Effect Of Pore Water Pressure Research Articles

Analysis of Pore Water Pressure Effect in Asphalt Patch Failure: An Integrated Modelling and Experiment Study

Moisture has been widely accepted as the main cause of asphalt patch failure, while the investigation of pore water pressure (PWP) in asphalt patch is limited. This research investigated the PWP generated in asphalt patch and quantified its impact on mechanical performance considering the inadequate compaction of asphalt patch. A hydro-mechanical model was first developed to analyze the PWP under field conditions, considering the effects of patch geometry, vehicle wandering, traffic loading, and temperature. The moisture-induced stress tester (MIST) was then used to condition patching materials under different PWP levels in a laboratory. After that, indirect tensile (IDT) and interface shear strengths of patching materials were evaluated to quantify the impact of PWP on material degradation. The numerical modeling results show that the maximum PWP can be within or at the interface of the asphalt patch, depending on the wheel path. The magnitude of PWP is related to the patch geometry, traffic load, and temperature. The maximum PWP, whether inside or at the interface of the asphalt patch, can be from 100 kPa to over 300 kPa under common field conditions. The laboratory experimental results suggest that the first 500 cycles of MIST conditioning led to over 30% decrease in both IDT and interface shear strengths of asphalt patch, and the strength ratio was much lower than the recommended level. These findings indicate that asphalt patches with poor compaction quality are very susceptible to moisture-induced damage, and the mechanical performance of patching material should be evaluated after conditioning with PWP effect.

Two-phase SPH numerical study of pore-water pressure effect on debris flows mobility: Yu Tung debris flow

A debris flow is one of the most dangerous types of geophysical mass flows moving downward mountain slopes and consist of a mixture of soil and rocks with a significant quantity of interstitial fluid. They can travel at extremely rapid velocities affecting long distances due to the significant influence of pore-water pressures on the propagation stage. In this paper, a novel two-phase depth-integrated SPH numerical model is applied to simulate a debris flow in which the effects of bed entrainment and pore-water pressure evolution, essential components to performed debris flows risk assessment, are included. The model was applied to simulate the dynamics of Yu Tung debris flow which is an appropriate benchmark case to examine the applicability of the developed model, as bed entrainment and pore-water pressure evolution were key aspects that affected the moving mass dynamics. The developed two-phase depth-integrated SPH-FD model is applied to assess the structural countermeasure of the bottom drainage screen, used to reduce the impact of debris flows. The reasonable results obtained from the analysis indicate that the model is capable to properly reproduce the propagation of the debris flow and more importantly to correctly perform the time-space evolution of pore water pressures during the whole deformation process from initiation through propagation, over an impermeable and permeable bottom boundary, up to deposition.

Effect of porewater pressure on the mechanical properties of red sandstone with different unloading rates

The red sandstone in the Luohe Formation in Shaanxi Province, China, contains a rich aquifer system. The excavation of coal mines and tunnels through the Luohe Formation affects the mechanical properties of the rocks in the surrounding environment, creating the need to determine the effect of the porewater pressure and unloading rate on the mechanical properties of the red sandstone. Using the constant axial pressure unloading method, triaxial unloading tests were performed under different unloading rates (0.1, 0.3 and 0.6 MPa s−1 and porewater pressure conditions (0, 1.0, 1.5 and 2.0 MPa). Based on the results, an unloading statistical damage model of red sandstone was established under the impacts of unloading rate and porewater pressure. During the loading stage, as the porewater pressure increased, the slope of the stress–strain curve and elastic modulus gradually decreased. During the unloading stage, lateral deformation larger than the axial deformation was observed owing to the influence of porewater pressure. The porewater pressure effect became significant as the unloading rate decreased. An increase in porewater pressure or a decrease in the unloading rate increased the confining strain flexibility. Unloading failure of rock samples was dominated by tensile shear failure, thus indicating that a faster unloading rate or larger porewater pressure causes more tensile cracks and severe fracture in the rock samples.

