蠕变对冻土塑性屈服应力的影响研究

发布时间：2018-10-08 15:31

【摘要】：对于融土,在荷载持续作用下,会发生水和空气的挤出以及土颗粒相应的移动,此过程即为融土的固结过程。融土的固结过程常常伴随着土体内部结构的改变,并将融土历史上所承受的最大固结压力称为前期固结压力,其实质是土体内部结构的宏观反应。与融土类似,冻土内部也存在各种不同形式的结构,那么也应当存在类似前期固结压力的指标,学者们称为“似前期固结压力”或“准前期固结压力”,在冻土中其实质为塑性屈服应力。不同于融土的是,在荷载作用下,由于冻土的渗透系数远远小于融土,并不存在像融土那样的固结过程,但在荷载持续作用下冻土会发生蠕变,导致冻土内部的结构或发生强化或发生弱化,可以料想在荷载持续作用下(蠕变),表征冻土结构性的力学指标——塑性屈服应力应当发生相应的变化。为了探究冻土蠕变对塑性屈服应力的影响规律,本文先介绍了几种典型的冻土蠕变理论,并指出它们各自的适用性,在此基础上介绍由融土发展而来的等速线模型。为了验证该模型在描述冻土蠕变时的适用性,本文做了不同温度条件下经过不同蠕变时间后的K0加载试验,通过理论计算和试验的对比发现该模型能很好的描述冻土的蠕变行为,并且具有参数较少、各参数都有明确的几何及物理意义,还能将冻土的蠕变过程和塑性屈服应力联系起来的优点。最终得出塑性屈服应力是初始塑性屈服应力、压缩系数和回弹系数的函数,且其对数与蠕变应变(时间)呈线性关系的结论。冻土与融土在力学性质和行为方面存在诸多差异,其中最明显的一个差异就是冻土对温度的敏感性,由于等速线模型由融土发展而来,故没有考虑到温度的影响,从试验中可以看到该模型最主要的三个参数回弹系数、压缩系数和蠕变速率参数均与温度相关,所以为了利用该模型更加准确和合理地描述冻土的蠕变过程,就需要对该模型参数修正成温度相关函数。结果发现修正后的模型能很好的预测冻土蠕变趋势,在知道冻土温度条件时直接可以确定模型参数,使该模型在描述冻土蠕变时更加简洁方便和准确。最后回归实际工程,模拟了块碎石护坡U形路基在15年间的蠕变沉降情况,结果发现U形路基上部土层蠕变量较大,在第13年左右后进入第三蠕变阶段,路基下部土层蠕变较小在该计算时间段内并没有进入第三蠕变阶段,与此同时,路基上部土层还出现了较大的水平位移,这是由于U形外围护坡不稳固所致。针对这些问题本文提出了在路基施工时分层填筑每层之间铺设土工隔栅或土工织物的办法使这些土工合成材料对路堤两侧的位移形成拖拽作用,从而使整个路堤更加稳固。
[Abstract]:For the thawing soil, the extrusion of water and air and the corresponding movement of soil particles will occur under the continuous load, which is the consolidation process of the thawing soil. The consolidation process of the thawed soil is often accompanied by the change of the internal structure of the soil, and the largest consolidation pressure in the history of the thawing soil is called the pre-consolidation pressure, which is essentially the macroscopic response of the internal structure of the soil. Similar to the thawing soil, there are also various forms of structures in the frozen soil, so there should also be indicators similar to the pre-consolidation pressure, which scholars call "quasi-prophase consolidation pressure" or "quasi-pre-consolidation pressure". In frozen soil, its essence is plastic yield stress. Different from thawing soil, the percolation coefficient of frozen soil is much smaller than that of thawing soil under load, so there is no consolidation process like thawing soil, but the frozen soil will creep under the continuous load. As a result of strengthening or weakening of the structure of frozen soil, it can be expected that the plastic yield stress, the mechanical index of the structure of frozen soil, should be changed correspondingly under the continuous load (creep). In order to investigate the influence of frozen soil creep on plastic yield stress, several typical creep theories of frozen soil are introduced in this paper, and their respective applicability is pointed out. On this basis, the isokinetic model developed from thawed soil is introduced. In order to verify the applicability of the model in describing the creep of frozen soil, a K _ 0 loading test under different temperature and different creep time has been done in this paper. Through the comparison of theoretical calculation and experiment, it is found that the model can well describe the creep behavior of frozen soil, and has fewer parameters, and each parameter has definite geometric and physical significance. It is also an advantage that the creep process of frozen soil can be connected with plastic yield stress. Finally, it is concluded that plastic yield stress is a function of initial plastic yield stress, compression coefficient and springback coefficient, and its logarithm is linearly related to creep strain (time). There are many differences in mechanical properties and behaviors between frozen soil and thawed soil. One of the most obvious differences is the sensitivity of frozen soil to temperature. Because the isokinetic model is developed from thawed soil, the influence of temperature is not taken into account. It can be seen from the test that the three main parameters of the model are the springback coefficient, the compression coefficient and the creep rate parameter, which are all related to temperature, so in order to describe the creep process of frozen soil more accurately and reasonably by using this model, It is necessary to modify the parameters of the model into a temperature correlation function. The results show that the modified model can predict the creep tendency of frozen soil very well, and the parameters of the model can be determined directly when we know the temperature condition of frozen soil, which makes the model more concise, convenient and accurate in describing the creep of frozen soil. At last, the creep settlement of U-shaped roadbed is simulated in the past 15 years. The results show that the creep of the upper soil layer of U-shaped subgrade is large, and the third creep stage is entered after 13 years. At the same time, the upper soil layer of the roadbed also appeared a large horizontal displacement, which was caused by the unstable U-shaped peripheral slope protection. In view of these problems, this paper puts forward a method of laying geotextile or geotextile between each layer in subgrade construction to make the geotechnical composite material drag and drag the displacement of both sides of the embankment, thus making the whole embankment more stable.
【学位授予单位】：兰州大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TU445

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