Pore Pressure Accumulation Research Articles

Enhancing sand liquefaction resistance through microbial-induced partial saturation: An experimental study

The liquefaction potential of saturated sand can be significantly reduced by inducing partial saturation in the soil. Conventional soil liquefaction mitigation methods, namely soil densification, drainage, cementing, and groundwater lowering, pose environmental concerns and are challenging to apply to pre-existing structures. However, the microbially induced partial saturation (MIPS) method is emerging as a novel and eco-friendly approach to mitigate liquefaction. The MIPS method involves microbial denitrification, which produces nitrogen gas and results in a desaturating effect in the saturated soil. The current study conducted a series of stress-controlled undrained cyclic triaxial tests on saturated sandy soil and microbially-desaturated sandy soil under different relative densities and loading conditions. In addition, the study systematically analyzed the effects of temperature and pH on bacterial activity and the denitrification process. Batch experiments were conducted to establish a relationship between the initial nitrate concentration in the bacterial media and the resulting desaturation.Comprehensive analyses of cyclic resistance curves were performed to gain a thorough understanding. Additionally, the study conducts detailed analyses of the accumulation of excess pore pressure and the resulting axial strains and deformation patterns in both treated and untreated sand. This study demonstrates that the MIPS treatment considerably enhances the liquefaction resistance of treated sand.

A generalized framework of two-way coupled numerical model for fluid-structure-seabed interaction (FSSI): Explicit algorithm

In a complex marine dynamics environment, the consideration of fluid-structure-seabed interaction (FSSI) plays a vital role in reliably analyzing the dynamic response of marine structures, and in assessing their structural dynamic stability. Currently, the predominant numerical analysis used worldwide for the problems of wave-seabed interaction and seawater-structure-seabed interaction is primarily the one-way coupling method. While only a few two-way coupled models are being developed. Consequently, two issues are brought up: (1) For the cases involving small deformation and displacement, the degree of discrepancy can't be quantitatively identified between the results obtained respectively from one-way coupling models and two-way coupled models which are more rigorous in mathematics and physics. (2) For the cases involving large deformation and displacement, one-way coupling models should be non-applicable. To address this problem, this study first proposes an explicit two-way coupling theory for the fluid-structure-seabed interaction. Then, a two-way coupled numerical model is developed by integrating the soil-structure dynamics software FssiCAS, and an OpenFOAM-based CFD solver OlaFlow by utilizing the data exchange library preCICE. This two-way coupled model has been embedded into the software FssiCAS. The reliability of the developed two-way coupled model is systematically validated through a rigorous verification process. Subsequently, a comparative study is conducted between the newly developed two-way coupled model and the existing one-way coupling model, to investigate the ocean wave-seabed interaction, as well as the interaction process between ocean wave, a breakwater, and seabed foundation. A comprehensive analysis is performed by comparing the differences in the wave profiles in fluid domain, dynamic displacement of structure and seabed foundation, seepage, pore pressure accumulation, and liquefaction in seabed foundation solved by the two-way and one-way coupled models. Finally, the suitability of the one-way and two-way coupled models in different applicable scenarios was discussed.

Influence of repeated surface surcharge loading on tunnel displacement considering the structural characteristics of soft clay

Shanghai soft clay is a typical marine clay with specific structural characteristics. However, the effects of surface surcharge loading on structured soft clay and existing tunnels remain unclear. In this study, the effect of repeated surface surcharge loading on tunnel displacement was numerically investigated, considering the structural characteristics of Shanghai Layer 4 soft clay. An elastoplastic constitutive model (Shanghai model) that describes the mechanical properties and structural characteristics of natural clay was used to simulate the soil response. The results indicate that the first cycle of repeated surface surcharge loading had the greatest effect on the displacement of the tunnel and overlying clay; subsequent loading cycles further increased the displacement. With repeated surface surcharge loading, the displacement of the overlying clay exhibited a significant decreasing trend with increasing depth. The maximum excess pore pressure of the clay exhibited a decreasing trend with increasing loading time. The displacements of the tunnel and overlying clay were greater when the effects of the soil structure were considered. However, the initial degree of the structure had no significant effect on the accumulation of excess pore pressure when the surface surcharge loading did not reach the ultimate bearing capacity of the clay.

