Accumulation Of Pore Water Pressure Research Articles

Behavior of group of geosynthetic encased granular piles with tire chips- aggregates mixture under static and cyclic loading – A model study

The current study addresses the dual challenges of improving the performance of soft soil beds subjected to static and cyclic stresses and managing the environmental impact of waste tire disposal. This research contributes valuable insights into the sustainable use of recycled tire chips in granular pile construction, coupled with the efficacy of combi-grid encasement for improved soft ground under static and cyclic loading conditions. A series of laboratory model tests were carried out on a group of granular piles to examine the principal parameters, such as the selection of geosynthetic materials and cyclic loading characteristics, including cyclic loading amplitude (qca) and cyclic loading frequency (f). The granular pile composition consists of (25% tire chips + 75% aggregates). The performance of granular piles on improved ground is assessed based on the settlement reduction ratio (Sc,r), accumulation of excess pore water pressure (Pexc), and stress concentration ratio (n). The key findings from static model tests are that the load-bearing capacity is significantly increased with installing a group of ordinary granular piles (= 58%) and substantially increased with combi-grid encasement (= 335%). The effectiveness of ordinary granular piles (OGP) in enhancing the performance of a soft soil bed becomes greater when subjected to lower cyclic loading frequencies (f) and smaller cyclic loading amplitudes (qca). The incorporation of combi-grid encasement greatly enhanced the cyclic performance of a group of granular piles by substantially minimizing cyclic-induced settlement (Sc) across both principle parameters f and qca. This study also examines the increased cyclic stresses on the improved soft bed, resulting in the accumulation of excess pore water pressure (Pexc) development, which is reduced to a greater extent with the help of combi-grid encasement across both principle parameters.

Experimental study on the influence of principal stress axes rotation on sand mechanical behavior and non-coaxiality effects

The soil around the foundations of offshore engineering experienced the principal stress axes rotation caused by the wave loadings. To investigate the influence of the principal stress axes rotation on the soil stress-strain relationship, a series of pure principal stress axes rotation tests under drained/undrained conditions were carried out with a hollow cylinder apparatus. The undrained test results show that the deviatoric stress, relative density, and starting point of rotation affect the mechanical properties of sand. Through the analysis of pore water pressure accumulation in the 1st cycle, it was observed that more pore pressure accumulates in Quadrants II and III, while relatively less accumulates in Quadrants I and IV. Under drained and undrained conditions, the non-coaxial behavior of the soil differs. The non-coaxiality can be significantly observed in the drained test, and the direction of strain increment tends to the stress increment direction when the cycle number increases.

Evaluating the influence of Cynodon dactylon on the wave force and wave erosion in the water-level fluctuation zone of the Three Gorges Reservoir Area

Wave erosion is the main erosion type in the water-level fluctuation zone (WLFZ) of the Three Gorges Reservoir Area (TGRA). Despite vegetation can effectively mitigate wave erosion in the WLFZ, its influence on the wave force and wave erosion remains unclear. Therefore, the wave experiments were conducted under 3 Cynodon dactylon coverage rates (0, 30% and 60%) and 9 wave conditions (3 wave heights of 4, 6 and 8 cm combined with 3 wave periods of 1, 2 and 3 s) to analyse the wave force (expressed as the wave pressure on the slope surface and the pore water pressure in the slope) and wave erosion rate, and the factors influencing wave erosion were identified. The results indicated that the wave pressure, pore water pressure and wave erosion rate increased by 19.14%–104.75%, 16.84%–65.04% and 23.33%–91.64%, respectively, as wave height increases. The wave pressure decreased by 1.50%–31.23% followed by an increase by 22.05% to 87.10% with the increase of wave period, whereas the pore water pressure and wave erosion rate decreased by 28.33%–53.59% and 20.46%–63.59%, respectively. However, these quantities decreased by 2.10%–50.84%, 17.06%–40.23% and 17.28%–82.18%, respectively, with the increase of Cynodon dactylon coverage rate. It was also discovered that the pore water pressure and Cynodon dactylon coverage rate attained the highest positive and negative correlation coefficients with the wave erosion rate, respectively. In addition, pore water pressure accumulation is the most critical influence factor on wave erosion, and Cynodon dactylon could effectively reduce the pore water pressure via its roots, thus improving the slope wave erosion resistance. This study could be useful to understand the mechanism of plants on controlling wave erosion and could provide a scientific reference for wave erosion control and the ecological construction in the WLFZ.

