岩溶顶板与桩基作用机理分析与模拟试验研究

【作者】 刘铁雄；

【导师】 彭振斌；

【作者基本信息】 中南大学 ， 地质工程， 2003， 博士

【摘要】 我国岩溶发育地区分布很广，并且近一些年来国家正在加大基础设施的建设，在工民建、修路、建桥等工程建设之中，经常会遇到复杂岩溶地层，这就给工程施工带来一系列的问题。在岩溶发育地区，灌注桩基的应用越来越广泛，如何选择合适的桩型、桩端持力层以及如何确定嵌岩桩的最佳嵌岩深度与岩溶顶板安全厚度，在工程实践中具有重要的理论意义和现实意义。 本文在参考大量的国内外文献资料的基础上，对岩溶地区嵌岩桩的研究现状进行归纳和总结。在岩溶发育地区，岩溶顶板的安全厚度以及嵌岩深度的取值，决定着岩溶地区嵌岩桩的设计、施工方法以及工程造价。因此，进行岩溶地区桩基与岩溶顶板作用机理分析和模拟试验研究是当务之急。本文在此方面取得了一定的成果。 首先对岩溶地区的狄岩岩性特征进行了综合分析，并对溶洞洞跨与空间几何形态分布规律进行统计，得出了溶洞洞跨与个数分布直方图。并讨论了多个工程实例中，岩溶顶板安全厚度和嵌岩深度的取值。 针对所研究的岩溶顶板与桩基作用系统，对岩溶顶板进行了合理的简化，并提出相应的假设，再根据溶洞顶板的状态以及边界条件，将桩基与溶洞顶板作用系统简化为边界条件和受力状况不同的十个力学模型。应用弹性力学和材料力学的理论，对简化后的桩基与溶洞顶板作用系统的十个力学模型进行了逐一的分析和讨论，并得出了相应的应力及应变表达式。同时讨论了泊松比对计算结果的影响。 综合讨论了嵌岩桩在竖向荷载作用下的承载力标准值的计算公式，并对岩溶发育地区的嵌岩桩竖向承载力特性进行分析和简化，将岩溶地区的嵌岩桩模型简化设计为短桩形式的端承桩。在假定溶洞顶板为一完整的平板状的岩体情况下，分别对基桩的抗冲切、抗剪及抗弯情况作出稳定性分析，认为岩体的抗弯强度是决定岩溶地区桩基稳定性的主要因素。 结合岩溶地区桩基与岩溶顶板的作用机理分析，运用相似理论，提出了建立室内桩基物理模型的试验方案，自主设计并研制了岩溶顶板与桩基模拟试验物理模型，同时结合模型编制了相应的自动测试系统软件。通过试验证明，该物理模型能满足试验要求，自动测试系统能正常地运行，所测参数的精度跟理论计算值吻合良好，证明自动测试系统的可靠性。 在岩性模拟试验阶段，根据现场灰岩的岩体特性以及相应的强度指标，结合相似理论，利用有约束的均匀设计法，选用细砂作为骨料和以水泥、石膏为胶结材料进行灰岩岩性的模拟试验，试验结果证明能基本满足相似条件，可用于制作岩溶顶板模拟试件。 采用不同的试验方案，通过对大量破坏性模拟试验结果的综合分析，提出了岩溶顶板安全厚度验算方法，得出岩溶项板和桩基竖向极限荷载的半经验半理论公式如下：其中修正系数ξ取0.8～1.3。 通过嵌岩桩模拟嵌岩深度的试验，给出了岩溶地区嵌岩灌注桩嵌岩深度建议值。在岩溶发育地区，嵌岩深度的选取应综合考虑侧摩阻力和顶板结构内力，一般岩溶地区，当岩面较为平整且上覆土层较厚时，嵌岩深度宜采用o.Zd或不小于0.2m。更多还原

【Abstract】 Karst area is widely in our country, all kinds of Karst condition always be come across as our country’s fundamental construction is greatly developed, which bring a series of problems in the building construction. Cast in-situ pile foundation is more popular in Karst area. How to choose suitable pile pattern and loading lays of pile-tip, how to determine the best socketed depth and roof safety thickness, would produce important theorem meaning and economic value.Based on a lot of international and our country’s papers, research level of socketed-rock piles in Karst area has been proposed and discussed. The socketed depth and roof safety thickness determine the cost of engineering projects .design method and construction of socketed-rock piles. Studying the mechanism of pile and cavity roof and doing simulated test become the most important thing which we should to do. In this paper, an extensive study is carried out and the results have been obtained as follows:Firstly, the paper synthetically analyses the limestone properties in Karst area, make a statistics for the roof width and space distribution laws, draw the distribution scheme of cavities, also discuss the socketed-depth and roof safety thickness in many engineering projects.Many reasonable hypothesizes have been proposed, the cavity roof has been simplified, the system of piles and cavity roof can be simplified for ten mechanics models according to the boundary conditions and mechanical station. The ten models have been discussed and analyzed respectively, the stress and strain formula has been deduced. At the same time, the influence of Poisson’s ratio also is discussed.With the vertical loading, the formula of loading capacity of socketed-rock piles’ has been talked over synthetically. The model of socketed-rock piles can be simplified to an only-tip loading pile. Taking the roof as a whole plate, the rock’s anti-bend strength is the key factor which determine the stableness of pile foundation.According to the analyses of the mechanism of pile and cavity roof, we design and make the test model of pile foundation by using the similar theory, also work out the auto-measure system. Through a lot of tests, the model and system can reach our purpose.The choice of similar material is the key problem in the simulated model test. With the uniform design methods, we choose the mixture of cement with gypsum and sand to simulate the limestone properties according to the similar theory. This kind of mixture is verified to be good.According to the test result and theoretical analysis, the paper has proposed the approach to examine the safety roof thickness, half-theory and half-experience formula associated with vertical loading is proposed as follow:In karst area, the socketed depth seems difficult to determine, considering synthetically the correlation between the pile skin friction resistance and the bedrock internal cohesion and the influences on loading capacity of pile, the mine socketed length is suggested 0.2 pile diameters or at least 0.2m when the rock surface is level and the upper soil is thick relatively.更多还原