Further development of distinct lattice spring model with Finn model for liquefaction analysis

The hazards associated with sand liquefaction induced by dynamic events are significant. The study of the dynamic stability in hydraulic structures presents an interdisciplinary challenge encompassing both geotechnical engineering and engineering seismology. This study was based on an actual hydraulic engineering project. To effectively predict changes in pore pressure caused by earthquakes, we integrated the Finn constitutive equations into a distinct lattice spring model (DLSM). In this study, the code was customized to accommodate multiple materials simultaneously participating in the calculations, thus simplifying the solution to complex engineering problems. Initially, we validated the DLSM’s liquefaction equation by comparing it with the finite difference method. Subsequently, we conducted a comparative analysis of liquefaction in an engineering project of sluice using the enhanced DLSM. Our analysis indicates that untreated sand has a severe risk of trending toward liquefaction, presenting a hazard to hydraulic engineering. The incorporation of a gridded concrete framework significantly mitigated the seismic-induced pore pressure accumulation and irreversible deformations caused by vibrations. A comparative study showed that concrete retaining walls with concrete supports are more effective at reducing liquefaction hazards and minimizing irreversible deformations in engineering structures.

Experimental Study on Consolidation Characteristics of Concentrated Full Tailings and Research on Pore Water Relief Methods of Piles

Surface harmless storage of concentrated full tailings (CFTs) involves the technology of adding a curing agent to the tailings slurry discharged from the thickener to realize the modification of the tailings and centralized storage of the tailings on the surface to realize the harmless treatment of the tailings. High water content of tailings is still the key technical problem that restricts the harmless storage of piles at present. Regarding the above problems, we implemented the consolidation test and numerical simulation of seepage-stress coupling consolidation of CFT, clarified the consolidation characteristics and parameters of CFT under different curing ages, and conducted a comparative analysis of pore water pressure in the whole cross-section of piles with different drainage schemes based on the results of the test. In addition, we also clarified the drainage effect of interlayer drainage on reducing the excess pore water pressure of piles and compared the simulation results of the pore water pressure of piles under different permeability coefficients. The results show that as the permeability coefficient of the concentrated tailings material decreases, the pore pressure accumulation inside piles under the same drainage scheme is more serious, and the length of time for consolidation and stabilization becomes longer. Therefore, it is recommended that the excess porous water pressure be relieved by means of increased drainage facilities under a small permeability coefficient.

Effect of stage amplitude ratio on dynamic behaviors of sand under wind turbine loading

Offshore wind turbines are subjected to more significant wave and wind environmental loads at extreme weather conditions, making subsoil experience various loading stages with different amplitudes. To investigate the coupling effect of both cyclic shear stress ratio (CSR) and stage amplitude ratio (Ar) between normal and extreme weather conditions, a series of bi-directional simple shear tests with five different Ar and three CSR values were conducted on marine sand using the variable-direction dynamic cyclic simple shear (VDDCSS) apparatus. In the tests, soil samples were compacted under vertical stress and then sheared in undrained conditions by applying two shear stresses acting in different horizontal directions. Test results indicated that the cyclic strain, pore water pressure ratio, and cyclic strength were significantly determined by the value of stage amplitude ratios and the CSRs: at the same CSR, cyclic strains, and pore water pressure increased while cyclic strength decreased with the Ar. Comparing the test data between various cyclic stress ratios found that the CSRs can accelerate shear strains, pore pressure accumulation, and cyclic strength attenuation.