In-situ Test and Calculation Method for Horizontal Deformation of Soil Induced by Multi-row Grouting

Multi-row grouting can be used to repeatedly mitigate the deformation of critical structures like tunnels. Nevertheless, no comprehensive investigation into the development patterns of soil deformation and excess pore water pressure, induced by multi-row grouting in soft soil, has been conducted to date. To address this gap, this study carried out a field test of multi-row grouting, systematically exploring the evolution and accumulation of soil horizontal displacement (SHD) and excess pore water pressure (EPWP) resulting from multi-row grouting. The findings demonstrated that the grouting process during multi-row grouting respectively exerted reaction and shielding effects on the subsequent grouting for the behavior of soil surrounding the grouting area. The reaction and shielding effects increased proportionally with the number of grouted rows. To predict the SHD induced by multi-row grouting, considering the reaction and shielding effects, this study provided a theoretical calculation method based on cavity expansion theory and the concept of upper and lower bounds, and proposed an optimal grouting scheme.

Experimental study on strain accumulation characteristics of saturated remolded loess under pure rotation of the principal stress axis

The study conducted a pure rotation test of 720° for saturated remolded Q2 loess under principal stress axis (PSA) rotation when deviatoric stress and the intermediate principal stress ratio are different, also paying attention to examining the changes of strain accumulation and pore pressure of such loess. According to the test, as the PSA rotates continuously, the pore water pressure exhibited by saturated remolded loess presents a normal cyclic accumulation elevation, and under the same deviatoric stress conditions, the pore water pressure accumulation trend of different principal stress ratios is the same; however, the magnitude is different. The increment of pore water pressure becomes smaller as the number of cycles increases. When the deviatoric stress level is low, the material hardening is in a cyclic stable state, the strain component remains stable with the continuous rotation of the PSA, and the strain path area is reduced, together with a stable final size. In case of higher deviatoric stress, the material strength cycle becomes weaker and the strain component accumulates slowly as the PSA (the spiral line of the strain path) continuously rotates in the γzθ−(ϵz−ϵθ) plane, and gradually expands until failure. The increased intermediate principal stress is accompanied by accelerated strain development speed and advanced failure.

Effect of cyclic shear stress ratio on cyclic behavior of clay under multidirectional shear loading

In offshore engineering, soft clay foundations supporting infrastructure are subjected to significant multidirectional cyclic shear stresses due to earthquake or storm loads. To explore the undrained multidirectional cyclic behavior of foundation clays under critical loading conditions, a series of tests were conducted using a variable-direction dynamic cyclic shear system. By controlling the amplitude ratio (η) of two mutually perpendicular shear stresses in the horizontal plane, both unidirectional and multidirectional cyclic shear stress paths were achieved. The shear strength, stiffness, and pore water pressure are presented and discussed by considering the effects of η and cyclic stress ratio (CSR). The results suggest that the increase of CSR and η accelerates the cyclic failure, stiffness degradation, and pore water pressure accumulation of the clay specimens. An η-independent cyclic strength curve is established by normalizing the CSR to 0.755 η . Although η hardly affects the shear modulus in the x direction (G x) during the first cycle and at failure, it affects the parameter (χ) governing the shape of the stiffness degradation curve. Additionally, a model is proposed for predicting the evolution of normalized residual pore water pressure with respect to the number of cycles, wherein the parameter d remains unaffected by CSR and η.

