Evaluating the stiffness of chemically stabilized marine clay

Cement-stabilized clay is widely used in soft clay improvement for deep excavation, underground construction, and land reclamation. This paper presents a study on the evaluation of elastic modulus for cement-stabilized marine clay. First, two types of cement-stabilized soils were studied through isotropic compression tests and cylinder split tensile tests. Specimens with different mix ratios and curing periods were used. Stress–strain behavior under isotropic compression was discussed, followed by an introduction and estimation of the stress-free bulk modulus. Empirical correlations between elastic moduli and functions for estimating elastic moduli were then proposed. Further estimation of elastic modulus was conducted with another data set. The results showed that the proposed function for estimating elastic modulus is effective for cement-improved marine clay. Finally, the proposed method and empirical functions were validated with other types of cement-stabilized clay.     

Laboratory and field test study on the improvement of marine clay slurry by in-situ solidification

Abstract

Soil solidification technology can create an artificial hard shell on a soft soil surface but the type and proportion of the curing agent, the construction technology, and the strengthening depth have large influences on the strengthening effect and engineering cost. This study introduces a new technology of soil solidification whereby an artificial hard shell layer is used as a new method to improve the soft ground. For the in-situ solidification technology, the soil and curing agent are mixed well by using a strong stirring machine so that the soil is strengthened rapidly and forms a hard crust. We introduce the key technology of the in-situ soil solidification method and determine the in-situ crust carrying capacity. The indoor experiment on the curing agent proportions is validated with field tests and a vane shear test, static penetration test, and plate loading test are used to evaluate the reinforcement effect. The experimental results show that the in-situ curing technology of dredged fill processing markedly reduced the reinforcement depth range of the soil water content, improved the physical and mechanical indices, and increased the bearing capacity and strength of the artificial hard shell layer, thereby fully meeting the requirements for the bearing capacity of construction machinery.     

Effect of nano-MgO on mechanical performance of cement stabilized silty clay

Abstract

A series of direct shear tests were performed on cement-admixed silty clay to investigate the effect of cement content and nano-magnesia (MgO) on its shear strength properties. For each normal stress, shear strength increased with cement content. However, an obvious increment in shear strength was achieved when the cement content was adjusted from 13% to 17%. Both cohesion and friction angle of cemented soil increased with cement content, and exponential function was adopted to correlate both the factors with cement content. For cement content of 10% investigated in this study, the optimum nano-MgO content was 10‰, wherein the cohesion could reach the peak value. The microstructure of the mixture revealed that the structure of the mixture was compacted for the optimum nano-MgO content. However, micro-cracks were formed when the amount of nano-MgO exceeded its optimum content.     

Primary yielding locus of cement-stabilized marine clay and its applications

ABSTRACT

Strength and stiffness properties of materials are widely studied and used in civil engineering practice. However, most studies are based on unconfined conditions, which are different from real status of soil. This study investigated the primary yielding and yield locus for cement-stabilized marine clay. In this study, two types of cement-stabilized soils were studied through isotropic compression, triaxial drained shearing, unconfined compression, and bender element testing. Specimens with 20–50% of cement content and 7–90 days of curing period were used for the tests. Stress–strain behavior and primary yielding were evaluated, followed by construction of the primary yield locus. The characteristics of the primary yield locus and its development with curing time then were studied. The results showed that the properties of the primary yield locus were dependent on the type of stabilized soil, but were independent of the cement content and curing period. Thus, the approach provides a way to estimate the primary yield stress and drained stress path before primary yielding for cement-stabilized soil under confined condition. An empirical function was used to fit the primary yield locus. The primary isotropic yield stress was correlated to unconfined compressive strength or maximum shear modulus. Three indirect methods were proposed to predict the primary yield stress for cement-stabilized marine clay. The results showed that the primary yield stress can be estimated with reasonable accuracy.     