Neural network models for seabed stability: a deep learning approach to wave-induced pore pressure prediction

Wave cyclic loading in submarine sediments can lead to pore pressure accumulation, causing geohazards and compromising seabed stability. Accurate prediction of long-term wave-induced pore pressure is essential for disaster prevention. Although numerical simulations have contributed to understanding wave-induced pore pressure response, traditional methods lack the ability to simulate long-term and real oceanic conditions. This study proposes the use of recurrent neural network (RNN) models to predict wave-induced pore pressure based on in-situ monitoring data. Three RNN models (RNN, LSTM, and GRU) are compared, considering different seabed depths, and input parameters. The results demonstrate that all three RNN models can accurately predict wave-induced pore pressure data, with the GRU model exhibiting the highest accuracy (absolute error less than 2 kPa). Pore pressure at the previous time step and water depth are highly correlated with prediction, while wave height, wind speed, and wind direction show a secondary correlation. This study contributes to the development of wave-induced liquefaction early warning systems and offers insights for utilizing RNNs in geological time series analysis.

Evolution of excess pore water pressures around monopiles subjected to moderate seismic loading

The sustained growth of the offshore wind sector is leading to the construction of offshore wind farms in highly seismic regions of the world. Hence, a large proportion of the potential sites may be exposed to liquefaction risk. Monopiles are directly affected by this phenomenon given their preference as a support system for offshore wind turbines over other foundation types. This paper includes a comprehensive study of the lateral response of monopiles against the combined action of earthquake induced liquefaction and environmental loading using centrifuge modelling. The experimental setup was designed to compare the amount of excess pore pressure generated within the soil adjacent to the monopile with and without operational wind/wave loading. The results revealed higher accumulation of excess pore pressure in the non-laterally loaded case, and significant differences in the amount of excess pore pressure recorded in the windward and leeward sides of the monopile in the laterally loaded scenario. In addition, the study provides data on the rotation experienced by monopile supported offshore wind turbines in liquefiable soils.

Research on the Accumulated Pore Pressure of Expansive Soil under Subway Loading

Taking the expansive soil near Hefei Xinqiao International Airport as the research subject, indoor dynamic triaxial tests were conducted to investigate the influence of different loading methods on the dynamic pore pressure of saturated remolded expansive soil on the basis of maximally simulating the real characteristics of subway loading. Furthermore, the action laws of three factors, namely intermittent loading ratio, static deviator stress, and cyclic stress ratio, on the accumulated pore pressure of saturated remolded expansive soil under intermittent subway cyclic loading were analyzed. The research results indicate that the loading method significantly affects the development trend of the accumulated pore pressure. Under similar conditions, a larger intermittent loading ratio leads to smaller accumulated pore pressure values and a slower initial development rate of pore pressure under the same cycle vibration. Increasing static deviatoric stress promotes the accumulation of pore pressure. The influence of the cyclic stress ratio is dependent on the intermittent loading ratio and does not follow a consistent pattern.

Joint analysis of macroscopic response and microfabric evolution of coral sand during earthquake-induced liquefaction

This study addresses the microfabric evolution of coral sand during earthquake-induced liquefaction using the joint tests of macroscopic shaking table and microscopic X-ray computed tomography. Based on the non-destructive scanning and image reconstruction in three-dimensional, the change in the grain–pore structure of sands under earthquakes is discussed. Furthermore, the special microstructural evolution of coral sand is studied by the contrast tests with Fujian sand. The results show that the coordination number of coral sand is larger than that of Fujian sand after liquefaction, which is consistent with that the reduction in acceleration of coral sand is smaller than that of the Fujian sand, and the coral sand foundation still has a larger shear strength due to the smaller excess pore pressure accumulation. Moreover, the contact index of Fujian sand and Reigate sand fluctuates in a similar range with the change of void ratio and coordination number compared with coral sand. This is due to the fact that the Fujian sand and Reigate sand are both general terrigenous sands (quartz sand), while coral sand has irregular particle shapes and larger surface friction caused by marine biogenesis. The test results deepen the understanding of the liquefaction mechanism of coral sand.

