Accumulation Of Excess Pore Pressure Research Articles

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.

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.

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.

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.

The role of geosynthetics in reducing the fluidisation potential of soft subgrade under cyclic loading

The instability of railway tracks including mud pumping, ballast degradation, and differential settlement on weak subgrade soils occurs due to cyclic stress from heavy haul trains. Although geotextiles are currently being used as a separator in railway and highway embankments, their ability to prevent the migration of fine particles and reduce cyclic pore pressure has to be investigated under adverse hydraulic conditions to prevent substructure failures. This study primarily focuses on using geosynthetics to mitigate the migration of fine particles and the accumulation of excess pore pressure (EPP) due to mud pumping (subgrade fluidisation) using dynamic filtration apparatus. The role that geosynthetics play in controlling and preventing mud pumping is analysed by assessing the development of EPP, the change in particle size distribution and the water content of subgrade soil. Using 3 types of geotextiles, the potential for fluidisation is assessed by analysing the time-dependent excess pore pressure gradient (EPPG) inside the subgrade. The experimental results are then used to evaluate the performance of selected geotextiles under heavy haul loading.

Centrifuge modelling on monotonic and cyclic lateral behaviour of monopiles in kaolin clay

This paper details a programme of centrifuge tests (100g) on the monotonic and cyclic lateral behaviour of large-diameter semi-rigid monopiles in kaolin clay. Piles of different diameters were used, with a maximum prototype value of 5·9 m. The pile–soil interaction was investigated in terms of the overall load–displacement behaviour, distribution of bending moment and pile deflection, p–y curves and excess pore pressure response. For monotonic loading, procedures were developed to fit the monotonic p–y curves obtained from different pile and soil properties using a hyperbolic tangent model. In addition, an explicit model was derived to assess the development of excess pore pressure within the surficial soil, which was found to approach the vertical effective soil stress at large displacement. For cyclic loading, the accumulation of lateral pile displacement could be accompanied by either increase or decrease in soil resistance, which strongly depends on the loading rate. Strong correlation between the development of peak soil resistance and a dimensionless velocity group was obtained, and was supported by the measured excess pore pressure. Under undrained conditions, the peak soil resistance decreases almost linearly with the accumulation of excess pore pressure.

Physical Modeling of the Stability of a Revetment Breakwater Built on Reclaimed Coral Calcareous Sand Foundation in the South China Sea—Regular Wave

In the past several years, a series of artificial islands have been constructed on the top of coral reefs by China in the South China Sea by way of reclamation. A large number of revetment breakwater also has been built along the margin of these artificial islands. The stability of these revetment breakwater is the precondition for the normal service performance of these reclaimed coral sand islands. In this study, taking the reclamation engineering in the South China Sea as the background, a series of wave flume physical model tests (geometrical similarity scale is set to 1:10) are performed to investigate the dynamics and the stability of the revetment breakwater and its reclaimed coral sand foundation under the impact of regular wave. Experimental results show that the revetment breakwater has a maximum final settlement of 6 mm if built on loose coral sand foundation. Furthermore, there is indeed excess pore pressure generated in the reclaimed coral foundation with a maximum magnitude of 1.5 kPa. It is found that the excess pore pressure has not caused liquefaction in the coral sand foundation due to the fact that the accumulation of excess pore pressure only occurred in the first 10 cycles of wave loading. Finally, it is concluded that the revetment breakwater and its reclaimed coral sand foundation basically are stable under regular wave impacting. However, excessive water overtopping would be a potential threat for the vegetation behind the breakwater, as well as for the desalinated underground water of the reclaimed lands.

Evaluation of various seismic response analysis methods for underground structures in saturated sand

Underground structures in liquefiable saturated sand are prone to earthquake damage. Various simplified pseudo-static seismic response analysis methods for underground structures have been recommended by current design codes and guidelines, but lack adequate systematic validation in liquefiable ground. In this study, the effectiveness of four categories of simplified pseudo-static methods in saturated sand are evaluated. These methods include a displacement-based method, a force-based method, a F(flexibility ratio)-R(racking ratio) method, and a detailed equivalent static method. High-fidelity dynamic analysis of circular and rectangular cross-section underground structures in non-liquefiable and liquefiable ground under different input ground motions are conducted, using a validated plasticity constitutive model for large post-liquefaction deformation of sand, to serve as a basis for the evaluation. Although the pseudo-static methods can generally provide adequate analysis results for underground structures in non-liquefiable ground, especially the displacement-based method, their performance in saturated sand are generally poor. The deviation of pseudo-static analysis results from reality stems from their inadequacies in considering the effects of dynamic soil-structure interaction in liquefiable ground. Temporal and spatial variability of soil-structure interaction in saturated sand, caused by the accumulation of excess pore pressure and subsequent decrease in soil effective stress and constraint on underground structures, are not considered in pseudo-static analysis methods. Modifications to the pseudo-static analysis methods and high-fidelity dynamic analysis methods are suggested to be adopted for the evaluation of the seismic response of underground structures in liquefiable ground.

