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.     

Experimental study on the phase transformation point of crushable and noncrushable soils

Two marine calcareous deposits as crushable soils and a siliceous sand as a noncrushable soil were used in this study to compare their monotonic response. Undrained monotonic triaxial tests were conducted on samples, which were prepared in different relative densities and consolidated under various confining pressures. The location of phase transformation point in undrained response of the sands in different initial conditions was evaluated. The effect of important parameters including relative density, confining pressure, particle shape, and particle breakage on phase transformation point was assessed. The input energy applied per unit volume of the soils was used to interpret the shearing response of crushable and noncrushable soils. The results showed that calcareous sands have more tendencies in contraction. Particle shape and breakage play a key role in engineering behavior of crushable soils.     

Behavior of Loose Silty Sand of Chlef River: Effect of Low Plastic Fine Contents and Other Parameters

This article presents a laboratory study of static behavior of silty-sand soils. The objective of this laboratory investigation is to study the effect of initial confining pressures and fines content on the undrained shear strength (known as liquefaction resistance) response, pore pressure, and hydraulic conductivity of sand–silt mixtures. The triaxial tests were conducted on reconstituted saturated silty-sand samples at initial relative density D_r = 15% with fines content ranging from 0 to 50%. All the samples were subjected to a range of initial confining pressures (50, 100, and 200 kPa). The obtained results indicate that the presence of low plastic fines in sand–silt mixture leads to a more compressible soil fabric, and consequently to a significant loss in the soil resistance to liquefaction. The evaluation of the data indicates that the undrained shear strength can be correlated to fines content (F_c), inter-granular void ratio (e_g), and excess of pore pressure (Δu). The undrained shear strength decreases with the decrease of saturated hydraulic conductivity and the increase of fines content for all confining pressures under consideration. There is a relatively high degree of correlation between the peak shear strength (q_peak) and the logarithm of the saturated hydraulic conductivity (k_sat) for all confining pressures.     

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。     

Undrained Monotonic Response and Instability of Medium-Dense Sandy Soil

A series of undrained monotonic triaxial compression tests were performed on natural, medium-dense (relative density (RD) = 50%) Chlef sand containing 0.5% of non-plastic fines, under different confining pressures of 50 kPa, 100 kPa, and 200 kPa. This article focuses on distinctive states of the monotonic undrained response of sands, namely the critical state, the phase transformation state, the quasi-steady state, and the state of undrained instability (onset of flow liquefaction). Specimens were prepared using dry funnel pluviation and wet deposition to investigate the effect of the initial sand fabric on these states. The present data suggest that the initial fabric of the sand appears to have a significant effect on the undrained behavior in the early stages of shearing, with its influence vanishing at large strains. Wet deposition specimens demonstrate considerably larger undrained instability state strength than their dry funnel pluviation counterparts, and a unique critical state locus is reached by both dry funnel pluviation and wet deposition.     

Shearing response of geotextile-reinforced calcareous soils using monotonic triaxial tests

Abstract

Different types of geosynthetic reinforcements have been widely used for improving shear strength of soil materials in recent decades. In this study, two different calcareous soils reinforced by geotextile layers were evaluated by monotonic compression triaxial tests. The calcareous soils were obtained from Hormuz Island and Boushehr Port located in the north coast of the Persian Gulf. Different reconstituted samples were prepared at dense conditions with and without geotextile layers and consolidated under different effective confining pressures ranging from 100 to 600?kPa. Different layers of reinforcements including one, two and three geotextile layers were placed in the reinforced samples and sheared under drained condition. The results show that confining pressure has a significant effect on the efficiency of reinforcements. Evidently, increasing the reinforcement layers decreased the particle breakage of the calcareous soils used in this study.     

Shear Strength and Dilatancy of Calcareous Sand in the South China Sea

The shear strength and dilatancy of typical uncemented calcareous sand from the South China Sea are investigated by soil lab tests. According to drained triaxial tests at various relative densities and confining stresses, it is found that the constant volume friction angle is approximated as 39° and the traditional Bolton’s equations can be modified to estimate the peak friction angle and dilation angle. The reliability of the equation proposed for the peak friction angle is verified in terms of calcareous sands from more onshore and offshore sites worldwide, while the errors of the predicted dilation angles scatter in a relatively large range. Totally, the dilation angles of sands in the South China Sea are estimated by the equation presented with an error of ±30%. The peak friction angle measured by the undrained is similar to that by the drained tests as the relative density smaller than 60%, while the former is slightly lower for denser samples.

