Research On Forming Characteristics Of Tapered Stiffening Core Cement-soil Composite Column

Based on the analysis and summary of the concrete core column and wedge column,Task Group put forward a new technique for strengthening soft soil foundation-tapered stiffening core cement-soil composite column.This paper uses the combination method of indoor soil test,large size model experiment and PFC2 D numerical analysis.Relatively systematic research on forming characteristics of tapered stiffening core cement-soil composite column,analyse the change regulation of cement-soil surface displacement,column strain of the inner core and cement-soil solumn,penetration force,and ect.the main research work and achievements include:1.Carried out indoor compressive strength tests to research the law of development of compacted cement-soil compressive strength.The results show: the strength of compacted cement-soil increases with the increase of age and mixing ratio,and the numerical relationship between the three was obtained by regression analysis;compacted cement-soil axial stress increases first and then decreases with the strain,and the axial peak stress increases with mixing ratio and age increasing;at the same time,mixing ratio and age influence and change the failure mode of cement-soil.When the cement content is low,the early strength of cement-soil is low,the plastic collapse occurs.With the mixing ratio and curing period increasing,failure mode gradually transformed into brittle failure.2.Based on the similarity theory,design model test to research the forming characteristics of tapered stiffening core composite column with cement-soil.The research shows: 1)The penetration force is influenced by the type of pile,wedge angle and replacement rate.When other conditions are the same,the penetration force decreases with the increase of moisture content of cement-soil,but increases with the increase of the inner core replacement rate,the wedge angle and the average diameter.Under the same conditions,the penetration force required to press into the wedge core is larger than that of the inner core;2)When the inner core with uniform cross section is pressed,the surface of cement-soil column will appear "uplift".The larger the diameter of the inner core and the replacement rate,the more obvious the phenomenon of "uplift".And the maximum surface uplift appears in the range of 1d~2d,which is about 1%~1.5% of the columun diameter;when the wedge core is pressed,the surface displacement of cement-soil columun increases continuously,and the increase amplitude increases with the increase of wedge angle,replacement ratio and water content of cement-soil;3)The pressure both at the bottom of the cement-soil column and at the end of the inner core increases with the increase of the vertical displacement of the inner core.They are concerned with the replacement ratio,water content of cement-soil and column type;4)The strain of cement-soil column increases first and then decreases with the increase of depth.The peak value appears in the depth of column and the peak strain increases;For the inner core with uniform cross section,the depth of the peak strain will continue to fall with the inner core sinking,and it is always within the range of about 10 cm,the distance from the end of inner core;compared with the inner core of the wedge and the uniform cross section,the strain is larger when the wedge core is pressed under the same conditions.3.Based on model test results,PFC2 D software was used for numerical analysis,and the influence of replacement ratio and column type of the inner core on the forming characteristics was discussed.Analysis shows: the lateral deformation of cement-soil is caused by pressing the inner core.When the replacement ratio is the same,the larger the diameter of the inner core(or the average diameter)is,the greater the lateral deformation is.Compared with the inner core of uniform cross section,the inner core of the wedge makes the lateral deformation of the cement-soil bigger,and the column forming effect is more obvious;The resistance of the inner core is composed of the column end resistance and the column side resistance,and both of them increase with the increase of the vertical displacement of the inner core in the process of column forming;The ratio of column side resistance(about 80%~95%)is much larger than that of the column end resistance in the process of pressing the wedge core.