基于冻融循环作用的基坑变形机理及支护方案优化研究
发布时间：2018-09-17 15:47
【摘要】：随着城市地下空间开发与利用,开挖深度大、平面形状复杂的深基坑数量不断增加,而深大基坑经常面临因施工周期长出现越冬的问题.由于现有基坑支护设计规范中并未考虑冻融作用的影响,依照规范施工,可能会造成支护结构和基坑土体的过大变形,甚至破坏失稳.虽然这一问题已引起人们的重视,但从机理上的研究尚未深入,尤其对于季节性冻土地区.由于考虑冻融作用的基坑土体应力场-温度场-渗流场三场间耦合作用与基坑土体支护方案优化的研究相对较少,所以研究冻融循环作用下基坑变形机理及支护方案优化是十分必要的.论文以毗邻沈阳市政府的东森CBD商务广场二期基坑工程为背景,以冻土力学、热力学、渗流力学、基坑工程学等相关知识为理论基础,通过室内土工试验、冻土三轴试验、土体冻融特性试验获得土层基本参数,并总结了季节性冻土在冻结和融化过程中,土体参数和力学性能随不同物理状态条件(围压、冻结温度、冻融循环)下各变量(含水率、孔隙比、干密度、压缩模量、弹性模量、强度等)的变化规律,为数值模型的构建和数值计算提供基本参数；采用冻融试验中考虑冻融作用影响所获得的土层参数,建立考虑冻融作用影响的水热力三场耦合数值计算模型,得出考虑冻融作用的基坑变形规律；设计基坑模型实验模拟冻融循环作用下基坑在越冬过程中的温度环境,揭示基坑支护在冻融作用下的变形和内力规律,并将模型实验结果与数值计算结果进行对比,验证考虑冻融循环作用影响的水热力三场耦合模型预测基坑变形规律的正确性,实现了考虑土体冻融循环作用的水热力三场耦合分析,从理论上完成了季节性冻土区冻融循环与水-热-力耦合作用下基坑变形规律研究，，对于越冬基坑的支护方案优化具有理论和应用价值.通过系统研究沈阳季节性冻土地区基坑变形机理及支护方案优化,得出以下主要结论：(1)最佳含水率是含水率在冻融循环中对压缩模量影响程度的界限标志：当小于最佳含水率时,首次冻融循环后压缩模量降低幅度较大；当大于最佳含水率时,压缩模量受含水量影响不明显：随着冻融循环次数的增加,弹性模量先降后增,第1次冻融后的变化幅度最大,经历5-9次冻融循环后,弹性模量降到最低点,并趋于稳定.(2)冻融作用对孔隙比随着含水率的增大先减后增的规律基本无影响,冻融后土样孔隙比减小,干密度的增大会减弱冻融作用对孔隙比影响程度,当干密度增大到一定程度后影响可忽略；试样含水率随着冻融循环次数的增加缓慢增大趋势在冻融循环超过3次后趋于稳定；随着冻融次数增加,粘聚力c呈现降低的趋势,而内摩擦角φ有增大趋势；(3)初次冻融循环对土的结构性扰动最大,多次冻融循环后土样强度有所恢复.当围压较小时,冻融后土的强度损失较大,随着围压提高,受冻融影响不明显；含水率的提高会使应力—应变曲线由硬化型向弱软化型转化,在高围压条件下尤为显著；冻结温度条件对土样冻融后的破坏强度影响显著,土样的破坏强度,在经历5-7次冻融后强度会达到最低值,建议沈阳地区冻土工程设计时可参考冻融循环7次时的力学指标；(4)通过基坑模型实验得出：桩的埋深对腰梁位移影响显著,整体趋势为埋深越浅,腰梁位移越大,支护结构越不稳定；在冻融循环下桩间和桩后土压力分布随桩的埋深不同差异显著：埋深越浅土压力分布越复杂,随冻融循环土压力不规则波动；桩间和桩后土压力随土温变化显著,在冻融循环前期、中期土温度较为稳定,但土压力波动明显；在冻融循环后期土压力基本稳定,温度也无较大改变；桩后土压力分布随深度不同分布规律不同.(5)建立了考虑冻融作用的三维数值模型,对越冬基坑降温过程进行数值模拟,从数值计算结果云图中显示,从0℃降至-10℃的降温过程的基坑位移量最大,基坑的变形受温度影响在基坑阳角处最为明显.整个降温过程中基坑最大位移量先减后增,但幅度较小.温度变化对于内力的影响相对较小.(6)提出基于冻融作用影响下基坑变形规律的基坑支护优化方案,根据土体冻融特性试验,获得考虑冻融作用后的土层参数,建立考虑冻融作用的数值分析模型,通过现场监测证明变形控制效果较好,对于指导季节性冻土地区基坑工程实践具有一定应用价值和参考意义.
