| The soft clay is recognized as clay with low undrained shear strength(s_u<50 k Pa),high compressibility,large void ratio,and natural water content higher than the liquid limit.The depositions of these clays are largely found in China as well as all around the world.The soft clays demonstrate a complex structural behavior;thus,their quantitative experimentation requires a large number of identical natural specimens;the collection of such specimens is indeed an expensive and challenging task.The artificial structured clays could be an alternative and cost-effective choice.Shanghai soft clay from the 4th layer of stratum was selected as basic material for this research.The current study proposes a method to regain the soil structure of natural soft clays from a disturbed state.The advantages of this method are the liberty to reconstitute the large-size specimens for laboratory and centrifuge modeling,reduction of the need for expensive natural specimens,and allowance of the reuse of obtained natural specimens for physical and mechanical testing.The main contents and achievements of this study are as follows:(1)A reconstitution method is proposed for artificial structured clays which based on the generation of bonding(cementation)due to mixing of cement in clay at low temperature,and applicability of proposed reconstitution method is verified by using the natural soft clay.The bonding among clay particles is an unstable component in soil structure,and is easier to disintegrate than fabric.The combination of cement and consolidation is used to generate cementation,similar to natural soft clay.Commonly,the void ratio of reconstituted clay is less than the natural soft clay.The addition of low cement content increases the void ratio due to flocculation as well as generates bonding.The consolidation pressure during reconstitution is used to add supplementary strength in soil structure.The 4th layer Shanghai soft clay was used to make the clay slurry.The clay-cement slurry was 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.Initially,the batches of artificially structured specimens were reconstituted with 1-6%cement content under the in-situ stress of 98 k Pa,and on behalf of tests results of these batches,the following batches of specimens were reconstituted with 1-3%cement content under pre-consolidation pressure(50 k Pa)of selected natural soft clay for detailed study.Based on the macro-and microstructure investigation,the influence of cement content on strength characteristics,compression characteristics,and other indicators are summarized to expand the application range of the research.(2)The mechanical characteristics of artificial structured specimens like unconfined compression strength,shear strength ratio,deformation modulus,yield stress,compression index,and swell index are investigated and compared with the natural specimen by performing unconfined compression and oedometer tests.Furthermore,the mechanical behavior of natural and artificial structured specimens is also investigated and compared under different triaxial loading and drainage conditions.The conventional consolidated undrained triaxial tests results show that with increasing cement content,the deviatoric stress ratio,excess pore water,and M-value have been increased;however,the effect of curing time on these parameters is insignificant at such low cement content.The drained true triaxial tests are performed to compare the mechanical behavior regarding shear strength,volumetric strain,and principal strains of natural and artificial structured specimens under three-dimensional loading conditions with(15)°to(21)(15)°lode angle.The final values of shear strength and volumetric strain against the deviatoric strain decrease with the increment of lode angle.Under the aforementioned testing conditions,the general mechanical behavior of the specimen with 2%cement is observed to be analogous with the natural soft clay in macro level investigation.However,only the deformation modulus,excess pore pressure,and initial void ratio of the specimen with 1%cement are equivalent to the natural specimen.(3)The scanning electron microscope(SEM)and mercury intrusion porosimetry(MIP)tests are chosen to investigate the soil structure development in artificial structured specimens.As per SEM analysis,the natural and artificial structured specimens have an open type dispersed structure with high silt content and different sizes of inter-aggregate pores.The effect of 1%cement on soil structure is not detected;however,some signs of reticulation and cementation are observed in case of 2%and 3%cemented specimens.Furthermore,the total pore volume of artificial structured clays increases with increasing cement content,and the main influence of cement is detected on the pores<10?m.However,the increment in curing time increases the pores>10?m.The pore size distribution on the natural specimen is identical to 2%cemented specimen.(4)An elastoplastic constitutive model is also proposed for artificial structured clays,and verify with the results of oedometer,undrained triaxial,and drained true triaxial tests.The model can simulate the tests results,using a single set of material parameters.The initial degree of structure,initial over-consolidation and their behaviors under different loading conditions are also predicted by this model.The model simulated results of structural and over-consolidation behaviors of both natural and 2%cemented specimens are observed to be similar to the tests results.The proposed reconstitution method substantiates that natural soil structure of soft clays can be regained up to a satisfactory limit.In the future,it is hoped that artificial structured clay specimens reconstituted using proposed method will use in various indoor geotechnical experiments,which will lay the foundation for revealing the relationship of soil structure and mechanical characteristics of natural soft clays. |
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