| Expansive soil is known as "engineering cancer" due to its super expansion and contraction,which has caused many disasters to the local area.Therefore,it is necessary to improve the expansive soil to reduce various disasters caused by it.Coal gangue is the largest solid waste in coal mining area.A large number of coal gangue piles not only do great harm to the environment,but also seriously endanger people’s health.Fiber reinforced soil is a new type of reinforced composite material developed in recent years.with the development of productivity,a variety of artificial synthetic fibers are gradually applied to soil improvement and have achieved good results.In order to solve the problems of expansive soil in practical engineering,and comprehensive treatment of coal gangue pollution pile problem,and try to use soil reinforcement technology,this thesis studies the use of polypropylene fiber and coal gangue to improve expansive soil.And considering that in winter in the north,expansive soil as subgrade filler,will be subject to the effect of frost heaving and thawing.Therefore,it is necessary to study the mechanical properties of fiber-gangue modified expansive soil under the action of freeze-thaw cycles.In this thesis,coal gangue and polypropylene fiber are mixed into expansive soil and the optimum content is determined by free expansion test and no-load expansion test.Then the direct shear test and unconfined compressive strength test under freeze-thaw cycle are carried out on plain expansive soil,polypropylene fiber and coal gangue modified expansive soil.Finally,COMSOL Multiphysics is used to conduct the water-heat-force three-field numerical simulation on the plain expansive soil and the fiber coal gangue modified expansive soil,so as to further confirm the improvement effect of the fiber and the coal gangue on the expansive soil.It was concluded as follows:(1)adding polypropylene fiber and coal gangue powder composite material can effectively improve the swelling and shrinkage properties of expansive soil,and finally get the optimal content of coal gangue is 20%,the optimal length of polypropylene fiber is 9mm,and the optimal content is 0.3%.Compared with the plain expansive soil,the no-load expansion rate of the composite modified soil is reduced by about 49.5%.(2)After 12 times of freeze-thaw cycles,the decrease amplitude of the cohesion of plain expansive soil is 87.7%,while that of fiber-coal gangue composite is only 46.9%,and the cohesion is greatly improved.The internal friction angle of the improved expansive soil is 10.44 after 12 times of freeze-thaw cycles,which is still larger than the internal friction angle of the plain expansive soil when it is not frozen and thaw.The addition of coal gangue greatly increases the friction between soil masses and has a good effect of inhibiting the frost heaving damage of soil mass.(3)The unconfined compressive strength values of plain expansive soil and fiber coal gangue modified expansive soil before freezing and thawing are 277 k Pa and 581 k Pa,respectively.After 12 times of freezing and thawing cycles,the strength values drop to160 k Pa and 385 k Pa,respectively,indicating that the addition of polypropylene fiber and coal gangue greatly increases the unconfined compressive strength of expansive soil and improves the expansion and contraction characteristics of expansive soil.(4)The mathematical model of frost heaving and thawing settlement of frozen soil is established by COMSOL Multiphysics software,and the migration process of water in soil is explained by using the thin film water migration theory,and the displacement,volume strain and stress of soil column under freezing and thawing are qualitatively and quantitatively analyzed,in which the stress range of plain expansive soil is 556 k Pa to1366.6k Pa;The stress range of expansive soil with fiber coal gangue is 1159.7k Pa to2118.6k Pa.The stress value increased by 752 k Pa and 603.7k Pa respectively.It is proved that the improvement effect of expansive soil by adding fiber gangue is obvious. |
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