Study On The Effect Of Seawater Intrusion On The Compressive Strength Of Soil And Cement And Its Meso-mechanism

In recent years,with the continuous expansion of the scale of sand-blown reclamation projects,dealing with the weak foundation under the dredger-filling site has gradually become one of the most difficult problems.Cement-soil is a cost-effective soft soil foundation reinforcement material.The cement-soil mixing method developed on this basis has significant advantages such as fast construction and low price.Therefore,it is widely used in foundation treatment projects in coastal areas.However,in the case of seawater intrusion,the corrosive ions in groundwater have a greater impact on the mineral composition and mesostructure of cement soil,which in turn affects its compressive strength.Therefore,it is very meaningful to use different test methods to study the mechanical properties of cement-soil under the environment of seawater intrusion.This article takes the soft soil foundation treatment project of Beibu Gulf in Guangxi as the research background.Through the unconfined compressive strength test to study the influence of different concentrations of seawater and age on the mechanical properties of cement soil,combined with X-ray diffraction test,ion chromatography test and scanning electron microscopy test,the influence mechanism of the compressive strength of cement-soil is revealed from the two aspects of mineral composition and mesostructure.Based on the above work content,the main research conclusions of this paper are as follows:(1)Compressive strength test: Through the unconfined compressive strength test,when the age is less than 60 d,the compressive strength and elastic modulus of cement-soil under the action of seawater environment both increase and then decrease with the increase of age.,The peak strain shows a trend of first decreasing and then increasing.The compressive strength and elastic modulus of cement soil increased with the increase of age under the action of0.5 times the seawater environment and freshwater environment,while the peak strain decreased with the increase of curing age.In addition,the compressive strength of cement-soil is the largest under 0.5 times the seawater environment,the next time it is in the freshwater environment,and the smallest under the seawater environment.Finally,based on the basic theory of damage mechanics and the Weibull distribution function,a piecewise damage evolution equation of cement-soil is established.Aiming at the elastoplastic deformation in the stress-strain curve of cement-soil under different test conditions,Combining the unconfined compressive strength test results,the parameters of the evolution equation are determined,and the relationship is established with the test variables to obtain a damage model considering seawater erosion.(2)The influence mechanism of mineral composition: Through X-ray diffraction test and ion chromatography test,the mineral composition of cement-soil with different seawater concentration and age was analyzed.The erosion effect of corrosive ions on cement-soil can be divided into swelling erosion and dissolution erosion.Expansive erosion mainly targets gypsum and ettringite in the soil.Dissolution type erosion mainly targets hydrated magnesium silicate(M-S-H)and hydrated calcium silicate(M-A-H).In the seawater environment,when the age t<15.51 d,the promoting effect of swelling erosion on compressive strength is greater than the inhibiting effect of dissolved erosion.Therefore,in this age range,the strength of the cement soil in the seawater environment is greater than that in the freshwater environment and 0.5times the strength in the seawater environment.When the age is t>15.51 d,the excessive ettringite will produce swelling stress to gradually disintegrate the soil.At this time,swelling erosion becomes an unfavorable factor.Furthermore,under the dual effect of dissolving erosion,the strength of cement soil decreases with age.Under 0.5 times the seawater environment,when the age t<60 d,the expansive erosion has the effect of enhancing the strength of the soil throughout the age range.In addition,the swelling erosion has always been greater than the inhibitory effect of dissolved erosion on the strength of cement soil.Therefore,under the setting of three kinds of seawater concentration environment,the strength of cement soil is the largest in the 0.5 times seawater environment.(3)The influence mechanism of pore structure: Scanning electron microscopy experiments on cement soils with different seawater concentrations and ages found that under seawater environment,with the growth of age,the internal structure of cement soil gradually changed from a dense flocculent shape to a loose structure with bare particles and pores penetrating.Cement-soil deteriorates seriously,and the average pore area and apparent pore ratio increase,which in turn leads to a decrease in compressive strength.Under 0.5 times the seawater environment,the cement-soil changes from flocculation to a dense gel network structure,the water tightness increases,the pores decrease,the apparent void ratio and the average perimeter of the pores are reduced,resulting in an increase in the compressive strength of the cement soil.