| The soft marine clay,distributed in North of Jiangsu,comprises montmorillonite minerals and high pore water salinity.In practice,the safety index is generally based on the in-situ soil strength in a stable environment.However,there could exist a potential risk,such as slope instability,the bearing capacity of foundation failing,and deformation increasing,without considering porewater salinity changes.To have a comprehensive understanding of marine clay,it is crucial to study on the salinity effect on hydro-mechanical behaviour of this material.In this study,the physical properties,consolidation behavior and shear strength of clays with a variety of porewater chemistry were investagted,and the major conclusions are listed as following:(1)To investigate the salinity effect on grading size distribution of clay,the laser analyser test and hydrometer test were conducted.The results show that increasing of ion concentration lead to a flocculation of clay particles and the median particle size increment.From the sedimentation test,it is found that the bentonite present a ’zone settling’ in NaCl solution,while a ’dispersed free settling’ in distilled water.Meanwhile,the sedimentation rate of bentonite was promoted in NaCl solution.According to Storck’s law,it indirectly proved that the high ionic concentration could cause clay particle size increasing.(2)The liquid limit of kaolinic clay is not apparently changed with salt concentration,in contrast,that of montmorillonitic clay significantly increased with ionic concentation increasing.When the liquid limit was high,a larger change of it with NaCl concentration was identified.The main mechniasm could be that ionic strength compressed the thickness of diffused double layer between clay particles,and water-holding capacity decreased.The relationship of wL(c)/wL(c=o)and cNaCl could be normalized to an empirical expression.(3)The induced swelling pressure and deformation of bentonite were measured,when the samples were injected by distilled water or different solutions(NaCl,MgCl2,CaCl2).Owing to different hydration radius,the swelling pressure and deformation decreased with ionic concentration and cation valence of injected liquid increasing.(4)The experimental desiccation tests were conducted on an initially slurry soil layer.The constant evaporation rate decreased with fraction of bentonite increasing.Besides,the increasing of porewater salinity promoted the evaporation rate for clays containing bentonite.By applying image processing techniques,the total area(Acrack),the maximum width(dmax),and the total length(L)of the crack were quantitatively described.The geometric or morphological characteristics of kaolinite were not influenced by pore water chemistry,while the total area(Acrack)and the maximum width(dmax)of crack decreased with pore water salinity increasing for clay containing bentonite.After wetting and drying cycles,the cracks of clays containing bentonite developed along the initial cracks.For kaolinite,there is a significantly difference of morphological characteristic before and after wetting-drying cycles.(5)With the NaCl concentration increasing,the Zeta potential of clay suspensions changes from-40～-35mV to-15～-10 mV.Due to the relationship between Zeta potential and colloid stability,the colloid tend to floccuate in high NaCl concentration.Therefore,the main mechanism attributed to salinity effect on physical properties was proved.(6)From the results of oedometer test,it is observed that the compression index C,of bentonite-kaolinite mixture exponentially decreased with the osmotic suction,while the swelling index was almost constant with salinity.Thus,an empirical relationship of Cc versus WL was proposed based on these observations.The compression behaviors after pre-yielding can be normalized in the logσ’v-Iv system proposed by Burland(1990),but it could be not appropriate before pre-yielding.(7)For the bentonite-kaolinite mixture,the starting of secondary consolidation and the ending of primary consolidation advanced with pore water salinity increasing.There is a negative correlation between the secondary consolidation coefficients and the osmotic suctions at an identical void ratio,furthermore,the ratio of the secondary consolidation coefficient to the compression index,Ca/Cc,decreased with the osmotic suction.(8)At an identical initial void ratio and applied pressure,the higher the osmotic suction of pore water is,the higher the permeability coefficient of montmorillonitic clay.The coefficient of Ck =Δe/Δlgk proposed by Tanenas(1983)is not only dependent on initial void ratio,but also pore water chemistry.(9)During the percolation procedure,a larger secondary consolidation was observed for the salt intrusion process.After percolation,the compression curves of two processes(desalination and salt intrusion)became close to each other.The initial permeability was larger for the samples blended with salt solutions.The permeability k decreased in the desalinition process and increases in the salt intrusion process during the percolation period,which can be explained that the higher osmotic suction of NaCl solutions and larger inter-aggregate pores.(10)Two methods were adopted to prepare samples using in tri-axial test:a large-scale oedemeter(15cm in height,20 cm in diameter)to pre-consolidate the soft clay;soil were pre-compressed to attain a certain dry density.For the pre-consolidation and remodeling of kaolinic samples,there is a slightly difference between the internal friction angle φ of them,whereas that of bentonite-kaolinite mixtures increased with higher pore water salinity.The increment of the internal friction angle owing to pore water chemistry was summarized as an empirical function of osmotic suction or liquid limit(represent the water holding capacity).For bentonite-kaolinite mixtures,the slope of critical state line in p’-q plane(M)increases with higher pore water salinity,while the slope of the normal consolidation line(NCL)and critical state line(CSL)λ in v-lnp’ system decrease with NaCl solutions.In contrast,M and λ were not significantly influenced by pore water for kaolinite.(11)The microstructure observation(SEM test)shows that:smaller aggregates were formed after normal consolidation;flake and acicular montmorillonite minerals tend to be aggregated when they were mixed with high salinity water.Soils formed by particles with a shape of bulky pellet conld have a better connectivity between pores,thus a higher permeability and a shorter consolidation time.MIP test results indicate that a ’mono-model’ was appreciated for samples prepared by two mentioned methods,and the pore size distribution curves were not significantly influenced by pore water.(12)A constitutive model of soft clay considering pore water salinity was proposed based on the following assumption:the influence of of osmotic suction on the yield surface was dependent on the current state of stress;the osmotic suction could be taken as the increment of current stress state;during osmotic suction was induced,the soil was firstly compressed along the compression line with a slope of λ(0),then linearly swelled with a slope of κ(π),finally compressed along the compression line with a slope of λ(π).By analyzing the sensitivity of parameters,the critical stress ratio M(π)mostly dominate the calculation results of this model.(13)With the help of SIGMA/W software,the critical slope safety of an embankment was calculated under three condition:filling process of the embankment,changes of seepage flow and coupled with salt migration.It was found that the critical slope safety reduced after salt migration,thus the potential risk caused by salt migration was verified. |
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