Effect of jet grouting and dewatering on an adjacent tunnel in a Shanghai deep excavation

In general, jet grouting is usually adopted in deep excavation projects to improve the soils inside and outside the pit to protect surrounding tunnels or building foundations. A cost-effective way is to keep the deformations caused by pit excavation within limitations with the minimum jet-grouting. In this study, different types of jet-grouting soils around tunnels or inside pits were compared and analysed in detail using three-dimensional finite-element modelling. Tunnels on one side of the pit or underneath the formation level were considered. A dewatering method was also introduced in some finite-element models to explore the effect of pore-water pressure (PWP) relief wells in soft clay with very low permeability. The finite-element modelling results indicated that enhancing the soils around tunnels is more profitable than only improving the soils between tunnels and retaining walls. The positive effect of jet-grouting soils above the tunnel may exert only when the excess PWP dissipates. This study may assist engineers who are seeking more cost-effective ways to protect surrounding tunnels or building foundations during deep excavation in soft clay.

Load-indentation testing to assess porewater pressure effects on linear drag and disc cutting of a saturated permeable brittle rock

In this study load-indentation tests with porewater pressure measurement were conducted to explore the unexpected differences owing to water saturation in the linear cutting of a permeable quartzose sandstone using drag and disc type cutters. This was done to evaluate the probable role of porewater pressure in rock fragmentation by drag and disc type cutters. The contrasting response to water saturation of these two types of cutters might be explained by the effects of their different fragmentation mechanisms, such as the relative size of the crushed zone that forms beneath the cutters. The relationship between cutting speed and rock permeability was expected to be a major factor influencing the effective pressure beneath a cutter in saturated rock. However, load-indentation tests with pore pressure measurement at the same speed showed that the pore pressure within the tested sandstone remained too low to affect the rock fracture process. Other possible mechanisms are discussed.

Numerical modeling of the 2008 Wenchuan earthquake-triggered Niumiangou landslide considering effects of pore-water pressure

Ring-shear experiments are commonly conducted to analyze the kinematic mechanisms of landslides triggered by earthquakes, particularly for landslides impacted by pore-water pressure near the grain-crushing sliding surface. However, pore-water pressure is rarely considered in numerical simulation of the post-failure behavior of earthquake-induced landslides. In this paper, a pore-water pressure model based on the results of ring-shear tests (Cui et al. Landslides 14(3):1–15, 2016) is incorporated into the discontinuous deformation analysis (DDA) for modeling and further understanding the initiation and motion behaviors of the Niumiangou landslide triggered by the 2008 Wenchuan earthquake. The pore-water pressure can be recalculated in each time step before the sliding block is detached from the base in the source area, and the friction coefficient of the sliding surface is simultaneously updated according to the pore-water pressure ratio in the modified DDA. The modified pore-water pressure DDA (PWP-DDA) is validated by means of comparison with the analytical results of the dynamic behaviors of a sliding block on an inclined plane under dynamic acceleration. The simulation results indicate that the pore-water pressure on the sliding surface of the Niumiangou landslide sharply increases within a short period, with small relative displacement of the landslide. Relative to the simulated results from the unmodified DDA, the pore-water pressure calculated by the PWP-DDA promotes higher velocity and longer run-out of the sliding mass. Moreover, the modeling run-out and deposit pattern from the PWP-DDA are in basic agreement with the topography of an actual survey.

Back-analysis of geophysical flows using three-dimensional runout model

Predicting the mobility and delineating the extent of geophysical flows remains a challenge for engineers. The accuracy of predictions hinges on the reliability of input parameters of runout models. Currently, limited field data for landslide case histories are available for benchmarking the performance of runout models. Key rheological parameters, such as the equivalent internal friction angle, cannot be measured directly using laboratory experiments and must instead be determined through back-analyses. A series of dynamic back-analyses was carried out for notable landslide case histories in Hong Kong, accounting for the effects of pore-water pressure on the equivalent internal friction angle, using a three-dimensional finite-element mobility model. The recorded and simulated run-out distances, as well as lateral spreading, were compared. Results reveal that the back-analysed equivalent internal friction angles resulting from open-hillslope failures and from channelized geophysical flows range from 25° to 30° and 15° to 20°, respectively. This is attributed to incised geophysical flow channels having an elevated water head and higher degree of saturation compared to open-hillside slope surfaces, wherein the induced elevated pore-water pressure profoundly lowers the equivalent internal friction angle. The back-calculated values may be useful for finite-element-based design of mitigation measures.

