Pore Water Pressure Distribution Research Articles

Semianalytical Model of Unloading-Induced Pore-Water Pressure and Consequent Consolidation Settlement of Surcharge Preloading Railway Subgrade

Surcharge preloading is a suitable foundation treatment for high-speed railways in soft soil regions. As a temporary preload on the foundation, the surcharge load must be removed before track construction. In this process, rebound deformation of the foundation and variation in the pore-water pressure have been widely observed. These phenomena significantly influence the further evolution process of foundation consolidation. Although Biot’s consolidation theory is suitable for unloading-induced soil consolidation, the conventional Laplace transform method for solving Biot’s equations entails a time-consuming procedure and cannot reflect the time-dependent characteristics of the preloading and unloading process. To address these issues, a time difference method is proposed in this work. The ordinary differential equation in the transformation domain is obtained using the backward two-step difference method, and a physical analytical solution of the plane strain problem of overloaded subgrade is obtained via the Fourier inverse transform method. The proposed solution is validated through a comparison with existing literature data and actual engineering practices. Finally, based on this calculation method, the influence of the surcharge preloading amount, loading time, and soil properties on pore-water pressure dissipation and distribution after surcharge preloading removal is investigated.

Stability Analysis of a High-Steep Dump Slope under Different Rainfall Conditions

The existing slope stability research, which is based on the fluid–solid coupling theory, is mainly focused on the slopes of central and eastern China. The impact of rainfall on the stability of the dump slope has often been ignored. It is worthwhile to reveal the mechanism of the fluid–solid coupling mechanics of dump slopes in the arid desertification area of northwest China under the maximum precipitation. The method of combining the seepage mechanics theory with the geomechanics theory was adopted. Darcy’s law and the mass conservation law were introduced to derive and establish the fluid–solid coupling analysis method. Taking the Xinxing Coal Mine in Wuhai City, China, as an example, the finite element software ABAQUS was used to construct the fluid–solid coupling model for slope stability analysis with unsaturated soil. The equivalent rainfall intensity of 68 mm/h for 1 h and 18 mm/h for 24 h was designed in the simulation, respectively. Four different types of initial water content (i.e., 1.72%, 7.34%, 14.69%, and 22.03%) of the dump slopes were defined as the initial conditions. The high-steep slope was compared to the standard slope. Therefore, a set of sixteen rainfall schemes was proposed. The variation regularity of slope stability was thoroughly discussed in regards to four areas: vertical deformation, pore water pressure distribution, equivalent plastic strain, and safety factor. As was expected, the research showed that the slope height and angle have a significant effect on the slope stability. When high-intensity rainfall occurs for a short duration, the slope tends to be more stable as the initial water content increases on the slope. When low-intensity rainfall occurs over a long period, the slope stability reduces if the initial water content is too high or too low in the slope.

Measurement of Physical Parameters for Transient Seepage Assessment of Levees

Abstract An embankment was instrumented and monitored as part of a multiyear project to assess appropriate pore-water pressure distributions and soil water retention curves for use as initial conditions in transient seepage analyses. The increased use of unsaturated soil mechanics and transient seepage has led to the need for practical methods of assigning accurate properties and initial conditions in these types of analyses. Monitoring results were validated against published pore-water pressure measurements collected from multiple embankment studies. The collected distributions showed nonlinear behavior in near-surface embankment soils due to infiltration and evapotranspiration processes. Seasonal variations in pore-water pressure and the soil moisture deficit were shown to correlate at the monitoring site. The seasonal response of the embankments exhibited a large variation in pressures between summer and winter. Assessment of flood occurrence showed the distribution of pore pressures during flood loading was typically near −10 kPa at a depth of 2 m at the onset of the flood. Laboratory soil water retention testing of undisturbed samples collected from the embankment showed that when compared with field measurements, they overestimated the field response for the considered initial water content and dry density values. Also, comparisons between hydrostatic pore pressures above the phreatic surface showed that at soil moisture deficits greater than 20 mm, the difference between the measured and hydrostatic pore pressures greatly increased. The laboratory and monitoring results were used to provide practical methods of assigning representative pore-water pressure distributions and material properties necessary for performing transient seepage analyses. These results will aid in more accurate transient seepage analyses of flood control embankments.