Discrete element numerical simulation on the accumulation of wave-induced excess pore pressure in silty seabed sediments

Marine geological disasters, such as seabed liquefactions, submarine landslides, debris flows and turbidity currents, are all closely related to the accumulation of wave-induced excess pore pressure in the seabed. Due to the limitations of physical model experiment and in-situ observations, numerical analysis has become an important method to explore the microscopic mechanism of the accumulation of excess pore pressure. Based on the discrete element porous density flow method, we simulated the changing process of excess pore pressure in seabed sediment in this study, comparing with laboratory flume experiment. The simulation results reproduce the changing process of excess pore pressure in the laboratory flume experiment. The excess pore pressure occurs in the surface of seabed and gradually transfers to the deep layer, tending to a stable value. Thus, the discrete element porous density flow method is well suitable for simulating the accumulation of wave-induced excess pore pressure. Furthermore, the method constitutes a promising tool to study the microscopic mechanism of seabed liquefaction.

Uplift of immersed tunnel in liquefiable seabed under wave and current propagation

In ocean environments, wave usually propagates in association with a current. The paper presents a cyclic characterization framework for elastoplastic consolidating behavior of liquefiable seabed around the immersed tunnel under combined wave and current loading, based on the Biot dynamic consolidation theory. Special attention is placed on modeling the nonlinear wave-current interaction, the cyclic plasticity behavior of marine deposits and the soil-structure contact effect. The paper extends an existing Masing model to large strain regime (in the order of 10−2) in the framework of cyclic plasticity for capturing the liquefaction-induced nonlinear behavior of marine deposits. Then the proposed cyclic plasticity model is implanted into an explicit time matching finite difference analysis platform, permitting a comprehensive simulation of the intensive response of the immersed tunnel in the seabed experiencing the excess pore pressure accumulation and residual liquefaction. Retrospective simulation of a well-documented centrifuge test by the proposed framework indicates satisfactory agreement, validating the reliability for capturing the excess pore pressure accumulation and residual liquefaction during wave propagation. Finally, the influences of current on the wave-induced liquefaction in the vicinity of an immersed tunnel are investigated by a numerical example. The results indicate that the ocean current significantly changes the original characteristics of wave-induced liquefaction as well as the induced uplift of the immersed tunnel, especially with consideration of the cyclic plasticity behavior of marine deposits. The mechanisms of seabed liquefaction and the induced uplift of the immersed tunnel under wave and current actions are also interpreted

Effect of particle shape on the liquefaction resistance of calcareous sands

The irregular particle shape is the significant feature of calcareous sands. It is known that calcareous sands generally show a higher liquefaction resistance than that of quartz sands, but the contribution of particle shape to the liquefaction resistance has not been well understood. In this paper, the particle shapes of calcareous sand, quartz sand and steel ball were quantitively assessed. Then, undrained monotonic and cyclic simple shear tests were conducted to investigate the influence of particle shape on the liquefaction resistance. The particle size and shape before and after simple shear were compared to assess the particle breakage. It is found that the degree of particle breakage is quite small, the surface grinding is the main mode, and the particle shape keeps almost unchanged during cyclic simple shear. Irregular sand particles show slow accumulation of excess pore pressure and need more cycles to trigger liquefaction in cyclic tests. It also shows the liquefaction resistance has a good linear correlation with the overall regularity of sand particles. Based on the fitting parameters, the liquefaction resistance of sand samples with different particle shapes can be directly obtained by the provided overall regularity of the sand particles.

Stability of immersed tunnel in liquefiable seabed under wave loadings

This paper presents a robust modeling method for fully coupled effective stress analyses of wave–induced liquefaction scenarios around immersed tunnels. The method is based on the Biot consolidation theory, and special emphasis is placed on modeling the high–strain behavior of the liquefiable seabed and the evolving boundary conditions at the tunnel–seabed interface. Motivated by the experimental results, the existing Masing model is extended to the high–strain regime in the cyclic plasticity framework to capture the liquefaction–induced nonlinear behavior of the sand. In particular, a newly–observed shear–volume coupling model is implemented into plastic flow rule to realize volumetric compaction by cyclic shearing and exercise as the source term for residual excess pore pressure accumulation in the Biot’s equation. Subsequently, the proposed cyclic plasticity model is implanted into an explicit time matching finite difference analysis platform, permitting a comprehensive simulation of the intensive response of the immersed tunnel in the seabed experiencing the excess pore pressure accumulation and residual liquefaction. Moreover, a model calibration procedure is presented based on the cyclic laboratory sample test data and prescribed loading paths. Finally, the liquefaction–induced uplift of an immersed tunnel under progressive wave action is studied numerically. The obtained results provide the cause of liquefaction and the resulting consequences for tunnel uplift in ocean wave environments.