An experimental study of deformation and fracture characteristics of shale with pore-water pressure and under triaxial cyclic loading

The deformation and fracture characteristics of shale in the Changning-Xingwen region were experimentally studied under triaxial cyclic loading with a controlled pore-water pressure. An RLW-2000M microcomputer-controlled coal-rock rheometer was used in the State key Laboratory of coal mine disaster dynamics and control in Chongqing University. These experimental results have indicated the following. (i) The shale softened after being saturated with water, while its failure strength decreased with the increase of axial strain. (ii) A complete cyclic loading–unloading process can be divided into four stages under the coupling action of axial cyclic loading and pore-water pressure; namely the slow or accelerated increasing of strain in the loading stage, and the slow or accelerated decreasing of strain in the unloading stage. (iii) The axial plastic deformation characteristics were similar when pore-water pressures were set to 2, 6 and 10 MPa. Nevertheless, the shale softened ostensibly and fatigue damage occurred during the circulation process when the pore-water pressure was set to 14 MPa. (iv) It has been observed that the mean strain and strain amplitude under axial cyclic are positively correlated with pore-water pressure, while the elastic modulus is negatively correlated with pore-water pressure. As the cycle progresses, the trends in these parameters vary, which indicates that the deformation and elastic characteristics of shale are controlled by pore-water pressure and cyclic loading conditions. (v) Evidenced via triaxial compression tests, it was predominantly shear failure that occurred in the shale specimens. In addition, axial cyclic loading caused the shale to generate complex secondary fractures, resulting in the specimens cracking along the bedding plane due to the effect of pore-water pressure. This study provides valuable insight into the understanding of the deformation and failure mechanisms of shale under complicated stress conditions.

Effect of Pore-Water Pressure on 3D Stability of Rock Slope

Effect of Pore-Water Pressure on 3D Stability of Rock Slope

An Experimental Study on the Wave-Induced Topographic Change in Artificial Shallows: Focusing on the Effects of Pore-Water Pressure on Sediment Transport

Dredging to maintain channels and berths produces large volumes of sand that need to be appropriately disposed of or used. The construction of artificial shallows and tidal flats has attracted attention as a way of effectively using dredged sand. However, there is still an issue to be resolved in that dredged sand with its fine sediments is vulnerable to wave action. In this study, the wave-induced topographic change in artificial shallows composed of fine sand was examined experimentally for bottom flow velocity on its surface and pore-water pressure on its surface layer. Experimental results showed that the relative vertical effective normal stress ratio calculated from pore-water pressure was negatively correlated with the signed Shields parameter defined as positive landward. Its relationship was unchanged despite a change in the surface profile of the shallows, implying an acceleration in seaward sediment transport and a reduction in landward sediment transport, regardless of the topographic change in the shallows. Furthermore, a signed Shields parameter modified to consider the effects of pore-water pressure gave a reasonable evaluation of the trend in the sediment transport. From these results, it was demonstrated that the effects of pore-water pressure must be considered when discussing the sediment transport.

Characteristics of seismic responses at liquefied and non-liquefied sites with same site conditions

To understand the characteristics of seismic response at liquefied sites, a liquefiable site and a non-liquefiable site were selected, separated by about 500 m and having the same site conditions as Class D in the National Earthquake Hazards Reduction Program (NEHRP). A suite of earthquake records on rock sites are selected and scaled to the spectrum of the Joyner, Boore, and Fumal (JBF) attenuation model for a magnitude 7.5 earthquake at a distance of 50 km. The scaled records were then used to excite the two sites. The effect of pore-water pressure was investigated using the effective stress analysis method, and nonlinear soil behavior was modeled by a soil bounding surface model. Comparisons for spectra, peak ground acceleration (PGA), peak ground displacement (PGD) and permanent displacement were performed. Results show that the mean ground response spectrum at the non-liquefied site is close to the estimated ground response spectrum from the JBF model, but the mean ground response spectrum at the liquefied site is much lower than the estimated ground response spectrum from the JBF model for periods of up to 1.3 s. The mean PGA at the non-liquefied site is about 1.6–1.7 times as large as that at the liquefied site, but the mean peak ground displacement (PGD) at the non-liquefied site has a slight difference with that at the liquefied site. The mean permanent displacements at the liquefied site are larger than those at the non-liquefied site, particularly at the liquefied layer.