[Abstract]:With the development and utilization of urban underground space, the depth of excavation is great, and the number of deep foundation pits with complex plane shape is increasing. However, deep and large foundation pits often face the problem of overwintering due to long construction period. Although this problem has attracted people's attention, the mechanism research has not been thorough, especially for the seasonal frozen soil area. Because of considering the effect of freezing and thawing, the coupling between stress field, temperature field and seepage field of foundation pit soil and the optimization of foundation pit soil support scheme are comparatively studied. So it is very necessary to study the deformation mechanism of foundation pit under freeze-thaw cycles and the optimization of supporting schemes.Based on the second-stage foundation pit project of Dongsen CBD Business Plaza adjacent to Shenyang Municipal Government, this paper takes the permafrost mechanics, thermodynamics, seepage mechanics, foundation pit engineering and other related knowledge as the theoretical basis, through indoor geotechnical tests, permafrost triaxial. The variation of soil parameters and mechanical properties with various physical conditions (confining pressure, freezing temperature, freezing-thawing cycle), including water content, porosity ratio, dry density, compressive modulus, elastic modulus and strength, during freezing and thawing of seasonal frozen soil is summarized. The construction of numerical model and numerical calculation provide the basic parameters; adopt the soil parameters obtained by considering the effect of freeze-thaw in freeze-thaw test, establish the hydrothermal three-field coupling numerical calculation model considering the effect of freeze-thaw, and obtain the deformation law of foundation pit considering the effect of freeze-thaw; design the foundation pit model experiment to simulate the effect of freeze-thaw cycle. The temperature environment of foundation pit in the process of overwintering reveals the deformation and internal force law of foundation pit support under the action of freezing and thawing, and compares the model experimental results with the numerical results, verifies the correctness of the three-field coupling model considering the effect of freezing and thawing cycles to predict the deformation law of foundation pit, and realizes the freezing-thawing cycle of soil. The deformation law of foundation pit under the action of freeze-thaw cycle and water-heat-force coupling in seasonal frozen soil region is studied theoretically by using the coupled analysis of hydrothermal and mechanical fields. It is of theoretical and practical value to optimize the supporting scheme of overwintering foundation pit. The main conclusions are as follows: (1) The optimum moisture content is the limit sign of the effect of water content on compressive modulus in freeze-thaw cycles: when the water content is less than the optimum, the compressive modulus decreases greatly after the first freeze-thaw cycles; when the water content is greater than the optimum moisture content, the influence of water content on compressive modulus is not obvious: with the increase of freeze-thaw cycles After 5-9 freeze-thaw cycles, the modulus of elasticity decreases to the lowest point and tends to be stable. (2) Freeze-thaw has little effect on the rule that the porosity ratio decreases first and then increases with the increase of water content. The porosity ratio decreases after freeze-thaw, and the effect of freeze-thaw decreases with the increase of dry density. The influence degree of porosity ratio can be neglected when the dry density increases to a certain extent; the water content of samples increases slowly with the increase of freeze-thaw cycles, and tends to be stable after more than three freeze-thaw cycles; the cohesion c decreases with the increase of freeze-thaw cycles, while the internal friction angle increases; (3) the initial freeze-thaw cycles. When the confining pressure is small, the strength loss of the soil after freezing and thawing is large, and with the increase of confining pressure, the influence of freezing and thawing is not obvious; the increase of water content will make the stress-strain curve change from hardening to weakening, especially under high confining pressure. Temperature conditions have a significant impact on the failure strength of soil samples after freeze-thaw, and the failure strength of soil samples will reach the lowest value after 5-7 freeze-thaw cycles. It is suggested that the mechanical indicators of freeze-thaw cycles for 7 times can be referred to in the design of Permafrost Engineering in Shenyang area. (4) Through the foundation pit model test, it is concluded that the depth of pile has a significant impact on the displacement of waist beam, and the overall trend. The shallower the buried depth is, the greater the displacement of the waist beam is, and the more unstable the supporting structure is; the difference of the earth pressure distribution between piles and behind piles is significant with the depth of the piles under freeze-thaw cycles: the more complex the earth pressure distribution is, the more irregular the earth pressure fluctuates with the freeze-thaw cycles; the earth pressure between piles and behind piles varies significantly with the soil temperature; at the early stage of the freeze-thaw cycles, in the middle stage of the freeze- The soil temperature in the period of freezing-thawing is stable, but the earth pressure fluctuates obviously; the earth pressure is basically stable and the temperature does not change greatly at the later stage of freezing-thawing cycle; the distribution law of the earth pressure behind the pile is different with the depth. (5) A three-dimensional numerical model considering the freezing-thawing effect is established to simulate the cooling process of overwintering foundation pit. The figure shows that the displacement of the foundation pit is the largest when the temperature is lowered from 0 C to - 10 C, and the deformation of the foundation pit is most obvious at the sunny corner of the foundation pit. According to the freeze-thaw characteristic test of soil, the soil parameters after considering the freeze-thaw effect are obtained, and the numerical analysis model considering the freeze-thaw effect is established. The field monitoring shows that the deformation control effect is better, which has certain application value and reference significance for guiding the foundation pit engineering practice in seasonal frozen soil area.
