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。     

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.     

Prediction of Settlements of Soft Clay Subjected to Long-Term Dynamic Load

—Presented is the numerical analysis of settlements of soft soil by a 2-D dynamic effective stressFEM method.The model based on the results of cyclic triaxial tests on the reconstituted soft Ariake clay isused to predict the wave induced excess pore water pressure and residual strain of soft clay.The settle-ments of two types of breakwaters on the soft clay under ocean wave load,a low embankment subjected totraffic load and the tunnel surrounded by soft clay in Shanghai subjected to locomotive load are calculatedas examples.     

Characteristics of Saturated Soft Clay in Dynamic Deformation

-This paper presents the research results of dynamic pore water pressure and the characteristics of dynamic deformation of saturated soft clay and the factors affecting the dynamic pore water pressure and dynamic deformation. Dynamic triaxial compression tests are carried out and the soil samples used in the tests are remoulded clay from the seabed of the Bohai Bay. The paper also deals with the dynamic deformation mechanism of saturated soft clay foundation without drain. A calculation model for permanent dynamic deformation of saturated soft clay foundation has been established.     

Response of Single Piles in Marine Deposits to Negative Skin Friction from Long-term Field Monitoring

The behavior of single piles subjected to negative skin friction in soft soil was conducted by analyzing the results from full-scale long-term field measurements and three-dimensional (3D) numerical analyses. A skin friction coefficient (α and β coefficients) of the instrumented piles is back-calculated at different degrees of consolidation (U) of soft marine clay. Back-calculated β-values ranged from 0.15 to 0.35 for clay, and from 0.30 to 0.55 for sand, respectively. In addition, back-calculated α-values ranged from 0.1 to 0.3 for coated pile, and from 0.2 to 0.8 for uncoated pile when undrained shear strength of the soft clay was about 30–60 kPa, respectively. Moreover, this study describes behavior of a pile based on full-coupled 3D finite element (FE) analysis. The appropriate parametric studies needed for verifying the pile-soil interaction with consolidation are presented in this paper. Compared to the results from the measurements, it is shown that the computed results are capable of predicting the pile-soil behavior under consolidation. The major parameters that influence the pile behavior are discussed for different soil-pile conditions.     

Investigation of the stability of submarine sensitive clay slopes underwave-induced pressure

The exploration and exploitation of marine georesources ordinarily disturbs the submarine soft clay surrounding construction areas and leads to a significant decrease in the shear strength of structured and sensitive clayey soils in submarine slopes. Under wave action, local slides can even trigger large-scale submarine landslides, which pose a serious threat to offshore infrastructure such as pipelines and footings. Therefore, accurately evaluating the stability of submarine sensitive clay slopes under wave-induced pressure is one of the core issues of marine geotechnical engineering. In this paper, a kinematic approach of limit analysis combined with strength reduction technique is presented to accurately evaluate the real-time stability of submarine sensitive clay slopes based on the log-spiral failure mechanism, where external work rates produced by wave-induced pressure on slopes are obtained by the numerical integration technique and then are applied to the work-energy balance equations. The mathematical optimization method is employed to achieve the safety factors and the critical sliding surfaces of submarine slopes at different time in a wave cycle. On this basis, the stability of submarine sensitive clay slopes under various wave parameters is systematically investigated. In particular, extreme wave conditions and special cases of slope lengths no more than one wavelength are also discussed. The results indicate that waves have some negative effects on the stability of submarine sensitive clay slopes.     

Settlement Analysis for An Embankment Considering Visco-Elasto-Plastic Characteristics

—In the paper.a visco-elasto-plastic constitutive model and a method for determining modelparameters for soft clay are presented.In this model total strain of soft clay is assumed to be divided intothree parts:instantaneous elastic.visco-elastic and visco-plastic.The characteristics of instantaneous andvisco-elastic deformation of soft clay are simulated by Merchant's model.the plastic is by a model withtwo yield surfaces.And related constitutive equation is conducted,A number of stress-controlled triaxialtests are performed to calculated the model parameters The visco-elasto-plastic model is used for analysisof the displacement of an embankment on soft ground by use of three-dimensional finite element method.The predicted settlements agree well with the measured data.It is indicated that the viscous characteristicsshould be taken into account in deformation analysis for soft clay.     

Strength Behavior of Naturally Cemented Marine Clay Due to Change in Ground Water Level

The change in strength and deformation behavior of a typical marine clay deposit formed under shallow water is presented and discussed for the conditions of changing water table. This is a costal marine clay deposit with moderate carbonates along the east coast of India. The soil samples were taken from tidal flats where the sea had receded some time back, and the behavior of this deposit had been studied for the conditions of 1976, with high water table, and for the conditions of 2001, with depleted water table. Undisturbed soil samples were taken from sheeted open test pits. Standard consolidation and triaxial shear tests were conducted. Consolidation tests conducted on the samples taken for 1976 conditions with high water contents indicate that cementation effect are erased out under moderate stresses. Beyond this stress range, it behaves like soft, normally consolidated clay. In contrast, the results obtained from samples taken during 2001, with depleted water table, clearly indicate that the soil behaves like an over-consolidated one, and the improvement in the system is due to the chemical bonding and desiccation.     

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.     