     

Strain-Rate Dependent Stress--Strain Behavior of Undisturbed Hong Kong Marine Deposits under Oedometric and Triaxial Stress States

This article presents a testing study on the strain-rate effects on the stress--strain behavior of natural, undisturbed Hong Kong marine deposits (HKMD) from three Hong Kong locations, including a one-dimensional (1-D) compressibility in a confined condition, and undrained shear strengths in triaxial compression and extension modes. The influences of the strain rates on the one-dimensional compressibility are studied by means of constant rate of strain (CRS) tests and multistage loading oedometer (MSL) tests, and those on the undrained shear strengths are studied by K_o-consolidated undrained compression and extension tests with step-changed axial strain rates (CK_oUC and CK_oUE tests), and with both step-changed axial strain rates and relaxation processes (CK_oUCR and CK_oUER tests). The strain-rate effects on the stress--strain behavior are generally examined by “apparent” preconsolidation pressures in the 1-D compressions and undrained shear strengths in the triaxial compression and extension stress states. The stress--strain behavior of the natural, undisturbed HKMD exhibits considerable viscous characteristics. In the CRS and MSL tests at a given strain, the higher the strain rate, the higher the effective stress, the higher the porewater pressure. In the undrained shearing tests, the higher the strain rate, the higher the undrained shear strength, but the lower the porewater pressure. For the CK_oUC and CK_oUE tests on the Tsing Yi site samples, the undrained shear strength increases by 8.5% and 12.1% for one order increment of axial strain rate of 0.2%/hr (i.e., ρ_0.2) for the compression and extension modes respectively. For the CK_oUCR and CK_o tests on the Tung Chung site samples of different compositions, average ρ_0.2 is increased by 6.2% for the compression and 9.5% for the extension, but by 18.8% for the extension on a higher plastic sample. The present study shows that the strain-rate effects on the stress--strain behavior of the undisturbed HKMD are larger for specimens in extension than those in compression.     

南中国海神狐海域天然气水合物上覆地层不排水抗剪强度预测

浅层沉积物不排水抗剪强度（Su）是深水作业的关键参数之一。为了获取南海神狐海域首次海域天然气水合物试采区W18-19框体的基本工程地质特征,试采工程准备阶段开展了原位孔压静力触探测试（CPTU）及大量的室内实验。本文将主要基于CPTU计算不排水抗剪强度的基本模型,采用微型十字板、电动十字板、袖珍贯入仪及不固结不排水三轴实验,确定该区域不排水抗剪强度的基本模式,并提出适用于南海神狐钙质黏土层的不排水抗剪强度纵向分布规律计算模型,对该区域水合物上覆层的不排水抗剪强度进行预测。 结果表明,基于总锥端阻力、有效锥端阻力、超孔隙压力的模型系数分为13.8、4.2、14.4。综合考虑地层压实效应和含气情况,本文提出的分段函数预测模型与室内结果的一致性较好,可用于工程设计阶段进行工区不排水抗剪强度纵向分布规律的预测。另外,基于有效锥端阻力的不排水抗剪强度经验模型适应于浅层极软-较硬压实的钙质粘土层,基于超孔隙压力的不排水抗剪强度模型适用于较硬-坚硬的不含气层,而基于总锥端阻力的不排水抗剪强度计算模型则适用于坚硬含气的钙质黏土层。本文提出的分段函数模型有效的提高了经验模型在南海神狐水合物赋存区的适用性,计算结果可为工程安全评价提供支撑。     

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.     

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.     

