Undrained Shear Strength from Field Vane Test on Busan Clay

An unusually thick and soft clay, which is called Busan clay, has been deposited throughout the Nakdong River estuary, South Korea. Although many reclamation works have been carried out in the area since early 1990s, large- and small-scale slope failures have taken place mostly during the soil placement on the clay. A case history briefly describes failure of a breakwater resulting from overestimation of the undrained strength, obtained from laboratory testing. An attempt has been made to examine the methods for determining the undrained strength from field vane testing. For this, the undrained strengths from field vane testing were obtained using different correction factors and compared with each other at five sites in the area. An alternative correction method applicable especially for Busan clay was found, using a range of corrected strength ratios. Further, relationships between the depositional environment and the undrained strength of the clay have also been investigated.     

Undrained Shear Strength from Field Vane Test on Busan Clay

An unusually thick and soft clay, which is called Busan clay, has been deposited throughout the Nakdong River estuary, South Korea. Although many reclamation works have been carried out in the area since early 1990s, large- and small-scale slope failures have taken place mostly during the soil placement on the clay. A case history briefly describes failure of a breakwater resulting from overestimation of the undrained strength, obtained from laboratory testing. An attempt has been made to examine the methods for determining the undrained strength from field vane testing. For this, the undrained strengths from field vane testing were obtained using different correction factors and compared with each other at five sites in the area. An alternative correction method applicable especially for Busan clay was found, using a range of corrected strength ratios. Further, relationships between the depositional environment and the undrained strength of the clay have also been investigated.     

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.     

Laboratory and Field Observations for Golden Horn Marine Clay

Istanbul, the largest city in Turkey and one of the major metropolitan areas in the world, cleaned one of its environmentally polluted areas—Golden Horn—by dredging 5 million m³ of the bottom sediments and pumping the resulting sludge to a storage area behind a dam built at an abandoned rock quarry site in Alibey district. The reclamation of the land that formed over the storage area of Golden Horn dredged material is socially and economically very desirable. In this paper, results from experimental studies that are focused on determining the shear strength behavior of the dredge material and undisturbed soil are presented. Slurry consolidometer test, large model tests and small model tests are used to consolidate the dredged soil samples from Halic to simulate the natural consolidation behavior of these soils. Shear strength parameters are determined by laboratory vane tests; unconfined compression tests; undrained-unconsolidated (UU) and consolidated-undrained (CU) triaxial tests on samples that are obtained through in situ undisturbed samples and laboratory model tank and slurry consolidation. Moreover, the effects of fly ash and lime additives on the undrained shear strength were determined by mixing the materials with the dredged clay from Golden Horn during the model experiments conducted in the laboratory. Based on these findings, equations are proposed that govern the relationships between undrained shear strength and water content value.     

Evaluation of Undrained Shear Strength and Consolidation Characteristics of Disturbed Marine Sedimentary Clay due to SCP Installation

The behaviors of the marine sedimentary ground improved by sand compaction pile (SCP) method are analyzed. To do this, the results of upheaval characteristics of the sea floor, undrained shear strength, and horizontal consolidation coefficient (consolidation) are investigated. Due to SCP installation on ground, as thickness of a soft clay layer increases, upheaval height increases and upheaval angle decreases. Undrained shear strength of disturbed ground due to SCP construction decreases in early stage after completion of construction, but it shows a trend of recovering as months elapse. As the result of piezocone penetration dissipation tests, consolidation delay phenomenon by the disturbance due to SCP installation clearly is identified and its degree is dependent on the replacement area ratio of SCP and the location of ground.     

Nonparametric Estimation of Undrained Shear Strength for Normally Consolidated Clays

For decades, various well-known empirical correlations have been established and commonly used to estimate the undrained shear strength of normally consolidated clays in geotechnical practice. However, there are still many contradictory cases regarding the validity of these correlations, so their applicability and reliability are questionable. In this study, an extensive geotechnical database for normally consolidated clays has been compiled. Next, a Bayesian nonparametric method is exploited to identify the most critical soil index properties in inferring the undrained shear strength and a new predictive model is proposed. Finally, the performance of the proposed predictive model is assessed and compared with two well-known correlations using an independent testing database. It confirms that the proposed model not only possesses high predictability, but also provides superior performance over other existing empirical correlations.     

Estimation of Residual Undrained Shear Strength from Gravity Corer Penetration Data

Residual undrained shear strength, s _{u_res} , is an important parameter for analyzing the response of structures buried within potentially unstable soil mass. A framework for estimating s _{u_res} of fine grained sediments from gravity corer penetration has been developed considering viscous drag during free fall of the corer through seawater and cohesive energy loss during sediment penetration. The procedure was used to estimate s _{u_res} using data from a submarine geotechnical investigation in western Canada. Comparison of the results with alternative estimates of s _{u_res} from miniature torvane tests on the gravity core samples and CPTs performed nearby reveals a reasonable agreement.     

