| Soft soil, distributed widely in our country, has the characteristics of low shearing strength, large deformation. If soft soil was treated in mistaken way, foundation would come into being local shear failure or whole shear failure. It is a virtual importance for soft-soil foundation to deal with, in order to guarantee freeway to run safely. The paper discusses some aspects below to the effective pile-length of cement power-sprayed piles in the background of the engineering of Nan-zhu highway of Nan-sha economic development area in Guangzhou at the entrance of zhujiang.In the paper the main work is carried out about the above. Base on the research progress of cement power-sprayed piles, which is used to deal with softy foundation of highway, the chemical and mechanical mechanism is explored and clarifies the necessity of improving soft-soil foundation. Afterwards, the author studies the mechanism of cement power-sprayed piles composite foundation from the chemical reaction between cement particle and soil grain. First effective length of cement-mixed single pile is computed by elastic theory - Mindlin Equation, then the relations between effective length of single pile and the modulus ratio of pile to soil, substitution ratio, the stress ratio of pile to soil, poisson's ratio, the diameter of pile are analyzed. At last its numerical simulation is conducted by large and common finite element software named Adina. It's results show that the effect of poisson's ratio on effective pile length is unobvious and the effect of other's factors on effective pile length is very obvious. In particular, by field loading tests and small strain measurement, the author studies the relationship of n (the stress ratio of piles to soil) to load p and summarizes the factors to effect n and puts forward the theory of calculating the value of n by the modified value of modulus of compressibility k, witch affords available method for engineering construction.
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