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.     

Application of T-bar in numerical simulations of a monopile subjected to lateral cyclic load

A series of centrifuge tests were performed to investigate the response of a free-head monopile due to cyclic lateral loading in normally consolidated clay. By linking the maximum reaction-force point of the final cycles in all tests with various amplitudes, a postcyclic reaction-force curve is obtained, which can be used to assess the postcyclic remolded lateral stiffness. To numerically analyze the tests, a strength degradation model of the clay is calibrated by the T-bar cyclic test. However, this model is T-bar-dependent, which is unable to capture the degrading behavior of the monopile stiffness. Thus, a modification approach is proposed based on the upper bound theory, and the modified model is further combined with finite element analysis to simulate the cyclic behavior of the model pile. The simulation results show similar degrading trend and consistent postcyclic remolded lateral stiffness with the model tests. This further demonstrates that the remolded lateral stiffness mainly depends on the soil remolded strength, which is one of the parameters calibrated by the T-bar tests. Based on this finding, a simplified numerical analysis is proposed, which can predict the postcyclic reaction-force curve by performing one monotonic loading instead of modeling the whole process of cyclic loading.     

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.     

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.     

In situ geotechnical characterization of sediments on the Nova Scotian Slope, eastern Canadian continental margin

A seabed 2-m-long cone penetrometer and coring system (Geotechnical Module) has been used at 17 stations in four transects on the Scotian Slope to characterise in situ shear strength and induced pore pressure on several different types of late Pleistocene and early Holocene failure. Study sites were selected using the SAR high-resolution deep-towed acoustic system equipped with a digital 160–190 kHz sidescan sonar and a 3.5 kHz subbottom profiler.

Several distinctive types of “geotechnical signature” were recognised from plots of cone resistance and induced pore pressure with depth in the sediment. Normally consolidated sediments show a progressive increase in cone resistance with depth (to about 75 kPa at 2 m subbottom). Holocene surficial muds show spectacular apparent overconsolidation, reaching a peak of 250 kPa at about 50 cm subbottom and then decreasing down to 1.5 m. This overconsolidation is associated with Zoophycos burrows. Late Pleistocene sediments exhumed by bedding plane slides show strong true overconsolidation consistent with the original depth of burial inferred from high-resolution seismic stratigraphy. Debris flows show only a slight shear stress gradient with depth (40–45 kPa over 0.5–1 m subbottom) with under-consolidation due to remoulding of sediment.     

Assessment of cyclic response to suction caisson in clay using a three-dimensional displacement approach

An investigation was made to present analytical solutions of cyclic response to suction caisson subjected to inclined cyclic loadings in clay using a three-dimensional displacement approach. A model representing the relationship between vertical load and vertical displacement and that between lateral load and lateral displacement along the skirt of suction caisson subjected to cyclic loadings is proposed for overconsolidated clay. For the effect of vertical load on cyclic load capacity of suction caisson, using the Mindlin solution in the case of a vertical point load, the vertical stress of soil under the base of suction caisson is presented. For the stress state of soil beneath the base of suction caisson subjected to cyclic loading, the Mohr–Coulomb failure line and critical state line are presented and the relationship between total stress, effective mean principal stress, stress difference, and pore-pressure is elucidated. The comparison of results predicted by the present method for a suction caisson subjected to cyclic loadings in clay has shown good agreement with those obtained from field tests. Cyclic behavior of clay up to failure is made clear from the relationship between cyclic tensile load, vertical and lateral displacements, and rotation and that between depth, vertical, and lateral pressures.     

Prediction and observation of a model gravity platform on Drammen clay

The results of five centrifuge tests of a stiff circular model platform on overconsolidated undisturbed Drammen clay are reported from Manchester University together with predictions computed at the Norwegian Geotechnical Institute. Towards failure the models developed high shear strain and softening local to the base, accompanied by a permanent settlement or shakedown. A total stress transfer from edge to centre caused a rapid increase of excess pore pressure under the centre, having a high vertical gradient. These features, and failure values which decreased with increase in eccentricity of loading, conformed with previous observations on reconstituted clays. Comparison of observations with predictions highlights the sensitivity of the analytical method to the fineness of the finite element mesh just below the base. The predictions also indicated a sensitivity of the failure load to the degree of overconsolidation associated with a given undrained shear strength. The elastic type analysis did not predict the degree of shakedown which was associated with plastic displacement of clay from just under the base.     