Numerical simulation of pore pressure development beneath suction anchor under undrained condition during uplift

Suction anchors are increasingly used in the deep-sea anchoring infrastructure, and the accurate evaluation of the uplift behavior is of great importance. In this paper, the development of pore pressure under the suction anchor lid during the undrained pullout process was investigated. The numerical model based on the solid-fluid two-phase theory and by updated Lagrangian method was employed. Laboratory experimental studies with three different aspect ratios were carried out to verify the effectiveness and accuracy of numerical method. Parametric studies were performed to analyze the effect of key soil properties on the accumulation of pore pressure. The results show that the numerical method in terms of capturing the failure pattern and simulating pore pressure evolution was significantly efficient. The development of excess pore pressure increased faster at the beginning, followed by a slower increase and then tended to level off. The maximum excess pore pressure appeared just beneath the suction anchor, and decreased with the increasing soil depth. Parametric analysis showed that a greater negative pore pressure needed a larger shear modulus, MCC swelling index and critical stress ratio, or a smaller porosity, permeability and MCC compression index. The softening coefficient had little effect on the pore pressure generation.

Loading rate influence on high-cyclic loading of clays

Experimental observations reported in the literature, based on cyclic triaxial and cyclic simple shear tests, show that the loading frequency influences the excess pore pressure and strain accumulation for fine-grained soils under undrained conditions. A high-cycle accumulation (HCA) model for clay was introduced by Wichtmann (2016) and Staubach et al. (2022) to quantify the strain/pore pressure accumulation as a function of the number of applied cycles, amplitude, void ratio, stress ratio, OCR and loading frequency. This article explores the convenience of including the loading frequency factor in the HCA clay model based on the interpretation of a series of cyclic undrained triaxial tests performed on three different clay materials, namely Dorfner kaolin, Lower Rhine clay and Onsøy clay. Analysis of results shows that for those specific three clay materials, the normalization of the pore pressure accumulation rate u̇acc with the amplitude function fampl purifies the data from the influence of loading frequency and thus no explicit frequency function ff is required.

Experimental investigation on cyclic behavior of geogrid-reinforced coral sand from the South China Sea

The present study provides insight into the cyclic behavior of unreinforced and geogrid-reinforced coral sand. To achieve these goals, stress-controlled undrained cyclic triaxial tests were conducted on medium-dense coral sand with and without geogrid reinforcements. The influence of geogrid reinforcement, cyclic stress ratios, and confining pressure on the deformation, pore pressure, liquefaction resistance, stiffness degradation, and particle breakage characteristics of saturated coral sand was investigated. The results indicated that coral sand reinforced with geogrid increased the liquefaction resistance and decreased the rate of pore pressure accumulation. The effectiveness of the geogrid inclusion on coral sand’s liquefaction resistance was found to be more pronounced under large effective confining pressure and a smaller cyclic stress ratio. Increasing the effective confining pressure increases the liquefaction resistance for both unreinforced and geogrid-reinforced coral sand, whereas increasing the cyclic stress ratios decreases the liquefaction resistance of the unreinforced and reinforced coral sand. The particle breakage was observed to be increased with increasing the effective confining pressure and cyclic stress ratio, and the breakage was more in the reinforced coral sand than the unreinforced coral sand.

