Research On Settlement Characteristic Of Saturated Loess Composite Foundation Reinforced By Cement And Fly Ash

With more and more railway engineering constructed in the loess area, reinforcement and treatment of loess foundation have become an important problem in the course of the construction of railway engineering. The construction process of foundation treatment methods have continuously improved, ensuring the construction quality and expanding the application range. In the past research mainly concentrated in collapsible loess foundation treatment aspect, and also made a lot of research results, but research of saturated loess foundation in railway engineering construction was less. Reinforcing saturated loess foundation by cement-soil mixing piles is a common foundation treatment methods, but the study of this method is not perfect at present, and regional experience is scanty, so bearing characteristics and settlement deformation characteristics of composite foundation formed by cement mixing saturated loess remain to be studied.Relying on Lanzhou west railway engineering, settlement of cement-soil mixing pile composite foundation under the embankment load was observed. Through the analysis of the measured settlement data of cement-soil mixing pile composite foundation under embankment load, influence of different blending ratio on the settlement of composite foundation was studied. Using different settlement prediction methods to make curve fitting analysis of the measured settlement of composite foundation with, we obtained suitable prediction methods of cement-soil mixing pile composite foundation settlement on condition of saturated loess, and predicted the final settlement of composite foundation. Settlement control effect of cement-soil mixing pile composite foundation was analyzed and settlement deformation characteristics of cement-soil mixing pile composite foundation under embankment load was discussed. By using numerical simulation software FLAC3 D we analyzed regular that settlement of cement-soil mixing pile composite foundation changes with the upper load comparing with the regular of the measured settlement development. The main conclusions are as follows:(1) As the railway embankment filling, settlement of cement-soil mixing pile composite foundation is increasing. When embankment filling is completed, settlement of composite foundation changes slowly, and gradually tends to a stable value. The settlement of composite foundation in same section is different, and the settlement of the center of the subgrade is slightly larger than the center of the left line.(2) Lanzhou West to Zhongchuan Airport railway engineering test section used cement-soil mixing piles to reinforce saturated loess foundation by cement and fly ash(referred to lime-ash) as curing agent. The study on the test section designed three different blend ratio of lime-ash of cement-soil mixing piles, and the analysis of the observed settlement data showed: lime-ash blend ratio is higher in a certain range, the settlement of soil-cement mixing pile composite foundation is smaller, the reinforcement effect of foundation is more significant, the settlement control effect is higher.(3)After analysis of settlement observation data, we had curve fitting analysis of the measured settlement using the logarithmic curve method, hyperbola method, three points, Asaoka method and quadratic polynomial curve method. Through comparing these several methods on curve intends collection effect, relative error, the sum of variance and regression correlation coefficient, it was concluded that the three point method has the highest precision, double curve method and Asaoka method followed by. Therefore, we used three points method to analyze settlement forecasting of settlement observation points, and compared and validated with double curve or Asaoka method.(4)Predicting the final settlement of cement-soil mixing piles composite foundation by three-point method, hyperbolic method and Asaoka method, for the sake of safety, bigger predictive value was selected as the final settlement of the point. It obtained the final settlement of composite foundation by cement-soil mixing piles with lime-ash blending ratio of 16%—the center of left line is 116.1mm, and the center of the subgrade is 114.7mm. It showed that the settlement of saturated loess foundation reinforced by cement-soil mixing piles with lime-fly ash blending ratio of 16% can meet the standard required for foundation settlement after construction of this railway. In addition, the settlement of cement-soil mixing piles composite foundation primarily occurred within the embankment and filling and 6 months after it’s completion, accounted for about 80% of final settlement, and the settlement of composite foundation has been basically completed when 24 months after construction of embankment.(5) The modeling analysis of simulating model on the working condition by numerical simulation software FLAC3 D concluded that the settlement of cement-soil mixing piles composite foundation increases as the embankment load increases. Due to the influence of complex and uncertain factors on composite foundation, thus it’s changing laws has a little bit inconsistent with the measured settlement. The settlement is the largest at the center of subgrade, and decreases outward gradually. With the increase of lime-ash blend ratio, the settlement of composite foundation is reduced, and when it continues to increase if it increased to a certain value, the subsidence control effect is not significant.