Undrained monotonic shear behavior of marine soft clay after long-term cyclic loading

Abstract

In the coastal area, nearshore and offshore structures have been or will be built in marine soft clay deposits that have experienced long-term cyclic loads. Therefore, the mechanical behavior of marine clay after long-term cyclic loading needs to be investigated. In this research, a series of monotonic and cyclic triaxial tests were carried out to investigate the postcyclic mechanical behavior of the marine soft clay. The postcyclic water pore pressure, shear strength and secant stiffness are discussed by comparing the results with the standard monotonic test (without cyclic loading). It is very interesting that the postcyclic behavior of marine soft clay specimen is similar to the behavior of overconsolidated specimen, that is, the specimen shows apparent overconsolidation behavior after long-term cyclic loading. Then relationship between the overconsolidation ratio and the apparent overconsolidation ratio is established on the basis of the theory of equivalent overconsolidation. Finally, a validation formula is proposed which can predict the postcyclic undrained shear strength of marine soft clay.     

Undrained behavior of natural marine clay under cyclic loading

To study the undrained behavior of natural marine clay under cyclic loading, two kinds of stress-controlled cyclic triaxial tests were conducted on natural K₀-consolidated Wenzhou clay. In the Series I tests, samples were cyclically sheared until failure, and the accumulative behavior was studied; based on the results, a suitable cyclic failure criterion is suggested for natural clays. The effect of loading frequency was also investigated, and it was observed that the loading duration t is a key factor in controlling the undrained cyclic behavior. In the Series II tests, cyclic undrained tests followed by strain-controlled monotonic compression tests were carried out, and special attention was given to changes in the undrained strength after cyclic loading. The degradation of the post-cyclic peak strength was affected by the accumulative behavior during cyclic shearing, but the deviatoric stresses at the critical state were nearly constant. Finally, the accumulative behavior of natural clays was simulated using a proposed anisotropic elastic viscoplastic model with a pseudo-static method of equivalent undrained creep, and the results indicate that this equivalent creep simplification is suitable in practice. By taking the apparent overconsolidation after cyclic loading into account, the post-cyclic strength degradation can also be explained by this model.     

海洋粉质粘土在波浪荷载作用后的不排水抗剪强度衰化特性

通过对南海重塑粉质粘土土样的大量动三轴试验结果分析,得到此种土在波浪荷载作用后不捧水抗剪强度衰化同动载作用引起的动应变幅及平均累积孔压之间的相互关系和预估公式;并通过与超固结土样的静三轴剪切试验结果的比较,发现动、静三轴两种试验结果具有很好的吻合关系。建议可用超固结土样的静三轴剪切试验同时结合部分动三轴试验来预估土样在波浪荷载作用后不排水抗剪强度衰化与平均累积孔压之间的关系。     

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.     

Field Evaluation of Undrained Shear Strength from Piezocone Penetration Tests in Soft Marine Clay

The use of the piezocone penetration test (CPTU) in a geotechnical site investigation offers direct field measurement on stratigraphy and soil behavior. Compared with some traditional investigation methods, such as drilling, sampling and field inspecting method or laboratory test procedures, CPTU can greatly accelerate the field work and hereby reduce corresponding operation cost. The undrained shear strength is a key parameter in estimation of the stability of natural slopes and deformation of embankments in soft clays. This paper provides the measurements of in situ CPTU, field vane testing and laboratory undrained triaxial testing of Lianyungang marine clay in Jiangsu province of China. Based on the literature review of previous interpretation methods, this paper presents a comparison of field vane testing measurements to CPTU interpretation results. The undrained shear strength values from both the field vane tests and cone penetration resistances are lowest at the mid-depths of the marine clay layers, and the excess pore water pressures are highest at the mid-depths of the marine clay layers, indicating that the marine clay layer is underconsolidated.     

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.     

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.     

