Characteristics of Pore Water Pressure of Saturated Silt Under Wave Loading

The characteristics of dynamic stress in the seabed under wave loading are constant principal stress and continuous rotation of the principal stress direction.Cyclic triaxial-torsional coupling shear tests were performed on saturated silt by the hollow cylinder apparatus under different relative densities,deviator stress ratios and vibration frequencies to study the development of pore water pressure of the saturated silt under wave loading.It was found that the development of pore water pressure follows the trend of "fast～steady～drastic".The turning point from fast to steady stage is not affected by relative density and deviator stress ratio.However,the turning point from steady to drastic stage relies on relative density and deviator stress ratio.The vibration cycle for the liquefaction of saturated silt decreases with increasing deviator stress ratio and increases with relative density.The vibration cycle for the liquefaction of the saturated silt increases with vibration frequency and reaches a peak value,after which it decreases with increasing vibration frequency for the relative density of 70%.But the vibration cycle for the liquefaction of saturated silt increases with vibration frequency for the relative density of 30%.The development of pore water pressure of the saturated silt is influenced by relative density and vibration frequency.     

Analysis of wave-induced liquefaction in seabed deposits of silt

The dynamic stress introduced in half elastic space by wave loading is characterized by the equation between the magnitude of half cyclic axial stress and cyclic torsion shear stress and the principal stress,whose direction rotates continuously and compression stress on seabed can be calculated by the use of small amplitude wave theory.With relationship curves of saturated silt of liquefaction cycles and cyclic stress ratios obtained by cyclic triaxial-torsional coupling shear tests and curve fitting method to different data points of relative density,it is suggested that the cyclic stress ratio corresponding to constant liquefaction impedance be taken as the critical cyclic stress ratio which implies liquefaction.There exists a linear relationship between critical cyclic stress ratio and relative density under different relative densities.Empirical formula for critical cyclic stress ratios of seabed liquefaction induced by wave loading under different relative densities is established.The possibility of seabed silt liquefaction and its influence factors are analyzed based on the small-amplitude wave theory and the data acquired in laboratory tests.     

Comparison Between Shear Behaviors of Overconsolidated Nakdong River Sandy Silt and Silty Sand

From this research, overconsolidated undrained and drained behaviors of specimens with high sand content were highly dilatant. According to the comparison results of laboratory tests, the deviator stresses of silty sand were greater than sandy silt due to high sand content under increasing OCRs, and both silty sand and sandy silt were presented strain softening tendency after failure under undrained loading. The pore water pressure increased with increasing fines content under increasing OCRs. Silty sand exhibited more dilatancy and increasing shear strength than sandy silt because pore water pressures of silty sand were lower than sandy silt under higher OCRs. In overconsolidated drained tests, silty sand is higher strength than sandy silt because silty sand has a lower volumetric strain and higher deviator stress than sandy silt under increasing OCRs. As the degree of overconsolidation increased, similar behaviors of silty sand and sandy silt observed that volumetric strain decreased to negative values due to dilatancy effect and low-cohesion under current effective confining pressures.     

杭州湾浅层粉土动力特性研究

选用杭州湾广泛分布的浅层粉土为研究对象,进行了一系列动三轴加载试验,研究了结构性、加载频率及动应力对杭州湾海底浅层粉土动应变、临界循环应力比和动强度的影响。试验结果表明,同一加载条件下,重塑样比原状样在更小振次内达到破坏,原状样动强度大于重塑样;结构性是影响土样动强度的重要因素,结构性对土样强度的影响随着动应力的增大而增强,随着频率的增大而减弱;加载频率的变化也对试样动力特性有较大影响,随着频率的增大,试样达到破坏所需振次加大,试样临界循环应力比也随之增大。     

Wetting-induced axial and volumetric strains of a sandstone mudstone particle mixture

