Research On The Effect Of Cement-soil Mixing Piles In Strengthening Loess Subgrade Under Rainfall Conditions

Subgrade treatment is a common problem encountered in civil engineering,and cement-soil mixing pile is a common method for subgrade reinforcement.The loess area is widely existed in our country,and long-term continuous rainfall will bring great damage to the subgrade.Therefore,it is necessary to study the stability of the subgrade under rainfall conditions in the loess area.Based on laboratory tests,various performance indices of loess were tested,including particle distribution,liquid limit and plastic limit,maximum dry density,optimal water content,cohesion,internal friction angle,compression modulus,and permeability coefficient.The results provided a basis for subsequent numerical calculations.By analyzing the failure mechanism of cement-soil blocks,the permeability characteristics of unsaturated soil and the principle of subgrade deformation under rainfall conditions,the numerical simulation software ABAQUS was used to study the influence of rainfall infiltration on the stability of loess subgrade.The main research contents were as follows:(1)According to the analysis of the 28d compressive strength of the cement-soil test with different mixing methods,it was showed that the compressive strength of fly ash cement-soil was reduced about 7.42%by the dry mixing method,and that of coal gangue cement-soil was reduced about 7.09%by the dry mixing method,and it was more reasonable to use the wet mixing method in combination with engineering practice.(2)Based on unconfined compressive strength test and scanning electron microscope test on cement-soil of different ages and cement substitution,it was analyzed the effects of age and cement substitution on the strength of cement-soil from both macroscopic and microscopic.Within 28 days,the increase of the strength of cement-soil was positively correlated with the age.As the content of cement substitution gradually increased from 0 to 60%,the strength of the cement-soil decreased continuously.(3)According to the particle analysis test of the soil,combined with the analysis of the permeability characteristics of the unsaturated soil,the SEEP/W module was used to estimate the relationship between the permeability coefficient and the matric suction,it provided parameters for the numerical calculation.(4)The composite subgrade model under the condition of rainfall infiltration was constructed by ABAQUS.The composite subgrade model was consisted of loess and cement-soil mixing pile with different contents of fly ash or coal gangue.The impact of rainfall duration and rainfall intensity on the horizontal displacement and settlement at the foot of the slope was analyzed,and the calculation results showed that the horizontal displacement of the slope foot with piles was greater than the horizontal displacement without piles,and the settlement was less than the settlement without piles.It showed that the cement-soil mixing pile had an obvious vertical enhancement effect on the subgrade.