| The area of seasonal permafrost accounts for about 50% of the total area of China.During the process of freezing and melting,roadbeds and foundations of construction generally undergo some engineering diseases like frost heave and thaw settlement,directly leading to structural instability and engineering disasters,and definitely large economic losses.In order to counterbalance the negative impacts of frost on engineering,it is pressing to discover modified materials that can effectively improve the performance of frost resistance of soils.This thesis,therefore,takes two categories of materials including Nano-silica and basalt fiber as soil amendments,aiming at exploring their effects on the improvement of the frost resistance of clay.Based on direct shear test and unconfined compressive strength test,this thesis comparatively analyzed to what extent the frost-thaw cycles affect the shear strength,compressive strength and failure can pattern of several kinds of soils,such as plain soil,Nano-silica modified soil and Nano-silica basalt fiber modified soil.Meanwhile,the mechanisms of how frost-thaw cycles influence the performance of modified-soils were also discussed.In general,the results are expected to provide theoretical and practical guidance for the improvement of clays in seasonal permafrost areas.After systematic experimental studies in laboratory,several main conclusions have been drawn,which are presented as below.(1)Under the general conditions,namely when modified soils do not undergo freezing and thawing effects,it is indicated that Nano-silica and basalt fiber are both favorable amendments to improve clay performances in compressive strength,shear strength,cohesion,elastic modulus and brittleness index,and reduce the soil friction angles.In addition,basalt fiber can prevent clay from brittle damage.(2)Once Nano-silica modified clay underwent frost-thaw cycles,cementation between Nano-silica and soil would be seriously damaged.During the former three times of frost-thaw cycles,shear strength of Nano-silica modified soils was lower than that of plain soils,while the cementation effects started to recover gradually after the three times of frost-thaw cycles because of the increase in water content.In the meantime,Nano-silica modified clays witnessed a growth in compressive strength,which was higher than the compressive strength of plain soils.For the Nano-silica basalt fiber modified clays,their compressive strength was stabilized after three times of frost-thaw cycles.More importantly,frost-thaw cycles exerted a relatively smaller effect on Nano-silica basalt fiber modified clays when compared with other two kinds of soils.In conclusion,Nano-silica basalt fiber could improve obviously the compressive strength of clays,especially after several times of frost-thaw cycles.(3)During the freeze-thaw cycle,brittleness index(I_B)of Nano-silica basalt fiber modified clays was smaller than that of the plain soil.Meanwhile,it is illustrated that was inert to times of frost-thaw cycles.On the contrary,the brittleness indexI_Bof Nano-silica modified clays was larger than that of plain soils,showing that basalt fiber could effectively enhance clay ductility.(4)Basalt fiber as a kind of amendment can alter failure patterns of clays during frost-thaw process,making clays experience plastic failure with large deformation.However,Nano-silica modified clays suffered brittle failure with soil peeling off from the surface.(5)Meanwhile,differences of compact effect in plain soil were larger than that of Nano-silica and basalt fiber modified soils during frost-thaw process.This is because Nano-silica and basalt fiber can enhance clay compactness and then reduce effects of frost-thaw and vertical loads on soil compressive strength.(6)In addition,Nano-silica and basalt fiber can effectively improve cohesion and reduce friction angle of soils. |
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