Correlations between CPT and FVT Results for Busan Clay

In this study, CPT and FVT tests are carried out on young, normally consolidated clay (Busan clay, which is an unusually thick deposit) at several locations in the Nakdong River deltaic plane in Busan, Korea. Two correlation techniques are applied for estimating the cone factors for which the uncorrected and corrected field vane strengths are used as the reference strength. First, due to the scattered data over the entire depth, a direct correlation between the tested results (a method generally used in practice), is applied for each depositional environment (facies) that is detected from the CPT profiles and the geological analysis. Secondly, a special attempt is made to determine the cone factors based on the fact that the ratios of the field vane strength and the (q _t  ? σ _v0) and (u ₂ ? u ₀) values to the effective overburden pressure respectively, are approximately constant for each facies (the indirect correlation). It appears that the values obtained from the two techniques are almost identical. However, the direct correlation is sensitively affected by the fabric effects especially in some of the uppermost and lowermost layers of the clay and hence the arithmetic average values of each facies sometimes result in lower correlation coefficients. It is therefore inferred that the indirect correlation is more proficient for estimating the cone factors at each facies and can be used for practical applications. From the results of the indirect correlation, the cone factors of Busan clay are N _kt  = 7.13–13.06, N _kt,corr  = 9.45–17.07, N _Δu  = 6.10–9.88 and N _Δu,corr  = 8.41–12.24, the corrected values of which are considerably smaller or larger than the general ranges recommended by Lunne et al. (1997a). However, good correlations between N _kt and I _P , and between N _Δu (or N _kt ) and B _q , are not found for the clay.     

Estimation of Residual Undrained Shear Strength from Gravity Corer Penetration Data

Residual undrained shear strength, s_{u_res}, is an important parameter for analyzing the response of structures buried within potentially unstable soil mass. A framework for estimating s_{u_res} of fine grained sediments from gravity corer penetration has been developed considering viscous drag during free fall of the corer through seawater and cohesive energy loss during sediment penetration. The procedure was used to estimate s_{u_res} using data from a submarine geotechnical investigation in western Canada. Comparison of the results with alternative estimates of s_{u_res} from miniature torvane tests on the gravity core samples and CPTs performed nearby reveals a reasonable agreement.     

Evaluation of the Strength Increase of Marine Clay under Staged Embankment Loading: A Case Study

This article presents a case history of the performance of a full-scale test embankment constructed on a marine soft clay deposit improved by prefabricated vertical drains (PVDs) in the east of China. For analyzing the subsoil behavior, a 2D FEM model is established, in which the PVD-improved effect is considered by a simplified method of equivalent vertical hydraulic conductivity. The calculated results can predict the settlement behavior well; however, the FEM gives an underestimate for the value of excess pore pressures and it predicts similar values for the dissipation rate of excess pore pressures. The measured undrained shear strength of subsoil, C_u, is compared with the predicted value based on Ladd’s empirical equation and the Modified Cam-Clay model (MCC). The shear strength predicted by Ladd’s equation agrees well with the measured value, whereas the MCC overestimates the ability to improve subsoil shear strength during consolidation. The undrained shear strength of subsoil, C_u, increased as the construction progressed, and the shear strength incremental ratio, ΔC_u/Δp′, decreased slightly with the degree of consolidation, U.     

Remolded Undrained Strength of Soils

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Study of the Shear Strength of Sediments in Main Sedimentation Stages

The shear strength properties of sediments are relevant to many practical problems, including those related to predicting the bearing capacity of the man-made crust lying over dredged disposal sites and those associated with estimating the erosion resistance and the bearing capacity of sediments. In this study, an experimental apparatus and method is developed for sedimentation. This apparatus consists of a settling column, pore measurement apparatus, shear vane apparatus, and multilayer extraction sampling apparatus. The change regulation of interface height, density, excess pore pressure, peak undrained shear strength, residual undrained shear strength, and sensitivity varies before and after the excess pore pressure dissipates to zero in the self-weight consolidation stage. The higher the water content, the greater the particle segregation degree. Particles are mainly segregated in the settling stage, and they are not segregated further in the self-weight consolidation stage. Before excess pore pressure dissipates to zero in the self-weight consolidation stage, shear strength is related to water content, effective stress, and the formed structure of sediments. After excess pore pressure dissipates to zero, peak undrained shear strength is mainly associated with the structure (thixotropy) of sediments. Residual undrained shear strength increases because of the slight decrease in water content. The mechanisms of thixotropy can be expressed as the increase in the original and curing cohesions of sediments with time as determined from microscopic aspects.     

Shear Strength of Jumunjin Sand according to Relative Density

The properties of sands pertaining to shear strength have been studied extensively by plane strain and triaxial tests for relative densities above 40%. Unfortunately, properties of sand have not been comprehensively studied for relative density below 40%. An experimental investigation of the shear strength of Jumunjin sand was carried out using a plane strain, triaxial and direct shear apparatus according to relative density ranges from 0 to 100%. Because of the complexity of the shear tests on coarse materials, correlations were developed that would be useful for practical purposes. This article presents a comparison of strength properties' results and their correlation with relative density.     