Numerical investigation of solute migration and release from sediments driven by wave-induced accumulation of pore water pressure

Sediments with poor drainage conditions are prone to build-up of excess pore water pressure when exposed to dynamic wave loading. This results in accumulated seepage flow that may have a significant impact on migration of solutes in marine sediments. In this study, the complex interplay between solute transport and wave-induced seepage process is investigated through a pioneering approach that combines and synchronously solves the two-dimensional advection-dispersion equation governing nonreactive solute transport with the governing equations for wave-induced oscillatory and residual pore pressures. The simulated results have demonstrated that the accumulated seepage flow significantly contributes to the advective and dispersive fluxes of solute, where the advection leads to movement of the solute layer and the hydrodynamic dispersion primarily enlarges the thickness of solute layer. The distribution of residual pore water pressure along depth exhibits two patterns that are primarily influenced by the thickness of the seabed. When the seabed is thinner than 0.3 times the wavelength, the residual pore pressure increases monotonically with depth (Pattern I). In this situation, the solute always migrates upwards to the seabed surface. Otherwise, Pattern II depicts the distribution of residual pore pressure, where the peak value of residual pore pressure attains at a certain depth and may advance downwards with wave action time. In this case, the direction of solute migration in regions shallower than a certain depth (e.g., 0.6 times the thickness of sediment) may alternate over time but the solute can eventually be transported upwards to seabed surface. The parametric study shows that when the seabed thickness is 0.2–0.3 times the wavelength, accumulated seepage flow has less effect on solute migration, whereas decreasing relative density and shear modulus, as well as increasing soil permeability can enhance the effect of accumulated pore water pressure on solute migration.

Influence of water content on clay deformation characteristics subjected to periodic intermittent loading

The influence of clay's drainage and reconsolidation behavior during the intermittent period on clay's deformation needs to be further studied. Dynamic loads, such as traffic and earthquake, etc., have a certain intermittent period during the loading process. This study examined the deformation behavior of the clay obtained from Nanchang under various cyclic stress ratios (CSR) and water content conditions. The results demonstrated that minor strain rebound occurs in all intermittent periods. Aside from that, drainage reconsolidation during intermittent periods alters the pattern of strain accumulation during subsequent loading periods, and cyclic stress levels and water content conditions can impact the intermittent effects. In contrast, the development of strain in the samples under intermittent loading is more negatively impacted by high CSR and low water content (deviation from optimal water content). The hysteresis curve continued to change throughout the test, the accumulation and dissipation of excess pore water pressure changed, and it was found that the samples' resilient modulus had increased, indicating an improvement in the samples' resistance to deformation.

Numerical analysis of energy piles in a hypoplastic soft clay under cyclic thermal loading

AbstractThis work is a numerical investigation of the effect of thermally induced volumetric collapse of normally consolidated clays on the performance of energy piles. A series of coupled thermo‐hydro‐mechanical Finite Element simulations were carried out using the commercial software ABAQUS. These examined a single free‐head energy pile embedded in a normally consolidated clay layer subjected to a constant mechanical load and to a number of heating/cooling cycles, to reproduce operating conditions. The soil behaviour was described with two advanced hypoplastic constitutive models for clays, one of which incorporates the thermally induced volumetric collapse using an ad‐hoc algorithm developed by the authors. Both models predict a cyclic accumulation of settlement and excess pore water pressure, especially when the thermal collapse effect is considered. While the excess pore pressure distribution stabilises within a few cycles, the rate of settlement of the pile head does not show any tendency to decrease from one cycle to another. These results are in agreement with data from small scale tests on an isolated energy pile in normally consolidated clay, indicating that the numerical model developed in this study can be used to investigate the complex soil/pile/raft interaction processes occurring in real piled foundations incorporating energy piles.

Seismic displacement of earth slopes incorporating co‐seismic accumulation of dynamic pore water pressure

