Experimental Study On Durability And Strength Degradation Mechanism Of Cement-solidified Soft Soil In Coastal Environment Of South China

The coastal areas of south China are distributed with a large area of coastal soft soil.Because of its adverse engineering geological characteristics such as large pore ratio,low strength,low water content and low permeability,it needs to be treated to meet the needs of the project.Hydro-soil soil mixing pile can effectively cure the soft soil,and has the advantages of maximizing the raw soil,which is widely used in the Marine soft soil foundation treatment of coastal engineering.Due to the location in the coastal environment,the large area of soft soil after curing is faced with the structural damage caused by the alternating dry and wet cycle of seawater and salt and fresh water,and the deterioration phenomena such as reduced strength and increased permeability occur,which affect the service life of hydro-soil and solid.This thesis with nansha coastal soft soil as the research object,using ordinary Portland cement as curing agent for curing,the curing soil set pure seawater immersion,seawater dry wet cycle,salt water and freshwater dry wet cycle,through the macro mechanical test and micro-structure and mineral composition analysis,studied the mechanical characteristics of cement curing soil in south China coastal environment complex and strength degradation mechanism.The main research results of this thesis are as follows:(1)ordinary Portland cement can effectively solidify/stabilize the coastal soft soil in south China(river phase silt,low organic matter content),and after the incorporation of ordinary Portland cement,the degree of solidified silt soil sample is significantly improved.Meanwhile,the unlimited compressive strength(UCS)of the cured sample increased with the increase of cement mixing.When the cement mixture was 10%,15%and 20%,the UCS of the cured sample was 1.44MPa,2.90MPa and 3.20MPa,respectively,increasing 0.94MPa,2.4MPa and 2.7MPa compared with the uncured sample.(2)The UCS value of the curing sample decreases with the increase of dry and wet cycles,but it shows different intensity reduction characteristics in different cycle stages and different dry and wet conditions.When the number of dry and wet cycles of curing samples is not more than 8,the UCS decreases rapidly with the increase of dry and wet cycles;When the number of dry and wet cycles is more than 8,the UCS of curing samples decreases slightly with the increase of dry and wet cycles.In addition,the strength deterioration of the solidified sample is the fastest in the dry and wet cycle under seawater conditions,and the next time,the fresh water condition is the slowest.For soaking test,the strength deterioration of the curing sample is significant under seawater immersion condition,and the UCS decreases with the increase of soaking days,but shows different characteristics.When the cement dosage is 10%and 15%,the UCS of the curing sample,the UCS decreases to 9.7%,20%and 20.7%,the UCS when the soaking days exceeds 60d.(3)When the cement content is the same,the partial stress of the solidified silt soil decreases with the increase of the circulation times.Under the same circulation conditions,the partial stress of solidified silt soil increases with the increase of cement content.Under different circulation conditions,the partial stress of solidified silt soil decreases greatly after6 times or 8 times.For example,under seawater dry and wet circulation conditions of 10%cement content sample,after 6 dry and wet cycles,the partial stress under 100KPa is reduced from 1.2MPa to 0.2MPa,a decrease of 83%.In addition,the partial stress and strain curves have different damage forms.It is mainly nuclear crack type and shear crack type,and some samples have slight strain softening characteristics.(4)The action rules of chloride ion and sulfate ion are different in seawater dry and wet cycle and salt water dry and wet cycle.The erosion of chloride ion on the curing sample runs through the whole dry and wet cycle process,and its destruction mode mainly produces the erosion product Friedel salt,while the erosion of sulfate ion is obvious in the initial stage of erosion,which produces sulfate compounds with expansion effect and destroys the internal structure of the curing sample.In addition,the peak strength of salty alternating dry and wet cycle sample is higher than the strength of seawater dry and wet cycle,mainly attributed to the high concentration of Na⁺,Mg²⁺weakened the adsorption and wrapping of hydrophilic gel C-S-H,salty alternating dry and wet cycle,freshwater will dilute chloride ion and sulfate ion,indirectly slow the erosion of seawater,make the strength of salty alternating dry and wet cycle condition higher.From the table to the inside,the content of chloride and sulfate ions gradually decreased,and with the number of cycles,the content of the two ions outside,middle and inside of the sample also increased.