Study On Flocculation And Filtration Characteristics Of Waste Slurry From Shield Tunneling

The amount of waste slurry from high water content pipe jacking is huge every year,and it is urgent to find a method to quickly reduce it."Flocculation pretreatment combined with mechanical filtration" method is a better method for treating this type of high water content slurry.However,due to the presence of additives such as bentonite,sodium carboxymethyl cellulose(CMC),polyanionic cellulose(PACE),pre-gelatinized starch(PGS)added in advance in the pipe jacking waste slurry,this makes the properties of the pipe jacking waste slurry more complex,which leads to the traditional "Flocculation+mechanical filtration" process in the treatment of pipe jacking waste slurry,A series of engineering problems have been encountered: the efficiency of the dehydration treatment project is low,and the water content of the dehydration product is still on the high side.Aiming at these engineering problems,this article summarizes and sorts out the existing scientific issues in the research on flocculation and filtration of pipe jacking waste slurry: the impact of additives and flocculation treatment methods in the slurry on the flocculation and filtration characteristics of the slurry and the mechanism are unclear.In response to these scientific issues,this article has carried out experiments such as flocculation and vacuum filtration,sedimentation,zeta potential,centrifugal separation,one-dimensional consolidation,and electron microscope scanning.The scientific issues have been studied from macro and micro perspectives,respectively.The specific research content and conclusions are summarized as follows:(1)In view of the influence of residual admixtures(bentonite,CMC,PACE and PGS)in the shield slurry on the flocculation characteristics of waste slurry and the unclear mechanism,according to the composition of the shield slurry in the actual project,this paper simulates the preparation of the shield project waste slurry with bentonite,CMC,PACE and PGS as the main residual admixtures,and uses the flocculation combination of Aluminium chlorohydrate(PAC),anionic polyacrylamide(APAM)and cationic polyacrylamide(CPAM),Conduct flocculation tests on the prepared waste mud.Measure the specific resistance(SRF)of the mud after flocculation and the moisture content of the mud cake after filtration through tests such as filtration,sedimentation,zeta potential,and centrifugation(ω Re).Moisture content of concentrated mud layer(ω SL),zeta potential and other indicators to study their impact and mechanism of action.Research has found that the presence of residual additives such as bentonite and polymers can worsen the flocculation and dehydration results of shield tunneling waste mud.With the increase of CBen,CCMC,CPACE,and CPGS in the mud,SRF increases by one to two orders of magnitude,ω Re increases by two to five times,and mud dehydration gradually becomes difficult.After using PAM to treat waste mud,the improvement in its dehydration effect is relatively limited,and SRF is still at the same level as the control group,ω Re is still as high as 170%～333%;The use of PAC and composite flocculation groups to treat waste mud achieved good dehydration results,with a decrease of one order of magnitude in specific resistance,corresponding to ω The minimum Re can be reduced to 66%.The mechanism by which bentonite and polymers worsen the dewatering results of mud filtration is related to their own properties.After adding bentonite to the slurry,a thick bound water film is formed on the surface of the bentonite particles,which blocks the permeable channels between the clay particles and reduces the dehydration efficiency;After adding polymer additives to the mud,their molecular chains will first adsorb onto the surface of soil particles through hydrogen bonding,which makes it difficult for subsequent PAM molecular chains to adsorb onto the surface of soil particles,thereby reducing dehydration efficiency;PAC has a flocculation mechanism of electrical neutralization,which can greatly improve the high potential characteristics of shield tunnel waste slurry,thereby greatly improving the dehydration characteristics of shield tunnel waste slurry;When PAM is used to treat mud,although there is a certain adsorption bridging flocculation mechanism that can cause mud to aggregate,it still cannot change the high potential characteristics of the mud.The mud has high stability and is difficult to dehydrate.(2)In response to the issue of unclear effects and mechanisms of residual additives in the slurry on the consolidation characteristics of the slurry,this article simulates the preparation of waste slurry for shield tunneling based on the composition of the slurry in actual engineering,with bentonite,CMC,PACE,and PGS as the main residual additives.PAC,CPAM,and APAM are used to conduct flocculation tests on the prepared slurry..Determine the compression coefficient(a1-2)and structural yield stress of waste mud through tests such as consolidation,zeta potential,and electron microscopy scanning(σ p)Consolidation coefficient(Cv),zeta potential,and scanning electron microscopy(SEM)were used to study their effects and mechanism of action.Research has found that the presence of additives in the mud can lead to changes in the consolidation characteristics of the mud.As the content of Cben,CCMC,CPACE,and CPGS in the mud increases,a1-2 increases by 46.03% to 55.37%,σ P increases by one to three times,while Cv decreases by 6.08% to 56.81%.The change mechanism of mud consolidation characteristics is related to the nature of the admixture itself.Bentonite has cation exchange and water swelling characteristics,which will increase the Compressibility and structure of the mud and bring about a decrease in the consolidation coefficient;The existence of polymer makes the mud form a network polymer structure,and the smaller soil particles in the mud gather to form a skeleton structure,which increases the Compressibility and structure of the mud and brings about a decrease in the consolidation coefficient;PAC and PAM respectively have the mechanisms of electrical neutralization and adsorption bridging,which can cause smaller soil particles in the mud to aggregate and form flocs,thereby increasing the pores in the mud and improving the dehydration characteristics of the mud.Ultimately,this is manifested as an increase in the mud compression coefficient,structural yield stress,and consolidation coefficient.