumetric water content to be monitored, respectively. A digital camera was installed allowing direct monitoring of different movements (ballast, ballast/subsoil interface, etc.). The effects of loading (monotonic and cyclic loadings) and degree of saturation of subsoil (w = 16 %, Sr = 55 %, and near-saturation state) were investigated. It was found that the development of pore water pressure in the subsoil is the key factor causing the migration of fine particles, hence, resulting in the creation of interlayer, as well as mud pumping. In particular, the camera exposed the pumping up level of fine particles during the test, showing that the migration of fine particles was not only a result of the interpenetration of ballast particles and subsoil, but also of the pore water pressure that pushed the fine particle upward. The quality of the recorded data showed that the physical model developed worked well and the test protocol adopted for studying the mechanisms of intermixing of ballast and subsoil and mud pumping was appropriate.

Insight into the Effects of Gradation on the Pore Pressure Generation of Sand–Silt Mixtures

Liquefaction of saturated sandy soils has been considered as the main cause of most geotechnical hazards during earthquakes. Generation of excess pore water pressures in saturated silty sands when subjected to monotonic, cyclic, and earthquake loading has been shown to cause the liquefaction, which can be defined as the transformation of stable soil structure into unstable liquid form. The proposed research investigated the effect of grading characteristics on the generated excess pore water pressure of sand–silt mixture samples in loose, medium dense, and dense states. The laboratory investigation aimed at understanding the extent or degree at which excess pore pressure of sand–silt mixture soil is affected by its gradation under static loading conditions. For the purpose of clarifying and evaluating the generated pore pressure characteristics of sandy soils, a series of undrained monotonic triaxial tests were carried out on different reconstituted samples of sand–silt mixtures with various gradations. The soil samples were tested under a constant confining pressure (σ3′ = 100 kPa) and at three relative densities (Dr = 20 %, 53 %, and 91 %). The results from this laboratory investigation were used to develop insight into the pore water pressure response of sand and sand–silt mixtures under monotonic loading conditions. The analysis of the obtained data revealed that the grading characteristics [D10, D50, Cu, effective size ratio (ESR), and mean grain size ratio (MGSR)] have significant influence on the generation of the excess pore water pressure. It was found that maximum positive excess pore water pressure (Δumax) can be correlated to the grading characteristics for the sand–silt mixture. The ESR and MGSR appear as pertinent parameters to predict the excess pore water pressure response of the sand–silt mixtures for soil gradation under study.

Simplified quantitative risk assessment of rainfall-induced landslides modelled by infinite slopes

Rainfall induced landslides vary in depth and the deeper the landslide, the greater the damage it causes. This paper investigates, quantitatively, the risk of rainfall induced landslides by assessing the consequence of each failure. The influence of the spatial variability of the saturated hydraulic conductivity and the nature of triggering mechanisms on the risk of rainfall-induced landslides (for an infinite slope) are studied. It is shown that a critical spatial correlation length exists at which the risk is a maximum and the risk is higher when the failure occurs due to a generation of positive pore water pressure.

월류에 의한 저수지 제체의 붕괴 거동

In this study, an experiment with large-scale model was performed according to raising embankment in order to investigate the behaviour of failure due to overtopping. The pore water pressure, earth pressure and settlement by high water level, a rapid drawdown and overtopping were compared and analyzed. Also, seepage analysis and slope stability analysis were performed for steady state and transient conditions. The pore water pressure and earth pressure for inclined core type showed high value at the base of the core, but they showed no infiltration by leakage. The pore water pressure and earth pressure by overtopping increased at the upstream slope and core, it is considered a useful data that can accurately estimate the possibility of failure of the reservoir. The behavior of failure due to overtopping was gradually enlarged towards the downstream slope from reservoir crest, and the inclined core after the raising embankment was influenced significantly to prevent the reservoir failure. The pore water pressure distribution for steady state and transient condition showed positive (+) pore water pressure on the upstream slope, it was gradually changed negative (-) pore water pressure on the downstream slope. The pore water pressure by overtopping showed a larger than the high water level at the downstream slope, it was likely to be the piping phenomenon because the hydraulic gradients showed largely at the inclined core and reservoir crest. The safety factor showed high at the steady state, and transient conditions did not show differences depending on the rapid drawdown.

Field instrumentation for monitoring of water, heat, and gas transfers through unsaturated soils

In this paper, measurement details of pore-water pressures, volumetric water contents, temperatures, and relative oxygen concentrations in unsaturated soil layers under asphalt pavement, aeration slab, and turf cover, as well as weather conditions are described. Measurement results show that positive pore-water pressures developed frequently in the soil layers under the asphalt pavement. Volumetric water contents of the soil layers near the ground surface fluctuated highly as compared to those of the soil layers below. Magnitude and fluctuation of temperatures in the soil layers under the asphalt pavement were the highest while those beneath the turf cover were the lowest. Oxygen concentrations in the soil layers under turf cover increased faster than those in the soil layers under the asphalt pavement and aeration slab. By conducting comprehensive and continuous measurements, relationship between pore-water pressures and volumetric water contents can be obtained and mechanisms of water, heat and gas transfers through unsaturated soil under different surface conditions can be better understood.