In this study, the deformation induced by the first wetting typically referred to as wetting deformation of a sandstone–mudstone particle mixture is examined. Wetting deformation can lead to collapse-inducing cracks. The cracks may weaken the strength of the structure. To investigate the first wetting-induced deformation of the sandstone and mudstone particle mixture, three triaxial tests were conducted. Two of the triaxial tests used saturated soil and dry soil. These tests were used to calculate the strain under the condition of minimized deviator stress ratio at wetting. The third triaxial test accesses various deviator stress ratios at wetting. Additionally, the influence on axial and volumetric strain of wetting at varying levels of the confining stress and deviator stress ratios at wetting was evaluated. The wetting axial and volumetric strains were found to increase with an increased confining stress and deviator stress ratio at wetting. The relationships between the wetting axial strain and deviator stress ratio at wetting yielded a logarithmic curve for various levels of confining stress. In contrast, the wetting volumetric strain yielded a direct relationship with the deviator stress ratio at wetting for various levels of confining stress. Based on these results, two equations for predicting the wetting axial and volumetric strains are suggested.     

Dynamic characteristics of marine soft clay under variable phase difference and initial static shear stress

Abstract

Under seismic loading, the soil layer is subjected to multidirectional cyclic shear stress with different amplitudes and frequencies because of the coupling of multiple shear waves and the soil element within a slope or behind a retaining wall is subjected to initial static shear stress before subjected to cyclic loading. Due to the complexity of seismic loading propagation, a phase difference exists between the initial static shear stress and cyclic shear stress. To investigate the influence of the phase difference and initial static shear stress on cyclic shear strain, cyclic modulus, and cyclic strength, a series of laboratory tests are performed on Wenzhou marine soft clay by multi-directional simple shear system, which can simulate the actual state better by controlling the horizontal cyclic stress in the x and y directions simultaneously. As the phase difference varies from 0° to 90°, the dynamic shear modulus increases and cyclic strain accumulation decreases with an increasing number of cycles. The shear strain increases with the initial shear stress.     

Experimental study on the cyclic behavior of loose calcareous sand under linear stress paths

Abstract

Hollow cylinder torsional shear tests on loose isotropically and anisotropically consolidated calcareous sand were conducted to investigate the cyclic behavior under three different linear stress paths, including horizontal line, oblique line, and vertical line stress paths, in a coordinate system of the normal stress difference and the horizontal shear stress. The dominant strain components of the isotropically consolidated specimens are affected by the stress paths. With increasing consolidation stress ratio, axial strain gradually becomes the dominant strain component under the three different stress paths. The cyclic strength of the isotropically consolidated specimens under the three different stress paths are almost the same, while for the anisotropically consolidated specimens, the cyclic strengths are strongly affected by the stress paths. These results indicate that conventional cyclic triaxial tests may overestimate cyclic strength in some cases. Irrespective of the stress paths and cyclic stress ratios, the terminal residual excess pore pressure ratio decreases with increasing consolidation stress ratio. Moreover, an empirical equation is proposed to describe the relationship between the normalized shear work and the normalized residual excess pore pressure ratio. The comparative study reveals that the relationship proposed for silica sand is not suitable for the dynamic analyses of calcareous sand.     

Evaluation of cyclic shear strength of mixtures with sand and different types of fines

Soils are classified as sandy soils and clayey soils in most soil classification systems, and appropriate equations are used for practical design, depending on soil type, to represent each soil behavior. Sand-clay mixtures, however, need a special standard to evaluate their specific behaviors since they are categorized as intermediate soils or transitional soils and typically show intermediate properties. In this article, a series of cyclic triaxial tests were conducted on specific sand-fines mixtures with three fine types and various fine contents. The behaviors of various soil mixtures (silica sand with Iwakuni natural clay, Tottori silt, and Kaolin clay) were investigated by considering a concept of granular void ratio expressed in terms of sand structure. The cyclic shear strengths of the soil mixtures were also evaluated by increasing the fine content but no more than the threshold fine content. In the results, the cyclic deviator stress ratio decreased in dense soils whereas it increased in loose soils by increasing the fine content. In addition, a simple equation was proposed to predict the liquefaction resistance of sandy soils by evaluating the cyclic deviator stress ratio with a concept of equivalent granular void ratio.     