Experimental study on mechanical properties of gas hydrate-bearing sediments using kaolin clay

A triaxial system is designed with a temperature range from-20 ℃ to 25 ℃ and a pressure range from 0 MPa to 30 MPa in order to improve the understanding of the mechanical properties of gas hydrate-bearing sediments.The mechanical properties of synthetic gas hydrate-bearing sediments (gas hydrate-kaolin clay mixture) were measured by using current experimental apparatus.The results indicate that:(1) the failure strength of gas hydrate-bearing sediments strongly depends on the temperature.The sediment’s strength increases with the decreases of temperature.(2) The maximum deviator stress increases linearly with the confining pressure at a low-pressure stage.However,it fluctuates at a high-pressure stage.(3) Maximum deviator stress increases with increasing strain rate,whereas the strain-stress curve has no tremendous change until the axial strain reaches approximately 0.5%.(4) The internal friction angles of gas hydrate-bearing sediments are not sensitive to kaolin volume ratio.The cohesion shows a high kaolin volume ratio dependency.     

On the compression behavior of remolded cement-admixed soft clay

Although extensive research has been performed on the mechanical properties of cement-stabilized clays, quite a few attempts have been made on the compression behavior of remolded cement-admixed clays. The results from oedometer tests have been discussed to investigate the compressibility of remolded cement-admixed clays, taking into consideration cement amount and curing time. The findings show that the difference in shape and position of compression curves is attributed to cement amount and curing time. Most compression index (C_c) values of remolded cement-admixed clays are greater than those of untreated clay due to the presence of remolded yield stress σ′_yr that is closely related to initial water content and clay fabric. Based on the obtained test data, the relationships of C_c vs. e₀, C_c vs. w₀, C_c vs. e₁, C_c vs. e_yr, and σ′_yr vs. e_yr are preliminarily discussed and quantitatively established. Especially, an important divergence of void index I_v at effective stress σ′_v less than remolded yield stress σ′_yr can be observed at different cement amounts and curing durations. Being independent on cement amount, curing time, and initial state of soil, an excellent convergence occurs at stress σ′_v greater than yield stress σ′_yr. The normalized compression curves of I_v vs. σ′_v at σ′_v?>?σ′_y can be expressed by a unique line that agrees well with intrinsic compression line (ICL) and extended ICL.     

Laboratory study on cement admixed marine silt bricks

For the purpose of efficient utilization of sediments dredged from harbor, a new method was proposed in this study. Marine silt bricks were made by mixing sediments with cement and gypsum, placing it in a cubic mold with 240 mm in length, 115 mm in width, and 53 mm in height, and curing for certain days. To investigate the effects of cement and initial water content of soil on the mechanical behavior of marine silt bricks, unconfined compressive and flexural strength tests were carried out. Given the same curing time and cement content, the higher the initial water content, the lower the compressive and flexural strength. After 60 days of curing, the compressive strength of marine silt bricks with cement content = 20% and water content = LL (liquid limit) reached approximately 5 MPa. The flexural strength was relatively low. The flexural strength of marine silt bricks with 20% cement and water content = LL was around 1.5 MPa. The compressive and flexural strength decreased with the increase of water/cement ratio. As for the curing time, longer curing time had a positive impact on the compressive strength. The ratio of flexural to compressive strength varied slightly in the range of 0.4–0.5.     

Influence of viscosity on one-dimensional thermal consolidation of marine clay

ABSTRACT

The elastic mechanical response of porous materials under a heat source has many applications in civil engineering and has received considerable attention in the geotechnical literature. In this paper, a Kelvin viscoelastic model is combined with the thermohydromechanical governing equations for marine clay and solved using a numerical inversion of the inverse Laplace transform in the time domain. After validation against existing analytical solutions, numerical parametric studies are conducted to investigate the influence of viscosity on temperature, excess pore pressure, and displacement. It is shown that viscosity has little influence on temperature, a modest influence on displacements, and a quite significant influence on excess pore pressure.     

