| High-speed railway has been in phase of large-scaly constructing, operation and maintenance in China, and how to ensure the safty of high-speed railway during long-term operation has become a serious problem. The requirements of foundation structure in strength, stability and durability under train load, gravity and environmental factors in the long-term operation of high-speed railway are much higher than that of the ordinary railway; For subgrade structure, the control of post construction settlement development is the key to keep the track structure in good geometric smoothness, provide long-term stability and maintain the safty during operation of high-speed railway; The deformation with time effect of natural clay foundation caused by the upper load is the main source of post construction settlement of the subgrade. As medium-low compressibility soil, engineering property of which is very good, is widely distributed in China, and reaserches about deformation and strength of it are so deficient. Therefore,reasonably calculating the thickness of the compressible stratum of foundation consisted of medium-low compressibility soil, is the core technology of controlling post-construction settlement of subgrade and ensure the long-term service performance of high-speed railway. Meanwhile, it also has great practical significance to optimize the foundation design of high-speed railway, and to obtain foundation treatment plan that unificating technology and economy.On the basis of summarizing, analyzing and absorbing the existing research results,with the goal of mastering time effect characteristic of long-term deformation of foundation consisted of medium-low compressibility soil under embankment load in high-speed railway, taking element filling model test, triaxial creep test and geotechnical centrifuge model test as technical means, reaserches on criterion of deformation evolution state, the thickness of compressible stratum and K30 test have been studied.The main work and achievements are as follows:1. Analysis and test reaserches on evolution state characteristics of time effect of soil deformationBased on small-size plate loading test of element filling model, which is in optimum moisture content and in compaction degrees of 90%, 95% and 100%, the evolution law ,changing with load, of time-effect characteristics of filling soil is been analyzed,and evolution state of soil deformation is been divided into four categories of fast convergence, slow convergence, slow divergence and fast divergence; Based on the attenuation law of negative power function of v(t)?t-? between deformation rate and time, fractal characteristic during convergence type decay creep is revealed, so that relationship of fractal dimention D and the power a is derived, what is more, threshold value ?-1 and ?-2 are obtained, respectively used to discriminate the deformation is convergence or divergence and wehter the deformation is with time effect or not; Based on mutation characteristic of the curve in shape of reversed"S" between a and loading level ?l , deformation in convergence state is divided into three subclass-states of time effect of weak, obvious and significant, according to the two phase transition of ?? and a?,"Power Criterion" that quantitatively distinguish soil deformation evolution state is put forwarded, according to ??, ?=1 and ?=0, which are thresholds of the subclass-states of weak and obvious, convergence and divergence deformation and attenuating or not of deformation rate; Via element filling model test and triaxial creep test, the time-effect characteristics of filling soil is been analyzed, so that three thresholds of loading level ?l?,?l? and ?l? under K30 test condition and three thresholds of deviatoric stress level of ?q?,?q? and ?q?, through which deformation state strength parameters are obtained referencing "Mohr-Coulomb crirerion".2. "time-effect method" to calculate the thickness of compressible stratum of foundation in high-speed railwayBased on deformation state strength parameters of soil, the crirerion equation of deformation states are estabilished using the deformation state equilibrium condition of point stress; Combining the distribution characteristics of dead-weight additional stress in foundation under the embankment load, the deformation state of foundation is divided into the four categories of fast divergence, slow divergence, slow convergence and fast convergence along the depth; In view of subgrade engineering aiming at post-settlement controlling, “time-effect method",taking the no-fast convergence layers with obvious time effect as compressible stratum of foundation, is put forwarded; taking the foundation soil strength that can ensure the thickness of compressible stratum no more than 3-5m as foundation condition of shallow layer treatment, the shear strength control equation,which makes the deformation of foundation under the ballastless track of high-speed railway double track embankment in fast convergence state, is established, and then, the shear strength controlling indexs meeting foundation condition that corresponding to embankment hight are disscussed, as well as the highest embankment that certain foundation can bear.3. Analysis on the influence of the embankment load on thickness of compressible stratum of foundation based on geotechnical centrifugal model test Using the technology of centrifugal model test, the deformation evolution state of medium-low compressibility soil foundation, the upper 10m of which is reinforced, under embankment load is reaserched, and the influence of the embankment load on thickness of compressible stratum of foundation is disscussed. Experiments show that, deformation rate attenuation ? reduces with the embankment hight H showing rapidly firstly and slowly subsequently, the time effect degree is positive correlation with embankment load;Using "Power Criterion",combining the mutation characteristic of the curve in shape of reversed"S" between a and H, it can be inferred that, when the H is beteween 4-5m,the thickness of compressible stratum of foundation is almost 10m, while H increases from 3m to 6m or 9m, the thickness of compressible stratum of foundation streches more than 10m from less than 10m; the influence law of the embankment load on thickness of compressible stratum of foundation that concluded from test matches the result that calculated by "time-effect method", Therefor the rationality of "time-effect method" is effectively verified.4. Deformation State characteristics of silt clay during loading process of K30 test Based on the data of small-size plate loading test, the change of the deformation state of the soil during the loading process of K30 test and the influence of the loading stability time are discussed. Research shows as followings: The evolution trend of deformation sub-class state is presented from weak to apparent, and when the settlement is more than 1.25mm, the deformation state changes from fast convergence into slow convergence, and the time effect of soil becomes obvious; To make sure the deformation is convergence, and the settlement is no less than 1.25mm, for loading system of five steps and 0.04MPa loading increment, it is suggested that the K30 value in the detection of compaction quality of fine grained soil filler should be between 60 -160MPa/m; According to the two control standards of "deformation in final 1min no more than (1%) "and "deformation rate (?s /min)”,step loading time respectively showed two different trends of shortening after increasing and accelerated increasing with time effect coefficient during K30 test, and the latter is more in line with the positive correlation between stability time and time effect of soil deformation; As is 0.01mm can reduce test time of 51%-58% with test error less than 6%, so that, "step-time controlling method" is proposed, namely loading time TN (min) can be obtained by loading step N as TN=2N+1?... |
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