Research on the Cusp-Catastrophic of the Stability of the Red Beds Landslide in Western Yunnan

The research is for the firs floods and the post-drought in Yunnan area in 2010. The strong rainfall and the hole pressure change are the red rock landslide most essential factor, so an apex post-modulus which indicates the effect of pore-water pressure on stress—strain curves transition is defined at the first. By combining with the stiffness criterion of landslide is analyzed. Consequently rainfall and other conditions changes have important effects induced landslides. It union research thinks of the elephant mountain landslide area, that red rock landslide slope body displacement and the rainfall amount relations and the slope body full water area compares and the stability of slope coefficient relations, establishes one under the strong rainfall function the red rock landslide cusp sudden change model, the quota and qualitative reveals the strong rainfall to the red rock landslide trigger action. This developed model may carry out some use full knowledge to understand the disaster mechanism and the evolutional or accumulative process of hazard factors in mountain red layer of the slope stability problem, which may mean the further to advance the technique of forecasting, it can provide important reference to the disaster prevention.

Conjunction effect of stream water level and groundwater flow for riverbank stability analysis

Variations in pore-water pressure determined by the groundwater table within a riverbank have been investigated and recognized as an essential factor in determining riverbank stability with respect to mass failure. However, the effect of pore-water pressure is taken into account for most of the existing riverbank stability models under some simplified assumptions, and the limitations of predicting ability may arise. To avoid the unrealistic estimation of pore-water pressure distribution, the new approach proposed here is to couple riverbank stability with groundwater flow modeling, and apply this to tackle the conjunction effect between river stage and groundwater table. Moreover, riverbank material characteristics and the influence of infiltration can be taken into consideration via groundwater flow modeling. Two hypothetical examples, stage rising and stage falling, are used to investigate the capabilities of the present study and two representative methods. The simulated results show that riverbank failure is triggered particularly during the falling stage, which has been pointed out by other researchers as well. Furthermore, the riverbank material characteristics predominantly control the occurrence of failure and should be considered regarding assessment of riverbank stability. Additionally, the effects of parameters indicate that riverbanks with soil properties of low permeability or high specific yield with great infiltration intensity during the falling stage have a tendency to riverbank failure.

The effect of pore-water pressure on NATM tunnel linings in decomposed granite soil

Tunnelling in a water bearing soil often produces a long-term interaction between the tunnel lining and the surrounding soil. With respect to lining design, infiltration and external pore-water pressures are often one of the most important factors to be considered. Development of pore-water pressure may accelerate leakage and cause deterioration of the lining. This can be particularly troublesome to structural and functional components of the tunnel and can often lead to structural failure. However, as a result of the complicated hydraulic boundary conditions and the long times often required for pore pressure equilibration, research on this subject is scarce. Consequently, most design approaches deal with the effects of pore-water pressure on the tunnel lining in a qualitative manner. In this paper, the development of pore-water pressure and its potential effects on the tunnel lining are investigated using the finite element method. In particular, the deterioration of a drainage system caused by clogging is considered. It is shown that the development of pore-water pressure on the lining is dependent on the lining permeability and the deterioration of the drainage system, particularly for a tunnel with both a primary and a secondary lining. The magnitude of pore-water pressure on a new Austrian tunnelling method (NATM) tunnel constructed in decomposed granite soil and the effect of tunnel shape are investigated. Design curves for estimating pore-water pressure loads on a secondary lining are proposed.Key words: numerical analysis, tunnel lining, decomposed granite.

Limit Analysis of Soil Slopes Subjected to Pore-Water Pressures

The limit-equilibrium method is commonly used for slope stability analysis. However, it is well-known that the solution obtained from the limit-equilibrium method is not rigorous, because neither static nor kinematic admissibility conditions are satisfied. Limit analysis takes advantage of the lower- and upper-bound theorems of plasticity to provide relatively simple but rigorous bounds on the true solution. In this paper, three-noded linear triangular finite elements are used to construct both statically admissible stress fields for lower-bound analysis and kinematically admissible velocity fields for upper-bound analysis. By assuming linear variation of nodal and elemental variables, the determination of the best lower- and upper-bound solution may be set up as a linear programming problem with constraints based on the satisfaction of static and kinematic admissibility. The effects of pore-water pressure are considered and incorporated into the finite-element formulations so that effective stress analysis of saturated slopes may be done. Results obtained form limit analysis of simple slopes with different ground-water patterns are compared with those obtained from the limit-equilibrium method.

