Research On Excavation Deformation Mechanism And Support Scheme Optimization Based On The Effect Of Freeze-thaw Cycles

With the urban underground space development and utilization, the number of deep excavation with complex shapes plane continues to improve, while the deep foundation pit often face the problems of long construction period result to overwinter. As the existing excavation design specification does not consider the impact of freezing and thawing, in accordance with existing excavation design specification to construct, it may cause excessive deformation of the supporting structure and the excavation of soil, even cause instability failure. Through this issue has attracted attention, but not yet in deeper study on the mechanism of it, especially for seasonal permafrost regions. Considering the study on the coupling of stress field-temperature field-seepage field and excavation support scheme optimization is relatively weak, so the study of foundation pit deformation mechanism and support scheme optimization on the effectt of freeze-thaw cycles is essential.Based on the background of Shenyang Dongsen excavation project next to the Shenyang municipal government, taking frozen soil mechanics, thermodynamics, fluid mechanics and other related knowledge as the theoretical basis, through the indoor soil test, frozen three triaxial test, soil freeze-thawcharacteristics test to get the basic parameters of the soil, and summarizes the parameters and mechanical properties change rule of seasonally frozen soil with different conditions (confining pressure, freezing temperature, freeze-thaw cycles) and physical state of variable (moisture content, void ratio, dry density, modulus of compression, modulus of elasticity, strength etc), to provide the basic soil parameters for construction of model and numerical calculation; Using the parameters obtained by freeze-thaw test considering the impact of freeze-thaw cycles to establish seepage-temperature-stress three fields coupling numerical model, to conclude the excavation deformation law considering the impact of freeze-thaw cycles; To design the excavation model experiment to simulate the temperature environment under the action of freeze-thaw cycle in the overwintering period to reveal the excavation deformation and the internal force rule under the impact of the freeze-thaw, and compared the results of the model experiments and numerical calculation to verify the deformation law prediction accuracy of the coupling model considering the impact of freeze-thaw cycle, to realize the seepage-temperature-stress three fields coupling analysis considering freeze-thaw cycles, theoretically complete the study on the deformation law of foundation pit under seepage-temperature-stress three fields coupling analysis considering the impact of freeze-thaw cycle in seasonal frozen soil area, it has theoretical and application values for the overwintering foundation pit support scheme optimization.Through the system research of the foundation pit deformation mechanism and support scheme optimization in Shenyang seasonal frozen soil area, and obtained the following conclusions:(1) The optimum moisture content is the boundarymarker of the impact of freeze-thaw cycle on the compression modulus, the compression modulus decrease greatly after the first cycle of freeze-thaw when moisture content is less than the optimum moisture content, when the moisture content is more than the optimum moisture content, the compression modulus affected by moisture content is not obvious. The elastic modulus depress initially and increase afterward in company with the increasing of freez-thaw cycles and it varies within wide limits in first freez-thaw cycle. The elastic modulus descends on to minimum point after five to nine freez-thaw cycles and to be stable.(2) The impact of freez-thaw does not effect the law of the porosity ratio reducsing initially and increasing with moisture content increasing, the porosity ratio of soil sample reduces after freeze-thaw. The increasing of dry density weaken the degree of the effect of freez-thaw on the porosity ratio,when the dry density increase to a certain degree the effect of it can be ignored;The trend of the moisture content of soil sample increase slowly with the increasing of freeze-thaw cycles is stable after three freeze-thaw cycles.In company with the increasing of freeze-thaw cycle the cohesion present depressed trend,while the angle of internal friction has increasing trend.(3)The first freeze-thaw cycle disturbs the soil structure heavily, the soil samples recovered strength after repeatedly freeze-thaw cycles, when the confining pressure is small, the strength of the soil sample losses largerly after freeze-thaw cycles, with the increase of confining pressure, the influence of freeze-thaw cycles is not obvious; The stress-strain curves transformed from the type of hardening to softening weak with the moisture content increases, it is particularly significant at high confining pressure; The effect of freezing temperature conditions on the failure strength of the soil samples is significantly after freeze-thaw cycles. The failure strength of soil samples reach the lowest value after 5 to 7 freeze-thaw cycles,so suggest that using the mechanics index of soil after 7 freeze-thaw cycles as reference in Shenyang area in design of frozen engineering.(4) Through the model experiment of foundation pit concludes that the buried depth of pile influence the waist beam displacement significantly, the overall trend is that buried depth of pile is shallower, the waist beam displacement is greater,the more unstable retaining structure is. The soil pressure between and behind piles distributes with buried depth of pile is different significantly in the freeze-thaw cycles; The shallower of the buried depth of the pile the soil pressure distribution is more complex,and the soil pressure irregular fluctuations with the freeze-thaw cycles;The soil pressure between and behind the piles changes with temperature significantly, In the prophase and metaphase of freeze-thaw cycles the soil temperature is stable, but the soil pressure fluctuates obviously; In the laterphase of freeze-thaw cycles the soil pressure is basically stable, the temperature, also changed little; the the soil pressure behind the piles distributes with the depth is different.(5) A three dimensional numerical model is established considering the effect of freeze-thaw cycles to simulate the cooling process of the overwintering excavation. From the numerical calculations show that the results of imagedisplacement of foundation pit, the cooling process of from 0℃ to -10℃ the displacements of the excavaton is maximum, the deformation of foundation pit is affected by temperature in the foundation pit Yang corner is the most obvious. The maximum excavation displacements firstly decrease and then increase in entire cooling process, but to a lesser extent. The temperature changes affect the internal forces. relatively small.(6) The excavation support scheme optimization is proposed based on the excavation deformation law considering the effect of freeze-thaw cycles. According to the freeze-thaw characteristics test of the soil to obtain the parameters considering the impact of freeze-thaw cycle and establish the simulation analysis model considering the impact of freeze-thaw cycles. Through the on-site monitoring to prove deformation control is better, it has a certain reference value and significance for the guidance of seasonal frozen ground area excavation practice.