Pore Pressure Accumulation Research Articles

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%.

On the “elastic stiffness” in a high-cycle accumulation model — continued investigations

The high-cycle accumulation (HCA) model proposed by the authors can be used to predict permanent deformations or stress relaxation due to a large number (e.g., several millions) of load cycles with relative small strain amplitudes (<10−3). The predicted stress relaxation depends on the isotropic “elastic stiffness”, [Formula: see text], used in the HCA model. To calibrate the bulk modulus, K, the rate of pore pressure accumulation obtained from an undrained cyclic test and the rate of volumetric strain accumulation measured in a drained cyclic test are compared. Poisson’s ratio, ν, can be determined from the shape of the stress relaxation path measured in an undrained test with anisotropic consolidation stresses and strain cycles. Unfortunately, the calibration of K shown for a medium coarse sand in a previous paper by Wichtmann et al. in 2010 was affected by membrane penetration effects. Consequently, all further studies have been performed on a fine sand for which membrane penetration is negligible. The present paper reports on the new results. The strong pressure dependence of K and its independence from amplitude found in the previous study could be confirmed by the new tests. In addition, the new experimental results reveal a density dependence of K, while the bulk modulus is rather independent of stress ratio. Furthermore, for the first time Poisson’s ratio, ν, used in the HCA model has been calibrated based on tests performed with different amplitudes, densities, and initial stresses.

Cnoidal Water Wave Induced Pore Pressure Accumulation in the Seabed

The pore water pressure accumulation in the seabed soil under cnoidal water waves is examined in detail. The cnoidal water wave theory is employed to calculating the water pressure on the double-layer seabed in shallow water region. The modified Terzaghi consolidation equation is the governing equation. A general analytical solution for the consolidation equation is given in an integral form. And a comparative study is carried out to verify the two previous pore pressure generation models: the linear relation and the hyperbola model. Accordingly, a joint model is presented to analysis the pore pressure accumulation in seabed and the soil liquefaction in a practical accident happened in China Shengli Oil Field. Key words : Cnoidal water wave; Pore pressure accumulation; Hyperbola model; Modified Terzaghi consolidation equation

Experimental Study on Mechanism of Wave-Induced Silty Seabed Fluidization

Abstract. In this study, process of wave-induced silty seabed response was explored in a rectangle flume. Based on the experimental phenomena, the variations of pore water pressure and theory analysis, discussions were made on the mechanism of wave-induced silty seabed instability. Results indicated that seabed under wave nodes would experience fluidization and oscillation under wave actions, obvious pore pressure accumulation was observed at the same time. The mode of wave-induced silty seabed instability was fluidization. The mechanism of wave-induced silty seabed fluidization was a combination of excess pore water pressure and shear failure, the chief factor of fluidization could be ascribed to wave-induced cycling shear stress. The steady state strength was somewhat an expression of the level of wave-induced decrease of soil strenth that could finally reach, therefore, it could be used for the estimation of wave-induced fluidization of silty seabed.

A study of freezing behavior of cementitious materials by poromechanical approach

The freezing behavior of cementitious materials is investigated in this paper through poromechanical approach after the Biot–Coussy theory. The material is taken as a porous medium saturated with water and subject to freezing. The involved thermodynamic laws are recalled to establish the constitutive equations for the phase change, mass transport and heat transfer processes. As a result, the pore pressure arising from freezing is converted to macroscopic effective stress through homogenization scheme. The established model is applied to predict the macroscopic freezing strain of a saturated cement paste and the theoretical prediction is compared to observed experimental results in ( Powers and Helmuth, 1953). The results show that the poromechanical model can reasonably capture the freezing behaviors from pore pressure accumulation, pore pressure relaxation as well as the thermal shrinkage associated with the freezing process.

Effects of Wave Non-Linearity on Residual Pore Pressures in Marine Sediments

A better understanding of the wave-induced pore pressure accumulations (i.e., residual pore pressure) is a key factor in the analysis of the wave-induced liquefaction in marine sediments. In this paper, the residual mechanism of nonlinear wave-induced pore water pressure accumulation in marine sediments is examined. Unlike previous investigations, the second-order Stokes wave theory is considered in this study. The new model is verified with experimental data and provides a better prediction of pore pressure accumulation than the previous solution with linear wave theory. The parametric study concludes that the influences of wave non-linearity increase as the following parameters increase (i) wave steepness (H/L) and (ii) residual parameter (β). However, an opposite trend is found for (i) wave period (T), (ii) relative water depth (d/L), (iii) seabed thickness (h/L) and (iv) another residual parameter (α). Furthermore, the effect of wave non-linearity becomes more significant in soft seabed (Soil C).