Strain-Rate Dependent Stress--Strain Behavior of Undisturbed Hong Kong Marine Deposits under Oedometric and Triaxial Stress States

ABSTRACT

This article presents a testing study on the strain-rate effects on the stress--strain behavior of natural, undisturbed Hong Kong marine deposits (HKMD) from three Hong Kong locations, including a one-dimensional (1-D) compressibility in a confined condition, and undrained shear strengths in triaxial compression and extension modes. The influences of the strain rates on the one-dimensional compressibility are studied by means of constant rate of strain (CRS) tests and multistage loading oedometer (MSL) tests, and those on the undrained shear strengths are studied by K _o-consolidated undrained compression and extension tests with step-changed axial strain rates (CK _oUC and CK _oUE tests), and with both step-changed axial strain rates and relaxation processes (CK _oUCR and CK _oUER tests). The strain-rate effects on the stress--strain behavior are generally examined by “apparent” preconsolidation pressures in the 1-D compressions and undrained shear strengths in the triaxial compression and extension stress states. The stress--strain behavior of the natural, undisturbed HKMD exhibits considerable viscous characteristics. In the CRS and MSL tests at a given strain, the higher the strain rate, the higher the effective stress, the higher the porewater pressure. In the undrained shearing tests, the higher the strain rate, the higher the undrained shear strength, but the lower the porewater pressure. For the CK _oUC and CK _oUE tests on the Tsing Yi site samples, the undrained shear strength increases by 8.5% and 12.1% for one order increment of axial strain rate of 0.2%/hr (i.e., ρ_0.2) for the compression and extension modes respectively. For the CK _oUCR and CK _o tests on the Tung Chung site samples of different compositions, average ρ_0.2 is increased by 6.2% for the compression and 9.5% for the extension, but by 18.8% for the extension on a higher plastic sample. The present study shows that the strain-rate effects on the stress--strain behavior of the undisturbed HKMD are larger for specimens in extension than those in compression.     

Study of dilatancy behaviors of calcareous soils in a triaxial test

Abstract

In this article, the dilatancy of calcareous soil is studied systematically based on triaxial consolidation drainage shear tests, and the difference in dilatancy between calcareous soil and siliceous soil is also investigated. It was found that: ① Calcareous soil experience obvious dilated deformation. Dilatancy tendency increases with increasing related density and decreases with increasing confining pressure. ② The volumetric strain rate initially increases from negative to positive. After it reaches a maximum, there is a small decrease in the volumetric strain rate, but it is still greater than zero, and the stress-strain curves are of softening type. ③ For the same condition, the dilatancy deformation of calcareous sand begins later than that of siliceous sand, and the volume compression before dilatancy is also larger for calcareous sand. ④ The critical state alone cannot accurately describe the entire deformation process of soil, and it is proposed that the phase transformation state be added to the standard method used to assess soil dilatation and contraction. ⑤ Based on the statistical analysis of experimental data, mathematical relationships were established between void ratio, relative density, and effective confining pressure of phase transformation state and critical state, respectively.

• Highlights

• Reports results from a well-designed experiment that includes a good amount of samples and data.

• Effects of relative density and effective confining pressure on deformation mode and mechanical properties of calcareous sand are evaluated.

• The difference in dilatancy between calcareous sand and siliceous sand was compared

• The phase-transformation state and critical state were compared with the axial strain, volumetric strain and deviatoric stress.

• Using phase-transformation void ratio and critical void ratio to describe the whole deformation process of calcareous sand is proposed.

• The mathematical expressions of phase-transformation void ratio and critical void ration were given, respectively.

     

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.     

In Memoriam

Abstract

The engineering properties of deep continental margin sediments were determined on a worldwide basis. Deep Sea Drilling Project (DSDP) core data and material were utilized from over 900 cores obtained from 89 sites, primarily on the continental margins. Cores were recovered from penetrations to 200 m in water depths averaging 3000 m. Supplementary laboratory testing on selected cores was directed toward determining index properties and shear strength properties of the sediments. The study included a literature review of deep‐sea soil properties, the results of which are to be used by DSDP to evaluate foundation conditions for reentry cones with long casing strings. The results will also be used for a feasibility study of an ultradeepwater marine riser and well‐control system. The marine sediments examined can be divided into three main types: clay, calcareous ooze, and siliceous ooze. Sediment distribution consisted of 48% calcareous ooze, 43% clay, 6% siliceous ooze, and 3% volcanic ash. Because of the sample disturbance inherent in the deepwater coring operation, emphasis was placed on analyzing sediment properties not significantly affected by changes in in situ stresses and structure caused by sampling. Averages and ranges in values of water content and unit weight are presented for the three main sediment types. Plasticity and strength characteristics are discussed in detail and the elastic and compression properties are outlined. The geotechnical properties of deep continental margin soils are summarized.     

