Experimental Research On Polypropylene Fiber Improving Fly Ash Soil Of Roadbed Under Freeze Thaw Cycles

The strength and stability of roadbed is essential for normal service of highway. Only we fully grasp its influencing factors when we carry out roadbed design in seasonal frozen region, we can guarantee it. In this thesis we conduct investigation and research on the disease of road in seasonal frozen region, we find that the instability of roadbed is the mainly influencing factor of road disease. In order to reduce the harmful effect, we put forward the method to use fly ash soil improved by polypropylene fiber as roadbed filling. Its dynamic strength property is analyzed by dynamic triaxial test.Currently, the research on fly ash soil improved by randomly distributed fibers is uncommon. In this paper we conduct the contrast of dynamic strength and modulus of fly ash soil before and after improvement by use of freeze-thaw cycle and dynamic tri-axial tests in order to study the influence of randomly distributed fibers on the property of fly ash soil. In order to study the dynamic characteristics of fly ash soil, it is necessary to understand the dynamics mechanism of soil and soil liquefaction. The second chapter introduces the theory of soil and frozen soil mechanics. As we all know, the strength of material is dependent on its shear capability.The data shows that the shear strength of frozen soil is related to many factors, its formula is a function with at least three variables as follows:τ= f ( ?θ, p , t).And its ultimate strength under plane shear is normal pressure-related, that is the strength is dependent on not only cohesive stress but also internal friction.It can be described by normal pressure and a variable parameter that related to the time of negative temperature and load when the soil is under lower pressure. The function is as follows:τnp = cθ, t + tg?θ,tp.The experimental results of frozen soil ultimate shear strength shows that the strength increases with the temperature of frozen soil owing to the improvement of internal friction angle and cohesive stress.This paper uses the dynamic load test of tension and compression symmetrical sinusoidal frequency of 1 Hz. We need to study the momentum indicators such as dynamic modulus, dynamic strength, clay cohesion and internal friction angle. This paper uses the hyperbolic curve as the dynamic strain - dynamic stress curve described by the equationσd /εd= 1/(a +bεd). By Coulomb's law, we can see that the ability of clay to resist reciprocating movement under cyclic loading largely depends on the effective internal friction angle and effective cohesion, as well as the growth of the pore water pressure.In the third chapter, we conduct the dynamic triaxial test of the improved fly ash soil. The conclusions are as follows: the dynamical modulus, the cohesive stress, the dynamic strength of the soil is respectively improved by 25.64%, 37.11%, 8.37%.And we also found that the dynamic strength increases with the increasing of confining pressure and decreases with the increasing of the number of failure cycles.After the analysis, the main reason of the improvement of strength is that the randomly distributed fiber can increase the effective cohesion. According to the formulaτ=σtan ?+c,theσincreased after the reinforcement of fibers, so the shear strength is improved too. On the other hand, the shape of fibers in the soil is crooked. When the soil suffers external force and the fiber is in a tensile station, the soil will endure compression and friction at the excavated side. At the same time, there are many crossing point of the randomly distributed fibers, these fibers interact with each other to generate a spatial force area. These all play the role of reinforcing the soil. As through the fiber reinforced soil tri-axial test Equivalent additional stress method advanced by Li Guang-xin, we can see that in the axial pressure, adhesive force will occur between fiber and soil, and the greater the pressure, the more closely fiber and soil contact. So if increasing the lateral pressure, with the result that the soil compression as well as shear strength will increase.In the fourth chapter, we conduct the dynamic triaxial test of the reinforced soil after several freeze-thaw cycles tests, the results are as follows: the dynamic strength of reinforced soil tend to be stable after six freeze-thaw cycles.When the freeze-thaw cycle reaches 8,the strength increases with the increasing of confining pressure. the strength is improved by 15.2% when the confining pressure increase from 200KPa to 300KPa and is improved by 35.87% when the confining pressure increase from 100KPa to 300KPa.When the confining pressure is respectively 100KPa, 200KPa , 300KPa,the strength decline rate of reinforced soil is 14.42%,21.94% and 19.35%, while the strength decline rate of fly ash soil is 35.27%,31.04% and 24.96%.This suggests that the strength of reinforced soil have little lose after several freeze-thaw cycles,this material has a good property of frost-resisting durability and is a ideal road material in seasonal frozen region.According to the research, we find that the dynamic shear strength, dynamic modulus of the fly ash soil reinforced by polypropylene fiber have been substantially improved .So randomly distributed polypropylene fibers reinforced fly ash soil is an effective measure to increase the strength of the fly ash soil. But we still don't have perfect formula to refer to so further study and improvement is needed.