Thermal volume change of saturated clays: A fully coupled thermo-hydro-mechanical finite element implementation

The creep and consolidation behaviors of clays subjected to thermal cycles are of fundamental importance in the application of energy geostructures. This study aims to numerically investigate the physical mechanisms for the temperature-triggered volume change of saturated clays. A recently developed thermodynamic framework is used to derive the thermo-mechanical constitutive model for clays. Based on the model, a fully coupled thermo-hydro-mechanical (THM) finite element (FE) code is developed. Comparison with experimental observations shows that the proposed FE code can well reproduce the irreversible thermal contraction of normally consolidated and lightly overconsolidated clays, as well as the thermal expansion of heavily overconsolidated clays under drained heating. Simulations reveal that excess pore pressure may accumulate in clay samples under triaxial drained conditions due to low permeability and high heating rate, resulting in thermally induced primary consolidation. Results show that four major mechanisms contribute to the thermal volume change of clays: (i) the principle of thermal expansion, (ii) the decrease of effective stress due to the accumulation of excess pore pressure, (iii) the thermal creep, and (iv) the thermally induced primary consolidation. The former two mechanisms mainly contribute to the thermal expansion of heavily overconsolidated clays, whereas the latter two contribute to the noticeable thermal contraction of normally consolidated and lightly overconsolidated clays. Consideration of the four physical mechanisms is important for the settlement prediction of energy geostructures, especially in soft soils.

Numerical modelling of pipe-soil interaction for marine pipelines in sandy seabed subjected to wave loadings

Accurate assessment of pipe-soil interaction under cyclic wave actions is of pronounced importance for the stability analysis of submarine pipelines in sandy seabed. This paper presents a plane-strain numerical study on such a problem using a finite element program DBLEAVES, which incorporates an elasto-plastic soil model that is capable of capturing the cyclic mobility behavior of sandy soils under cyclic loadings. A detailed validation against analytical solution and model test results was provided to demonstrate the robustness of the present numerical model to mimic both pre- and post-liquefaction behavior of sands, before an extensive parametric study was introduced. It was found that the accumulation of excess pore pressure in the vicinity and far field of a pipeline was strongly affected by the existence of it, with an influential range of about two pipe diameters. The influences of wave and seabed properties (e.g. relative densities) on the uplift response of pipelines were then investigated, based on which an explicit model was developed to quantify the degradation effect of waves on the uplift bearing capacity of pipelines against thermally-induced buckling.

Discussion of “formation mechanism of large pockmarks in the subaqueous Yellow River Delta”

The excess pore pressure accumulation is a key factor when estimating the formation mechanism of large pockmarks, as it determines the liquefaction potential of marine sediments due to water waves. The governing equations for excess pore pressure may have different forms for various types of sediments and then shall reflect the cyclic plasticity of the soil. For water waves propagating over a porous seabed, the liquefaction area induced by waves is generally progressive, which indicates that the liquefaction area will move forward following the wave train. Therefore, the excess pore pressure accumulation can be used to explain the occurrence of the large pockmarks, but the dimension of the pockmark may be related to the heterogeneity of sediment or the wave properties affected by the topography in the subaqueous Yellow River Delta.

Numerical investigations on pore-pressure response of suction anchors under cyclic tensile loadings

The suction anchor is an effective option for the anchor foundations of floating offshore wind turbines (FOWTs). During its long-term service, in addition to the static pretension load, the suction anchor is subjected to a series of cyclic loads that are caused by waves, currents and the continuous motions of the floating structure. Thus, excess pore-pressure will accumulate within the soil around the embedded anchor, and the anchor capacity tends to be reduced. In this paper, by introducing the oscillatory and residual mechanisms, a novel numerical model is proposed to predict the instantaneous variations and accumulations of excess pore-pressures around a suction anchor that is subjected to long-term vertical cyclic loads. The results indicate that excess pore-pressure builds up mainly in the shallow soil near the external anchor wall. As a consequence, the effective soil stress in this region decreases along with the interface friction between the external wall and the soil. Detailed parametric studies reveal that the accumulation of excess pore-pressure is obvious for a larger load magnitude and smaller load period. With a lower permeability, smaller shear modulus or smaller relative density of the seabed soil, the pore-pressure accumulation outside the anchor increases significantly.

Interaction between catenary riser and soft seabed: Large-scale indoor tests

Nowadays, steel catenary riser (SCR) has become the most favorable method for oil and gas transportation in deep water. Accurate analysis of riser fatigue is heavily dependent on the interaction between riser and the seabed soil, which is a research focus in recent years. This paper aims to simulate the 3D interaction between SCR and typical clay seabed through large-scale indoor tests in 1g condition. The dynamic pipe•soil interaction is modeled through applying cyclic motion at one end of the pipe. The trench formation, pipeline behavior and the excess pore water pressure beneath the pipe invert are all analyzed in detail.The apparently dynamic embedding process and the ladled shape trench at touchdown zone (TDZ) were observed, which can be attributed to soil softening. The suction at the pipe/soil interface was captured and the accumulation of excess pore pressure was visualized. The results based on this study indicate that: the excess pore water pressures at different positions along the pipe axis vary with different trends, which may be attributed to their corresponding pipe trajectories. Therefore the accurate loading history simulation is very important for conventional 2D plane strain tests. It is found that, after 200 cycles, the maximum dynamic embedment factor fdyn along the axis of the model riser was up to 1.6, the pipe embedment depth increased by up to 60% and the average bending moment increased by up to 31.0%.