Pore Water Pressure Changes Research Articles

Deformation and pore water pressure change during static and cyclic loading with subsequent shearing on soils with different textures and matric potentials

Soil deformation on arable lands due to heavy machine traffic always results in compaction and stress dependent shearing. The deteriorated effects of compaction on soil pore functions have been often reported, but less is known about the shearing-induced deformation. To better understand the combined effect of compaction and shearing on soil deformation and pore water pressure uw, measurements were performed on repacked soils with three textures (silt loam, sandy loam, and sand) and two matric potentials (-60 hPa and -300 hPa). Soils were sheared after two types of loading (static loading and cyclic loading) with three compaction levels (50, 100, and 200 kPa). Compaction was the main stress accounting for the loss of soil void ratio e and volume, on the contrary, shearing slightly increased void ratio and volume due to particle rotation in the shear zone. At -60 hPa, soil water ratio stepwisely decreased with compaction and subsequent shearing process. The soil volume change induced by compaction and shearing resulted in an obvious increase in uw. The higher the stress levels and initial matric potential, the more prominent changes in soil deformation and uw. Compared with static loading, further decrease of soil volume and increase of uw were observed under cyclic loading. Soils pre-compacted by cyclic loading always showed high and relatively constant uw values during shearing. The results demonstrated that compaction increased uw by decreasing e and increasing the degree of water saturation while shearing enhanced these effects by means of rearranging particles and blocking the water-filled pores. Therefore, after shearing, soil pore functions were further deteriorated compared with the initial state before compaction.

The failure characteristics around shallow buried tunnels under rainfall conditions

The instability of weak surrounding rock around shallow buried tunnels induced by rainfall is a challenge for tunnel engineering and threatens construction safety. To reveal the influence of rainfall on the rock surrounding a tunnel, the results of experiments on the failure of surrounding rock under different rainfall intensities and durations using a self-developed test system are presented. First, the stress and seepage behaviors of the surrounding rock under rainfall were analysed, and the results showed that the stress increased while the pore water pressure decreased as the rainfall intensity decreased. It was more difficult to maintain vault collapse as rainfall intensity increased, and the particle erosion in the model intensified as the rainfall duration increased. The change in pore water pressure was greater than the change in stress, but the stress and pore water pressures influenced each other during rainfall. The influence of rainfall on the rock at the top of the tunnel was greater than that at the two sides of the tunnel because the seepage channels at the top of the tunnel were more developed and collapsed more easily due to smaller stresses. According to the phenomena of the surrounding rock observed during the test, the evolution of progressive failure under rainfall was roughly divided into eight stages: stability, water drop, continuous water flow, partial collapse, mud and water mixture, vault collapse, throughgoing collapse, and massive water and mud gushing. Furthermore, the final failure pattern of the surrounding rock consists of an excavation disturbance zone, vault damage zone and collapse failure zone. This research helps to reveal the progressive failure of surrounding rock around tunnels under rainfall conditions and can be used as a reference for designing disaster prevention strategies in practical engineering.

Unsaturated seepage and stability of double-layer slope under rainfall infiltration

Taking a homogeneous double-layer soil slope as an example, the SEEP/W module and SLOPE/W module in the finite element analysis software GeoStudio were used in this paper. Then, the changes of pore water pressure and stability under different rainfall patterns and soil parameters were studied. Finally, the variation curves of pore water pressure and slope safety factor with rainfall time were obtained. The results show that: Soil parameters a and m are directly proportional to the slope safety factor, while n is inversely proportional to the slope safety factor. Under the condition of continuous rainfall, the decreasing rate of slope safety factor is directly proportional to the rainfall intensity.Under different rainfall patterns, the continuous rainfall in the advanced and normal rainfall patterns will cause the slope stability to decline and then gradually recover, while delayed and averaged rainfall patterns rainfall will cause the slope stability to decline continuously.In addition, there is a lag period in the change of slope safety factor, and the whole lag period lasts about 6 hours. During the lag period, the pore water pressure inside the soil began to decrease, while the slope safety factor continued to decrease. The safety factor starts to recover after the lag period ends.

