Research On The Mechanical Properties Of Seasonally Frozen Soil Based On Accurate Temperature Measurement And Control

The seasonally frozen region covers more than half of the territory of China.Roads,railways and other transportation infrastructure are widely distributed on the seasonally frozen soil.The subgrade in the seasonally frozen region is subjected to cyclic effects of freezing and thawing during the service period,and its strength,stability and durability gradually decrease because of the deterioration of filling.The subgrade is susceptible to excessive plastic deformation and instability under the traffic load,especially those heavy vehicles,causing diseases in the pavement.Laboratory experiments are often used to investigate the mechanical properties of subgrade soil in seasonally frozen areas,and there are many research results about the evolution of soil mechanical properties after freeze-thaw cycles.However,there are still many shortcomings in terms of the influence of specimen states and test conditions on soil properties,especially the accuracy of temperature measurement and control.In this thesis,the change law and influencing factors of the static and dynamic properties of seasonally frozen soil are taken as the research objective.Laboratory tests were conducted,during which the initial state of the soil specimens and the testing conditions were fully considered.The static properties of frozen soil in the sensitive temperature range among which the mechanical properties of frozen soil change dramatically,and the static and dynamic properties of soil after freeze-thaw cycles under the most unfavorable conditions with the effects of multiple factors were investigated.The research work and achievements are concluded as follows:(1)A new method of precise temperature measurement and control for the frozen soil triaxial test was proposed.The specimen temperature was accurately measured and controlled without the blind zone and time lag by this way.The calibration formulas and parameters of the temperature sensor inside the specimen,the temperature sensor on the surface of the specimen and the one inside the freezing chamber were given.And the setting table for the specimen temperature was given,too.By performing comparative tests,the reliability of this system and method was verified from three aspects: freezing efficiency,temperature stability and the test results.(2)Under the precise temperature measurement and control,the change law of mechanical properties of seasonally frozen soil in the sensitive temperature range was studied.The effect of freezing time on the static properties of frozen clay was given.The change law of stress-strain relationship,elastic modulus and compressive strength of the frozen clay in the sensitive temperature range were revealed,and the triaxial compressive strength calculation formula for frozen clay was established.(3)Unconsolidated and undrained triaxial tests were conducted on the seasonally frozen soil which undergoing freeze-thaw cycles.The most unfavorable thawing duration when the strength of frozen soil is at a low value after thawing was given.The effect of negative temperature,initial moisture content and compaction degree on the stress-strain relationship,elastic modulus,static strength and shear strength parameters of the seasonally frozen soil after freezing and thawing cycles were revealed.The concept and determination of the most unfavorable conditions for seasonally frozen soil were proposed.(4)Through triaxial tests conducted on seasonally frozen soil under the most unfavorable conditions,the change law of stress-strain relationship of the seasonally frozen soil after freezing and thawing cycles was given.And the influence of freezing and thawing on the elastic modulus was given.The development of static strength of seasonally frozen soil with freeze-thaw cycles were obtained,and the change law of shear strength parameters of seasonally frozen soil was revealed(5)On the basis of the most unfavorable conditions after freezing and thawing cycles,dynamic triaxial tests were conducted.The influence of dynamic stress amplitude on the dynamic properties of seasonally frozen soil was given.The change law of dynamic characteristics with vibration frequency was pointed out.The effect of confining pressure on the dynamic properties of seasonally frozen soil was obtained,and the dynamic characteristics change of seasonally frozen soil with freeze-thaw cycles was revealed.The research method of this thesis will provide a reference for the study of mechanical properties of other soils in the seasonally frozen area.The results have enriched the knowledge about mechanical properties of the seasonally frozen soil.Meanwhile,it will provide references for the mechanism exploration of roadbed diseases,the road design and disaster prevention and mitigation in the seasonally frozen regions.