海洋粉质粘土在波浪荷载作用后的不排水抗剪强度衰化特性

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

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.     

Effects of frequency and cyclic stress ratio on creep behavior of clay under cyclic loading

The mechanical behavior of clay subjected to cyclic loading is important to consider in the design of the foundations of many types of structures that must resist cyclic loading, such as subgrades and offshore foundations, because clay undergoes greater settlement under cyclic loading than under static loading. The difference in settlement between these two loading patterns due to creep behavior is affected by the cyclic frequency and the cyclic stress ratio. This study investigated the effects of the frequency and cyclic stress ratio of cyclic loading on the creep behavior of a natural clay in China using stress-controlled triaxial tests. The assessed the following parameters: three frequencies, four cyclic stress ratios, and six vertical stresses. The test results indicate that the soft clay displays accelerated creep behavior under dynamic loads. A specific “limit frequency” (in this case, 0.2 Hz) and a “safe load” at which the strain of the soft clay increases very slowly were observed. The effect of the effective axial stress on the creep behavior increases with the increase in the cyclic stress ratio. Based on the tests, the critical cyclic stress ratio is 0.267 at a certain effective axial stress and frequency.     

Experimental study on the cyclic behavior of loose calcareous sand under linear stress paths

Abstract

Hollow cylinder torsional shear tests on loose isotropically and anisotropically consolidated calcareous sand were conducted to investigate the cyclic behavior under three different linear stress paths, including horizontal line, oblique line, and vertical line stress paths, in a coordinate system of the normal stress difference and the horizontal shear stress. The dominant strain components of the isotropically consolidated specimens are affected by the stress paths. With increasing consolidation stress ratio, axial strain gradually becomes the dominant strain component under the three different stress paths. The cyclic strength of the isotropically consolidated specimens under the three different stress paths are almost the same, while for the anisotropically consolidated specimens, the cyclic strengths are strongly affected by the stress paths. These results indicate that conventional cyclic triaxial tests may overestimate cyclic strength in some cases. Irrespective of the stress paths and cyclic stress ratios, the terminal residual excess pore pressure ratio decreases with increasing consolidation stress ratio. Moreover, an empirical equation is proposed to describe the relationship between the normalized shear work and the normalized residual excess pore pressure ratio. The comparative study reveals that the relationship proposed for silica sand is not suitable for the dynamic analyses of calcareous sand.     

Numerical Analyses of Caisson Breakwaters on Soft Foundations Under Wave Cyclic Loading

A caisson breakwater is built on soft foundations after replacing the upper soft layer with sand. This paper presents a dynamic finite element method to investigate the strength degradation and associated pore pressure development of the intercalated soft layer under wave cyclic loading. By combining the undrained shear strength with the empirical formula of overconsolidation clay produced by unloading and the development model of pore pressure, the dynamic degradation law that describes the undrained shear strength as a function of cycle number and stress level is derived. Based on the proposed dynamic degradation law and M-C yield criterion, a dynamic finite element method is numerically implemented to predict changes in undrained shear strength of the intercalated soft layer by using the general-purpose FEM software ABAQUS, and the accuracy of the method is verified. The effects of cycle number and amplitude of the wave force on the degradation of the undrained shear strength of the intercalated soft layer and the associated excess pore pressure response are investigated by analyzing an overall distribution and three typical sections underneath the breakwater. By comparing the undrained shear strength distributions obtained by the static method and the quasi-static method with the undrained shear strength distributions obtained by the dynamic finite element method in the three typical sections, the superiority of the dynamic finite element method in predicting changes in undrained shear strength is demonstrated.     