AbstractThe earthquake‐induced displacement of sloping soil mass is an important indicator of co‐seismic landslide initiation. In this paper, an improved Newmark displacement model that considers the accumulation of dynamic pore water pressure (DPWP) in the soil caused by both vertical and horizontal ground motions is proposed. The model can quantitatively describe the dynamic changes of the seismic yield acceleration of near‐saturated infinite soil slopes. Three different types of ground motion time histories are selected to compare the performance of the proposed Newmark displacement model, and the influence of DPWP accumulation on the slope yield acceleration and on the seismic displacement is obtained. The seismic slope displacement analyses indicate that the weakening effect of slope yield acceleration caused by bidirectional earthquake excitation‐induced DPWP is more obvious than when only considering the horizontal ground motion, when the slip surface soils are in near‐saturated state. The effect of initial saturation (Sr0) on the DPWP accumulation caused by vertical ground motion is also investigated. Furthermore, the accumulated seismic displacement can be reasonably explained by the frequency distribution characteristics of the ground motions. Finally, the numerical results of this paper show that the seismic displacement model seldomly considering the effect of DPWP, or only considering the DPWP induced by horizontal ground motion, can significantly underestimate the displacement value when the slip surface soils are in a near‐saturated state.

Monotonic and cyclic undrained behaviour and liquefaction resistance of pumiceous, non-plastic sandy silt

Experimental data related to the mechanical behaviour of crushable pumiceous soils are limited compared with those for hard-grained soils. The main focus of previous studies has been on pumiceous sands, whereas pumiceous silts have not been investigated to date. In this paper, several series of monotonic and cyclic triaxial tests were performed to investigate the undrained behaviour and liquefaction resistance of a natural volcanic-ash derived, non-plastic pumiceous silt from northern New Zealand. Particle crushing due to sample reconstitution and triaxial testing was analysed by quantifying the changes in grain-size parameters and pumice contents. The main results can be summarized as follows. (1) The pumiceous silt showed a contractive response, even at medium to high relative densities, leading to high flow liquefaction susceptibility. (2) When subjected to cyclic undrained loading, the silt exhibited similar trends in terms of excess pore water pressure and axial strain accumulation to those established for hard-grained soils as opposed to those for pumiceous sands. (3) The liquefaction resistance of both medium-dense and dense samples was within the lower range compared to published cyclic resistance curves of both hard-grained soils and pumiceous sands. (4) The material did not undergo significant particle crushing after testing. Result (4) was considered to be the main factor that contributed to the fact that, in general, the cyclic undrained behaviour of the pumiceous silt was close to the trends established for those of hard-grained soils.

Model tests of geosynthetic-reinforced soil walls with marginal backfill subjected to rainfall

A series of model tests were conducted to investigate the performance of geosynthetic-reinforced soil (GRS) walls with marginal backfill subjected to rainfall infiltration. The effectiveness of improvement measures—such as decreasing reinforcement spacing and increasing sand cushion thickness—to prevent the GRS wall failure due to heavy rainfall was evaluated. The distribution and variation of the volumetric water content, porewater pressure, wall deformation, and reinforcement tensile strain were monitored during the test. The advancement of the wetting front and the drainage function of sand cushions were visually observed using the fluorescent dyeing technique. For the baseline case, the wall began to deform as rainfall proceeded, causing the potential failure surface to gradually move backward. When the potential failure surface moved beyond the reinforced zone, the pullout of the topmost reinforcement layers occurred, resulting in the collapse of the GRS wall in a compound failure mode. Decreases in reinforcement spacing and increases in sand cushion thickness effectively reduced wall deformation and enhanced wall stability. The placing of sand cushions between the reinforcement layers can also delay water infiltration and reduce the accumulation of porewater pressure inside the wall. Suggestions for designing rain-resistant GRS walls are also proposed based on the findings.

The influence of cyclic preloadings on cyclic response of Zbraslav sand

This article presents a comprehensive experimental investigation on Zbraslav sand under monotonic and cyclic loading. Initially, the material response at large strains was investigated by means of undrained monotonic triaxial tests with different initial mean effective stresses and relative densities. A set of oedometric compression tests with multiple unloading–reloading cycles and different initial relative densities was as well included to evaluate the material compressibility. In addition, several undrained cyclic triaxial tests were performed and analyzed under a wide range of initial conditions. In the aforementioned tests, the undrained cyclic resistance was investigated considering different types of drained or undrained cyclic preloadings and reconsolidation stages. The experimental results suggested that drained cyclic preloadings remarkably modified the subsequent rate of pore water pressure accumulation and increased the number of cycles to reach the defined failure criterion without affecting the stress–strain response. On the other hand, undrained cyclic preloadings presented a threshold which separated positive and negative effects in the subsequent undrained cyclic resistance. A comparison of the undrained cyclic resistance variation depending on the number of drained or undrained cyclic preloadings was also included.

