Stiffness Degradation of Undisturbed Saturated Soft Clay in the Yangtze Estuary Under Complex Stress Conditions

Stiffness degradation will occur due to the generation of accumulated pore pressure in saturated soft clays under cyclic loading. The soil static-dynamic multi-purpose triaxial and torsional shear apparatus in Dalian University of Technology was employed to perform different types of test on the saturated soft marine clay in the Yangtze Estuary. Undisturbed samples of the clay were subjected to undrained cyclic vertical and torsional coupling shear and cyclic torsional shear after three-directional anisotropic consolidation with different initial consolidation parameters. Investigated were the effects of the initial orientation angle of the major principal stress, initial ratio of deviatoric stress, initial coefficient of intermediate principal stress and continuous rotation of principal stress axes on the stiffness degradation. It is found that the degradation index decreases （or degradation degree increases） significantly with increasing initial orientation angle of the major principal stress and initial ratio of deviatoric stress. Compared with the effects of the initial orientation angle of the major principal stress and initial ratio of deviatoric stress, the effect of initial coefficient of intermediate principal stress is less evident and this trend is more clearly reflected by the results of the cyclic torsional shear tests than those of the cyclic coupling shear tests. At the same cycle number, the degradation index obtained from the cyclic torsional shear test is higher than that from the cyclic coupling shear test. The main reason is that the continuous rotation in principal stress directions during cyclic coupling shear damages the original structure of the soil more than the cyclic torsional shear does.Based on a series of experiments, a mathematical model for stiffness degradation is proposed and the relevant parameters are determined.

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 stress FEM method. The model based on the results of cyclic triaxial tests on the reconstituted soft Ariake clay is used to predict the wave induced excess pore water pressure and residual strain of soft clay. The settlements of two types of breakwaters on the soft clay under ocean wave load, a low embankment subjected to traffic load and the tunnel surrounded by soft clay in Shanghai subjected to locomotive load are calculated as examples.

Settlement and Stability Analysis on Soft Clay Under Cyclic Loading

The results of undrained cyclic triaxial tests on three types of clays are collected and a relationship among the accumulated strain, the initial stress state, cyclic stress in the soil, as well as the number of cycles is established based on the experimental data. With this relationship, a procedure is proposed for subsidence and stability analysis on soft clay under the action of cyclic loads.

Non-mechanical behaviors of soft clay in two-dimensional electro-osmotic consolidation

To investigate the soil behaviors in a direct current field on both spatial and temporal scales, a 1: 5 scale model test was conducted in laboratory to simulate the two-dimensional (2D) electro-osmotic consolidation of soft clay foundation. Volume of drainage, intensity, voltage, water content and pH value of water collected in the cathodes were monitored. The pH values of soil and the mass of anodes were measured before and after the test. The test results indicate that the unsaturated state, resultant from fissures induced by the differences in water contents, is favorable to dynamic compaction of soil during electro-osmotic drainage. The results also demonstrate that water content, degree of saturation and electric potential distributions can be used to deduce the electro-osmotic drainage process. Water content of soil decreases first near electrodes, while keeps nearly constant in the center of the model. The area with constant water content is larger than half of the sample surface. Moving anodes towards cathodes by about one third of the electrode spacing is effective to improve the treatment effect after electro-osmosis stops due to the large resistance. Moreover, it is observed that during electro-osmosis, the corrosion rate of anodes becomes smaller, while the variation in pH values of soil near anodes becomes larger.

Stabilization effects of surplus soft clay with cement and GBF slag

Utilization of industrial waste and surplus construction soft clay as construction material was recommended, and many attempts at geotechnical waste utilization were undertaken. This study aimed at the application of cement and a kind of industrial wastes, i.e. granulated blast furnace slag, on stabilization of surplus soft clay. The results showed that the cement and slag can successfully stabilize Ariake clays even though this high organic clay fails to be stabilized by lime and cement. Addition of slag in cement for stabilization induces higher strength than cement alone for longer curing time. The application of the cement with slag is more suitable than cement alone for stabilization because of economical consideration.

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.