风暴浪导致的黄河口水下土体破坏试验研究

本文试验利用取自黄河水下三角洲的样品 ,重塑后铺设水槽底床进行水槽试验 ,并利用原状土进行动三轴试验 ,2种试验均测定土体内的孔隙水压力。根据各种情况下孔隙水压力的变化记录 ,表明土体破坏同时其孔隙水压力产生骤变。将本文试验结果与在黄河水下三角洲不稳定区的原位沉积动力学试验孔隙水压力测试结果对照 ,说明黄河三角洲水下斜坡某些土体的破坏 ,未出现波浪循环荷载作用下孔隙水压力积累升高所导致的土体液化破坏 ,而是风暴浪对海底的强切应力作用致使土体产生剪切破坏     

Degradation in Cemented Marine Clay Subjected to Cyclic Compressive Loading

The influence of cyclic loading on the strength and deformation behavior of cemented marine clay has been studied. This marine clay is of recent Pleistocene origin and deposited in a shallow water marine environment. Open pits were dug in sheeted enclosures and from these pits, undisturbed samples were taken for strength testing. A series of standard triaxial shear tests and stress controlled one-way cyclic load tests were conducted at consolidation stress ranges below and above the preconsolidation pressure. For the stress levels below the preconsolidation pressure, the cyclic loading has brought about the collapse of the cementation bond through an increase in strains, and at higher pressure ranges, the soil behaves like typical soft clay. This experiment studied the rate of development of strain and pore water pressure and shows that rate is a function of number of cycles, applied stress, and stress history. In addition, soil degradation during cyclic loading is studied in terms of Degradation Index. Attempt has been made to predict stain, pore water pressure, and degradation index through an empirical model.     

Test and Numerical Analysis of the Constitutive Relation of a Diatomaceous Soft Rock

A kind of diatomaceous soft rock and its constitutive model are studied in this article. Based on Yin and Graham's (1999) 3-D elastic viscoplastic constitutive model, the formulation under a triaxial stress state is established, and it is used to simulate the time-dependent stress-strain behavior and effective stress path of consolidated undrained triaxial tests of both normally consolidated and overconsolidated soft rock. The comparison between the model predictions and measured results shows that their agreement is good. This demonstrates that the constitutive equation established here can be used to simulate the time-dependent stress-strain behavior of the soft rock under triaxial stress condition.     

Test and Numerical Analysis of the Constitutive Relation of a Diatomaceous Soft Rock

A kind of diatomaceous soft rock and its constitutive model are studied in this article. Based on Yin and Graham's (1999) 3-D elastic viscoplastic constitutive model, the formulation under a triaxial stress state is established, and it is used to simulate the time-dependent stress-strain behavior and effective stress path of consolidated undrained triaxial tests of both normally consolidated and overconsolidated soft rock. The comparison between the model predictions and measured results shows that their agreement is good. This demonstrates that the constitutive equation established here can be used to simulate the time-dependent stress-strain behavior of the soft rock under triaxial stress condition.     

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.     

Quantifying subaqueous slope stability during seismic shaking: Lake Lucerne as model for ocean margins

Lakes can be used as model basins to investigate subaqueous slope stability under static and dynamic loading conditions. This study combines geophysical, sedimentological and in situ geotechnical methods with limit equilibrium calculations in order to discuss (i) the geological and sedimentological processes acting on submerged non-deltaic lateral slopes in perialpine, fjord-type Lake Lucerne (Central Switzerland); (ii) their control on physical and geotechnical properties that eventually affect the subaqueous stability conditions and slope failure initiation, and (iii) the quantitative assessment of subaqueous slope stability. Three detailed case studies are presented to describe and quantitatively reconstruct stability conditions of slopes that failed during a well-documented historic earthquake in 1601 A.D. and during a prehistoric Late Holocene earthquake around 2220 cal yr BP (both M_w > 6).

