【作者】 冯晓莹；

【导师】 栾茂田； 徐泽平；

【作者基本信息】 大连理工大学 ， 岩土工程， 2008， 博士

【摘要】 黏土心墙高土坝的水力劈裂是国内外工程界普遍关注又亟待解决的关键性问题之一,同时也是最具有争议的问题之一。迄今为止,人们对水力劈裂的发生机理、发生条件、影响因素及判别方法等尚存在不同的看法。近年来,随着世界范围内高土石坝建设的迅速发展,关于水力劈裂发生的可能性及发生条件研究在土石坝设计和建设中的重要性越来越突出。在系统总结了以往研究成果的基础上,本文对几种简化的黏土心墙模型进行了水力劈裂离心模型试验,对黏土心墙水力劈裂的发生机理和发生条件进行了分析研究。首次通过试验手段在试验室再现了均质黏土心墙的水力劈裂现象。试验结果表明,当心墙上游侧的水压力大于心墙土体压力时,心墙土体将会产生水力劈裂裂缝,最终在水压力作用下发展为土体的渗透破坏。在进行了水力劈裂离心模型试验的同时,本文还采用有限元数值分析方法对心墙水力劈裂进行了进一步分析,验证了有限元方法在水力劈裂分析中的有效性和可靠性。首先对ABAQUS软件进行了用户材料子程序的二次开发,添加了Duncan-Chang非线性弹性模型的E-B模式及沈珠江弹塑性模型,并通过算例和工程实例进行了验证。其次,在简化心墙离心模型试验和ABAQUS二次开发的基础上,对心墙水力劈裂的机理进行了进一步的分析、论证。计算分析结果表明,在水荷载的作用下,当上游侧的水荷载大于心墙土体压力时,均质心墙上游侧表面相应于离心模型试验中发生水力劈裂裂缝的位置出现了有效小主应力的拉应力区,从而具备了产生水力劈裂现象的条件。心墙两侧的应力方向由于边界约束作用而发生了偏转,蓄水后,水压力作用导致应力方向偏转更为明显,由于偏转后的有效大主应力方向接近水平,有效小主应力小于零,因此可以推断心墙上游侧土体发生水力劈裂破坏的劈裂缝将沿着水平方向发展,这与实际工程的水力劈裂破坏裂缝方向是一致的。水力劈裂是否发生取决于心墙上游的水体进入心墙裂缝后的裂缝发展过程,裂缝贯穿,形成透水通道后的渗透冲蚀破坏是水力劈裂破坏的最终表现形式。在对试验结果的数值分析中采用了有效应力法和总应力法两种方法进行水力劈裂的判断,从判断水力劈裂的发生条件看,采用有效应力方法更为直接,但总应力方法本质上与有效应力判别方法并无矛盾。在系统分析了水力劈裂发生机理和发生条件的基础上,本文还针对实际工程,采用二维和三维有限元数值分析方法,对坝体在施工期和满蓄期的应力变形特性进行了分析,同时,采用极端参数的处理办法分析了心墙坡度、心墙与堆石模量差、水库蓄水速度等因素对心墙发生水力劈裂的影响。通过计算分析可以发现,坝壳堆石对心墙的“拱作用”是导致心墙发生水力劈裂的重要原因,而拱作用大小与心墙的厚度(坡度)、心墙与堆石的模量差有着直接的相关关系,同时,水库的蓄水速度也对心墙的有效应力值有一定的影响。从二维和三维计算分析还可以看出,心墙上游侧在蓄水期的高剪应力区也会对心墙的水力劈裂有一定的影响,同时,岸坡对坝体的约束作用也会在心墙与岸坡的接触部位产生水力劈裂的可能性。此外,黏土心墙坝的坝坡稳定是大坝安全的一个重要因素,本文对此进行了数值分析,评价坝体的整体稳定性。更多还原

【Abstract】 Hydraulic fracturing of earth core rockfill dam is one of the most concerned problems in dam engineering and also the most disputed issue in geotechnical engineering. Although a lot of research works had been done, there are still different opinions on the mechanism, conditions, impact factors and judgement criteria of hydraulic fracturing. In recent years, the construction of high rockfill dam has been developed rapidly. The research on the occurring possibility of hrdraulic fracturing and the method for determining the conditions of hydraulic fracturing become more and more important.Based on the systematic summarization of the previous research works, the centrifugal modeling tests on the simplified clay core models were conducted to study the mechanism and occurring conditions of hydraulic fracturing. The process of hydraulic fracturing of the homogeneous clay core was firstly observed in the laboratory by centrifugal model test. The tests results show that: when upstream water pressure of the model is larger than the earth pressure of the clay core, hydraulic fracturing will be happened. The ultimate failure mode of hydraulic fracture will be seepage failure of the soil.For studying the mechanism of hydraulic fracturing by numerical analysis method, further development of the ABAQUS software system was conducted. Supplementary material models such as Duncan’s E-B model and Shen Zhujiang’s elasto-plastic model were added in the original system. The reliability of the added models was verfied by testing sample and real engineering project.Based on the centrifugal modeling test and the secondary development of the ABAQUS software, further analysis on the mechanism of hydraulic fracturing were conducted by numerical analysis method. From the analysis, it can be found that: during water impoundment, when upstream water load is larger than the earth pressure of the core, tensile stress zone of the minor effective principle stress was developed at the upstream surface of the core, where is the position of the cracks of hydraulic fracture in centrifuge model test. The direction of principal stress of the soil elements of upstream and downstream sides will be deflexed due to the constraint of the boundary. After water impoundment, the deflexion of the direction of the principle stress of upstream elements become more significant. As the direction of the effective major principle stress is almost in horizontal direction and the effective minor principle stress less than zero, it can be sure that the cracks caused by hydraulic fracture will be developend in horizontal direction. This is agreed with the real situation. Whether the hydraulic fracture could finally lead to the failure of the earth core will depend on the crack development after upstream water enters into the crack of the core. The final failure caused by hydraulic fracture is the pass through of the crack from upstream to downstream and the soil erosion by water flow. The numerical analysis was conducted by effective stress method and total stress method. For the judgement of the condition of hydraulic fracturing, the effective stress method is more directly. But in nature, there is no conflict between the method of effective stress and total stress.On the bases of the analysis of the mechanism of hydraulic fracturing, the stress and deformation properties of a real high earth core rockfill dam were analyzed by 2D and 3D numerical analysis method. Besides, the impacts of several factors such as the earth core slope, modulus difference of rockfill and soil, the reservoir impoundment speed, etc. on the hydraulic fracturing of earth core were studied. From the analysis, it can be found that the arching effect of rockfill to the earth core is the most important cause of hydraulic fracturing. The extent of arching effect is directly related to the width of the core and the material properties of rockfill and soil. Besides, the speed of reservoir impoundment will also have a certain impact on the hydraulic fracturing of the core. From the 2D and 3D analysis, it could also be noticed that the high shear stress zone at the upstream side of the core may has certain impact on the occurring of hydraulic fracture. At the same time, the constraint of abutment on dam body may lead to the possibility of hydraulic fracturing at the contacting area of the earth core and the abutment.As the stability of dam slope is one of the most important factors on dam safety, the numerical analyis based on the secondary development of ABAQUS was conducted to assess the 3D slope stability of the dam.更多还原