Experimental Study on the Negative Skin Friction of the Pile Group Induced by Rising and Lowering the Groundwater Level

Negative skin friction (NSF) has been one of the important factors in the design of pile foundation; especially, the influence of water level on the pile negative skin friction should be paid attention. In this paper, a series of model tests were carried out to analyze the bearing characteristic of the pile group influenced by groundwater level. The pile axial force and negative skin friction, settlement, and soil pore pressure were investigated. The results showed that both the water level rising and lowering cycle could increase the axial force of the pile along the upper part of the pile, yet reducing it along the lower part of the pile; both the axial force and the negative skin friction of the pile presented a feature of time effect; the value of negative skin friction was positively correlated with that of the pile head load, and the neutral plane ranged from 0.57 L to 0.64 L as the water level changed; the soil featured settling in layers, and the change of pore water pressure was accordant with the water level changing regulation.

Analysis of Influence Factors of Pore Water Pressure Change in Frozen Soil

In this article, changes of pore water pressures (PWP) in silty clay subjected to freezing and thawing were measured under an open‐system condition. A total of five soil samples were tested, with water contents of 10.70%, 18.28%, 23.98%, 31.00%, and 37.65%, respectively. Each experienced a first‐step freezing stage, a thawing stage, and a second‐step freezing stage. The results showed that changes in PWP depended on the water content, soil type, salinity, ice content, air, pressure, temperature, and others. The PWP minimum with initial water content has a “w‐shaped double‐valley” characteristic, which described two PWP minima existing in two optimum water contents as initial water content increased. An influence‐factor analysis of PWP was proposed and gave a reasonable interpretation on the “w‐shaped double‐valley” characteristic of PWP. In addition, the tensiometer method to measure PWP in frozen soil was further discussed with regard to its advantages and disadvantages.

The effect of rainfall on the slope stability with numerical simulation on Tawaeli-Toboli road, Central Sulawesi

Tawaeli-Toboli is an arterial road that often experiences landslides due to rainfall, which increases pore water pressure and thus, decreases the soil’s shear strength. This research analyzes the effect of rainfall on Tawaeli-Toboli road with numeric simulation to determine the characteristics such as changes in pore pressure and slope stability. Frequency analysis with HAVARA software was used to determine the maximum rainfall, while the dominant rain duration was analyzed using WindRose. Furthermore, the SoilVision software was used to fit grain size distribution, and the results are used to estimate the soil-water characteristic curve (SWCC). Analysis of pore water pressure changes was carried out using the SEEP/W program. The results were used as input data to determine the slope stability analysis using the SLOPE/W program. The frequency analysis result showed that the maximum amount of rainfall on Tawaeli-Toboli road is 205.698 mm after 11 days. During normal rainfall (20 mm/day), the Western slope remains stable with a safety factor value of 1.50. Simultaneously, in heavy rains, it decreases significantly to 1.350 after 11 days of rain. The eastern slope is quite stable because it has a safety factor value of 1.721 during heavy rain conditions.

Effect of climate change on earthen embankments in Southern Ontario, Canada

Soil embankments are an integral part of transportation networks and are vulnerable to climate change due to their continuous exposure to the environment. The direct and indirect costs associated with an embankment failure can be significant and it is critical to ensure against the adverse effects of a changing climate. This research aims to quantify the probable stability effects of regular and extreme climate change on a highway embankment in Southern Ontario, Canada. The impact of climate change was quantified for two different embankment fills (sand and silt) by comparing the stability results of historical climate with a 90-year future climate data set. This analysis was accomplished by coupling changes in pore-water pressures (using a two-dimensional (2D) transient unsaturated seepage finite-element model) with a 2D limit equilibrium unsaturated slope stability model. The results indicate that future climate could increase the cumulative annual net infiltration by as much as 41%, which would decrease the embankments’ factor of safety by as much as 30%. It was also found that hydraulic properties play a critical role in embankment stability for different climate scenarios and suitable fill material based on the geographic region should be selected to ensure the safety of the embankments against climate change.