The effect of cyclic loading on the undrained shear strength of a silty clay

Abstract

The strength of saturated clays subjected to low‐frequency cyclic loading is of considerable importance in the design of offshore structures. A series of postcyclic undrained triaxial strength tests were carried out on samples of a silty clay (Keuper Marl), which had not failed under cyclic loading, to investigate the effect of this loading on the postcyclic monotonie effective stress path and strength of the soil. Undrained cyclic loading of normally consolidated and lightly overconsolidated samples by generating positive pore pressures caused the apparent degree of overconsolidation to increase. Samples subsequently subjected to monotonie testing followed effective stress paths commensurate with this apparent overconsolidation and failed on the “Hvorslev” surface on the “dry” side of the stable state boundary surface rather than on the critical state as would be expected of samples of this initial stress history. Samples which were initially heavily overconsolidated did not generate large pore pressures under cyclic loading and did not therefore exhibit large changes in postcyclic monotonie strength. The reduction in strength after cyclic loading therefore is greater for normally consolidated and lightly overconsolidated samples. The pore pressures generated under postcyclic monotonie loading are related to the degree of apparent overconsolidation achieved during cyclic loading.     

Analysis of sediment stability on the Peru‐Chile continental slope

Abstract

Potential sediment mass movement was analyzed at ten locations on the continental slope off Peru and northern Chile, using samples obtained from up to 3 m below the seafloor. Shear strength parameters were obtained from consolidated‐undrained triaxial compression tests. Sediment behavior in these tests reflects the influence of organic matter, which is concentrated in the slope deposits by coastal upwelling. High water content of the organic‐rich sediments and the high de‐formability of organic matter contribute to the prevalent ductile behavior. Aggregation of clays by organic matter is apparently responsible for the high friction angles, up to 44°, displayed by the slope deposits. Sediment stability was assessed using infinite slope analyses. These analyses indicate that gravitational forces alone are not sufficient to cause sediment failure at any of the slope locations. Sediment accumulation on the slope is not rapid enough to generate excess pore pressure and reduce the resistance to gravitational sliding. Effects of earthquakes on slope stability were evaluated by modeling earthquake‐induced inertia forces as static forces and estimating pore pressures developed during cyclic loading. This analysis shows that sediments of the lower slope off Peru possess the highest susceptibility to failure during earthquakes. Earthquake accelerations on the order of 0.2 gravity are sufficient to trigger slumping at all ten slope locations. Indirect evidence suggests that creep and mass flows initiated at shallower water depths are factors that might contribute to sediment failure on the slope.     

Geotechnical Properties of Surface Sediments in the INDEX Area

As a part of the environmental impact assessment studies, geotechnical properties of sediments were determined in the Central Indian Basin. The undrained shear strength and index properties of the siliceous sediments were determined on 20 box cores of uniform dimension collected from various locations in five preselected sites. The maximum core length encountered was 41 cm and most of the sediments were siliceous oozes consisting of radiolarian or diatomaceous tests. The shear strength measurements revealed that surface sediments deposited in recent times (0-10 cm) have a shear strength of 0-1 kPa; this value increases with depth, reaching 10 kPa at 40 cm deep. Older sediments have greater strength because of compaction. Water content varies in the wide range of 312-577% and decreases with depth. The clay minerals such as smectite and illite are dominant and show some control over water content. Wet density, specific gravity, and porosity do not indicate any notable variation with depth, thereby indicating a uniform, slow rate of sedimentation. The average porosity of sediments is 90.2%, specific gravity 2.18, and wet bulk density 1.12 g/cm 3 . Sediments exhibit medium to high plasticity characteristics, with the average plasticity index varying between 105% and 136%. Preliminary studies on postdisturbance samples showed an increase in natural water content and a decrease in undrained shear strength of sediments in the top 10- to 15-cm layer.     

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.