Modeling Liquefaction by a Multimechanism Model

An anisotropic constitutive model was recently presented for describing the stress–strain behavior of granular materials with considerations for the initial and induced anisotropy. The model was developed within the framework of a microstructural theory known as the sliding–rolling theory. The resulting model falls within the definition of multimechanism models. The model was shown to satisfactorily represent the drained and undrained behaviors under monotonic loading. The framework used in the model allows extension to describe the behavior under cyclic loading, which is the subject of the present paper. Specifically, the model is further developed for representing the undrained behavior of granular materials under one- and two-way cyclic loading, some of which cause liquefaction resulting in large strain accumulations and the others lead to limited pore pressure and strain accumulations. The validity of the model is verified using triaxial data on Nevada sand.

The Effect on Stability of Slope under Drawdown Conditions

The program of PLAXIS was adopted to analyze the slope stability. The ideal elastic-plastic model and the yield criterion of Mohr-Coulomb theory were simulated numerically for the realistic project. The drainage condition and undrained condition of the soil body were compared. The calculation shows that the safety factor declines with the drawdown of the water level under the drainage condition. However, the safety factor would rise after the water level drops by 20 meters, which is caused by the vertical acting force of the pore pressure, and its influence on the safety factor of the slope is more obvious under the undrained condition. The safety factor with the excess pore pressure is 10% lower than that without regard to the pressure. As a result, the accumulation and dissipation of the excess pore pressure should be taken into consideration in the practice. What’s more, the safety factor should be checked according to the “least favorable water level”. The program of PLAXIS can simulate the effect of seepage on the slope stability when the water level drawdown in calculation.

Geo-Seismic Environmental Aspects Affecting Tailings Dams Failures

Seismic performance evaluations of tailings dams are essential for characterizing the geo- environmental risks posed by these earthen structures, which should include the geotechnical hazards implied by slope instability failure, free board loss and the potential release of contaminants. The observed damage is more important when liquefaction occurs on the dam body and foundation, which often leads to cracking, settlements, tilting and general distortion of dam geometry. Analyses based on limit equilibrium are generally sufficient to establish hazard zones. However, numerical models with solution schemes formulated in the time domain, which are capable of taking into account the kinematics of soil movement more realistically, are needed to quantify the geotechnical risk. This paper reviews the main geotechnical earthquake engineering aspects to account for when designing tailings dams and describes the application of a practice-oriented simplified constitutive model, which implemented in a lagragian finite difference platform, is capable of predicting the accumulation of pore pressure in fine-grained saturated materials due to earthquake loading, the reduction of shear strength and the corresponding permanent displacements. The model uses the Mohr-Coulomb failure criterion coupled with an incremental pore pressure generation scheme. Pore pressure is accumulated as a function of the number of stress cycles. The secant soil stiffness and hysteretic damping change with loading history. The numerical simulation is able to properly capture the kinematics of dam failure and provides the parameters to assess potential environmental impacts on the nearby areas of the dam.

カーボネイト系地盤上に設置された海底パイプラインの耐波安定性解析について

The purpose of this paper is to estimate the stability of an unburied offshore pipeline resting on a calcareous sand under severe storm condition. Pore pressure accumulation and pipeline movement during cyclic loading of pipeline have been numerically investigated from a view point of a non-linear relationship between pipeline and seabed. Both drag and lift forces acting on the pipeline are evaluated for 100 years return period storm condition using the Fourier decomposition method. Pipeline movement during cyclic loading greatly depends on the stiffness of non-linear spring element, and pore water pressure response as well as shear strain are more sensitive to the initial unit tangential stiffness of non-linear spring element.

A new approximation for pore pressure accumulation in marine sediment due to water waves

AbstractThe residual mechanism of wave‐induced pore water pressure accumulation in marine sediments is re‐examined. An analytical approximation is derived using a linear relation for pore pressure generation in cyclic loading, and mistakes in previous solutions (Int. J. Numer. Anal. Methods Geomech. 2001; 25:885–907; J. Offshore Mech. Arctic Eng. (ASME) 1989; 111(1):1–11) are corrected. A numerical scheme is then employed to solve the case with a non‐linear relation for pore pressure generation. Both analytical and numerical solutions are verified with experimental data (Laboratory and field investigation of wave– sediment interaction. Joseph H. Defrees Hydraulics Laboratory, School of Civil and Environmental Engineering, Cornell University, Ithaca, NY, 1983), and provide a better prediction of pore pressure accumulation than the previous solution (J. Offshore Mech. Arctic Eng. (ASME) 1989; 111(1):1–11). The parametric study concludes that the pore pressure accumulation and use of full non‐linear relation of pore pressure become more important under the following conditions: (1) large wave amplitude, (2) longer wave period, (3) shallow water, (4) shallow soil and (5) softer soils with a low consolidation coefficient. Copyright © 2006 John Wiley & Sons, Ltd.