Geological methane emissions and wildfire risk in the degraded permafrost area of the Xiao Xing\u2019an Mountains, China

With global warming, the carbon pool in the degradation zone of permafrost around the Arctic will gradually be disturbed and may enter the atmosphere in the form of released methane gas, becoming an important factor of environmental change in permafrost areas. We selected the northwestern section of the Xiao Xing'an Mountains in China as the study area, located in the degradation zone on the southern margin of the permafrost region in Eurasia, and set up multiple study monitoring areas equipped with methane concentration sensors, air temperature sensors, pore water pressure sensors and soil temperature sensors for long-term monitoring of data changes using the high-density electrical method, ground penetrating radar and on-site drilling to survey the distribution of frozen soil and geological conditions in the study area, combined with remote sensing images of Sentinel-2 L1C and unmanned aerial vehicle photographs and three-dimensional image reconstruction, analysis of fire activities and related geological environmental factors. The results show that since 2004, the permafrost thickness of the marsh wetland in the study area has gradually reduced and the degradation rate obviously accelerated; the organic matter and methane hydrate (metastable methane hydrate and stable methane hydrate) stored in the permafrost under the marsh wetland are gradually entering the atmosphere in the form of methane gas. Methane emissions show seasonal changes, and the annual methane emissions can be divided into three main stages, including a high-concentration short-term emission stage (March to May), a higher-concentration long-term stable emission stage (June to August) and a higher-concentration short-term emission stage (September to November); there is a certain correlation between the change in atmospheric methane concentration and the change in atmospheric pressure and pore water pressure. From March to May every year (high-concentration short-term emission stage), with snow melting, the air humidity reaches an annual low value, and the surface methane concentration reaches an annual high value. The high concentration of methane gas entering the surface in this stage is expected to increase the risk of wildfire in the permafrost degradation area in two ways (increasing the regional air temperature and self-combustion), which may be an important factor that leads to a seasonal wildfire frequency difference in the permafrost zone of Northeast China and Southeast Siberia, with the peak in spring and autumn and the monthly maximum in spring. The increase in the frequency of wildfires is projected to further generate positive feedback on climate change by affecting soil microorganisms and soil structure. Southeastern Siberia and northeastern China, which are on the southern boundary of the permafrost region of Eurasia, need to be targeted to establish fire warning and management mechanisms to effectively reduce the risk of wildfires.

The mechanism of fluidization in mud beds under progressive waves

ABSTRACT Wave-flume laboratory experiments were conducted to study the mechanism of fluidization of partially consolidated mud beds, under wave propagation. The mixture of commercial kaolinite and tap water was allowed to partially consolidate under its self-weight before initiating the progressive wave propagation. The vertical changes in pore water pressure in the mud bed were monitored by using sensitive pore pressure transducers, at three levels across the mud layer depth. The experiments revealed that the wave characteristics and bed properties strongly affect the complex fluidization process, in which the accumulated pore water pressure develops through four transitional stages. The accumulated pore water pressure increases sharply at the first stage, which is followed by further gradual increases in the next two stages, with different rates. The breakup of aggregated mud particles, i.e. the generation of fluid mud, is observed at the last stage, resulting in a partial dissipation of accumulated pore pressure and wave height attenuation. The measurements also revealed that the fluidization starts from the top of the mud layer, and proceeds to the underlying layers.

Assessing the Effectiveness of TRIGRS for Predicting Unstable Areas in a Tropical Mountain Basin (Colombian Andes)

Some physically based landslide models analyse pore water pressure changes due to rainfall infiltration and its effects on slope stability. The physically-based model TRIGRS has been successfully used in rainfall-induced shallow landslide assessments in different studies around the world; nevertheless, evaluating its performance in tropical mountain terrains, such as the Colombian Andes, is necessary. In this study, the TRIGRS model was applied to the La Arenosa basin (San Carlos, Colombia) and ROC (receiver operating characteristic) analysis was used to assess its effectiveness (performance) at predicting areas susceptible to shallow landslides in this tropical mountainous area. The results were compared with those obtained using the SHIA_Landslide (Simulacion Hidrologica Abierta, or SHIA, in Spanish) and SHALSTAB models in the same case study. The three models performed well, especially TRIGRS and SHIA_Landslide. The predictive results using TRIGRS were thoroughly analysed, describing the effect of the slope angle and its relationship with the estimated soil depth on the variation of the pressure head and the factor of safety (FS) during the simulated rainfall event. The high dependence of FS on soil thickness demonstrated that defining this variable must be carefully accomplished. The results suggest that TRIGRS can be a valuable tool in tropical mountain terrains, such as the Colombian Andes basins, and it can be useful despite the lack of data and the high parameter uncertainty that is common in many study areas.