Cumulative cyclic response of offshore monopile in sands

Satisfactory performance of offshore wind turbines is governed by the cyclic response of its foundation under wind and wave. Most methods of evaluating the lateral cumulative deformation of monopile due to cyclic loading are based on small-diameter pile tests installed in dry sand. This paper investigates the cumulative deformation of large-diameter monopiles installed in saturated sand. A numerical model was developed employing FLAC3D and was validated by comparing its predictions with the results of triaxial tests and centrifuge tests. The validated numerical model was used to evaluate and compare the cyclic responses of monopiles between saturated case and no pore pressure case. To evaluate the cumulative deformation for offshore monopile due to cyclic loading, the numerical model was refined to account for the effects of number and amplitude and frequency of cyclic loading, soil permeability coefficient, soil relative density and pile diameter. It was found that the larger subsidence and the less capacity of soil around the pile is caused by accumulation and dissipation of transient excess pore pressure, thus the monopile in saturated sand would experience the larger cumulative deformation. Correspondingly, based on this parametric study, an analytical model was developed to calculate the pile-top cumulative displacement of offshore large-diameter monopile under lateral cyclic loading considering the effects of pile diameter. This analytical model was validated against the results of centrifuge tests and model pile tests in saturated sands.

Pore pressure built-up in hydrate-bearing sediments during phase transition: A poromechanical approach

Due to the density contrast between the hydrate and methane gas, the pore pressure is accumulated in the sediment during the decomposition process of methane hydrate. This accumulation of pore pressure decreases the magnitude of effective stress, further triggering potential geological disasters such as landslide. This paper establishes a theoretical framework to investigate the evolution of fluid pressure in the hydrate-bearing sediments during the decomposition process. This model consists of two parts: an unsaturated thermo-poromechanical constitutive law as well as a phase equilibrium equation. Compared with the existing studies, the present work incorporates the effect of pore volume change into the pressure built-up model. In addition, the capillary effect is considered, which plays a nontrivial role in fine-grained sediments. Based on this model, the evolution of fluid pressure is investigated in undrained conditions. It is shown that four mechanisms mainly contribute to the pressure built-up: the density contrast between decomposing hydrate and producing fluid, the variation of pore volume, the compaction of hydrate due to variation of capillary pressure, and the thermal deformation of pore constituents induced by temperature change. Among these mechanisms, the density contrast dominates the pore pressure accumulation. Under the combined effect of these contributions, the evolution of fluid pressure exhibits a strong nonlinearity during the decomposition process and can reach up to dozens of mega Pascal. Nevertheless, this high-level pressure built-up results in a significant tensile strain, yielding potential fracturing of the sediment.

Cyclic Capacity of Monopiles in Sand under Partially Drained Conditions: A Numerical Approach

Cyclic loading of saturated sand under partially drained conditions may lead to accumulated strains, pore pressure buildup, and consequently reduced effective stress, stiffness, and shear strength. This will affect the ultimate limit state capacity of monopile foundations in sand for offshore wind turbines. This paper calculates the performance of large-diameter monopile foundations, which are installed in uniform dense sand, subjected to storm loading using the partially drained cyclic accumulation model (PDCAM). The simultaneous pore pressure accumulation and dissipation is accounted for by fully coupled pore water flow and stress equilibrium (consolidation) finite element analyses. Drainage and cyclic load effects on monopile behavior are studied by comparing the PDCAM simulation results with simulation results using a hardening soil model with small strain stiffness. At the end, a simplified procedure of PDCAM, named PDCAM-S, is proposed, and the results using this approach together with PLAXIS 3D and the NGI-ADP soil model are compared with the PDCAM results.

Insights from in-situ pore pressure monitoring around a wind turbine monopile

Offshore wind turbine monopiles transfer large bending moments into the soil. Due to the large diameter of the foundation, a significant volume of soil is mobilised to absorb the incoming forces from wind, waves and current. If excess pore pressures are generated during load cycles, the long drainage path length could lead to accumulation of pore pressures and loss of strength and stiffness in the soil. During monopile design, undrained behaviour of the soil during cyclic loading is often assumed to conservatively assess the lateral pile resistance during storms. In-situ monitoring of pore pressures in the soil surrounding a monopile foundation allows the assumption of undrained behaviour to be checked for several episodes during the operational life of the foundation. A 3-year period of monitoring data was analysed for a North Sea monopile which included normal operation, storm episodes and rotor stops. Pore pressure and earth pressure sensors were positioned on the pile at two different depths in the most intensely loaded layer of surface sand. The data provides insight into the amount of excess pore pressure accumulation with storm loading not leading to significant accumulation of excess pore pressure. The only episodes showing a significant development of excess pore pressures were rotor stops with suction pressures being developed on the unloaded side of the monopile. The observed response suggests that significant pile-soil stiffness reductions in the top layer due to cyclic loading are not expected for the monitored location. The study shows the value of monitoring pore pressures in the soil mass surrounding offshore foundations for checking design assumptions.