Evaluation of the uplift behavior of plate anchor in structured marine clay

The uplift behavior of a plate anchor in a structured clay (soft Ariake clay) is investigated through a series of laboratory tests and method of finite element analysis. The tests are adopted to identify the factors influencing the behavior of the anchor, including the thixotropic nature of Ariake clay, consolidation time, and embedment ratio of the anchor. A finite element method (FEM) is used to analyze and predict the uplift behavior of the anchor plate well in the elastic region and the yield load. The results from both the laboratory tests and the FEM analysis suggest that the embedment ratio for a deep anchor in Ariake clay is close to 4. Further increase in embedment ratio improves the capacity to a lesser extent. FEM overestimates the failure load of the uplift anchor in soft Ariake clay by about 20%. This may be ascribed to the hypothesis in the FEM analysis that there is continuous contact between the clay and the anchor until failure. Vesic’s theory for deep anchors, which may be used to predict the ultimate pullout resistance of the plate anchor in reconstituted Ariake clay, is verified to be applicable. In this paper, the plastic flow zone around the anchor is discussed using FEM which makes the behavior of anchor more understandable during the design stage.     

Immersed tunnel foundation on marine clay improved by sand compaction piles

The sand compaction pile (SCP) method can be applied to soft marine clay ground that is a reinforcement of composite ground consisting of compacted sand piles and surrounding clay. The application of SCP method in the immersed tunnel of Hong Kong–Zhuhai–Macao Bridge verify SCP method is a robust solution to limit the total settlement and differential longitudinal settlement and to promote smooth transition from immersed tunnel to artificial island. The SCP method has significant settlement reduction effect on marine clay. The SCPs can also function as a drainage path to accelerate the consolidation process in marine clay. It is also found that the consolidation rate of SCP-improved ground is delayed compared with that predicted program which is most probably because of the soil disturbance effect during the installation of SCPs.     

Microstructure analysis on the dynamic behavior of marine clay in the South China Sea

Abstract

The aim of this article was to study the dynamic behavior and microstructural variation of undisturbed marine clay from the South China Sea. First, dynamic cyclic triaxial tests were employed to investigate the dynamic stress–strain-pore pressure paths of the undisturbed clay. Then, scanning electron microscopy and mercury intrusion porosimetry were used to measure the variations of the micromorphology and pore size distribution between before and after the dynamic cyclic tests. Through these tests, the dynamic failure process and microstructure variation of the marine clay were quantitatively analyzed. In particular, their relationships are qualitatively established from the macro-micro perspective. Furthermore, by comparing the tests of the remolded clay with those of the undisturbed marine clay, the influence of the microstructure on the dynamic behavior is systematically investigated. The results show that the microstructural variation of the marine clay is caused by the compression deformation of the mesopores among the granular clusters into the small pores between individual particles. The study provides an effective reference for the selection of the microstructural parameters of marine clay.     

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.     

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 deformation characteristics of soft marine clay involved the change of b-values and stress direction

In actual engineering, soft clay foundations are in drained or partial drained conditions, it would be useful to establish reasonable constitutive relationship and provide guidance for engineering projects. A hollow cylinder apparatus is used to investigate the anisotropic deformation behavior of natural soft marine clay influenced by intermediate principal stress coefficient b and principal stress direction α. Tests were conducted by maintaining a fixed principal stress direction α relative to the vertical direction, while keeping the intermediate principal stress coefficient b constant. It was found that the anisotropic deformation behavior of natural soft clay is merely influenced by major principal stress direction α, but significantly influenced by intermediate principal stress coefficient b.     

