| Based on the project of SDDC treatment for collapsible loess foundation in Bailu Plateau,Baqiao,this paper summarizes the application status of DDC and SDDC treatment for collapsible loess foundation.In-situ tests were carried out in the study area,and the changes of compaction coefficient and gravity subsidence coefficient before and after soil treatment were compared and analyzed.The lateral compaction law and influence range of SDDC construction method under certain design parameters were obtained,and the optimal pile spacing suitable for the site was determined.Based on PFC software,the influence of different factors on lateral compaction effect and reinforcement scope of soil was studied.The specific research results are as follows:(1)DDC and SDDC construction method in the foundation treatment of collapsible loess region,the piles are usually filled with plain soil,2:8 lime soil,3:7 lime soil,1:5 cement soil,gravel soil and residual soil.Compared with DDC method,SDDC method has more ramming energy,diameter of hole,diameter of pile,spacing of pile and better treatment effect.Commonly used tamping energy of 800～3240kJ;The pore diameter is 0.4～1.2m.Pile diameter is 0.6～1.8m;The pile spacing is 0.9～4.5m,which is 2～3 times of the hole diameter.Pile length is basically consistent with collapsible loess depth of the site.The arrangement of piles is equilateral triangle.(2)Through borehole sampling test,the characteristics of stratum structure in the study area are as follows:non-collapsible Q3 loess within 4m below the surface,weak and medium collapsible Q2 loess between 9-12m,weak collapsible Q2 loess and non-collapsible Q2 loess interval distribution in soil layer below 12m;The maximum dry density of the test site is 1.64g·cm-3,and the optimal water content is 18.1%.(3)The single pile test of in-situ SDDC treatment foundation shows that the soil compaction effect of pile side is uneven with depth.The effective reinforcement range of pile side soil within the range of 1～5m below the surface increases from 0.5m to 1.0m,the effective reinforcement range of 6～8m soil layer is 2.5～4.0m,the effective reinforcement range of 9～10m at the bottom is about 0.5m.The compaction effect of soil 6～10m below the surface is the best,and the compaction coefficient of soil 0～4m away from the pile axis is increased by 11～21%.(4)Through the PILE group test of SDDC construction method,it is found that the foundation treatment effect is better when the pile spacing is 2.8～3.2m in the range of treatment depth.At 1～5m below the surface and 9～10m at the bottom,the compaction coefficient of each pile spacing is not up to the standard,and the soil effect between piles at 6～8m below the surface is up to the standard.(5)Based on PFC software,the reinforcement effect of SDDC single pile treatment on loess foundation under different influencing factors is studied.The numerical test considers eight influencing factors,including particle size ratio Rr,particle size Rp,soil initial porosity n,shear modulus G,Poisson’s ratio v,compaction energy E and pore diameter D.The influence law of the average variation of porosity Nave,effective reinforcement width B and influence depth H after compaction is analyzed,and the empirical formula between them is obtained.(6)Based on particle flow PFC software,the reinforcement effect of SDDC construction method pile group treating loess foundation under different influencing factors is studied.The numerical test considers the influence of pile spacing L,pile diameter D,tamping energy E and other factors on the foundation treatment effect.The results show that the soil compaction effect between piles is negatively correlated with pile spacing L and pile diameter D,and positively correlated with tamping energy E.The empirical formula of soil compaction effect between piles is put forward. |
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