A unified plasticity model for cyclic behaviour of clay and sand

This paper presents the development and an experimental evaluation of a simple unified bounding surface plasticity theory for modelling the stress–strain behaviour of sand and clay under both drained and undrained cyclic loading conditions. The model concerned is called CASM-c, which is based on the unified critical state model CASM developed by Yu [Yu, H.S., 1995. A unified critical state model for clay and sand. Civil Engineering Research Report No. 112.08.1995. University of Newcastle, NSW 2308, Australia; Yu, H.S., 1998. CASM: a unified state parameter model for clay and sand. International Journal of Numerical and Analytical Methods in Geomechanics 22, 621–653]. CASM is a relatively simple model as it only requires seven model constants, five of which are the same as those used in the modified Cam-clay model. All these constants have clear physical meanings and may be easily determined from the results of triaxial tests. A key advantage of CASM over many other existing critical state models lies on its simplicity and unified nature as it can model the behaviour of both clay and sand. The extension of the model CASM presented in this paper consists of adopting the bounding surface plasticity theory and treating the reloading and unloading processes differently when calculating the hardening modulus. As a result, a smooth transition of stiffness and gradual accumulation of permanent strain and/or pore pressure in unload–reload cycles as well as the hysteretic behaviour can be reproduced. The results of model simulations show an encouraging agreement with experimental data from triaxial tests subjected to both one-way and two-way cyclic loading conditions.

Developments in Modeling Liquefaction of Granular Soils, Caused by Cyclic Loads

The aim of this review article is to present historical developments of mechanics of saturated granular soils in relation to the liquefaction phenomenon, as well as to analyze the present state of this subject in connection with practically important problems. The first part is an introduction to mechanics of liquefaction, in order to make this paper self-contained. Then, some basic empirical findings are described. In the third part, the development of theoretical approaches to liquefaction-related problems, such as cyclic loading compaction and pore pressure accumulation, or cyclic loading degradation of shearing resistance, is presented. The fourth part deals with the presentation of the methods applied for solving practically important problems and those that have not been solved satisfactorily. The last part of this paper presents some of the most important conclusions and suggestions regarding further research. There are 152 references cited in this review article, and a supplementary bibliography of 45 publications is also included.

A mechanism of pore pressure accumulation in rapidly sliding submerged porous blocks

The generation and dissipation of pore pressure excess due to the rapid, mutual sliding of two water-saturated blocks which are composed of glued arrays of cylindrical rods is herein theoretically examined. The mathematical model consists of three ordinary differential equations describing the motion of a solid mass coupled with the continuity of a fluid mass. These equations are numerically solved for the following conditions: (a) motion induced by the constant horizontal velocity of the sliding mass; (b) constant drag force acting in a horizontal direction; (c) gliding on the irregular, inclined surface that envelops the rods. Theoretical predictions for case (a) favorably compare with experimental results reported in the literature from an investigation carried out on a physical model. Primary results show that the pore pressure excess depends upon physical and geometrical parameters, and furthermore, that displacement of the rods and pore pressure excess are strongly coupled. Maximum pore water pressure occurs when “fluidization”, i.e. the complete balancing of submerged block weight by pore pressure excess, is achieved. The latter does not coincide with the rising of the upper block (hydroplaning). A cyclic motion is reached under condition (a), when dynamic pore water pressure increases from a negative to a maximum positive value and then falls again to a negative value. On the other hand, non-cyclical motion and monotonically increasing pore water pressure are obtained under sliding conditions (b) and (c). The response of the model under condition (c), which is of particular interest for run-out analyses of flow slides, is compared to the sliding response of ordinary submerged blocks on a slope, showing significant differences and pointing out the roles of particle diameter, layer extent and medium permeability. The occurrence of different pore pressure states for fluidization and hydroplaning is highlighted. This, hopefully, can open the way to a reliable treatment of the mechanics of sub-marine flow slide propagation.

The minimum depth of fault failure in compressional environments

The absence of very shallow seismicity in thrust faults could be due to either a layered pore pressure/depth profile or to heterogeneous lithology of the crust. Poroelasticity equations were solved by the finite element discretization of a two‐dimensional plane strain cross‐section simulating a thrust environment. Although the modeled faults are actively compacting during thrusting, our numerical results show that the pore pressure accumulation in the basement is faster than in the fault zone and therefore the fluid flow is directed into the fault. The combination of heterogeneous rheology, with the existence of hydrostatic fluid, retards failure near the land surface. A layered pore pressure/depth profile, where the pore pressure is hydrostatic in the sediments and near lithostatic in the basement, magnifies this result and failure occurs faster. In fact, with this pore pressure/depth profile, failure is reached at the same depth regardless of the crustal structure we assume.