Secondary Compression Features of Jiangsu Soft Marine Clay

The results of one-dimensional compression tests conducted on undisturbed specimens of Jiangsu soft marine clay is presented. Because of its high in situ void ratios and natural water content, Jiangsu soft marine clay displays high values of both the virgin compression index, C_c, and the secondary compression coefficient, C_α. The laboratory data indicates that the value of the ratio C_α/C_c for Jiangsu soft marine clay is constant. However, neither C_α nor C_c are constant: they both depend upon the natural water content (or void ratio) and thus are also dependent on the deformation (or compression) of Jiangsu soft marine clay. Settlement analyses show that the secondary settlement of Jiangsu soft marine clay is a significant component of the field settlement. The concept of a constant value for C_α/C_c is used to predict the secondary settlement of a surcharged embankment founded upon Jiangsu soft marine clay. The predictions are in agreement with the limited post-construction field measurements of the embankment settlement.     

黏性土淹没射流破土实验研究

海底水力喷射开沟技术在海底管道、光缆埋设等工程中得到广泛应用,然而黏土尤其是硬质黏性土中的水力开沟施工较为困难,为阐明其中的破土机制,开展了二维垂向淹没射流开沟物理模拟实验。通过观察射流破土的过程,发现:软黏土与硬质黏性土中冲坑发展的特点有所不同,破土机制也存在差异。射流对硬质黏性土的破坏中,压力导致的剪切破坏起到很大作用,在土力学原理上近似于地基土整体剪切破坏,据此建立了硬质黏性土射流破土模型,同时提出冲坑颈口直径在硬质黏性土冲坑形态中的重要性,并以此取代原有的冲坑直径作为一个预测指标。     

Probabilistic analysis for wave‐induced submarine landslides

Abstract

When waves propagate over the ocean floor, they induce a change of hydrodynamic pressure, positive under the crest and negative under the trough. These pressure changes may cause shear failure in soft sediments and lead to submarine landslides. This paper presents a general analytical procedure for evaluating the probability of wave‐induced failure in offshore clay sediments. Both the wave and the un‐drained shear strength of clay sediments are considered random. Numerical results of some analyses are also presented.     

Strengthening of Soft Marine Clay Using Bioencapsulation

Dredged or excavated soft marine clay can be improved by mixing it with cement or lime. However, these treatments are usually expensive. It is shown in this paper that soft marine clay can be strengthened through a bioencapsulation method in which the shear strength of clay aggregates can be substantially increased after the aggregates are treated with urease-producing bacteria, calcium chloride, and urea. We found that the bioencapsulation had increased the unconfined compressive strength of marine clay aggregates with a size of 5 mm from almost zero to more than 2 MPa. The strength of the bioencapsulated clay aggregates decreases with the increase in the size of the aggregate when the size is greater than 5 mm.     

Stability of the Guiding Dike in Yangtze Estuary Under the Wave Load

1 .IntroductionIt has beenrealizedthat constructing gravitystructures onsoft soil foundations is a difficult task,especiallyinthe marine area where boththe geological and environmental conditions are severe .Thisisnot only because the bearing capacity of the soft foundation is relatively low, but also because thestrength of the soft soil will befurther weakened bythe waveloading (Andersenet al .,1988 ;Ander-sen and Lauritzsen,1988 ; Yasuhara ,1988) .The strength weakening may cause serious d…     

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.     

A Nonlinear Rheological Model of Soft Clay and Its Application in Settlement of Sunk Ship

A one-dimensional consolidation-creep model test on the creep deformation of soft muddy clay in the littoral area of Tianjin is performed. A nonlinear rheologic model is established and the model coefficients are determined, in consideration of the characteristics of soft muddy clay. Furthermore, a settlement equation is deduced from the theologic model and verified by the field settlement measurements of Beitang Reservoir dam in Tianjin littoral area. Finally, the settlement e- quation is applied in calculating the settlement of ＂FAIRWAY-＂ suction dredger, which sunk in the external channel of Tianjin Port, induced by the soft clay consolidation of seabed. These results provide useful information for the decision of salvage plan.     

Degradation of lateral bearing capacity of piles in soft clay subjected to cyclic lateral loading

Abstract

In this article, the degradation of the lateral bearing capacity of piles in soft clay subjected to cyclic lateral loading is studied numerically. A modified kinematic hardening constitutive model is employed to simulate the degradation of soft clay after cyclic loading. The modified model is verified by comparing the numerical simulation results with the results of centrifuge model tests. Furthermore, the modified model is applied to numerical simulations for evaluating the lateral bearing capacity of piles in soft clay subjected to cyclic lateral loading. The degradation of the lateral bearing capacity of piles in soft clay after different cyclic displacement levels and different numbers of cycles is investigated. The study reveals that the modified kinematic hardening constitutive model can effectively estimate the cyclic degradation behavior of piles in soft clay subjected to cyclic lateral loading. The degradation of the ultimate lateral bearing capacity progresses slowly with increasing cyclic displacement level for fewer cycles, and the degradation develops significantly at higher levels of cyclic displacement after applying a larger number of cycles.     

Prediction to the Efficiency of Soft Clay Sample Preparation with Vacuum Loading

Vacuum loading has been examined as a way of preparing uniformly consolidated soft claysamples.The facility and loading procedure are described in this paper.An analytical solution to the threedimensional consolidation equation is derived for estimating the degree of consolidation of the soil samplewith vacuum loading.The given example shows that the predicted degree of consolidation of a soft claybulk with vacuum loading is close to that measured in the consolidation process.     

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

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

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.