Stiffness Degradation of Undisturbed Saturated Soft Clay in Yangtze Estuary under Complex Stress Conditions

Stiffness degradation will occur due to the generation of accumulated pore pressure in saturated soft clays under cyclic loading. The soil static-dynamic multi-purpose triaxial and torsional shear apparatus in Dalian University of Technology was employed to perform different types of test on saturated soft marine clay in the Yangtze estuary. Undisturbed samples of the clay were subjected to undrained cyclic vertical and torsional coupling shear and cyclic torsional shear after three-directional anisotorpic consolidation with different initial consolidation parameters. Investigated were the effects of the initial orientation angle of the major principal stress, initial ratio of deviatoric stress, initial coefficient of intermediate principal stress and continuous rotation of principal stress axes on the stiffness degradation. It is found that the degradation index decreases (or degradation degree increases) significantly with increasing initial orientation angle of the major principal stress and initial ratio of deviatoric stress. Compared to the effects of the initial orientation angle of the major principal stress and initial ratio of deviatoric stress, the effect of initial coefficient of intermediate principal stress is less evident and this trend is more clearly reflected by the results of the cyclic torsional shear tests than those of the cyclic coupling shear tests. At the same cycle number, the degradation index obtained from the cyclic torsional shear test is higher than that from the cyclic coupling shear test. The main reason is that the continuous rotation in principal stress directions during cyclic coupling shear damages the original structure of the soil more than the cyclic torsional shear does. Based on a series of experiments, a mathematical model for stiffness degradation is proposed and the relevant parameters are determined.     

Estimation of undrained shear strength for saturated clay using shear wave velocity

Undrained shear strength is a fundamental parameter for estimating the stability of soft soils. This study explores the relationship between undrained shear strength, void ratio, and shear wave velocity for saturated and normally consolidated clay specimens. The undrained shear strength void ratio-shear wave velocity relationship was correlated to empirically determined parameters of selected marine clay specimens. To verify the proposed relationship between undrained shear strength and shear wave velocity, in situ flat dilatometer tests were used for determining the undrained shear strength, and downhole tests were used to assess the shear wave velocity on a natural soil deposit at various depths. The undrained shear strength estimated from the in situ shear wave velocities was compared to the undrained shear strength obtained in the field. The results show that the inferred undrained shear strength yield similar values and follow the same trends as the in situ undrained shear strength data. This method using shear wave velocity can help to nondestructively estimate the undrained shear strength of soft soils in the field and be used in both on-shore and off-shore geotechnical engineering projects.     

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。     

海洋井架结构非线性分析

根据海洋钻井井架实际结构特点,建立弹性基础上的三维有限元模型.按9种工况对某井架的有限元模型进行加载,并且在传统线弹性理论基础上加以改进,考虑非线性P-△效应的影响.应用有限元分析软件StruCAD进行分析计算,给出可靠的计算结果;并且系统分析结构的受力特点和影响因素,得出对于井架这类高耸结构应该考虑P-△效应的影响.     

Rate-Dependent and Long-Term Yield Stress and Strength of Soft Wenzhou Marine Clay: Experiments and Modeling

This paper focuses on the rate-dependent yield stress, undrained strength, and long-term undrained strength of clay. Various oedometer and triaxial tests were performed on Wenzhou marine clay. The uniqueness of rate-dependent and long-term yield stress and strength is discussed. A new anisotropic elastic viscoplastic model is proposed by adopting the concept of anisotropy and isotropy processes. The determination of model parameters is discussed, demonstrating how all model parameters can be determined in a straightforward way requiring the same basic tests as for the Modified Cam Clay model. Experimental verifications are carried out using the constant strain-rate and creep tests on the Wenzhou marine clay.     

Macro-and Micro-Properties of Two Natural Marine Clays in China

In this paper,macro- and micro- properties of natural marine clay in two different and representative regions of China are investigated in detail.In addition to in-situ tests,soil samples are collected by use of Shelby tubes for laboratory examination in Shanghai and Zhuhai respectively,two coastal cities in China.In the laboratory tests,macro-properties such as consolidation characteristics and undrained shear strength are measured.Moreover,X-ray diffraction test,scanning electron microscope test,and mercury intrusion test are carried out for the investigation of their micro-properties including clay minerals and microstructure.The study shows that:(1) both clays are Holocene series formations,classified as either normal or underconsolidated soils.The initial gradient of the stress-strain curves shows their increase with increasing consolidation pressure;however,the Shanghai and the Zhuhai clays are both structural soils with the latter shown to be more structured than the former.As a result,the Zhuhai clay shows strain softening behavior at low confining pressures,but strain hardening at high pressures.In contrast,the Shanghai clay mainly manifests strain-hardening.(2) An activity ranges from 0.75 to 1.30 for the Shanghai marine clay and from 0.5 to 0.85 for the Zhuhai marine clay.The main clay mineral is illite in the Shanghai clay and kaolinite in the Zhuhai clay.The Zhuhai clay is mainly characterized by a flocculated structure,while the typical Shanghai clay shows a dispersed structure.The porous structure of the Shanghai clay is characterized mainly by large and medium-sized pores,while the Zhuhai clay porous structure is mainly featreed by small and medium-sized pores.The differences in their macro- and micro- properties can he attributed to different sedimentation environments.     