The role of creep deformation in pit lake slope stability

Large-scale open-pit mining activities have profound impacts on the surrounding landscape and environment. At the cessation of open-pit mining, the rehabilitation of large void spaces can be achieved by pit-lake filling, where the water body provides a confining pressure on surrounding mine surfaces, reducing both the likelihood of slope failure and the need for ongoing slope maintenance. Although pit-lakes present a range of long-term benefits, the geotechnical performance of mines containing soft soils that are susceptible to creep under increasing loads due to pit-lake filling is seldom considered. From a geotechnical standpoint, creep induced failure is commonly associated with slow, downslope movements, prior to critical slope failure events. In this research, time-dependent slope stability analyses based on creep-sensitive materials are presented for an open-cut mine undergoing pit-lake filling. Numerical simulation provides a mechanism for the assessment of materials exhibiting soft soil creep constitutive behaviour under various loading conditions due to pit-lake filling. The response of mine surfaces is investigated for various filling regimes, highlighting location-dependent deformation rates, pore pressures and slope Factors of Safety for a large Australian open-pit brown coal mine. Results are presented for two separate creep-sensitive materials, identifying the ability to achieve final, stable landforms for a range of long-term pit-lake conditions.Article HighlightsTime-dependent creep deformation behaviour is investigated for a large Victorian open-pit brown coal mine undergoing pit-lake rehabilitation.The soft soil creep model is implemented for a large open-pit rehabilitation model, to assess long-lasting creep movements of a specific mine slope.Mine void filling rates are simulated for a range of rehabilitation scenarios over a 5 to 40 year period, identifying the excess pore water pressure distributions in addition to vertical and horizontal deformations rates.The long-term behaviour of 8 cross-section profiles is presented, identifying the effect of pit-lake filling for silt and clay interseam materials.

Analysis of Surrounding Rock Pressure of Deep Buried Tunnel considering the Influence of Seepage

This paper takes the surrounding rock of deep tunnel as the research object and considers the action mechanism under the influence of seepage. Based on the Mohr-Coulomb criterion, the stress mechanism of surrounding rock of deep buried tunnel is analyzed by a convergence constraint method. Based on the elastic-plastic solution, the nonlinear elastic-plastic solution of the interaction between surrounding rock and lining structure considering the effect of seepage force is proposed, and the radius of surrounding rock plastic zone is obtained. The relationship between surrounding rock stress and displacement, radial deformation of lining, and support reaction force was observed. At the same time, considering the effects of seepage, strain softening, and intermediate principal stress, the surrounding rock is divided into a plastic residual zone, plastic softening zone, and elastic zone, and the stress distribution expressions of the plastic zone and each zone of surrounding rock of circular tunnel are derived. The results show that with the change of nonuniform permeability coefficient, the seepage shows anisotropy in different directions, and the closer to the horizontal or vertical direction, the more obvious the influence of nonuniform permeability coefficient on pore water pressure distribution. Seepage and material softening have different effects on the distribution of surrounding rock stress field and the size of plastic zone. Material softening is more unfavorable to the stability of surrounding rock than seepage. The intermediate principal stress coefficient has a significant impact on the tangential stress and plastic zone of surrounding rock. When the intermediate principal stress effect is not considered, the calculation results are relatively conservative and cannot give full play to the strength of surrounding rock effectively. The research conclusion can provide a theoretical reference for studying the stability of surrounding rock in tunnel excavation under water-bearing rock.

Excess Pore Water Pressure Generation, Distribution and Dissipation in Self-boring Pressuremeter Test: Numerical Analysis

Excess Pore Water Pressure Generation, Distribution and Dissipation in Self-boring Pressuremeter Test: Numerical Analysis

Long-Term Performance of the Water Infiltration and Stability of Fill Side Slope against Wetting in Expressways