【学位授予单位】：辽宁工程技术大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TU753
本文编号：2246393
[Abstract]:With the development and utilization of urban underground space, the depth of excavation is great, and the number of deep foundation pits with complex plane shape is increasing. However, deep and large foundation pits often face the problem of overwintering due to long construction period. Although this problem has attracted people's attention, the mechanism research has not been thorough, especially for the seasonal frozen soil area. Because of considering the effect of freezing and thawing, the coupling between stress field, temperature field and seepage field of foundation pit soil and the optimization of foundation pit soil support scheme are comparatively studied. So it is very necessary to study the deformation mechanism of foundation pit under freeze-thaw cycles and the optimization of supporting schemes.Based on the second-stage foundation pit project of Dongsen CBD Business Plaza adjacent to Shenyang Municipal Government, this paper takes the permafrost mechanics, thermodynamics, seepage mechanics, foundation pit engineering and other related knowledge as the theoretical basis, through indoor geotechnical tests, permafrost triaxial. The variation of soil parameters and mechanical properties with various physical conditions (confining pressure, freezing temperature, freezing-thawing cycle), including water content, porosity ratio, dry density, compressive modulus, elastic modulus and strength, during freezing and thawing of seasonal frozen soil is summarized. The construction of numerical model and numerical calculation provide the basic parameters; adopt the soil parameters obtained by considering the effect of freeze-thaw in freeze-thaw test, establish the hydrothermal three-field coupling numerical calculation model considering the effect of freeze-thaw, and obtain the deformation law of foundation pit considering the effect of freeze-thaw; design the foundation pit model experiment to simulate the effect of freeze-thaw cycle. The temperature environment of foundation pit in the process of overwintering reveals the deformation and internal force law of foundation pit support under the action of freezing and thawing, and compares the model experimental results with the numerical results, verifies the correctness of the three-field coupling model considering the effect of freezing and thawing cycles to predict the deformation law of foundation pit, and realizes the freezing-thawing cycle of soil. The deformation law of foundation pit under the action of freeze-thaw cycle and water-heat-force coupling in seasonal frozen soil region is studied theoretically by using the coupled analysis of hydrothermal and mechanical fields. It is of theoretical and practical value to optimize the supporting scheme of overwintering foundation pit. The main conclusions are as follows: (1) The optimum moisture content is the limit sign of the effect of water content on compressive modulus in freeze-thaw cycles: when the water content is less than the optimum, the compressive modulus decreases greatly after the first freeze-thaw cycles; when the water content is greater than the optimum moisture content, the influence of water content on compressive modulus is not obvious: with the increase of freeze-thaw cycles After 5-9 freeze-thaw cycles, the modulus of elasticity decreases to the lowest point and tends to be stable. (2) Freeze-thaw has little effect on the rule that the porosity ratio decreases first and then increases with the increase of water content. The porosity ratio decreases after freeze-thaw, and the effect of freeze-thaw decreases with the increase of dry density. The influence degree of porosity ratio can be neglected when the dry density increases to a certain extent; the water content of samples increases slowly with the increase of freeze-thaw cycles, and tends to be stable after more than three freeze-thaw cycles; the cohesion c decreases with the increase of freeze-thaw cycles, while the internal friction angle increases; (3) the initial freeze-thaw cycles. When the confining pressure is small, the strength loss of the soil after freezing and thawing is large, and with the increase of confining pressure, the influence of freezing and thawing is not obvious; the increase of water content will make the stress-strain curve change from hardening to weakening, especially under high confining pressure. Temperature conditions have a significant impact on the failure strength of soil samples after freeze-thaw, and the failure strength of soil samples will reach the lowest value after 5-7 freeze-thaw cycles. It is suggested that the mechanical indicators of freeze-thaw cycles for 7 times can be referred to in the design of Permafrost Engineering in Shenyang area. (4) Through the foundation pit model test, it is concluded that the depth of pile has a significant impact on the displacement of waist beam, and the overall trend. The shallower the buried depth is, the greater the displacement of the waist beam is, and the more unstable the supporting structure is; the difference of the earth pressure distribution between piles and behind piles is significant with the depth of the piles under freeze-thaw cycles: the more complex the earth pressure distribution is, the more irregular the earth pressure fluctuates with the freeze-thaw cycles; the earth pressure between piles and behind piles varies significantly with the soil temperature; at the early stage of the freeze-thaw cycles, in the middle stage of the freeze- The soil temperature in the period of freezing-thawing is stable, but the earth pressure fluctuates obviously; the earth pressure is basically stable and the temperature does not change greatly at the later stage of freezing-thawing cycle; the distribution law of the earth pressure behind the pile is different with the depth. (5) A three-dimensional numerical model considering the freezing-thawing effect is established to simulate the cooling process of overwintering foundation pit. The figure shows that the displacement of the foundation pit is the largest when the temperature is lowered from 0 C to - 10 C, and the deformation of the foundation pit is most obvious at the sunny corner of the foundation pit. According to the freeze-thaw characteristic test of soil, the soil parameters after considering the freeze-thaw effect are obtained, and the numerical analysis model considering the freeze-thaw effect is established. The field monitoring shows that the deformation control effect is better, which has certain application value and reference significance for guiding the foundation pit engineering practice in seasonal frozen soil area.
【学位授予单位】：辽宁工程技术大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TU753
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