Excess pore pressure accumulation in sands – A shear strain threshold concept for optimization of a laboratory testing programme

In particular during storm events, an accumulation of excess pore pressures may occur in the soil around cyclically loaded offshore foundations which can negatively affect the structural integrity of the foundation. As input for analytical or numerical calculation methods, high-quality laboratory test results are needed. Numerous tests are usually necessary to describe the soil behaviour over the full range of stress and strain boundary conditions. Therefore, an optimization of the test programme in terms of the number of required tests is highly desirable. In the paper, results of an extensive test programme, comprising Resonant Column (RC) and displacement-controlled cyclic Direct Simple Shear (DSS) tests on three sands are presented. Based on the findings, a shear strain threshold concept is developed in which different general soil behaviour of sands under cyclic loading is distinguished by shear strain ranges. The thresholds can be determined by RC tests and multistage cyclic DSS tests. As a main result, the soil response over the full range of possible boundary conditions can be quantified with a combination of only a few RC, multistage DSS and DSS tests. Finally, equations to parametrize the experimental results are discussed.

Wind-wave combined effect on dynamic response of soil-monopile-OWT system considering cyclic hydro-mechanical clay behavior

Based on advanced clay-monopile-OWT numerical model, the wind-wave coupling effect was investigated. Compared to the case of wind load only, both the nacelle top displacement and the maximum monopile’s bending moment are smaller when the wind-wave load coupling is considered. The monopile-head displacement cumulation can cause the drift of the nacelle top displacements. Moreover, the wind and wave loads may cause opposite accumulative direction of the monopile-head displacement due to the difference in load nature. The displacement cumulative magnitude is positively correlated to the load magnitude. Hence, the monopile possesses better energy-dissipating capacity in the coupled load case. The equivalent damping ratio of the soil is about 3.2 ∼ 4.9% under the wind-wave combination, 1.2 ∼ 1.4% under the wind only and 2.3 ∼ 3.5% under the wave only, which agrees with superposition principle. Under the wind-wave loads, noticeable accumulation of excess pore pressure exists in the seabed soil. Compared to the separate load cases, the excess pore pressure in the coupling case may be smaller. Analysis also showed that mean effective stress of soil decreases with loading time due to pore pressure buildup, although the soil mean stress may increase when total stress method is considered.

The stability of the submarine slope due to hydrate re-formation

The dissociation of gas hydrate leads to the accumulation of excess pore pressure and the decrease of sediment strength, which may cause the instability of submarine slopes. Aiming at the risk caused by gas hydrate dissociation in offshore engineering, this study is conducted to propose a novel excess pore pressure model by improving the Grozic-Nixon model. Combined with the actual seismic and topographic data of the submarine slope in the northern South China Sea, the proposed excess pore pressure model is employed to numerically investigate the submarine landslide induced by gas hydrate dissociation. The corresponding safety factor is also obtained with the strength reduction method. The effects of water depth and the total amount of hydrate dissociation are discussed in detail on the stability of submarine slopes. The results show that the excess pore pressure caused by hydrate dissociation increases rapidly at the beginning, then increases slowly, and finally dissipates gradually. The hydrate re-formation of hydrate blocks the pore channels and slows down the dissipation of excess pore pressure to a certain extent. Hydrate dissociation amount, water depth, and sediment depth are crucial factors affecting the stability of submarine slopes.