Different settlements and instabilities of unsaturated subgrade subjected to wetting have been paid increasing attention in the southeast coastal areas of China. However, the treatments are costly when they are used in engineering. In addition, the long-term performances of the treatments are unclear. Based on seepage theory for unsaturated soils, a novel subgrade using a capillary barrier was proposed in this study to reduce the different settlements and stabilities. Compared with previous studies, a capillary barrier was merely applied in the landfill. The long-term performance and feasibility of a capillary barrier applied in a tilted subgrade slope is worthy of study, particularly in humid climates. Using Geo-Studio, the feasibility was verified by comparing a conventional subgrade with a subgrade using a capillary barrier in southeast coastal areas in terms of pore-water pressure, water content, settlement, and the safety factor. The numerical results showed that the subgrade using a capillary barrier could provide significant improvements in the performance of reducing the impact of pore-water pressure distribution it suffered from, so as to lead to smaller different settlements. The vertical settlement of the pavement using a capillary barrier over a 1 year period was 1 cm. Compared with a conventional subgrade, the settlement fell by 94%, and the safely factor increased by 15% for the subgrade using the capillary barrier.

Influences of freezing\u2013thawing actions on mechanical properties of soils and stress and deformation of soil slope in cold regions

Freezing–thawing actions can affect the mechanical features of soil greatly, which is vital for the stability of soil slope in cold regions. Firstly, triaxial compression tests on sand samples under undrained conditions were performed to investigate the influences of freezing–thawing cycles, which shows that the freezing–thawing actions can weaken their strength and stiffness, and with the increasing freezing–thawing cycles, both the deviatoric stress and pore water pressure decrease gradually. Then, the double hardening constitutive model was revised to model the influences of freezing–thawing cycles in consideration of the influences of freezing–thawing actions, and the model was also validated by the test results. Finally, the proposed constitutive model was incorporated into a finite element code to numerically simulate the distribution of displacement and pore water pressure of sand slope subjected to freezing–thawing cycles, which shows that the freezing–thawing actions accelerate the dissipation of the pore water pressure and enlarge the displacement of the slope. The study here can provide a help in designing and construction of civil engineering in cold regions.

Experimental investigation on water release and gas emission of evapotranspirative capillary barrier landfill covers

AbstractEvapotranspirative (ET) landfill covers release soil moisture stored therein by grass root–water uptake during the drying process. The reduction of soil water content during this process, however, would cause an increase in gas permeability and landfill gas emission. Few studies have focused on the mechanisms of water release and gas emission in ET covers under drying conditions and how different cover configurations might affect the water–gas transport mechanisms. In this study, a series of soil column tests were conducted to investigate the water and gas transport in three configurations of ET covers, namely monolithic cover, two‐layer capillary barrier cover (CBC), and three‐layer CBC. All columns were subjected to the same drying treatment for 42 h followed by the application of a constant gas pressure at the column base. Vertical distributions of pore water pressure, volumetric water content, and pore gas pressure, as well as any gas emission rate from the top of each cover were measured. Compared with the monolithic ET cover, the two‐layer capillary barrier ET cover did not adversely affect the cumulative amount of water release, but the presence of the coarse‐grained layer, which has relatively high gas permeability, caused a higher gas emission rate. In the three‐layer cover, the addition of a low‐permeability layer underneath the capillary barrier effectively minimized gas emission. On the other hand, the upper two‐layer capillary barrier can avoid desiccation cracks of the low‐permeability layer during the drying period.

Rainfall Triggered Slope Instability Analysis with Changing Climate

Slope instability triggered by rainfall is common in tropical countries like India, where the soil of hill slopes are generally in unsaturated condition, having higher strength than in the saturated condition due to matric suction. This study aims to investigate the change in pore water pressure (PWP) with rainfall infiltration into the slope and the corresponding effect of the slope stability on different hill slopes from different regions of India. By applying rainfall on the surface of slope as a flux boundary condition, a transient seepage analysis is performed using SEEP/W, and the corresponding results in the form of PWP distribution within the slope are used as input for SLOPE/W program of Geostudio 2018 R2, to check the stability of slope. This study involves the use of CMIP5 (Coupled model intercomparision project phase 5) simulations for rainfall data of different regions for the duration of 2015–2050, 2051–2075 and 2076–2100; which is completely based on non-stationary approach (i.e., the statistics of extremes changes with time) rather than the conventional methods of extracting rainfall data that are based on stationary approach. The stability of different slopes is checked for 95 percentile rainfall intensity as well as for maximum rainfall intensity for a period of 2015–2100, and the analysis is performed for 5 days for which the duration of rainfall is considered to be 24 h. It is concluded that due to climate change, slopes which may be marginally stable under historical and present rainfall events can fail under projected precipitation estimates, and this can be understood by the results of Haflong slope stability analysis as the 30° slope, which is marginally stable under maximum rainfall intensity for the period 2015–2050 having factor of safety (FOS) value 1.0013, will fail under the maximum rainfall intensity for the period 2051–2075 and 2076–2100 having FOS value 0.9935 and 0.9989, respectively. A continuous decrease in FOS with increasing rainfall intensity is observed for all the study areas under consideration.

