圆形基坑主动土压力计算方法研究

发布时间：2018-08-11 21:04

【摘要】：受环向拱效应的影响,圆形基坑的主动土压力小于直边基坑的主动土压力。但在实际工程设计中,由于缺乏完善的理论基础及相应的计算方法,设计人员多采用平面土压力理论计算圆形基坑的土压力,导致成本增加、资源浪费。本文对圆形基坑弹塑性问题的计算模型进行改进,运用轴对称滑移线法和极限分析的上限解法推导了其主动土压力的计算公式,采用数值模拟方法计算分析了圆形基坑土体中主应力系数的影响因素。具体内容如下:(1)在圆形基坑的弹塑性空间轴对称问题中,环向应力即为中间主应力。本文引入中主应力系数b,即假设σ2=σθ=σ3+b(σ1-σ3),改进了目前计算中所广泛采用的σ2=σ1或σ2=σ3的HaarKarman完全塑性假定,使其更符合工程实际情况。(2)运用滑移线场理论,推导了空间轴对称问题的滑移线方程。通过将墙后土体的滑移曲线简化为直线形式,求解滑移线方程,得到圆形基坑土压力的计算公式,并分析了土压力的分布模式。计算结果表明,圆形基坑土压力随着深度的增加呈非线性分布,但在较浅的范围内可以认为是线性分布形式。此外,分析了基坑半径对土压力的影响,结果表明土压力随半径的增大而增大,最终趋于平面Rankine 土压力。(3)采用极限分析中的上限法,同时引入中主应力系数b,并考虑环向应力做功引起内能耗散,推导了圆形基坑土压力合力的计算公式。计算结果表明,不同中主应力系数b对应的土压力明显小于平面情况下土压力。当半径趋于很大时,b所对应的结果趋于相同,该值即为在平面情况下Rankine 土压力值。此外,圆形基坑破坏面倾角受中主应力系数影响很大,b值越大时,土体破坏面倾角越大;b值越小时,土体破坏面倾角越小;随着圆形基坑半径的增大,土体破坏面倾角将减小。当开挖半径足够大时,破坏面倾角趋于45°+φ/2。(4)通过有限元计算,分析了圆形基坑中主应力系数的影响因素及其变化规律。结果表明,基坑半径及土体内摩擦角是中主应力系数取值的主要影响因素,而黏聚力的影响较小。基坑半径及内摩擦角越小,中主应力系数越大且趋于1.0。为便于实际工程应用,作者通过大量计算分析,给出了圆形基坑土体中主应力系数的建议取值。
[Abstract]:Under the influence of circumferential arch effect, the active earth pressure of circular foundation pit is smaller than that of straight side foundation pit. However, in the practical engineering design, due to the lack of perfect theoretical basis and the corresponding calculation method, the designers often use the plane earth pressure theory to calculate the earth pressure of circular foundation pit, which leads to the increase of cost and the waste of resources. In this paper, the calculation model of elastic-plastic problem of circular foundation pit is improved, and the calculation formula of active earth pressure is derived by using axisymmetric slip line method and upper limit method of limit analysis. The influence factors of principal stress coefficient in circular foundation pit soil are calculated and analyzed by numerical simulation method. The main contents are as follows: (1) in the elastic-plastic space axisymmetric problem of circular foundation pit, the toroidal stress is the intermediate principal stress. In this paper, the intermediate principal stress coefficient b, which is assumed to be 蟽 2 = 蟽胃 = 蟽 3 b (蟽 1- 蟽 3), is introduced to improve the HaarKarman complete plasticity assumption of 蟽 2 = 蟽 1 or 蟽 2 = 蟽 3, which is widely used in the calculation, so that it is more suitable for engineering practice. (2) the slip line field theory is used. The slip line equation of space axisymmetric problem is derived. By simplifying the slip curve of the soil behind the wall into a straight line form and solving the slip line equation, the calculation formula of the earth pressure of the circular foundation pit is obtained, and the distribution model of the earth pressure is analyzed. The results show that the earth pressure of circular foundation pit is nonlinear distribution with the increase of depth, but it can be regarded as a linear distribution form in a shallow range. In addition, the influence of foundation pit radius on earth pressure is analyzed. The results show that the earth pressure increases with the increase of radius and eventually tends to plane Rankine earth pressure. (3) the upper limit method of limit analysis is used. At the same time, the calculation formula of the earth pressure force of circular foundation pit is derived by introducing the coefficient of intermediate principal stress b and considering the internal energy dissipation caused by the work of circumferential stress. The calculated results show that the earth pressure corresponding to different principal stress coefficients b is obviously smaller than that under plane condition. When the radius tends to be very large, the corresponding result of b tends to be the same, that is, the value of Rankine earth pressure in plane case. In addition, when the slope of the failure surface of circular foundation pit is greatly affected by the principal stress coefficient, the larger the slope angle of soil failure surface is, the smaller the slope angle of soil failure surface is, and the smaller the slope angle of soil failure surface is with the increase of radius of circular foundation pit. When the excavation radius is large enough, the slope of the failure surface tends to be 45 掳蠁 / 2. (4) through finite element calculation, the influencing factors of principal stress coefficient and its variation law in circular foundation pit are analyzed. The results show that the radius of foundation pit and the internal friction angle of soil are the main factors affecting the value of the coefficient of principal stress, but the influence of cohesion is small. The smaller the radius of foundation pit and the angle of internal friction, the larger the coefficient of principal stress is and tends to 1.0. In order to facilitate practical engineering application, the author gives the suggested value of principal stress coefficient in the soil of circular foundation pit through a lot of calculation and analysis.
【学位授予单位】：西南交通大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TU432

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