Effects Of Repeated Freezing-thawing On Loess In Winter Irrigation Area

Due to the low temperature in Ningxia,the depth of frozen soil is large.And the average extreme minimum temperature in past years is-28.4?in Ningxia.Loess is widely distributed in Ningxia.There are a lot of loess engineering,such as ditch,storage reservoir and so on.The loess of Ningxia is characterized by strong frost heaving and high capillary water rise.And winter irrigation makes shallow soil moisture content increased.Based on above-mentioned reasons,the engineering properties of loess are seriously deteriorated.Based on the above problems,combined with the local characteristics of climate and loess,the effect of freeze-thaw on loess in winter irrigation zone was experimentally studied by using the soil mechanics test.It analyzed the effect of freeze-thaw on frost heaving and thawing settlement characteristics and compression,shearing strength and collapsibility of Ningxia loess.Then the mechanism of action and the leading influencing factors are revesled.And relationship models of freeze-thaw effects on loess in winter irrigation were established.The test results have important significance for the evaluation and prevention of frost damage of loess in Ningxia irrigation area.The results show that:(1)The increment of compression coefficient,frost heaving ratio,cohesion loss,increment of collapsibility coefficient increase with the increase of the freeze-thaw cycles,and the change of internal friction angle has no obvious law.It is related to water conditions and initial dry density that thawing settlement coefficient changes with freeze-thaw cycles.The frost heaving ratio,thawing settlement coefficient,increment of compression coefficient,cohesion loss,internal friction angle and increment of collapsibility coefficient caused by freeze-thaw change greatly in the previous several freeze-thaw cycles,and tend to be stable after several freeze-thaw cycles.(2)Under the condition of no water supply,forst heaving ratio of unsaturated loess increases with the increase of initial dry density and water content;thawing settlemrent of coefficient decrease with initial dry density,and increases with the increase of water content.Under the condition of water supply,the frost heaving ratio decreases with the increase of the initial dry density when the freeze-thaw cycles are less than 10 times,and then it turns to increase with the increase of the initial dry density.Thawing settlement coefficient decreases with the increase of the initial dry density.There are more cycles of freezing and thawing when freeze-thaw deformation is stable.(3)Under the condition of no water supply,because of freeze and thaw,the unsaturated loess's increment of compression coefficient increases with the increase of initial dry density and water content.Under the condition of water supply,the effect of freeze-thaw on compression decreases with the increase of the initial dry density when the freeze-thaw cycles are less than 10 times,and then it turns to increase with the increase of the initial dry density.The effect of freeze-thaw on compression of loess with water supply is more obvious than that without water supply.And there is a perfect linear correlation between increment of compression coefficient and freeze-thaw deformation rate.The frost heaving and thawing settlement canbe used to predict the change of compression after freeze-thaw.The increase in compressibility of loess after freeze-thaw is mainly caused by the increase of its void ratio.(4)Due to the shear strength parameters of Ningxia saturated loess is very small,the space for the loss of shear strength parameters is very small,and the effect of freeze-thaw on it is not obvious.Therefore,this paper only studies the effect of freeze-thaw on shearing strength of unsaturated Loess.As water content is fixed,the cohesion loss caused by freeze-thaw increases with the increase of initial dry density.As initial dry density is fixed,the cohesion loss caused by freeze-thaw decreases with the increase of water content.(5)As water content is fixed,the effect of freeze-thaw on coefficient of collapsibility increases with the increase of dry density.As initial dry density is fixed,the effect of freezing and thawing on coefficient of collapsibility first increases,and then decreases with the increase of water content.The worst water content is about 22.0%.The effect of freezing and thawing on coefficient of collapsibility has one of worst water content,and loess engineering should avoid suffering cold under the worst water content,especially the compacted loess.(6)Initial dry density,moisture content,water condition and freeze-thaw cycles are all important factors affecting the freezing and thawing effect of loess.Under the condition of no water supply,increasing the compactness of soil can effectively lessen freeze injury.Under the condition of water supply,this method becomes more and more infeasible with the increase of the freeze-thaw cycles.The membrane materials should be set up to separate water from the freezing layer,or to take measures such as dewatering and drainage and so on.