Study of wave-induced seabed response around twin pipelines in sandy seabed through laboratory experiments and numerical simulations

Wave-seabed-pipelines interaction is of critical importance in the design of submarine pipelines. Previous studies mainly focus on investigating the characteristics of flow fields and hydrodynamics around a single pipeline. In this study, laboratory experiments and numerical simulations have been performed to examine the effect of burial depth and space between the centers of twin pipelines on the wave-seabed-twin pipelines interaction subject to waves. In the mathematical model, the Volume-Averaged Reynolds-Averaged Navier-Stokes (VARANS) equations are used to describe the wave motion in the fluid domain, while the seabed domain is described by using the Biot's poro-elastic theory. Numerical models are validated using these experimental measurements and available relevant experimental data. Experimental and numerical results indicate that the burial depth and relative position of twin pipelines can significantly affect the wave-averaged flow velocity field and the pore-water pressure distribution as well as effective stress.

Effects of long-term leakage of shield lining on tunnelling-induced ground consolidation movements

Limited mathematical analytical studies for tunnel leakage-induced surface and subsurface soil movements and tunnel responses incorporated the distribution of excess pore water pressure, in which the concrete lining was often assumed perfectly permeable or impermeable. This mechanical closed-form model will bring calculation errors and inconsistencies in engineering practice. This article introduces a new set of mathematical model for analyzing tunnelling-induced dissipation of excess pore water pressure and ground consolidation settlement considering long-term leakage influence through a semi-permeable lining, in which the soil-lining permeability ratio is used as the relative parameter to measure the influence of lining permeability on consolidation settlement. The interesting point is that the theoretical model conducts coupling with the Terzaghi-Rendulic consolidation theory through solving of the conformal mapping technique in complex variable theory and the separation variable method in classical mathematical physics equation. The accuracy of presented solutions is verified by comparisons with numerical simulation and in-situ measurements of engineering profiles and reasonable agreements are obtained. Furthermore, a sensitivity analysis is implemented on the subsurface consolidation settlement and the distribution of excess pore water pressure, and the results indicate that the soil-lining permeability and the soil Young’s modulus notably affect the tunnel leakage-induced ground responses.

Monopile responses to monotonic and cyclic loading in undrained sand using 3D FE with SANISAND-MSu

Monopile response under undrained conditions in sand is gaining increasing interests owing to the recent development of offshore wind farms in seismic regions. Pore pressure evolution in liquefiable soil can significantly reduce the strength and stiffness of the soil which in turn affects the structural dynamic response. Several numerical models have been developed in the last two decades to enhance understanding of the mechanism of monopile–soil interaction with the existence of pore water pressure. In this study, the effects of geometry and static vertical load on monopile lateral response were studied using three-dimensional finite element methods that consider the existence of lateral cyclic load-induced pore water pressure. To achieve reliable simulation results of pore pressure development and pile displacement accumulation during cyclic loading, the simple anisotropic sand model with memory surface for undrained cyclic behavior of sand was adopted. For piles with the same diameter, a accumulated pile head displacement during lateral cyclic loading decreased linearly with increasing pile embedded length but increased with increasing eccentricity. Static vertical load had minor effects on pile cyclic lateral response. The distributions of mean effective stress and pore water pressure in the soil domain were presented. The pile reaction curve (cyclic soil reaction against pile defection) of the monopile was extracted. The numerical results aim to provide reference for optimized engineering design procedures.