未胶结钙质砂静力和循环强度的单剪试验研究

通过等体积的单调和循环单剪试验研究南海未胶结钙质砂的静、动力反应,讨论应力水平和相对密实度对钙质砂静、动力强度的影响,并与典型的石英砂性质进行比较。在单调单剪试验中,中密和密实钙质砂在100～400 k Pa范围的初始竖向应力下都表现出应变硬化的性质,有效内摩擦角随剪应变增大。在循环单剪试验中,钙质砂的反应与相对密实度和初始竖向应力密切相关,但中密和密实钙质砂中的等效孔压都能达到初始竖向应力的85%～90%,此时剪应变突增,试样发生破坏。与相近密实度的石英砂相比,钙质砂抵抗液化的能力更强。提出了南海钙质砂动强度的归一化表达式,建立了不排水静强度、不排水动强度和循环次数之间的关系。     

Effects of frequency and cyclic stress ratio on creep behavior of clay under cyclic loading

The mechanical behavior of clay subjected to cyclic loading is important to consider in the design of the foundations of many types of structures that must resist cyclic loading, such as subgrades and offshore foundations, because clay undergoes greater settlement under cyclic loading than under static loading. The difference in settlement between these two loading patterns due to creep behavior is affected by the cyclic frequency and the cyclic stress ratio. This study investigated the effects of the frequency and cyclic stress ratio of cyclic loading on the creep behavior of a natural clay in China using stress-controlled triaxial tests. The assessed the following parameters: three frequencies, four cyclic stress ratios, and six vertical stresses. The test results indicate that the soft clay displays accelerated creep behavior under dynamic loads. A specific “limit frequency” (in this case, 0.2 Hz) and a “safe load” at which the strain of the soft clay increases very slowly were observed. The effect of the effective axial stress on the creep behavior increases with the increase in the cyclic stress ratio. Based on the tests, the critical cyclic stress ratio is 0.267 at a certain effective axial stress and frequency.     

Pore Water Pressure Buildup Under Cyclic Rotation of Principal Stress and Stability Evaluation of Seabed Deposit

- The cyclic rotation of principal stress direction with a constant amplitude is the characteristics of cyclic stress in seabed deposit induced by travelling waves. Presented in the paper are the results obtained from tests simulating the cyclic stress characteristics, with emphasis laid on the buildup of pore water pressure in soil samples. Regression analysis of test data shows that the pore water pressure can be expressed as the function of the number of cycles of cyclic loading, or as the function of generalized shear strain. Using the results thus obtained, the possibility of failure of seabed deposit under cyclic loading induced by travelling waves can be evaluated. The comparison with the results of conventional cyclic torsional shear tests shows that neglect of the effect of the cyclic rotation of the principal stress direction will result in considerable over-estimation of the stability of seabed deposit.     

Experimental investigation on cyclic deformation behavior of soft marine clay involved principal stress rotation

To investigate cyclic deformation behavior of natural soft marine clay-involved principal stress rotation, a series of undrained tests were conducted by using GDS hollow cylinder apparatus. The principal stress rotates 5000 cycles while the deviator stress was kept at a constant level. The tests results show that the deformation behavior of the tested samples are significantly dependent on cyclic stress ratio (CSR). Furthermore, different type of generation of axial strains occur under different CSRs. With the same CSR, the type of axial strain is different between that considering and ignoring principal stress rotation. When CSR is larger than CSR = 0.42 under principal stress rotation, the axial strain grows rapidly after a few cycles. Compared with the results conducted by cyclic triaxial results, the effect of principal stress rotation on the axial strain is significant.     