Numerical Modelling for Dynamic Instability Process of Submarine Soft Clay Slopes Under Seismic Loading

Marine geological disasters occurred frequently in the deep-water slope area of the northern South China Sea,especially submarine landslides,which caused serious damage to marine facilities.The cyclic elastoplastic model that can describe the cyclic stress-strain response characteristic for soft clay,is embedded into the coupled Eulerian-Lagrangian(CEL)algorithm of ABAQUS by means of subroutine interface technology.On the basis of CEL technique and undrained cyclic elastoplastic model,a method for analyzing the dynamic instability process of marine slopes under the action of earthquake load is developed.The rationality for cyclic elastoplastic constitutive model is validated by comparing its calculated results with those of von Mises model built in Abaqus.The dynamic instability process of slopes under different conditions are analyzed.The results indicate that the deformation accumulation of soft clay have a significant effect on the dynamic instability process of submarine slopes under earthquake loading.The cumulative deformation is taken into our model and this makes the calculated final deformation of the slope under earthquake load larger than the results of conventional numerical method.When different contact conditions are used for analysis,the smaller the friction coefficient is,the larger the deformation of slopes will be.A numerical analysis method that can both reflect the dynamic properties of soft clay and display the dynamic instability process of submarine landslide is proposed,which could visually predict the topographies of the previous and post failure for submarine slope.

Cyclic deformation behaviour of natural K_0-consolidated soft clay under different stress paths

Characteristic of cyclic loading due to passing wheels is associated with one-way loading without stress reversal,which includes a simultaneous cyclic variation of vertical normal stress and horizontal normal stress lasting for a long period of time and generally takes place in partially-drained conditions.Therefore,it is of great practical relevance to study the deformation behaviour according to the characteristic of traffic loading.In this work,a series of one-way stress-controlled cyclic triaxial tests with a simultaneous variation of the vertical and horizontal stress components during cyclic loading were conducted to investigate the deformation behaviour of natural K_0-consolidated soft clay in partially-drained conditions.Test results demonstrate that not only the deviator part of the stress rules accumulation but also the volumetric part significantly contributes.While the deviator part of the stress amplitude is held constant,the increase amplitude of cyclic confining pressure will promote the development of both permanent volumetric strain and axial strain significantly.Furthermore,the effects of cyclic confining pressure on the deformation of natural K_0-consolidated soft clay was quantified.Finally,an empirical formula for permanent axial strain considering the effects of cyclic confining pressure was proposed which can be used for feasibility studies or for the preliminary design of foundations on K_0-consolidated soft clay subjected to traffic loading.

Pore Pressure Accumulation of Anisotropically Consolidated Soft Clay Subjected to Complex Loads Under Different Stress Paths

Owing to different influence factors of foundation soil,the initial stress state of the soil under various working conditions is complex.To simulate this situation,in this paper,a series of tests on undisturbed soft clay under pure principal stress axis rotation were carried out by using the hollow cylinder apparatus(HCA).The influence of initial consolidation angle ζ(the angle between the vertical direction and direction of the applied load in consolidation)and intermediate principal stress coefficient b on pore water pressure accumulation of undisturbed soft clay were mainly studied.The test results show that,during pure principal stress axis rotation,the pore water pressure accumulation of the undisturbed soft clay fluctuates and increases with the rotation of the major principal stress;the values of major principal stress anglesα,corresponding to the peak value of the pore water pressure in a certain cycle,are different with different initial consolidation angles;the pore water pressure accumulation of soft clay is greatly affected by the intermediate principal stress coefficient b.With the fixed initial consolidation angle ζ,the variation trend of the maximum pore water pressure for each cycle is appropriately the same with different b values.With the increase of cycles,the difference value of pore water pressure between b=0 and b=1 in each cycle increases gradually with different initial consolidation angles ζ.While with different initial consolidation anglesζ,the increase of the pore water pressure when b increases from 0 to 0.5 is different with that when b increases from 0.5 to 1;the variation of maximum pore water pressure withζis significantly affected by the value of b;the value of maximum pore water pressure increases with the cycle number increases under all test conditions,but the growth rate decreases gradually.And the variation of maximum pore water pressure with the cycle number N is obviously influenced by both ζ and b.

Theory and Practice of Soft Clay Engineering

-The shear strength and deformation properties of soft clay are discussed first. Then some methods for predicting the performance of soft clay foundation are proposed. Finally, case histories are presented to illustrate some discussed aspects of soft clay.

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.

A Normalized Stress-Strain Model for Soft Clays

-A nonlinear model for the stress-strain behaviour of normally consolidated clays is presented based on the experimental results. It is indicated that the volume strain under pure shear is a power function of stress ratio and the normalized stress-strain curve is a standard hyperbola. According to the model, the coefficient of pore pressure induced by shear stress and the critical stress ratio which governs the influence of the negative dilatancy are suggested. It is shown by some triaxial tests that the proposed model can be used to study the negative dilatancy and to describe the stress-strain-pore pressure adequately for soft clays.