Influence of Layer Transition Zone on Rainfall-Induced Instability of Multilayered Slope

Abstract Although numerous studies have been paid much attention to rainfall-induced instability of multilayered slopes, the interface between layers is generally considered to be “zero thickness”, and the layer transition zone between layers is neglected. In this study, the influence of the layer transition zone on the rainfall-induced instability of multilayered slope was investigated. A model was developed to simulate the rainfall infiltration process, the distribution of pore water pressure, and the stability of multilayered slope by coupling the unsaturated seepage model and the slope stability analysis method. Based on the analysis of the multilayered slopes with the different thickness ratios of the layer transition zone, a method for determining the critical thickness of the layer transition zone was proposed. The results showed that the layer transition zone had a significant influence on the stability of multilayered slope. It was found that the presence of the layer transition zone in the multilayered slope reduced the hydraulic conductivity of the slope and increased the rate of formation of transient saturated zone, which contributed to excess pore water pressure at the toe of the slope. The analysis of the local factor of safety (LFS) showed that when the thickness ratios of the layer transition zone were between 2.5% and 5%, the corresponding hydraulic conductivity of the slope decreased by 1%-2.5% and the maximum failure area of the slope during the rainfall was 25% of the slope. Our study highlighted the importance of the layer transition zone for the rainfall-induced instability of the multilayered slope.

Characterization of dynamic response of asphalt pavement in dry and saturated conditions using the full-scale accelerated loading test

Asphalt pavement exhibits different dynamic responses to vehicle loading in saturated conditions and in dry conditions. It is essential to directly conduct field full-scale tests to systematically investigate comprehensive influence of water on dynamic response of asphalt pavement. However, real vehicles which were often applied in field tests might result in a certain deviation in dynamic responses measured from each field loading test due to the inevitable influence of the driver’s subjective operation and the tread pattern on vehicle wheel. This study adopted the full-scale accelerated loading test system to comprehensively characterize dynamic response of asphalt pavement in both dry and saturated conditions based on the strict control of loading vehicle’s path. The results indicated that dynamic responses showed relatively larger magnitudes in saturated condition than those in dry condition. Strain and stress respectively showed slowly increasing and decreasing trends with the increase of vehicle load and speed correspondingly. Pore water pressure was apparently sensitive to vehicle speed compared with vehicle load. Pore water pressure generated by the front axle wheels showed obviously larger magnitudes than those produced by the rear axle wheels due to the change in thickness of surface runoff. The transverse distribution of pore water pressure obtained from both field tests and numerical simulation exhibited consistent variation trends. Decline degree of pore water pressure magnitudes was obtained at 86.1% for the two adjacent testing points with a distance of 100 mm under the vehicle wheel, which proved the great influence of the tread pattern of vehicle wheel on the transverse distribution of dynamic pore water pressure. A prediction model of positive pore water pressure that considered vehicle load and speed was proposed.

Effect of several patterns of floating stone columns on the bearing capacity and porewater pressure in saturated soft soil

One of the common geotechnical problems is the construction on soft soil and the improvement of its geotechnical properties to meet the design requirements. A stone column is one of the well-known techniques used to improve the geotechnical properties of soft soils. Sometimes thick layers of soft soil imposed the designer to use floating stone columns for improvement of such soil; in this case, the designer will be lost the end bearing of the stone column. In this study, the effects of several patterns of floating stone columns distribution under footing on the bearing capacity of soil and the distribution of excess porewater pressure are investigated. The soft soil used in this study has a very low undrained shear strength (cu) of 5.5 kPa and improved by several patterns of stone columns (single, two linear, triangular, square, and quadrilateral). The stone column has a length of 180 mm and a diameter of 30 mm. The material of the stone column is poorly graded sand has an angle of internal friction (48.5°) at a relative density of 65%. The results indicated a significant increase in the ultimate bearing capacity of soft soil when treated with floating stone columns despite the small ratio of area replacement and reducing the excess porewater pressure and settlement. Also, the ultimate bearing capacity of soil calculated from experimental work is compared with the corresponding values obtained from the proposed equations in the previous studies to evaluate the validity of using such equations.