Plasticity Model for Sand under Small and Large Cyclic Strains

A plasticity constitutive model for sands is proposed, which combines a bounding surface framework for large cyclic strains with a Ramberg-Osgood-type hysteretic formulation for relatively smaller strains. The distinction between small and large cyclic strains is based on the volumetric threshold cyclic shear strain γtv, a well-established geotechnical parameter. The state parameter ψ is used explicitly to interrelate the critical, peak, and dilatancy deviatoric stress ratios. The plastic modulus is expressed as a particular function of accumulated plastic volumetric strain, which simulates empirically the effect of fabric evolution during shearing. Extensive comparisons with experiments show accurate simulation of the basic aspects of cyclic behavior for a wide range of cyclic strain amplitudes, specifically, (1) the degradation of shear modulus and increase of hysteretic damping with cyclic shear strain amplitude; (2) the evolving rates of shear strain and excess pore pressure (or volumetric strain) accumulation with number of cycles; and (3) the resistance to liquefaction. The 14 model parameters are proven independent of initial and drainage conditions, as well as the cyclic shear strain amplitude. The simulation of monotonic shearing is equally accurate.

Solutions of pore pressure build up due to progressive waves

AbstractThe analytical solution of soil pore pressure accumulations due to a progressive wave is examined in detail. First of all, the errors contained in a published analytical solution for wave‐induced pore pressure accumulation are addressed, and the correct solution is presented in a more general form. The behaviour of the solution under different soil conditions is then investigated. It is found that the solution for deep soil conditions is sensitive to the soil shear stress in the top thin layer of the soil. However the solution is significantly influenced by the shear stress in the thin layer of soil near the impermeable base, for shallow and finite depth soil conditions. It is also found that a small error in the soil shear stress can lead to a large error in the accumulated pore pressure. An error analysis reveals the relationships between the accuracy of the pore pressure accumulation and the accuracy of the soil shear stress. A numerical solution to the simplified Biot consolidation equation is also developed. It is shown that the error analysis is of significant value for the numerical modelling of pore pressure buildup in marine soils. Both analytical and numerical examples are given to validate the error estimation method proposed in the present paper. Copyright © 2001 John Wiley & Sons, Ltd.

Posttesting Correction Procedure for Membrane Compliance Effects on Pore Pressure

Isotropic triaxial compression tests were conducted on five different, fairly uniform sands, with mean grain sizes between 2.8 and 0.105 mm, to determine the effects of effective confining pressure, mean grain size, and relative density on unit membrane penetration. Volume changes due to membrane penetration were determined using sand specimens with identical void ratios and height to diameter ratios but with different sizes, surface areas, and volumes. A semiempirical exponential relationship with respect to effective mean stress was used to model unit membrane penetration, and the results were compared with some other models proposed in the literature. A posttesting correction procedure was formulated to calculate pore-pre~sure accumulation in monotonic and cyclic tests, taking into consideration coupling between membrane complIance and pore-pressure accumulation. The procedure developed is based on membrane flexibility and soil com­ pressibility expressed as semiempirical exponential relationships with respect to effective mean stress. The applicability of the model is shown based on undrained monotonic and cyclic tests and with respect to some methods proposed in the literature.

Stability Criteria for Marine Clay under One‐Way Cyclic Loading

Results are presented from a series of one‐way cyclic undrained triaxial tests on Ariake clay, a highly plastic, normally consolidated marine clay from Kyushu in southwest Japan. Considering first pore water pressures developed during cyclic loading, it is shown that the buildup of pore water pressure is a time‐dependent phenomenon, independent of frequency. The rate of pore pressure accumulation decreases exponentially with elapsed time of cyclic loading and at any given time it is a function of the applied cyclic stress. A similar creep type of relationship also exists for the accumulation of plastic strain with time, and from this a relationship is developed between pore pressure and strain. Effective stress paths for samples taken to failure suggest that the projection of the critical state line may be taken as a failure criterion under cyclic loading. Combining the exponential relationship for pore pressure against time with this failure criterion, related stability criteria are developed for pore pressure and strains at different cyclic stress levels.

Random Wave-Induced Pore Pressure Accumulation in Marine Soils

Random wave-induced pore pressure is examined using linear superposition, Miner’s method and a single representative wave. Solutions are developed for deep and shallow soils. A deep soil has a higher liquefaction potential than a shallow soil for the same wave conditions. Linear superposition, Miner’s method and using the rms wave height are in general agreement for the deep soil. The significant wave overestimates the liquefaction potential and should not be used. There was poor agreement among the methods for a shallow soil.