Seasonal movement and groundwater flow mechanism in an unsaturated saprolitic hillslope

Numerous field monitoring programs have been conducted to investigate the performance of an unsaturated soil slope subjected to rainfalls in wet seasons. Most case histories focus on the response of matric suction, which is one of the two stress-state variables governing unsaturated soil behaviour. However, effects due to another variable, net normal stress, are often ignored. Also, slope performance under alternative wet and dry seasons is rarely reported and analysed. In this study, a saprolitic hillslope situated in Hong Kong was instrumented heavily to investigate its seasonal movement due to changes of the two variables and also groundwater flow mechanism. Two-year seasonal variations of matric suction and net normal stress were monitored by tensiometers together with heat dissipation matric water potential sensors and earth pressure cells, respectively. During heavy rainstorms in wet season, there was a substantial recharge of the main groundwater table, causing a significant increase of positive pore-water pressure in deeper depths. Rupture surface likely developed at depths between 5.5 and 6 m, hence resulting in a “deep-seated” mode of downslope movement. The downslope movement resulted in a peak increase of horizontal stress. In dry seasons, matric suction of up to 190 kPa was recorded, and the associated soil shrinkage led to substantial upslope rebounds. The stress built up in wet seasons hence reduced. After monitoring period of 2 years, downslope ratcheting is identified. Up to 40 % of the downslope displacements were recovered by the upslope rebounds.

Behavior of Coarse Widely Graded Soils under Low Confining Pressures

Colluvial soils are usually coarse and widely graded. Shallow-seated failures occur frequently in colluvial soil deposits during rainfall infiltration. This paper investigates the behavior of coarse, widely graded soils under very low confining pressures of 5–25 kPa encountered in shallow-seated failures. Isotropic consolidation tests, drained triaxial tests, and undrained triaxial tests were conducted on several widely graded soils with different coarse contents but with the same void ratio of 0.62. With increasing coarse content, the soil microstructure changes from a fines-controlled structure to a coarse-controlled structure after a critical coarse content of approximately 70%. Silty sand with gravel with a coarse content close to the critical value exhibits the highest compressibility because of the presence of large interaggregate pores. Even under very low confining pressures, such soil still shows strong contractive behavior during drained loading, and generates large positive pore-water pressures during undrained loading. This explains why shallow-seated failures occur frequently in colluvial soil deposits caused by rainfall infiltration. Soils with lower or higher coarse contents than the critical value may show dilative behavior under the same low confining pressures. The critical state friction angle increases with the coarse content.

Effects of soil spatial variability on rainfall-induced landslides

This paper presents a probabilistic framework to assess the stability of unsaturated slope under rainfall. The effects of soil spatial variability on the probability of rainfall-induced slope failure (landslides) are investigated. Soil spatial variability is considered by modeling the saturated hydraulic conductivity of the soil ( k s ) as a stationary lognormal random field. Subset simulation with a modified Metropolis–Hastings algorithm is used to estimate the probability of slope failure. It is demonstrated numerically that probabilistic analysis accounting for spatial variability of k s can reproduce a shallow failure mechanism widely observed in real rainfall-induced landslides. This shallow failure is attributed to positive pore-water pressures developed in layers near the ground surface. In contrast, analysis assuming a homogeneous profile cannot reproduce a shallow failure except for the extreme case of infiltration flux being almost equal to k s . Therefore, ignoring spatial variability leads to unconservative estimates of failure probability. The correlation length of k s affects the probability of slope failure significantly. The applicability of subset simulation with a modified Metropolis–Hastings algorithm to assess the reliability of problems involving spatial variability is highlighted.

Shear behaviour in numerical triaxial compression tests by 3D fluid-coupled DEM: a fundamental study on mechanisms of landslide initiation

Shear behaviour in triaxial compression tests was numerically examined by 3D fluid-coupled DEM in a fundamental study on the mechanisms of landslide initiation. Changes in pore-water pressure because of undrained compression were calculated by introducing a measurement sphere surrounding a ball which represented the soil particles. The pore-water pressure was assigned to the measurement sphere as the product of the changes in volumetric strain and modulus of compressibility of water, and resulting flow due to the pressure differences among neighbouring measurement spheres was given on the basis of Darcy's law. Specimens for the numerical triaxial compression tests were prepared with wide ranges of initial void ratio by use of samples with three different grain-size distributions. In the loose specimens, positive pore-water pressure build-up and subsequent liquefaction were numerically reproduced. In contrast, in the dense specimens, negative pore-water pressure was observed, showing upgrade of soil structure. For the medium specimens, phase transformation, which is a typical characteristic of granular soils, was successfully reproduced. Although steady-state lines were found for each sample on a diagram, there was a gap at a specific initial void ratio. Although the angles of internal friction were inversely proportional to the initial void ratio in both the undrained and drained tests in the medium to dense specimens, the observed angles of internal friction were smaller than the particle-to-particle friction angle. This result may be attributable to the fact that the balls could not form the complicated soil structures created in reality by particles with complex shapes.