Bentonite homogenisation during the closure of void spaces

In a geological repository, the disposal of radioactive waste will result in the creation of engineering voids. Bentonite is commonly proposed as a sealing material as a result of its high swelling capacity. As the bentonite expands, the non-uniform development of porewater pressure and its coupling to total stress within the bentonite, may impair homogenisation. In this study we present results from five laboratory tests performed on sodium- and calcium-based bentonites to examine their swelling potential and capacity to homogenise over extreme bentonite-to-void ratios. Results demonstrate that even under these extreme ratios, the bentonite is able to swell and ultimately fill each void, creating a small swelling pressure. The swelling pressure development is spatially complex and time-consuming, and does not appear to be influenced by friction. Instead, it is characterised by plastic yielding of the clay with 70%–80% of the volume change associated with clay expansion adjacent to the void. This leads to heterogeneity illustrated by the presence of persistent differential stresses and the non-uniform distribution of moisture contents. Increases in the moisture content were measured but did not always correlate with the development of swelling pressure. This disequilibrium of the system is likely a reflection of the test durations and the slow evolution in the rates of change in swelling and porewater pressure beyond 130 days. Given the length of the experimental tests presented here, the time required to achieve full homogenisation of the clay is likely to be many years, if it occurs at all. Gravity segregation was also present in horizontal tests, further impairing clay homogenisation. However, as presented in this paper, it is possible to define functional relationships describing the bentonite swelling potential across engineering voids of differing size. This information will assist in establishing a safety case for bentonite usage in geological radioactive waste disposal.

Study on Seepage Characteristics and Stability of Core Dam Under the Combined Action of the Variation of Reservoir Water Level and Rainfall

To study the influence of the variation of reservoir water level and rainfall on the seepage characteristics and stability of the core dam, considering the coupling effect of seepage field and stress field, finite element simulation of seepage and stability of dam slope when the core dam encounters the variation of reservoir water level and rainfall was carried out based on the unsaturated seepage principle, different rainfall intensities, different rainfall types and different variation rates of reservoir water level were selected for finite element simulation. The results show that: The variation rate of water level mainly affects the time when pore water pressure tends to be stable. The rainfall intensity and rainfall type mainly affect the pore water pressure in the upper part of the dam, and the higher the rainfall intensity, the more drastically the pore water pressure changes in the upper area of the core dam. Rainfall type and rainfall intensity are the main factors that affect the safety factor of the downstream slope of the core wall dam, but have little effect on the safety factor of the upstream slope. When the safety factor of the downstream dam slope tends to be stable, the order of the safety factor of different rainfall type is: forward type > = center type > average type > post-peak type. Whether the water level rises or falls, rainfall will reduce the safety factor of downstream dam slope. The results of this study provide a reference for correctly understanding the seepage and stability law of the slope of core dam and a reference for risk analysis and emergency management when the core dam encounters extreme working conditions.

Effects of coastal erosion on landslide activity revealed by multi‐sensor observations

AbstractCoastal erosion is becoming an increasingly serious consequence of climate change. This study demonstrates the effects of coastal erosion on landslide activity while considering the amount of erosion and changes in pore water pressure. To determine the factors related to landslide slip generation, we specifically measured the displacement, deformation, pore water pressure, and amount of erosion with high temporal resolution (1 s–1 h) for a coastal landslide in Hokkaido, north‐eastern Japan, for 7 months. It has been determined that landslides occur simultaneously with high pore water pressure. Toe erosion events also occurred several times, while the landslide exhibited major displacement. Because toe erosion and the increase in pore water pressure occurred simultaneously, we tried to determine which of the two contributed majorly to the landslide displacement by conducting a stability analysis that incorporates the effects of the two factors. From the actual observed data, toe erosion and the increase in pore water pressure had comparable effects on the destabilization of the studied landslide. Specifically, the time series for the safety factor shows that the landslide in the case with toe erosion was destabilized more than that in the case with no erosion, with a difference of more than 5% in the safety factor. The model with toe erosion provided a better explanation for the landslide displacement. Furthermore, the inclination data suggested that erosion took place at least 1 month before the landslide displacement. This implied that coastal erosion played a role in the preparation and ongoing displacement of the coastal landslide. Inland landslides with toes that are subject to undercutting due to river incision or artificial construction have geomorphological settings that are similar to those of the studied landslide. The knowledge obtained here can contribute to the understanding of destabilization mechanisms and terrain changes related to such landslides. © 2020 John Wiley & Sons, Ltd.