On the influence of drained cyclic preloadings on the cyclic behaviour of Zbraslav sand

This article experimentally evaluates the influence of directional drained cyclic preloadings on the subsequent undrained monotonic and cyclic response of Zbraslav sand. The cyclic tests were performed on dense samples and under triaxial conditions. The experiments considered several different preloading directions, magnitude of the maximum deviatoric stress reached during the drained preloading stage and deviatoric stress amplitudes during the undrained shearing stage. The experimental results suggested that the rate of strains and pore water pressure accumulation, and therefore, the number of cycles to reach initial liquefaction or failure conditions are remarkably affected by the direction of the drained preloading history. In addition, the preloading direction plays a major role on the contractive/dilative response during the subsequent undrained monotonic loading. In particular, directional drained cyclic preloadings toward positive deviatoric stresses at constant mean effective stress and isotropic compression led to more dilative responses. Contrary, preloadings toward negative deviatoric stresses at constant mean effective stress led to more contractive responses. The results suggest that these major variations cannot be associated to the rather small changes in the relative density of the samples but likely due to fabric changes and induced anisotropy caused by the drained preloading history.

Pore-water pressure accumulation during closely spaced earthquakes: Fundão Tailings Dam

ABSTRACT This paper focuses on pore-water pressure accumulation in loose sandy tailings during three (3) earthquakes within four minutes and the accompanying decrease in effective stress to assess the dynamic stability of the Fundão Tailings Dam, which failed on 5 November 2015. Results of laboratory cyclic direct simple shear tests are used to illustrate the accumulation of pore-water pressures with closely spaced low-level cyclic events. The seismically-induced pore-water pressures in the loose sands below the left abutment downstream slope and setback area are used to estimate the decrease in factor of safety with time until failure occurred within 20–30 minutes of the earthquakes. Field cone penetration and standard penetration test conducted prior to the failure are used to calculate the factor of safety against liquefaction to estimate pore-water pressures generated during the fore, main, and aftershocks in the sandy tailings. The effective stress stability analyses utilise a liquefied shear strength ratio for the sand tailings below the left abutment downstream slope and toe of the plateau because the sand tailings reached the critical state condition due to a reduction in the effective stress. SITE LOCATION: 20.206500°S, 43.467111°W

Effects of physical properties and undrained cyclic shear conditions on the pore water pressure responses of saturated sands and clays

For clarifying the effects of relative density (Dr) and Atterberg’s limits on the cyclic shear-induced pore water pressure properties of soils, sandy soils with similar index properties and clayey soils with different Atteberg’s limits were collected from Vietnam and Japan and used for this study. Specimens at Dr = 50% of Nam O sand and Dr = 70% of Toyoura sand, and those of Hue clay and Japanese Kaolin clay were consolidated under the vertical stress of σvo = 49 kPa. They were then subjected to undrained cyclic shear for various cyclic shear directions and wide ranges of the number of cycles and shear strain amplitudes. Under the same cyclic shearing conditions, specimens of sand at higher Dr (Toyoura sand) and clay with higher Atterberg’s limits (Kaolin) show a lower pore water pressure ratio. The number of cycles and the cumulative shear strain at the starting point of pore water pressure generation were observed for different soils and testing conditions. In addition, using the cumulative shear strain, a new strain path parameter, the effects of shear strain amplitude and cyclic shear direction can be captured, resulting in a unique uacc/σ’vo - G* relation on each soil. Based on this, fitting lines can be drawn and referred to promote a prediction of the cyclic shear-induced pore water pressure accumulation for the used soils under different cyclic shear conditions.