Vertical Vibration of Pile in Vibration-Induced Excess Pore Pressure Field

Vertical vibration of pile generates the cyclic shear in soil and can induce excess porewater pressure if the soil is submerged and cohesionless. The induced porewater pressure softens the soil and reduces the maximum skin friction allowed at the soil-pile interface. The behavior of a pile foundation under such an environment is numerically studied herein. A soil-pile interaction model and its formulations are first developed in the frequency domain adopting Winkler's hypothesis. They are computationally efficient yet capable of taking into account the effects of porewater pressure rationally. Parametric studies show that the most significant effects associated with porewater pressure are the reduction of the maximum skin friction allowed at the interface and the drastic alteration of the pile response due to slippage. It is found that the skin friction can be fully mobilized far more easily with porewater pressure than without it. This suggests a great advantage in utilizing vibration for driving a pile in submerged cohesionless soil.

Progressive failure of the Carsington Dam: a numerical study

The mechanism of progressive failure is well understood as one which involves nonuniform straining of a strain-weakening material. Traditional limit equilibrium analysis cannot be used alone to obtain a rational solution for progressive failure problems because the deformation of the structure must be taken into account in the analysis. The failure of the Carsington Dam during construction in 1984 has been attributed to progressive failure of the underlying yellow clay and the dam core materials. The dam was monitored extensively prior to failure, and an elaborate geotechnical investigation was undertaken after failure. The limit equilibrium analysis indicated that the factors of safety were over 1.4 using peak strength of intact clay material or 1.2 based on reduced strength accounting for preshearing of the yellow clay layer. Factors of safety were found to be less than unity if residual strengths were used. The actual factor of safety at failure was, of course, equal to one. By using the finite element analysis with strain-weakening models, the extent and degree of weakening along the potential slip surface were calculated. The calculated shear strength was then used in the limit equilibrium analysis, and the factor of safety was found to be 1.05, which is very close to the actual value of 1.0. More importantly, the mechanism of failure and the initiation and propagation of the shear zones were captured in the finite element analysis. It was also found that accounting explicitly for pore-water pressure effects using the effective stress approach in the finite element and limit equilibrium analyses provides more realistic simulations of the failure process of the structure than analyses based on total stresses. Key words : progressive failure, strain softening, finite element analysis, dams.

A computational model for progressive cracking in large dams due to the swelling of concrete

The paper presents a computational model for the analysis of large concrete gravity dams subjected to severe damage due to internal actions. It has been observed in several operating concrete dams that the combined effects of water intrusion and concrete expansion produce a time-advancing deteriorating process that may endanger the global stability of the construction. The present study was undertaken to simulate numerically the observed phenomena and to enable to predict future developments. Tensile cracking of the concrete is modelled using an elasticfracturing constitutive model. The model is able to simulate in a realistic manner the phenomena of primary and secondary crack initiation, elastic degradation, crack closing and reopening. The triggering of the volumetric expansion due to water intrusion is linked to the onset of cracking at each point, assuming that water enters the dam mostly through the cracks. Temperature and pore-water pressure effects are included using assumed distributions based on available field data. Construction joints are modelled using a frictional joint element, although a “constitutive alternative” is outlined. Close surveillance of the behaviour of a dam that presented this sort of problem was used to calibrate the numerical model and to confirm the obtained results.

A Constitutive Equation for Mass-Movement Behavior

A phenomenological constitutive equation can serve as a basis for modeling and classifying mass-movement processes. The equation is derived using the principles of continuum mechanics and several simplifying assumptions about mass-movement behavior. These assumptions represent idealizations of field behavior, but they appear highly justifiable in light of the geomorphological insight that can be gained through modeling application of a mathematically tractable constitutive equation. The equation represents coupled pressure-dependent plastic yield and nonlinear viscous flow deformation components. The plastic yield component is a generalization of the Coulomb criterion to three-dimensional stress states, and the effect of pore-water pressures is accounted for by treating normal stresses as effective stresses. The nonlinear viscous flow component is a dimensionally homogeneous form of a three-dimensional power-law equation. Straightforward laboratory and field experiments can be used to estimate all plastic...

Effects of Earthquakes on Ground (I)

An empirical formula for the hypocentral distance of damaged railroad subgrade in earthquakes is given. Ground cracks caused by earthquakes are classified into those of tensile type, normal-fault type, and strike-slip type. After description of soil-liquefaction phenomena in earthquakes, the mechanism of spontaneous liquefaction of soils is reviewed critically. The solid-mass sliding and flow slide of soil slopes, and the fragment failure and rock-mass sliding of rock slopes are described. The effect of pore-water pressure on the stability of slopes in cases of the solid-mass sliding and the flow slide is discussed.