Anisotropic Drained Deformation Behavior and Shear Strength of Natural Soft Marine Clay

The deformation behavior and shear strength of soft marine clays subjected to wave or traffic loads are different from that in triaxial loading due to the changes of major principal stress direction β and intermediate principal stress coefficient b. To investigate the anisotropy affected by β and b in natural soft marine clay, a series of drained tests were conducted by hollow cylinder apparatus. The principal stress direction relative to vertical direction were maintained constant under an increasing shear stress, with fixed intermediate principal stress coefficient b. The influence of the b and β on anisotropy of typically Wenzhou intact clay is discussed. It was found that octahedral stress–strain relationships expressed anisotropy with different b and β. The friction angle and deviator stress ratio with different b and β were presented to provide guidance for engineering projects in the coastal zone.     

The effect of periodic intermittency on the cyclic behavior of marine sedimentary clay

Abstract

For subway systems built in coastal areas, the marine sediments are subjected to regular load sequences of waves and intermittencies, resulting in more complex reactions in their cyclic behaviors compared with those under the uniform cyclic loading applied in common studies. This research involved a series of experimental investigations into the undrained behavior of undisturbed saturated marine sedimentary clay subjected to cyclic loading with periodic intermittency considering the initial deviator stress and the conventional uniform cyclic triaxial tests for comparison. The results indicate that periodic intermittency significantly increases cyclic resistance, manifested by weakening of the long-term response and decreases in the number of vibration times required to achieve a steady state. The effect is greater with longer intermittency durations. Furthermore, changes in the pore water volume during cyclic loading were analyzed via nondestructive detection based on nuclear magnetic resonance. A conversion from bound water to free water was observed, referring to vibration magnitude and times. Lastly, the macroscopic results observed in triaxial tests and the microscopic results obtained in the nuclear magnetic resonance test appear to be closely related, indicating that the use of the variation in pore water is an applicable approach to delineate microchanges.     

Effect of Cyclic Loading Frequency on Undrained Behaviors of Undisturbed Marine Clay

Based on a series of cyclic triaxial tests, the effect of cyclic frequency on the undrained behaviors of undisturbed marine clay is investigated. For a given dynamic stress ratio, the accumulated pore water pressure and dynamic strain increase with the number of cycles. There exists a threshold value for beth the accumulated pore water pressure and dynamic strain, below which the effect of cyclic frequency is very small, but above which the accumulated pore water pressure and dynamic strain increase intensely with the decrease of cyclic frequency for a given number of cycles. The dynamic strength increases with the increase of cyclic frequency, whereas the effect of cyclic frequency on it gradually diminishes to zero when the number of cycles is large enough, and the dynamic strengths at different frequencies tend to the same limiting minimum dynamic strength. The test results demonstrate that the reasons for the frequency effect on the undrained soil behaviors are beth the creep effect induced by the loading rate and the decrease of sample effective confining pressure caused by the accumulated pore water pressure.     

Experimental simple shear study of composite soil with cemented soil core

Abstract

Cement soil mixing piles are an effective treatment method for marine soft clay. To investigate the static and dynamic characteristics of the composite soil with cemented soil core, a series of experiments are carried out by using the cyclic simple shear test. The result shows that, the static shear strain showed strain hardening, cemented soil core can improve static shear strength of composite soil, vertical stress can enlarge reinforcement of cemented soil core. The tendency of strain development of composite soil with different area replacement ratios under cyclic loading is the same as that of pure clay, existing critical cyclic stress ratios corresponding to different area replacement ratios. In addition, improving area replacement ratio can increase cyclic strength. At same time, adding of cemented soil core does not change shape of hysteresis curve compared with it for clay either. Moreover, cemented soil core can also obstruct stiffness softening. Through regression analysis of the experimental data, relationship between cyclic number and soil softening index is proved to be linear. The results can give a reference for the dynamic characters of the marine soft clay foundation with cement soil mixing piles.     