Improve Geotechnical Design Parameter of Some Soft Clayey Soils

The shallow Soft Clayey deposit is common in Alexandria-Egypt. Most soft clays in their natural state are unsuitable for supporting any structure. Thus, improvement treatments exist to strengthen these soils so that improved soil can have adequate bearing capacity without undergoing failure or producing substantial excessive settlement post construction and applied loads to them. This paper presents a case study of an improved site in the city center, which reclaimed part of Maryout Lake, where the highly compressible clay with water content varies from 200% near the surface to 90% at the base of the shallow clay deposit. A prefabricated vertical drain with preloading has been used to improve this soft soil. Values of shear parameters and consolidation coefficient back-calculated from field measurements and have been compared with the values from lab and in situ tests. The study provides different relationships from comparisons of prediction and estimation compressibility and consolidation settlement from laboratory studies and particularly field case studies. Also, some correlation related to the compressibility with index properties of soft clay is presented. The results display that a substantial improvement is noticeable in the compressibility properties.

Inflow and outflow permeability tests in a very soft clay under low stresses

In situ inflow and outflow permeability tests with the BAT probe at SarapuíII soft clay test site are presented.A description of the BAT permeability test is provided,discussing its advantages and shortcomings,especially in the case of very soft clays under low stresses.Pore pressures were monitored during probe installation and were found to be slightly lower than piezocone u2 pore pressures,consistent with the position of the filter.The role of filter tip saturation was investigated after the usual saturation procedure provided an unsatisfactory pore pressure response during probe installation.Results show that the vacuum saturation procedure provides adequate response during installation and increases the reliability of the coefficient of permeability determination in early measurements.Both inflow and outflow tests yielded similar results,indicating that careful execution of the test can lead to good test repeatability regardless of the loading condition.Various sequences of alternated inflow and outflow tests have yielded similar results,indicating that soil reconsolidation and filter clogging were negligible in the tests performed.Data are presented concerning the relationship between index parameters and the in situ coefficient of permeability for SarapuíII clay,which plot outside the range of existing databases.

Case study of post uplift in deep excavation of a subway station in thick soft clay using long pile foundations

There is growing engineering concern about the base heave and post uplift phenomena in deep excavation in soft clay,which may pose a risk of instability of retaining systems.The purpose here is to conduct a detailed case study on the post uplift observed in a 17.6-meterdeep braced excavation of a subway station in thick soft clay(total thickness up to 42 m)in Shanghai.In this case,a large uplift up to 87 mm unexpectedly developed for the post founded on a 30-meter-long pile foundation.Efforts were first made to examine the complex relationships between the post uplift with the excavation depth(H),Terzaghi’s safety factor against base heave(Fs)and maximum deflection of retaining wall.A simplified approach for soil-post-strut interaction analysis was then proposed and used for quantitative research.The working characteristics of the long pile foundation under low safety factor against base heave(Fs<1.5)are summarized as following:(a)the back-analyzed neutral plane,where soil uplift equals the post uplift,lies at approximately 0.68 times the pile length from the pile top;(b)deep soil movement below the neutral plane results in the observed post uplift;(c)strut reaction plays a minor role in the restriction of post uplift.The influence of base treatment and excavation/construction procedures on post uplift and the principles of pile foundation design are also discussed in this paper.

Numerical investigation of the ultimate lateral resistance of piles in soft clay

The paper presents a numerical study on the undrained lateral response of a single, free-head, reinforced concrete pile in soft clays. Soil conditions simulating normally consolidated clays are examined undrained shear strength increasing with depth--and the pile-soil interaction under static lateral loading is analyzed. The nonlinear p-y curves proposed in literature for soft clays are imported into a beam-on-nonlinear-Winkler-foundation simulation in order to predict the pile head lateral load--displacement curve and the distribution of the horizontal displacement and bending moment along the pile. The striking differences among these methods require further investigation via 3D finite element analyses. The determination of the ultimate soil resistance Pult from the results of the finite element analyses aims at providing the estimation of a range of values for the ultimate soil resistance coefficient Np with depth and the comparison of the derived values to the corresponding ones proposed by existing methodologies.