Spatially distributed landslide triggering analyses accounting for coupled infiltration and volume change

Rainfall infiltration in unsaturated slopes alters the effective stress through pore water pressure changes, thus causing ground deformation. Although important to assess the timescale over which the margin of safety of a slope decreases, such coupled processes are rarely accounted in the context of spatially distributed hazard assessment procedures. In this paper, a physically based, spatially distributed model accounting for full hydro-mechanical coupling is discussed. The model relies on a vectorized finite element (FE) solver to calculate the stability of deformable unsaturated infinite slopes subjected to transient flow. First, the FE solver is used to study the response of individual slopes to a prolonged rainfall for three scenarios (i.e., rigid, swelling, and collapsible soil). Then, the model is used in the context of spatially distributed computations to assess spatiotemporal variations of factor of safety over a large area. For this purpose, a series of shallow landslides occurred in a mountainous landscape covered by collapsible loess deposits in northwestern China was used as test site. The analyses show that hydro-mechanical couplings affect the performance of the model in terms of computed failure time and areal extent of the unstable zones. Specifically, volume collapse due to suction decrease is found to reduce the time of failure compared with uncoupled computations obtained for a rigid soil scenario. The most substantial advantages of using coupled analyses have been reported with reference to gentle slopes, for which the higher rate of suction reduction driven by volume change was crucial to capture landslide source areas that would otherwise be overlooked by uncoupled analyses. The proposed methodology offers a complete tool for landslide hazard assessment, in that it incorporates sources of coupling between hydrology and mechanics that are crucial to replicate the physics of landslide initiation.

Dilation of subglacial sediment governs incipient surge motion in glaciers with deformable beds

Glacier surges are quasi-periodic episodes of rapid ice flow that arise from increases in slip rate at the ice–bed interface. The mechanisms that trigger and sustain surges are not well understood. Here, we develop a new model of incipient surge motion for glaciers underlain by sediments to explore how surges may arise from slip instabilities within a thin layer of saturated, deforming subglacial till. Our model represents the evolution of internal friction, porosity and pore water pressure within the till as functions of the rate and history of shear deformation, and couples the till mechanics to a simple ice-flow model. Changes in pore water pressure govern incipient surge motion, with less permeable till facilitating surging because dilation-driven reductions in pore water pressure slow the rate at which till tends towards a new steady state, thereby allowing time for the glacier to thin dynamically. The reduction of overburden (and thus effective) pressure at the bed caused by dynamic thinning of the glacier sustains surge acceleration in our model. The need for changes in both the hydromechanical properties of the till and the thickness of the glacier creates restrictive conditions for surge motion that are consistent with the rarity of surge-type glaciers and their geographical clustering.

Field Test of Excess Pore Water Pressure at Pile-Soil Interface Caused by PHC Pipe Pile Penetration Based on Silicon Piezoresistive Sensor.

Prestressed high-strength concrete (PHC) pipe pile with the static press-in method has been widely used in recent years. The generation and dissipation of excess pore water pressure at the pile–soil interface during pile jacking have an important influence on the pile’s mechanical characteristics and bearing capacity. In addition, this can cause uncontrolled concrete damage. Monitoring the change in excess pore water pressure at the pile–soil interface during pile jacking is a plan that many researchers hope to implement. In this paper, field tests of two full-footjacked piles were carried out in a viscous soil foundation, the laws of generation and dissipation of excess pore water pressure at the pile–soil interface during pile jacking were monitored in real time, and the laws of variation in excess pore water pressure at the pile–soil interface with the burial depth and time were analyzed. As can be seen from the test results, the excess pore water pressure at the pile–soil interface increased to the peak and then began to decline, but the excess pore water pressure after the decline was still relatively large. Test pile S1 decreased from 201.4 to 86.3 kPa, while test pile S2 decreased from 374.1 to 114.3 kPa after pile jacking. The excess pore water pressure at the pile–soil interface rose first at the initial stage of consolidation and dissipated only after the hydraulic gradient between the pile–soil interface and the soil surrounding the pile disappeared. The dissipation degree of excess pore water pressure reached about 75–85%. The excess pore water pressure at the pile–soil interface increased with the increase in buried depth and finally tended to stabilize.