Seabed Liquefaction around Pipeline with Backfilling Trench Subjected to Strong Earthquake Motions

As an indispensable part of the lifeline for the offshore gas and oil industry, submarine pipelines under long-term marine environmental loadings have historically been susceptible to earthquakes. This study investigates the impact of trench backfilling on the residual liquefaction around a pipeline and the induced uplift of a pipeline under the combined action of an earthquake, ocean wave and current loading. A fully coupled nonlinear effective stress analysis method, which can consider the nonlinear hysteresis and the large deformation after liquefaction of the seabed soil, is adopted to describe the interaction between the seabed soil and the submarine pipeline. Taking a typical borehole in the Bohai strait as the site condition, the nonlinear seismic response analysis of the submarine pipeline under the combined action of seismic loading and ocean wave and current is carried out. The numerical results show that trench backfilling has a significant impact on the seismic response of the pipeline. The existence of trench backfilling reduces the accumulation of the residual excess pore water pressure, so that the seabed liquefaction around the pipeline is mitigated and the uplift of the pipeline is also decreased.

Load history idealisation effects for design of monopiles in clay

Evaluation of the characteristic behaviour of clay under cyclic loading is desirable and essential for an optimised monopile design of offshore wind turbines (OWTs). In current design practice, an irregular storm history is transformed into an idealised regular cyclic history with loading packages of uniform amplitude in ascending order. This paper investigates the effects of load representations by performing a series of load-controlled cyclic triaxial tests. The load histories investigated consist of a calculated 1 h storm loading on an OWT monopile foundation in the North Sea and its representations with different orders of the idealised load packages. The role of the average shear stress is discussed by comparing the shear strain evolution, pore water pressure accumulation, soil stiffness and strength. Cyclic degradation obtained with and without average shear stress components is studied. The experimental investigations in this paper indicate that the current design practice of load idealisation may not necessarily be conservative. The average shear stress affects the strain accumulation significantly but has relatively minor effects on the evolution of pore water pressure. It is suggested that the soil element response subjected to irregular load history should be investigated further from both experimental and numerical perspectives to develop an improved procedure for monopile design.

Poro-elastic and poro-elasto-plastic modeling of sandy seabed under wave action

The seabed could be liquefied due to the wave-induced accumulation of residual excess pore water pressure (EPWP), resulting in the instability of offshore and marine structures. Thus, it is of importance to consider both the buildup of EPWP and the associated deformation behavior of the seabed. In this study, a dynamic poro-elasto-plastic seabed model, integrated with a state-dependent plasticity model, is developed in COMSOL Multiphysics. The numerical model of seabed is validated against the analytical solutions and model tests. The elliptical trajectories of soil particle at different depths of the poro-elastic seabed under the progressive wave are illustrated. The ratio of the amplitude of vertical displacement to that of horizontal displacement generally decreases as the depth increases for the poro-elastic seabed. In the poro-elasto-plastic seabed, however, the soil particles tend to move following the direction of the wave propagation. Moreover, the soil particles at the shallow part of the seabed go downwards before the initial liquefaction, while the soil particles at the lower position initially go upwards and then turn to go downwards when the initial liquefaction occurs. Particularly, the surface seabed with lower relative density is more prone to lose the stability, resulting in a dramatic upward vertical displacement.

Evolution process and hardening mechanism of consolidated silt in silty seabed subject to waves

In this study, the entire evolution process of consolidated silt in a silty seabed under the action of waves is reproduced via a flume experiment. The response of pore water pressure in the seabed, variation of soil strength, and change in characteristic parameters of the soil particles are studied. From the experiment, the silty seabed is inferred to undergo two stages, i.e., failure and reconsolidation, finally forming a high-strength consolidated silt layer. The rapid accumulation of pore water pressure in the seabed under the action of waves provides the initial conditions for the formation of consolidated silt. The shear of the wave and seepage of pore water pressure provide power for the reconsolidation of the seabed. Macroscopically, reconsolidation of an unstable seabed results in consolidated silt. Microscopically, consolidated silt is the product of coarsening, homogenization, and embedment of particles. The hardening mechanism of consolidated silt is driven by increases in the internal friction angles of the particles and existence of hardening forces between the particles. The empirical formula for the internal hardening force of consolidated silt is obtained experimentally.