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。     

Applicability to Korean Marine Clay of the Mobilized Undrained Vane Shear Strength Using Correction Factors

The undrained shear strength normalized by the yield consolidation pressure, su/pc', is presented for four coastal sites: Busan/Gwangyang and Incheon/Gunsan, having the characteristics of high and low plasticity, respectively. The field vane shear strengths, su(FVT), were compared with unconfined compressive strength, qu/2 which has been used as a representative testing method in Korea. Many researchers have suggested that the undrained shear strength normalized by the yield consolidation pressure, su/pc', depends on Ip. However, the undrained shear strength normalized by the yield consolidation pressure, su/pc' is in the range of 0.25–0.35, independently of the plasticity index, Ip except for su/pc' using qu/2 values in the case of soils having a low plasticity, such as Incheon and Gunsan intermediate soils.

Bjerrum's correction factor has been commonly applied to evaluate mobilized undrained shear strength using the field vane test in Korea. However, the corrected undrained shear strengths using Bjerrum's correction factor, including Morris and Williams' method, were considerably underestimated for Korean marine clay when compared with the qu/2 values that have been used as the mobilized undrained shear strength for practical design in Korea.     

Comparison of vane shear and fall cone strengths of soft marine clay

A total of 1,014 measures of sediment shear strengths were measured by means of miniature vane shear and fall cone tests on five gravity cores collected in Eckernfo‐erde Bay, Baltic Sea. Paired t test was used to compare the shear strengths measured by the two methods. It was found that fall cone strength calculated with Wood's K₆₀value (0.29) overestimates the vane shear strength by 0.15 kPa (a = 0.001) and the sample mean of the fall cone strength is 4.1% higher than the mean of the vane shear strength. However, fall cone strength calculated with Hansbo's K₆₀ value (0.24) underestimates the vane shear strength by 0.88 kPa (a = 0.001), and the sample mean of the fall cone strength is 13.8% less than the mean of the vane shear strength. Both calculated fall cone strengths are significantly different from the vane shear strength, with a p value of less than 0.001. Regression analysis of the Echernfoerde Bay data indicates that a new K₆₀ value is 0.275 with a confidence interval (a = 0.01) from 0.2704 to 0.2786. Paired t test shows that there is no significant difference between miniature vane shear and fall cone tests for these samples if the fall cone strength is calculated with K₆₀ = 0.275.     

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.     

The blooms of a cyanobacterium, Microcystis cf. aeruginosa in a severely polluted estuary, the Golden Horn, Turkey

The distribution of toxic cyanobacterium Microcystis cf. aeruginosa in the severely polluted Golden Horn Estuary was studied from 1998 to 2000. Microcystis persisted at the upper estuary where the water circulation was poor and values ranged between 2.9 × 10⁴ and 2.7 × 10⁶ cells ml⁻¹ throughout the study. Simultaneously measured physical (salinity, temperature, rainfall and secchi disc) and chemical parameters (nutrients and dissolved oxygen) were evaluated together with Microcystis data. Although the Microcystis blooms generally occur in summer due to the increase in temperature, the blooms were recorded in winter in the present study. The abundance of Microcystis depended on the variations in salinity and both blooms were recorded below S = 2. A moderate partial correlation between Microcystis abundance and salinity was detected in the presence of temperature, dissolved oxygen and precipitation data (r = −0.561, p = 0.002). The M. cf. aeruginosa abundance was low in the summer when the salinity was higher than winter. A remarkable increase in the eukaryotic phytoplankton abundance following the improvements in the water quality of the estuary occurred, whilst the Microcystis abundance remained below bloom level.     

Comparison of Remolded Shear Strength with Intrinsic Strength Line for Dredged Deposits

Chandler proposed the intrinsic strength line to correlate the undrained shear strength of samples one-dimensionally consolidated from slurry with the void index proposed by Burland. The undrained shear strength on the intrinsic strength line is different from the remolded undrained shear strength that is an important parameter for design and construction of land reclamation. The void index is used in this study for normalizing the remolded strength behavior of dredged deposits. A quantitative relationship between remolded undrained shear strength and void index is established based on extensive data of dredged deposits available from sources of literature. Furthermore, the normalized remolded undrained shear strength is compared with intrinsic strength line. The comparison result indicates that the ratio of undrained shear strength on the intrinsic strength line over remolded undrained shear strength increases with an increase in applied consolidated stress.     

Behavior of Marine Clay Subjected to Cyclic Loading with Sustained Shear Stress

Foundations of offshore structures are designed to withstand a combination of static and cyclic loads due to ocean waves. Wave action on offshore structures can cause a significant amount of cyclic horizontal and vertical forces to be transmitted to the soil through the foundation. In all these cases, these cyclic loads are considered to be superimposed over the initial sustained static stress due to the self-weight of structures. This study considers various factors that influence the development of deformation and pore water pressure in a typical cemented marine clay. These results show that the sustained static shear stress significantly influences the strength and deformation behavior of marine clay under cyclic loading. Up to a certain range of sustained static stress, there is an improvement in strength during cyclic loading and the cyclic strains are greatly reduced.