兰州大学机构库 >土木工程与力学学院
蠕变对冻土塑性屈服应力的影响研究
Alternative TitleInfluence of creep on plastic yield stress of frozen soil
乔响平
Thesis Advisor张豫川 ; 慕青松
2017-05-01
Degree Grantor兰州大学
Place of Conferral兰州
Degree Name硕士
Keyword冻土 蠕变 塑性屈服应力 模型参数 有限元模拟
Abstract

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。

Other Abstract

There will be extrusion of water and air and corresponding movement of soil particles under sustained load as for thawed soil, which process is the consolidation of thawed soil. The consolidation process often accompanied by the change of the internal structure of the soil. The maximum consolidation pressure in the history of soil consolidation is called the pre consolidation pressure, which of essence is the macroscopic response of soil internal structure. Similar to the thawed soil, different forms of internal structure also exist in frozen soil, similar index like the preconsolidation should exist in frozen soils, the scholars called "quasi preconsolidation pressure", which of essence is the plastic yield stress for frozen soil. There is not consolidation process like thawed soil under the load because the permeability coefficient of the frozen soil is much smaller than that of the soil. However, under the action of load there exist creep for frozen soil which can lead to internal structure strengthening or weakening.It can be expected that the plastic yield stress, which represents the structural characteristics of frozen soil, should be changed accordingly under the continuous load.

In order to explore the influence of creep on frozen soil plastic yield stress, this paper first introduces several typical frozen soil creep theory, and their respective applicability, and then employed an isotache model. In order to verify the applicability of the model in describing the creep of frozen soil, creep experiment for different durations at different tempreture was conducted at first and then step loadings under K0 condition were applied. By comparing the theoretical calculation and the experiment, it shows that the model can well describe the creep process of frozen soil, and it has fewer parameters, both geometric and clear physical meaning, and also the creep process of frozen soil and plastic yield stress linked advantages. Finally, it is concluded that plastic yield stress is a function of initial plastic yield stress, compression and unloading index and the logarithm of plastic yield stress is linearly related with creep strain (time).

One of the most obvious differences among many difference in mechanical properties and behavior between frozen soil and thawed soil is the temperature sensitivity for frozen soil. Since the isotache model is developed from the thawed soil, it does not take into account the effect of temperature, in order to use the model to describe the creep process of frozen soil more accurately and reasonably, it is necessary to modify the model parameters into temperature correlation function and it can be seen from the experiment that the three main parameters of the model— compression index, unloading index and creep rate are all temperature correlation. The results show that the modified model can well predict the creep tendency of frozen soil, and the model parameters can be determined directly when the frozen soil temperature is known which make the model more simple , convenient and accurate.

Finally, in this paper, the creep settlement of U-shape crushed-rock embankment in 15 years is simulated. The results showed that the upper soil layer had larger creep and the creep process come into the third creep stage after thirteenth years’ creep. The results also showed that the lower soil layer of the subgrade has less creep settlement and the creep process just at the second stage. At the same time, there is a large horizontal displacement in the upper soil layer, which is due to the instability of the U-shape peripheral slope. To solve these problems this paper proposes setting geotextiles at every layer to restrict displacement on both sides of embankment when layered filling in subgrade construction which can make the whole embankment more stable.

URL查看原文
Language中文
Document Type学位论文
Identifierhttps://ir.lzu.edu.cn/handle/262010/226017
Collection土木工程与力学学院
Recommended Citation
GB/T 7714
乔响平. 蠕变对冻土塑性屈服应力的影响研究[D]. 兰州. 兰州大学,2017.
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