Degradation in Cemented Marine Clay Subjected to Cyclic Compressive Loading

The influence of cyclic loading on the strength and deformation behavior of cemented marine clay has been studied. This marine clay is of recent Pleistocene origin and deposited in a shallow water marine environment. Open pits were dug in sheeted enclosures and from these pits, undisturbed samples were taken for strength testing. A series of standard triaxial shear tests and stress controlled one-way cyclic load tests were conducted at consolidation stress ranges below and above the preconsolidation pressure. For the stress levels below the preconsolidation pressure, the cyclic loading has brought about the collapse of the cementation bond through an increase in strains, and at higher pressure ranges, the soil behaves like typical soft clay. This experiment studied the rate of development of strain and pore water pressure and shows that rate is a function of number of cycles, applied stress, and stress history. In addition, soil degradation during cyclic loading is studied in terms of Degradation Index. Attempt has been made to predict stain, pore water pressure, and degradation index through an empirical model.     

循环应力下饱和黏土剪切变形特性试验研究

针对饱和重塑黏土,利用土工静力-动力液压三轴-扭转多功能剪切仪,在不固结不排水(UU)条件下进行了应力控制式循环扭剪和竖向-扭转耦合试验,通过对试验结果的对比分析探讨了初始预剪应力和应力反向对应力-应变关系特性的影响,并阐述了不同加荷模式下孔隙水压力发展特性。以此为基础,综合考虑剪切变形和正向偏差变形的共同效应,同时为了能够反映平均残余变形和循环变形的影响,建议了一个综合应变破坏标准的算式。进而通过利用试验数据与目前常用的应变标准比较,表明这种破坏标准具有普遍适用性和较好的稳定性,适用于判定各种应力条件下黏土试样破坏及其强度。     

Undrained strength behaviour of a cemented marine clay under monotonic and cyclic loading

The paper presents the results of a series of monotonic and cyclic triaxial shear tests carried out to study the influence of the strain effect and load cycles on the undrained shear strength of a cemented marine clay from the East coast of India. The undrained shear strength of Indian coastal marine clay has been established from a detailed shear testing carried out in three phases. Undisturbed soil samples taken out from the seabed were used in the test. In the first part, a comprehensive monotonic shear testing has been carried out under both stress-controlled and strain rate-controlled conditions. In the second phase on identical soil specimen, undrained cyclic shear tests were carried out at various cyclic stress ratios (CSR) and these stress levels are chosen in such a way so that no failure occurred during testing. In the final phase post cyclic monotonic shear testing was conducted to qualitatively evaluate the damage caused by cyclic loading. The monotonic shear test results bring out the influence of cementation that can be detected by the stress-controlled test. The cyclic stress results are analysed in terms of CSR. Further, the results are correlated in terms of stress path.     

Effects of Mudstone Particle Content on Shear Strength of a Crushed Sandstone–Mudstone Particle Mixture

The present study focuses on the shear strength, deformation, and particle crushing of sand which is mixed by crushed sandstone and mudstone particles. Two triaxial tests—one for unsaturated specimens and the other for saturated specimens—were performed, along with sieve analysis tests. Results obtained from the triaxial tests indicate that, with the increment of the mudstone particle content by weight, the angle of internal friction decreases, the cohesion increases and then decreases, and both the initial angle of shearing resistance at the atmospheric pressure and decremental angle increase and then decrease. Compared to the unsaturated specimens, the deviator stress or normalized deviator stress of the saturated specimens at the same axial stress may decrease due to the wetting action. Thus, the shear strength of the tested materials may be reduced by the wetting action. Results of sieve analysis tests indicate that the mixing of some mudstone particles into the sandstone particle mixture may reduce the amount of particle crushing, but the wetting action may increase the amount. The amount of the crushed particle may arrive at a minimum of 40% of mudstone particle content by weight.