A New Method of Combination of Electroosmosis, Vacuum and Surcharge Preloading for Soft Ground Improvement

As a rapid and effective ground improvement method is urgently required for the booming land reclamation in China＇s coastal area, this study proposes a new combined method of electroosmosis, vacuum preloading and surcharge preloading. A new type of electrical prefabricated vertical drain （ePVD） and a new electroosmotic drainage system are suggested to allow the application of the new method. This combined method is then field-tested and compared with the conventional vacuum combined with surcharge preloading method. The monitoring and foundation test results show that the new method induces a settlement 20% larger than that of the conventional vacuum combined with surcharge preloading method in the same treatment period, and saves approximately half of the treatment time compared with the vacuum combined with surcharge preloading method according to the finite element prediction of the settlement. The proposed method also increases the vane shear strength of the soil significantly. The bearing capacity of the ground improved by use of the new proposed method raises 118%. In comparison, there is only a 75% rise when using the vacuum combined with surcharge preloading method during the same reinforcement period. All results indicate that the proposed combined method is effective and suitable for reinforcing the soft clay ground. Besides, the voltage applied between the anode and cathode increases exponentially versus treatment time when the output current of power supplies is kept constant. Most of the voltage potential in electroosmosis is lost at electrodes, leaving smaller than 50% of the voltage to be effectively transmitted into the soil.

Deformation mechanism of roadways in deep soft rock at Hegang Xing’an Coal Mine

Engineering geomechanics characteristics of roadways in deep soft rock at Hegang Xing'an Coal Mine were studied and the nature of clay minerals of roadway surrounding rock was analyzed. This paper is to solve the technical problems of high stress and the difficulty in supporting the coal mine, and provide a rule for the support design. Results show that mechanical deformation mechanisms of deep soft rock roadway at Xing'an Coal Mine is of ⅠABⅡABCⅢABCD type, consisting of molecular water absorption (the ⅠAB -type), the tectonic stress type + gravity deformation type + hydraulic type (the ⅡABC -type), and the ⅢABCD -type with fault, weak intercalation and bedding formation. According to the compound mechanical deformation mechanisms, the corresponding mechanical control measures and conversion technologies were proposed, and these technologies have been successfully applied in roadway supporting practice in deep soft rock at Xing'an Coal Mine with good effect. Xing'an Coal Mine has the deepest burial depth in China, with its overburden ranging from Mesozoic Jurassic coal-forming to now. The results of the research can be used as guidance in the design of roadway support in soft rock.

Probabilistic seismic design of pile-supported building structure incorporating inherent variability of subsoil and ground motion uncertainty

Seismic failure of structures supported on pile foundation has revealed the importance of seismic soil-foundation-structure interaction(SSFSI)for ensuring safe design.The uncertainties in subsoil properties and seismic loading may lead the problem to be more redundant.In this context,the present study attempts to assess the seismic reliability of pile foundation-supported building structure embedded in inhomogeneous clay layer considering inertial interaction.Shear strength of clay and earthquake loading is considered as spatially variable uncertain parameters.A non-linear soil-pile-structure system was assumed,and Monte Carlo simulation(MCS)was adopted to obtain probabilistic response of the system.First-order reliability method(FORM)is used for reliability assessment.The study indicates significant influence of uncertain parameters on the seismic response of building structure.Further,the influence of material and load uncertainty parameters on the probabilistic seismic response of structure designed following older version of code is higher than counterpart structure designed following recent version.FORM based reliability analysis infers thatserviceability criterion may be the governing parameter for pile foundation design.Moreover,the study also indicates that the curvature ductility demand of pile may be considered another crucial design parameter to assess the reliability of pile foundation.

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

During the construction of the guiding dike in the Yangtze Estuary, some of the caisson structures sank into the soil for 1 -5 m or slid about 20 m away from the original place when a strong storm attacked this area. Dynamic triaxial tests were carried out to simulate the behavior of foundation soils under wave loading. The test results show that the excessive settlement and lateral movement of the caissons are due to the weakening of the soft clay strength during the strong storm. The test results also show that the ability of the soft clay to resist the wave force will be greatly increased when the soft soil samples are suitably improved. Based on the test results, an improvement method combining vertical drains with surcharge loading was proposed to strengthen the foundation soil. On the improved soil foundation, the dike has been reconstructed and undergone several strong storms without any damage. A finite element approach has been developed for analyzing the settlement and stability of the dike under the action of strong storm.