Study On The Influence Of Cations In Pore Water On The Bound Water Of Marine Cohesive Soil And Its Mechanical Effects

With the development of the times,especially in coastal areas,the rapid economic development and the gradual expansion of space required for human life and production have had an impact on marine cohesive soil areas that cannot be ignored.The high water content and high compressibility of cohesive soils make the normal use of above-ground as well as underground buildings a great challenge.In the practice of civil,energy and environmental engineering,the chemical components of pore water solutions also have a crucial influence on the mechanical behavior of cohesive soils.For example,when saline pore fluid developed in the leachate of landfill comes in contact with the highly plastic clay liner,the resulting change in the properties of the soil increases the permeability of the clay liner,which may cause malfunction of the liner.There is also the problem of stabilization of oil and gas wells arising from the entry of drilling fluids into the soil.Thus,it seems that the physical and mechanical properties of cohesive soils under changing pore fluid chemistry conditions are of great practical importance.Due to the special characteristics of cohesive soils,their clay mineral composition and grain size composition have a great influence on the state of water presence in the soil,and the surface adsorption of bound water has a crucial role in the stability of cohesive soils.The thickness,type and existence of bound water directly affect the physical and electrochemical properties and mechanical properties of cohesive soils,such as plasticity,contraction and expansion,electrical properties and strength.The variation of bound water content is influenced by its own mineral composition,particle morphology and size,and also by the chemical composition and concentration of the medium.These factors interact at different scales,making the final behavior of cohesive soils quite complex.Cohesive soil particles are usually negatively charged,so the effect of pore water solution cation changes on the bound water content as well as mechanical behavior of cohesive soils is worthy of deeper study.Therefore,it is very important to investigate the effect of pore water solution cation change on the adsorption of bound water and mechanical effect of cohesive soils with certain mineral composition,and it is an important theoretical basis for studying the mechanical properties of cohesive soil and solving soft soil engineering problems.This paper selects Qingdao marine cohesive soil near the Yellow Sea and Ningbo marine cohesive soil near the East Sea in China as the research objects.By changing the type and concentration of cations in the pore water solution,we combine the macroscopic and microscopic levels,and use experimental tests and theoretical analysis to obtain the change of the content of adsorbed bound water in marine cohesive soils and explore the effect of the change of inorganic salt cations on the mechanical strength of cohesive soils.In this paper,the following aspects of work were carried out to study the influence of cations in pore water on the bound water content and mechanical effects of marine cohesive soils:（1）The basic physical,chemical and hydraulic properties of natural marine cohesive soils in Ningbo and Qingdao were tested and analyzed,and the chemical composition of their groundwater and surrounding sea was investigated to provide a basis for the selection of cation types and concentrations of pore water solutions.（2）The particle size composition of the cation-activated marine cohesive soil was tested,and microstructure and pore characteristics of cation-activated marine cohesive soil were analyzed by SEM（scanning electron microscopy）and MIP（mercury-in-pressure）tests.（3）Determination of bound water content and differentiation of bound water types after the action of cations using isothermal adsorption and thermogravimetric analysis.（4）Compression tests and triaxial consolidation and drainage tests on cohesive soil after the addition of different aqueous solutions of cations to obtain the variation patterns of their parameters of shear strength and compressibility.（5）BP neural network was used to establish the model of the relationship between the basic physical,hydraulic and microscopic parameters and mechanical parameters of the marine cohesive soil after the action of cations in pore water.The research results of this paper are as follows:（1）The pore water cations disperse the particle agglomerates of cohesive soils,as shown by the increase in the content of cohesive grains,the clay flake minerals are arranged in an edge-face manner,showing a flocculated structure,and the structure of cohesive soils is gradually compacted with the increase in concentration.The dominant clay mineral in Ningbo and Qingdao clay minerals is illite.Under the weak acidic environment of chloride salt,the surface of clay flake minerals carries negative charge and both ends carry positive charge.The overall effect of cations is Ca²⁺>Mg²⁺>K⁺>Na⁺.The diffusion of clay particles into the pore space,under the effect of the attraction between the lamellar minerals in the edge-face contact,showing a"poker house"type,so that the structure of the clay to the flocculation structure,the large pores of the cohesive soil is filled.The results showed that the content of macropores（4-40μm）decreased and that of medium pores（0.4-4μm）increased.With the increase of concentration,the fractal dimension of pore morphology of cohesive soil increases,which proves that the spatial complexity of cohesive soil becomes larger.（2）It is mainly the increase of clay content and the development of flocculation structure in cohesive soils that make the cohesive soils with increased bound water content under the action of cations in pore water.Combined with the results of the bound water test for cohesive soils.It was found that the increase in mucilage content and the development of flocculation structure increased the specific surface area of cohesive soils and the adsorbable bound water content,which was proportional to the activity index A’,and the flocculation structure also facilitated the retention of more water in the pores.The parameters of adsorbed bound water mass,volume,density,and assumed thickness of adsorbed bound water film of cohesive soil particles at different relative water-gas equilibrium pressures were obtained according to the isothermal adsorption method.It was found that with the increase of cation concentration and cation exchange capacity,the volume V_w and thickness h of adsorbed bound water increased（but in the weakly bound water adsorption stage,the larger the hydration radius the smaller the thickness h）,and the densityρ_w of adsorbed bound water increased（in the strongly bound water adsorption stage,the densityρ_w of adsorbed bound water fluctuated around 1.3 g/cm³）,and the densityρ_hc of hydrated cohesive soil decreased,and the larger the concentration,the larger the corresponding decrease.（3）The increase of clay content,the development of flocculation structure and the increase of bound water content under the action of pore water cations are determining for the improvement of compressive deformation capability and the increase of shear strength of the cohesive soil.The experiments in this paper set the cohesive soil under a certain volume skeleton,i.e.,the water content and dry density conditions are kept constant,the type and concentration of cations in the pore water are changed,the increase of the bound water content of cohesive soil,the increase of the complexity of the clay particle arrangement and the more compact structure of the cohesive soil make the drainage channels unobstructed,and the resistance of the cohesive soil to compressive deformation is increased,which is expressed by the decrease of the compression and resilience index.The increase of adsorbable bound water content of cohesive soil means the relative decrease of free water and weakly bound water content between particles,the increase of both inter-particle attraction and bound water viscosity increase the value of cohesion,the flocculation structure also makes the inter-particle arrangement more complex and the contact points between particles increase,the cumulative effect is additive,that is,the increase of cation concentration improves the shear resistance,which is expressed as the rise of internal friction angle.（4）A prediction model of parameters of shear strength and compressibility of cohesive soil was established based on BP neural network.After the action of cations in pore water,24 sets of specimens were taken as the object of study,and 10 representative engineering geological parameters were selected:initial moisture content,dry density,and percentage of dominant clay minerals of cohesive soil,cation species and cation concentration;clay particle content and fractal dimension of pore morphology;strong and weak bound water content and free water content,etc.These parameters were taken as independent variables,and the parameters of shear strength and compressibility of cohesive soil obtained from triaxial shear and compression tests were selected as dependent variables.The shear strength and compressibility parameters obtained from triaxial shear and compression tests were selected as dependent variables for constructing the model,which reflects the mapping relationship from microscopic properties,physicochemical properties to macroscopic mechanical properties of cohesive soils.The BP neural network predicted parameters of the shear strength and compressibility of cohesive soils with high accuracy and good performance.