A simple experimental method to regain the mechanical behavior of naturally structured marine clays

Quantitative laboratory studies on the structural behavior of natural intact marine clays require a large number of identical natural samples leading to an expensive and challenging task. This study proposes a simple method to reconstruct an artificial structured marine clay as the state of its natural intact clay at both macro and micro levels. For this purpose, the Shanghai marine clay is selected and mixed with low cement contents (1–6%). The clay-cement slurry is mixed in a container with the ice-covered sides at a low temperature about 0 ± 2 °C to postpone the hydration reactions until consolidation began. The purpose of adding cement is to generate the inter-particle bonding and structure in reconstituted samples. Initially, the reconstituted samples are consolidated under the in situ stress of 98 kPa and then under the pre-consolidation pressure of 50 kPa. Mechanical characteristics such as compression index, yield stress, unconfined compression strength, shear strength ratio, and the stress paths from triaxial tests are compared with natural intact clay accordingly. Scanning electron microscope and mercury intrusion porosimetry analyses are also performed to analyze the microstructure of clays for comparison. Furthermore, the proposed method is also examined by using natural intact marine clays of different locations and characteristics.     

Compressibility behaviour of lime-treated marine clay

The necessity to tap natural marine resources from the ocean beds represents a considerable challenge for the construction of offshore structures on weak marine deposits. The use of lime to improve the behaviour of soft clays is not new. The present investigation examines lime-induced changes in the compressibility of marine clay. The test results indicate a reduction of 1/2 to 1/3 in the compressibility of the soil system within 30 to 45 days of treatment. The formation of various cementation compounds due to soil–lime reactions improves the soil characteristics with time. The results encourage the application of lime column and lime injection techniques to improve the engineering behaviour of soft marine clayey deposits. However, one has to be cautious in applying the lime technique to marine clays that contain sodium sulfate.     

Effect of angle between initial and cyclic shear stress on behaviors of marine clay

An angle exists between the initial static shear stress and cyclic shear stress when embankment and retaining walls are subjected to cyclic loadings. To investigate the influence of this angle on the dynamic properties of marine soft clay, tests were performed on Wenzhou soft clay. When the angle was varied from 0° to 90°, the shear strain and excess pore pressure decreased as θ increased while increased as θ increased from 120° to 180°. Shear strain developed more rapidly when θ was 120°, 150°, or 180° than that when θ was 0°, 30°, or 60°. These results indicate that the number of cycles to failure at the larger angles was greater than at the smaller angles. When θ was 90°, the strain in the x-axis direction increased as the number of cycles increased. The development of the excess pore pressure associated with specimen failure was different for different cyclic shear stress ratios and shearing angles. The effect of θ on the strain and excess pore pressure increased as the cyclic shear stress ratio increased.     

Permeability characteristics of lime treated marine clay

An attempt has been made to investigate the lime induced permeability changes in the permeability and engineering behavior of different lime column treated soil systems. Lime columns treated marine clay shows an increase in permeability up to a maximum value of 15–18 times that of untreated soil with time. The shear strength of the treated soil systems show an increment up to 8–10 that of untreated soil within a period of 30–45 days curing. In the case of lime injection systems, the permeability has been increased up to 10–15 times that of untreated soil, whereas the strength of the soil has been higher by 8–10 times that of untreated soil. Further, consolidation tests show a reduction in the compressibility up to 1/2–1/3 of original values. The test results revealed that both lime column and injection techniques could be used to improve the behaviour of underwater marine clay deposits.     

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.     

Experimental and theoretical methods on determination of stress-dependent natural frequency of marine sedimentary clay

Excited by the vibration sources in dynamic engineering, the natural frequency and damping factor of the saturated marine sedimentary clay are key dynamic parameters that influence the responses under cyclic loads. Experimental and theoretical methods are proposed in this paper to analyze the natural frequency and the stress-dependent nonlinearity. The experimental method shows that the natural frequency of soils with specific stress state subjected to large cyclic shear strain can be estimated from the data of dynamic triaxial tests based on the amplitude–frequency response curve. Trial and error by the criterion from the half-power bandwidth method is used to determine the optimal fitting. The results of a theoretical study on the free vibration of soil layers are then presented to derive the analytic solution of natural frequency. In addition to the two frequency-independent elements (a lumped mass matrix and a stiffness matrix), the system’s equivalent damping coefficient matrix is iteratively determined based upon the forced vibration experimentally. Finally, the impacts of the resonance phenomenon on the dynamic shear modulus and hysteretic loop are discussed.