Investigation of drainage function of geosynthetics for basal-reinforced embankments

This study investigates the behaviour of basal-reinforced embankments under seepage flow. For this purpose, physical embankment models with dimensions of 195 cm long, 100 cm wide and 110 cm high were constructed in the laboratory. The seepage gradient within the embankment was generated by applying a constant water head of 100 cm at the water compartment located behind the models. The unreinforced embankment experienced seepage erosion at the toe section which triggered deep-seated global failure. Due to the limited transmissivity of the non-woven geotextile, the geotextile-reinforced embankment exhibited similar pore-water pressure distribution to that of the unreinforced embankment, which resulted in lateral spreading failure between the base of the embankment and the non-woven geotextile. In contrast, the geocomposite drain, which has a higher in-plane flow capacity compared to that of the geotextile, significantly alleviated the pore-water pressures within the embankment. The behaviour of the physical models was verified numerically by variably saturated flow modelling conducted for a wide range of in-plane permeabilities for the basal reinforcement. The effect of increasing in-plane permeability of the basal reinforcement layer on pore-water pressure reduction was quantified irrespective of the geosynthetic type.

Active earth pressure of 3D earth retaining structure subjected to rainfall infiltration

Rainfall infiltration is a key factor that determines the active earth pressure on an earth-retaining structure. Previous investigations on the stability of earth retaining structures were mainly conducted under the assumption that the backfills was dry, saturated, or subjected to a steady unsaturated flow, ignoring the time-varying characteristics of the rainfall infiltration process. Based on the upper bound theorem of limit analysis, this study conducted a stability analysis of a three-dimensional (3D) earth retaining structure (ERS) subjected to various rainfall infiltration patterns. An analytical model that was, capable of calculating the pore water pressure distribution due to rainfall infiltration, was employed to calculate the time-dependent pore water pressure change of unsaturated soil. A horizontal slice method was employed to take into account the change of effective unit weight of soil. Afterward, an analytical expression of the active earth pressure coefficient is derived from the energy balance equation. The effects of the 3D characteristics of ERSs and rainfall patterns on the active earth pressure coefficient and the failure pattern of ERSs were investigated. It was found that compared with plain strain conditions, the 3D characteristics of ERSs lead to a lower value for the active earth pressure and better stability. The rainfall patterns not only directly determined the stability and active earth pressure of an ERS, but also played a dominant role in critical failure pattern of the ERS.

Numerical simulation of suction bucket foundation response located in liquefiable sand under earthquakes

The suction bucket foundation is considered as an effective option for offshore wind turbines because of its advantages in rapid installation and recyclability. However, the depth of the suction bucket inserted into the seabed is shallower compared to the monopile. Earthquakes and liquefaction threaten the safety and stability of suction bucket foundations in the North Sea of Europe and the Chinese offshore where earthquakes are active and the seabed is prone to liquefaction. This paper investigated the behavior of the suction bucket foundation located in the liquefiable sand under earthquakes using an advanced liquefaction model. The FE-FD method with the cyclic mobility constitutive model for soil was used to carry out the nonlinear dynamic analyses. Responses of the excess pore water pressure ratio, acceleration, displacement, and rotation were studied under different earthquake magnitudes and sand density. The distribution of excess pore water pressure of the suction bucket foundation from the results show strong space-time characteristics. The failure mechanism of the suction bucket foundation after liquefaction was revealed. The numerical results indicate that offshore wind turbines will be subjected to permanent displacements and tilt to reach 40% of the safety threshold due to earthquakes and liquefaction. Thus, seismic load and liquefaction of the foundation must be considered in the design.

Refined 3D modelling of spatial-temporal distribution of excess pore water pressure induced by large diameter slurry shield tunneling

Tunneling induced excess pore water pressure (EPWP) is crucial to face stability, time-dependent ground deformation and structural performance of tunnel lining. This paper numerically investigates the short-term spatial–temporal distribution of EPWP induced by large diameter slurry shield tunneling in saturated soils. A refined 3D finite element model is developed to consider the construction process including tunnel advance, segment installation, tail void grouting and grout material hardening. The commonly neglected shield conicity is fully considered in the refined modelling. The proposed fluid–solid coupled numerical model is well validated by the field monitoring data of Shanghai Yangtze River Tunnel after considering shield conicity. Temporal distribution of EPWP is equivalent to a certain spatial distribution in the longitudinal direction. Longitudinal distribution of EPWP is separated into five parts (Δua ~ Δue) induced by different construction process. The fives of EPWP are largely influenced by shield conicity induced overcut, soil compressibility, grouting pressure and tunnel diameter. The distribution of EPWP ahead of tunnel face is mainly determined by slurry support pressure and is hardly influenced by other factors.