Mechanical Properties And Numerical Simulation Of Fiber Modified Expansive Soils Under Freeze-thaw Cycles

Expansive soils are known as"engineering cancers"due to their strong expansion and contraction characteristics,which can easily cause slopes in the area to collapse or form roadbed subsidence and other diseases.With the development of productivity,fiber reinforcement technology is a new type of composite material developed in recent years,various fibers are gradually used in soil improvement research,and achieved very good results.This thesis examines the use of three fibers to improve the engineering properties of swelling soils by attempting to apply soil reinforcement techniques.Considering that in the northern winter,the expansive soil as road base filler is subject to freeze-thaw cycles and loadings,the strength change of expansive soil is significantly affected by freeze-thaw cycles,and it is necessary to study the strength change law and strengthening effect of fiber modified expansive soil under the action of freeze-thaw cycles.In this thesis,fibers of different types,doses and lengths were incorporated into the expansive soils,and the optimum ratio of fibers was determined by measuring the load expansion rate,unload expansion rate and expansion force of the fiber-improved expansive soils at each ratio.The freeze-thaw cycle test,the shear strength test and the unconfined compressive strength test of the expanded soil were compared with the freeze-thaw cycle test and the unconfined compressive strength test of the expanded soil with polypropylene fibers at the optimum mix ratio,and the mechanical strength enhancement of the expanded soil with fibers was analysed using finite element numerical simulation techniques:(1)The addition of polypropylene fiber,basalt fiber and glass fiber can effectively improve the swelling and shrinking properties of expanded soils,and the effect of polypropylene fiber improvement is better,and the optimum amount of polypropylene fiber is determined to be 0.3%and the optimum length is 12 mm.the unloaded swelling rate,400 k Pa loaded swelling rate and swelling force of polypropylene fiber improved expanded soils under the optimum ratio are reduced by 38.8%,118.2%and 17.2%respectively.(2)The incorporation of polypropylene fiber has a significant effect on the improvement of the unconfined compressive strength of the expanded soil,from153.5k Pa to 212.7k Pa,an increase of 38.6%,and the stress-strain relationship curve is of a softening type;freeze-thaw cycles cause freeze-thaw damage to the plain expanded soil and the modified expanded soil and lead to deterioration of their mechanical properties,which has a significant effect on the soil.When the length of polypropylene fiber is12mm and the dosing amount is 0.3%,the freeze-thaw damage is the smallest and the effect of freeze-thaw resistance is the best.(3)The mechanical properties of polypropylene fiber-amended expanded soil after freeze-thaw cycles were numerically simulated by using finite element numerical simulation method,and the stress-strain changes of the fiber and soil were analyzed by combining the fiber toughening theory.The rest of the lengths or doses are mainly for toughening.The strength deterioration law of modified expanded soil after freeze-thaw was simulated with different fiber parameters,and the strength prediction function of polypropylene fiber reinforced expanded soil was established,and the results showed that both plain expanded soil and fiber modified expanded soil conformed to the freeze-thaw strength deterioration functionY=a-bc~x.