Post-Cyclic Behavior of Carbonate Sand of the Northern Coast of the Persian Gulf

The post-cyclic behavior of biogenic carbonate sand was evaluated using cyclic triaxial testing through a stress control method under different confining pressures between 50 to 600 kPa. The testing program included a series of isotropically and anisotropically consolidated, undrained triaxial compression and extension tests on samples of remolded calcareous Bushehr sand. Grading analyses (before and after each test) were used to examine the influence of particle breakage on post-cyclic behavior of Bushehr sand. The particle breakage commonly occurred in these soils even in lower values of confining pressure, yet there was not a clear correlation between the post-cyclic responses and particle breakage. Based on the present study, a concept is suggested for post-cyclic behavior of carbonate sand. It was observed that post-cyclic strength has a good correlation with cyclic stress ratio, type of consolidation, and value of residual cyclic strain. For all specimens, it is clear that the post-cyclic strength is greater than monotonic strength, irrespective of confining pressure and relative density.     

Monotonic and cyclic behaviors of loose anisotropically consolidated calcareous sand in torsional shear tests

ABSTRACT

The behavior of loose anisotropically consolidated calcareous sand obtained from an island in the South China Sea was investigated under undrained monotonic and cyclic loading in a hollow cylinder torsional apparatus. The tests were conducted on specimens which consolidated under various initial effective confining pressures and consolidation stress ratios. The monotonic test results show that the failure and phase transformation line are essentially independent of the consolidation conditions, while the initial contractive tendency of the specimens decreases with an increasing consolidation stress ratio. During monotonic loading of the anisotropically consolidated specimens, a same major principal stress direction is observed at the constant stress ratio lines up to the phase transformation line, irrespective of initial effective confining pressure. The cyclic strength of the sand increases with an increasing consolidation stress ratio. Moreover, a pronounced stress dependence is observed in the sand with higher consolidation stress ratio. During cyclic loading, the generated excess pore water pressure presents considerable fluctuations. The normalized terminal excess pore water pressure is described as a function of consolidation stress ratio. The tests show that the particle shape, rather than particle crushing, plays an important role in the monotonic and cyclic behaviors of the calcareous sand.     

Cyclic behavior of natural organic clay under variable confining pressure that match traffic loading conditions

ABSTRACT

One-way cyclic loading is more typical for traffic loading and cyclic triaxial test has been recognized as a useful method for solving many engineering problems. Under traffic loading, the influence of variable confining pressure on cyclic behavior of natural organic clay subjected to cyclic traffic loading is rarely reported in the literature. In this study, a laboratory investigation on undrained cyclic behavior of natural organic clay is presented and conducted by cyclic triaxial apparatus. Tests are conducted by both constant confining pressure and variable confining pressure, to simulate the loading conditions induced by passing vehicles in actual engineering. Different stress levels are also considered in this study. By comparing between the results of constant confining pressure tests and variable confining pressure tests, it shows that the one-way cyclic behavior of organic clay is influenced significantly by variation of confining pressure, in terms of pore water pressure, permanent axial strain and stress–strain hysteretic loops.     

Settlement and Stability Analysis on Soft Clay Under Cyclic Loading

The results of undrained cyclic triaxial tests on three types of clays are collected and a relationship among the accumulated strain, the initial stress state, cyclic stress in the soil, as well as the number of cycles is established based on the experimental data. With this relationship, a procedure is proposed for subsidence and stability analysis on soft clay under the action of cyclic loads.     

基于流变仪测试海底浅表层软黏土不排水强度研究

鉴于海底浅表层软黏土强度测试精细化程度不足的现状,引入流体测试中的流变仪,对青岛海域海底浅表层软黏土开展多组原状和重塑试样的不排水剪切强度试验,通过对比静力触探和微型十字板测试结果,验证了流变仪测试方法的有效性。基于流变仪试验结果,揭示了海底软黏土原状和重塑状态下不排水剪切破坏模式,探讨了海底软黏土不排水剪切强度和灵敏度随埋深及液性指数的发展演变趋势,评价了软黏土的结构性特征。最后,引入含水率与液限之比对海底浅表层软黏土重塑不排水剪切强度进行了归一化分析,为近海海洋开发活动提供技术支撑。     

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.