Glacio-lacustrine sedimentation dominated by suspension settling from meltwater plumes and slight overconsolidation from ice-grounding during small readvances of a generally retreating glacier lead to a peculiar glacial-to-postglacial lithologic slope succession that eventually was buried by the Holocene sediment drape. During past earthquake shaking, the slopes that were stable under static loading conditions (factor of safety of 1.5–2) failed along planar sliding surfaces that developed at the lithological boundary between fine-grained, thinly-laminated, slightly underconsolidated cyclic plume deposits with low undrained shear strength values above and overconsolidated, glacially-deformed, glacio-lacustrine deposits with excessive formation pore pressure below. Measured in situ shear strength characteristics and sediment geometries were implemented into limit equilibrium models that allow for quantitative reconstruction of critical ground accelerations of past earthquakes in Central Switzerland. Results reveal seismic peak ground acceleration (PGA) of  0.08 g and  0.14 g for the historic 1601 A.D. M_w  6.2 earthquake and the prehistoric,  2220 cal yr B.P. earthquake, respectively. Additionally, results reveal that stability conditions change over relative short geological time scales because the postglacial sedimentation rate, which mainly controls the static weight of the slope sediment acting on the critical lithological boundary, turns out to be a key parameter in “charging” slopes susceptible to sliding.     

Fatigue analysis of offshore well conductors: Part I – Study overview and evaluation of Series 1 centrifuge tests in normally consolidated to lightly over-consolidated kaolin clay

An important aspect of deepwater well integrity assurance is conductor fatigue analysis under environmental loads acting on the riser system during drilling operation. Fatigue damage arises from stress changes in a structure due to cyclic loading. In practice, the lateral cyclic soil response is typically modelled using Winkler p–y springs. An appropriate soil model for conductor–soil interaction analysis is the one based on which the absolute magnitudes of stresses and their changes can accurately be predicted for well integrity evaluation. The API recommendations for p–y curves, which are often used for conductor–soil interaction analysis, have originally been developed for piled foundation and are inappropriate for well fatigue analysis. To that end, an extensive study involving four series of centrifuge model tests and FE numerical analyses was conducted to fundamentally study conductor–soil interaction under a wide spectrum of loading conditions. The tests simulated conductor installations in normally to over consolidated clays, and medium-dense clean sands. Soil models were developed specifically for conductor fatigue analysis for each of the soil types. The test results and soil models are presented in two papers. The first paper, Part I, presents an overview of the study and first series tests in normally to lightly over-consolidated kaolin clay and discusses the observations made with regards to monotonic and cyclic soil resistances and their relationship to conductor fatigue modelling. The second paper, Part II, presents centrifuge test results in normally to lightly over-consolidated Golf of Mexico (GoM) clay, over-consolidated natural clay and medium-dense clean sands along with the respective soil models developed for conductor fatigue damage prediction. Overall, the accuracy of fatigue life predictions using these novel soil models is very high – generally within about a few percentage of the measured values.     

Strength Behavior of Marine Sands at Elevated Confining Stresses

A testing program was initiated to determine the stress-strain and strength behavior of two very different marine sands (a calcareous sediment from South Australia and a siliceous sediment from the United Kingdom) at elevated confining pressures. The testing matrix consisted of a series of isotropically consolidated, undrained (CIU) and drained (CID), triaxial compression tests on samples of naturally deposited calcareous and siliceous sediment and remolded calcareous sediment. It was found that the calcareous samples displayed little cemented behavior during shear. For tests conducted at pressures up to 1.5 MPa, a significant amount of particle crushing occurred in the calcareous samples but not in the siliceous samples. Particle degradation and reorientation facilitates transitions from dilative to contractive behavior with increases in confining stress. The calcareous sediment exhibited contractive behavior at confining pressures above approximately 500 kPa and the siliceous sediment remained dilative at stresses up to 1 MPa during undrained loading. Comparison with data collected by the University of Sydney (CID tests with confining pressures up to 60 MPa) showed that most of the variations in strength behavior occurred within the low stress range (up to 2 MPa) tests conducted at URI. This was evident in the friction angle data and in the reloading Young's modulus data.