| The loess distribute extensivly in arid and semi arid seasonal frozen soil area of China.It is sensitive to the effect of wet load-freeze-thaw, By freeze-thaw cycles,it induced all kinds of diseases during construction process of shallow and ultra shallow buried high water content loess tunnel engineering. Taking as Yangqu loess tunnel NO.1 in Shan xi province for research object,the combination method of field investigation with laboratory test and theoretical analysis, field monitoring, numerical simulation to study deformation law and stability of surrounding rock for tunnel with different water content.The reaseach has important theoretical significance and practical engineering value.The main work is as follows:(1)Physical characteristics test for loess with different water content is carried out,the results show that:When the moisture content is less than freeze-thaw damage threshold moisture content(8%),the freeze-thaw cycles only have little influence on engineering properties of soil mass.With the increase of water content, cementation material between soil particles dissolved in water induced the link between the soil particles gradually became weak or even damage.The physical properties such as particle size distribution, void ratio and so on are changed.When the moisture content is more than freeze-thaw damage threshold moisture content, the freeze- thaw cycles caused change greatly in physical properties,such as the soil pore ratio, liquid limit, plastic limit, dry density and so on. Soil samples has obvious damage in the interior,the soil water with strong absorption and weak water capacity has changed greatly,so,the soil physical properties have great changes.(2)Based on the concept of the water sensitive potential, perturbation potential and the degree of structure of loess, the concept of freeze-thaw potential and the degree of freezethaw is proposed,the effects of coupling water and heat on soil elastic modulus, cohesion and internal friction angle were studied.Results show: The strength of Undisturbed and reconstituted soil samples were decreased with the increase of water content, the freeze-thaw cycles made the trend more obvious.When the moisture content is invariable,the freeze- thaw cycles have great influence on the soil damage with the increase of the number of freeze-thaw cycles. When the water content increases, the effect of freeze-thaw on soil strength damage is strengthened.When the water content is increased to saturation, it can be considered that the freeze-thaw effect is the strongest on the soil strength damage.The increase of water content makes the soil sample in the dense phase of pressure, the freeze- thaw cycles decrease the peak strength of soil samples.With the increase of water content. The soil water potential is decreased, the disturbance potential is reduced, the freeze-thaw potential is increased, and the freeze-thaw structure is reduced. With the increase of the number of freeze-thaw cycles, the sensitivity of soil water is increased, the disturbance potential is decreased, the freeze-thaw potential is decreased, and the degree of freeze-thaw is decreased.(3)The mechanism of water content and freeze- thaw cycles on soil damage is studied, the study found that: The damage of freeze-thaw cycles and water content changes on soil samples is mainly caused by the temperature taking free water in the soil as a medium, the water in the soil producing frost heaving and thawing and water transfer.When the temperature is reduced to below 0 degrees Celsius for a period of time, volume expansion of in the soil produces about 9% of the in situ freezing, electric field balance changed in soil. And it is affected by the temperature gradient, except the volume expansion caused by free water freezing, but also caused the free water migrating to freezing front.When the ambient temperature rises above 0 degrees Celsius for a period of time, the water in the soil will occur with the reverse migration of freezing. Both volume expansion and water migration can affect the link and arrangement of soil particles, and cause damage or destroyed of the link of soil particles, particle alignment and physical characteristics change, mechanical properties is deteriorated. Repeated freeze-thaw cycles will induce soil particle damage.(4)CT technique was used to observe the apparent damage of soil samples with different water content under different freeze-thaw cycles, the influence law of CT value was analyzed. It is found that:the first 5 times of freezing and thawing cycles, soil sample CT value has the maximum variation. When the number of freeze-thaw cycles is 6~20, CT value continues to decrease, but the change range is relatively small. After freeze-thaw is 21 times, the CT value continued to decrease, but the decrease of the amplitude continued to decrease, and tend to be the minimum. After 30 times of freeze-thaw, the CT value has no change basically, it can be considered that freeze-thaw cycle has no effect on the CT value after 30 times.(5)The concept of resistivity difference is proposed, using the difference of resistivity between after freeze-thaw cycles and before freeze-thawing cycles to reflect the influence of freeze-thawing cycles on the resistivity value of same water content soil sample. The experimental study of the influence of freeze-thaw cycles on the difference of soil resistivity was done, it is found that The relationship between resistivity difference and water content can be expressed with the power function equation,the relationship between the nonuniformity coefficient and uniform correction factor and the number of freeze-thaw cycles can be expressed with logarithmic function.(6)Considering the influence of moisture content change, damage variable is defined with elastic modulus, the damage constitutive relation was established under the combination effect of freeze-thaw cycles and loading according to the concepts and methods of macro phenomenological damage mechanics and non-equilibrium statistics,The theoretical calculation results and the experimental results are compared and analyzed, the correctness of the damage constitutive model is verified in this paper.(7)The numerical simulation of stress and deformation for surrounding rock in Yangqu NO.1 tunnel was carried under different freeze- thawing and water content conditions.The relationship between the safety factor of surrounding rock of No. 1 tunnel in Yangqu and the water content and the number of freeze-thaw cycles is obtained. The simulation results and monitoring results are verified. The results show that: When the water content of the surrounding rock in loess tunnel is less than 18%, monitoring should be strengthened to deal with adverse factors at any time.When the water content is 20%, anti freezing measures should be strengthened to prevent the deterioration of the surrounding rock mass by freeze-thaw.When the water content is 22%, the initial safety factor of surrounding rock is near the critical point. Water drainage and anti freezing measures should be adopted while the safety factor is reduced to below 1 after repeated freeze-thaw cycles. When the water content is 35%, the safety factor of surrounding rock is less than 1 while the surrounding rock has not experienced freeze-thaw cycles. The surrounding rock is in dangerous state, it is need to take drainage measures immediately. After the water content decreased significantly, and then taking anti freezing measures to ensure the safety of construction. The conclusions provides an important basis for the field construction. |
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