Machine learning for pore-water pressure time-series prediction: Application of recurrent neural networks

Knowledge of pore-water pressure (PWP) variation is fundamental for slope stability. A precise prediction of PWP is difficult due to complex physical mechanisms and in situ natural variability. To explore the applicability and advantages of recurrent neural networks (RNNs) on PWP prediction, three variants of RNNs, i.e., standard RNN, long short-term memory (LSTM) and gated recurrent unit (GRU) are adopted and compared with a traditional static artificial neural network (ANN), i.e., multi-layer perceptron (MLP). Measurements of rainfall and PWP of representative piezometers from a fully instrumented natural slope in Hong Kong are used to establish the prediction models. The coefficient of determination (R2) and root mean square error (RMSE) are used for model evaluations. The influence of input time series length on the model performance is investigated. The results reveal that MLP can provide acceptable performance but is not robust. The uncertainty bounds of RMSE of the MLP model range from 0.24 ​kPa to 1.12 ​kPa for the selected two piezometers. The standard RNN can perform better but the robustness is slightly affected when there are significant time lags between PWP changes and rainfall. The GRU and LSTM models can provide more precise and robust predictions than the standard RNN. The effects of the hidden layer structure and the dropout technique are investigated. The single-layer GRU is accurate enough for PWP prediction, whereas a double-layer GRU brings extra time cost with little accuracy improvement. The dropout technique is essential to overfitting prevention and improvement of accuracy.

Influence of pore water pressure change on consolidation behavior of saturated poroelastic medium

Abstract. There are many factors causing land subsidence, and groundwater extraction is one of the most important causes of subsidence. A set of coupled partial differential equations are derived in this study by using the poro-elasticity theory and linear stress-strain constitutive relation to describe the one-dimensional consolidation in a saturated porous medium subjected to pore water pressure change due to groundwater table depression. Simultaneously, the closed-form analytical solutions for excess pore water pressure and total settlement are obtained. To illustrate the consolidation behavior of the poroelastic medium, the saturated layer of clay sandwiched between two sand layers is simulated, and the dimensionless pore water pressure changes with depths and the dimensionless total settlement as function of time in the clay layer are examined. The results show that the greater the water level change in the upper and lower sand layers, the greater the pore water pressure change and the total settlement of the clay layer, and the more time it takes to reach the steady state. If the amount of groundwater replenishment is increased, the soil layer will rebound.

Soil Response to Repetitive Changes in Pore-Water Pressure under Deviatoric Loading

AbstractSoils often experience repetitive changes in pore water pressure. This study explores the volumetric and shear response of contractive and dilative sand specimens subjected to repetitive ch...

An Improved Unsaturated Triaxial Device to Characterize the Pore-Water Pressure and Local Volume Change Behavior of a Compacted Marl

Abstract Compacted fills in earthworks are in an unsaturated state, with suctions (negative pore-water pressures) usually ranging from several kPa to several MPa. Characterizing the variation of pore-water pressure is a major concern in the estimation of embankment stability and, at the same time, a big challenge when its value falls below –80 kPa. This technical note presents an improved triaxial system based on the standard triaxial cell, equipped with a tensiometer, a thermocouple psychrometer, and local strain gauges. The main objective of these improvements is to measure the pore-water pressure and local volume changes during isotropic loading. In this study, three compacted specimens were tested under undrained conditions. Pore-water pressure was continuously measured by means of the psychrometric probe in the range of –3,000 to –100 kPa and by the tensiometer in the range of –80 to 1,100 kPa. Volume change was measured by the local strain gauges. Analysis and development of the results from both this study and the literature lead to the conclusions that i) specimens with occluded air bubbles (e.g., at the standard Proctor optimum) behave like a saturated soil, as Bw is close to unity when pore-water pressure becomes positive, and ii) a pore-water pressure coefficient Bw close to 1 is not necessarily a sign of saturation, as soil in an unsaturated state may also have a pore-pressure coefficient close to unity, especially at the beginning of the mechanical loading.