Study On Crack Development And Dynamic Characteristics Of Rubber Reinforced Expansive Soil Considering Temperature Gradient

Expansive soil problems exist in a wide range of frozen soil areas in China,which causes huge losses every year.At present,there are many ways to improve the expansive soil.In this thesis,waste tyre rubber is used to regenerate the expansive soil reinforcement material.Expansive soil is tested in three aspects: crack evolution mode of rubberreinforced expansive soil,pore change characteristics in temperature change and dynamic characteristics of rubber-reinforced expansive soil.Based on the study that rubber can improve the crack evolution of expansive soil,the dynamic characteristics of rubberreinforced expansive soil with different rubber forms and rubber content under temperature change are further considered.At the same time,in order to determine the mechanism of action of temperature on expansive soil,the pore evolution characteristics of expansive soil during unidirectional freezing and freezing-thawing cycles are explored by using Nuclear Magnetic Resonance(NMR)equipment and the failure mechanism of expansive soil under temperature is summarized.The research provides theoretical basis for the improvement of expansive soil bearing dynamic load in cold region by using regenerated rubber,and has certain engineering significance for highway and railway construction in Northeast China and Tibet.The main research achievements in this thesis are as follows:(1)According to the different thickness of soil sample,there are different evolution trends of cracks in expansive soil.The fissure evolution of expansive soils shows obvious three-stage change during the drying and cracking process,which is suitable for fitting Boltzmann curve and predicting the fissure evolution trend of expansive soils.With the increase of soil sample thickness,there is an evolutionary feature of wider cracks and fewer cracks in expansive soil.The effect of rubber reinforcement on thickness effect of expansive soil is obvious.After rubber reinforcement,the crack of expansive soil tends to be smaller and more uniform,and the thickness effect decreases.(2)Temperature has significant influence on the porosity and unfrozen water content of expansive soil.During the monotonous cooling process,the NMR test results show that the pore of expansive soil has double-peak evolution characteristics.Most of the pore of expansive soil decreases,but the residual large pore size pore is caused by water loss shrinkage of expansive soil.The unfrozen water content of the expansive soil during unidirectional freezing is in accordance with the power function variation.During the freeze-thaw cycle,the pore in the melted state of expansive soil decreases with the increase of the number of freeze-thaw cycles,and the pore in the frozen state decreases first and then increases with the number of freeze-thaw cycles.(3)Modification of expansive soil with appropriate rubber proportion and form can increase its dynamic load resistance.The dynamic properties of rubber-reinforced expansive soils at different temperatures were determined by using three temperatures:20 ℃ ,-5 ℃ and-15 ℃ .The results show that the effect of 10 % rubber content is better at normal temperature and the rubber reinforced expansive soil with 5% rubber content at low temperature can withstand dynamic load better.From the point of view of particle size,smaller rubber particle size has better effect on soil improvement.The backbone curves of plain expansive soil and rubber reinforced expansive soil under different temperatures are suitable for H-D model fitting.(4)The dynamic shear modulus of rubber-reinforced expansive soil increases with decreasing temperature,and can be improved at different test temperatures by using appropriate rubber additives.However,excessive rubber additions during negative temperature test will result in a decrease of dynamic shear modulus.Damping ratio of rubber-reinforced expansive soil is more affected by temperature.In positive temperature measurement,the damping ratio increases with the change of dynamic shear strain,while in negative temperature environment,the damping ratio decreases with the change of dynamic shear strain.At the same time,for very low temperatures(-15 ℃ ),hardening of the rubber material reduces the damping effect of the modified soil.(5)Rubber reinforced expansive soil has significant advantages in the project of frozen soil area.Rubber itself can limit the expansion and contraction of expansive soil.At the same time,rubber modification can make the cracks of expansive soil evolve uniformly and provide certain energy consumption.It is a method of expansive soil improvement with broad application prospects.A certain proportion of rubber can be added as cushion in expansive soil area,which has broad application prospects in highway,railway and other engineering construction.