周期温度边界条件下冻土融化固结特性研究

发布时间：2018-08-23 10:44

【摘要】：冻土的融化固结沉降是造成寒区工程病害的主要原因之一。以我国修建的青藏铁路和青藏公路为例,近年来的监测数据显示：多年冻土层的融化下沉会引起一系列的工程病害,例如：路面开裂、路面凹陷、道路翻浆、涵洞开裂、桥跳以及地基的不均匀沉降等。针对冻土的融化固结沉降,研究者进行了大量的试验和理论研究。就目前来看,这些工作主要针对常温边界条件下土体的融化固结特性。在实际工程中,冻土地基的融化沉降往往伴随着地表温度的周期性变化而呈现出季节性的变化规律,即：在暖季随着地表温度的升高,地基内孔隙水融化后在外压力的驱动下排出,地基沉降持续发展；在冷季地基内孔隙水冻结,由孔隙水的排出造成的固结沉降也随之停止。很明显,周期温度边界条件下土体的融化固结规律完全有别于常温边界的情况。因而,先前基于常温边界的研究结果并不适用于实际工程问题。基于这样的考虑,本文从试验和理论两个方面对周期边界条件下冻土的融化固结规律及其影响因素进行了以下几方面的研究：(1)根据实际工程中边界温度变化遵循正(余)弦函数的规律,进行了不同周期温度和含水量条件的一维融化固结试验。研究了融化固结变形和融化深度随时间的发展变化规律。试验结果表明,一维周期温度边界条件下土体的融化固结变形随时间的发展呈现出周期性的发展规律,这是区别于常温边界条件的最显著特征。同时,随着冻融次数的增加,周期温度边界条件下的融化固结变形和融化深度均逐渐趋于常温边界的情况。(2)基于FLAC数值软件建立了适用于周期温度边界条件下融化固结的数值模拟平台。为了避免计算单元的温度状态反复改变导致的存储变量丢失的问题,本文将反复冻融的区域始终作为融化区域,并在该区域内进行固结计算。对于反复冻融的区域,通过改变其渗透系数以模拟冻结和融化状态对排水通道的影响。(3)通过对比分析一维状态下融化固结变形和融化深度的试验和理论计算结果验证了该数值平台的正确性。同时基于该平台计算分析了融化固结度随时间的变化规律。结果表明,融化固结度呈周期性衰减的变化趋势。这是周期温度边界条件下,土体融化固结特性有别于常温边界的最显著特点(常温条件下,固结度为常数)。同时,融化固结度的衰减速率正比于融化固结率。(4)对冻土路基的计算分析表明,冻土路基的周期性融化固结规律主要源于周期温度边界条件下深层冻土的融化固结,即：在每年的8至11月之间多年冻土层持续融化时,相应的固结变形也随之产生。对比路基沉降的计算和实测结果表明,本文所建立的针对周期温度边界条件下冻土的融化固结数值模拟能够较好地预测冻土路基的周期性融化沉降规律。因此,该平台可以作为计算实际寒区工程问题的可靠工具。(5)冻土路基的融化固结度随时间的发展呈现出先增大后减小的趋势。这表明路基浅层土体在运营初期已完成固结,后期固结度的降低主要是深层冻土的融化压缩所致。进一步的计算分析表明融化固结度的减小速率正比于冻土路基的融化固结率。通过拟和分析,路基融化固结的降低速率和融化固结率之间存在良好的幂函数关系。本研究表明：基于周期温度边界条件下冻土融化固结规律建立的数值模拟平台能够较好的预测土体的固结融化规律；对于较为理想的一维状态和边界条件复杂的实际工程问题,周期温度边界条件下土体的融化固结特性完全不同于常温边界的情况；同时表征土体热学和力学特性的融化固结率对土体的融化固结规律有着重要影响,在研究周期温度边界条件下冻土的融化固结时必须加以考虑。
[Abstract]:The thawing and consolidation settlement of frozen soil is one of the main causes of Engineering diseases in cold regions. Taking the Qinghai-Tibet Railway and Qinghai-Tibet Highway built in China as examples, the monitoring data in recent years show that the thawing and subsidence of permafrost layer can cause a series of Engineering diseases, such as pavement cracking, pavement depression, road grouting, culvert cracking, bridge jumping and so on. For the settlement of frozen soil, researchers have done a lot of experiments and theoretical research. At present, these works mainly focus on the characteristics of soil thawing and consolidation under normal temperature boundary conditions. There is a seasonal variation law, that is, in the warm season with the increase of surface temperature, pore water melted in the foundation under the driving of external pressure, the foundation settlement continued to develop; in the cold season, pore water frozen in the foundation, the consolidation settlement caused by the discharge of pore water also stopped. Therefore, the previous research results based on the normal temperature boundary are not applicable to practical engineering problems. Based on this consideration, this paper studies the regularity of thawing and consolidation of frozen soil under periodic boundary conditions and its influencing factors from both experimental and theoretical aspects: ( 1) According to the law of sinusoidal (cosine) function of boundary temperature in practical engineering, one-dimensional melting and consolidation tests under different periodic temperature and water content conditions were carried out. The development of melting and consolidation deformation and melting depth with time were studied. At the same time, with the increase of freezing-thawing times, the melting and consolidation deformation and the melting depth under the periodic temperature boundary condition tend to the normal temperature boundary gradually. (2) Based on the FLAC numerical software, a suitable periodic temperature boundary is established. In order to avoid the loss of storage variables caused by repeated changes in the temperature state of the calculation unit, the freeze-thaw region is always regarded as the melt region and the consolidation calculation is carried out in this region. For the freeze-thaw region, the freeze and thaw are simulated by changing the permeability coefficient. (3) The validity of the numerical platform is verified by comparing and analyzing the experimental and theoretical results of the deformation and depth of melting consolidation in one-dimensional state. This is the most remarkable characteristic of the soil thawing and consolidation under the periodic temperature boundary condition (the degree of consolidation is constant at room temperature). At the same time, the attenuation rate of the degree of thawing and consolidation is proportional to the rate of thawing and consolidation. (4) The calculation and analysis of the frozen soil roadbed show that the periodic thawing and consolidation law of the frozen soil roadbed is the main. The thawing and consolidation of deep frozen soil originated from the periodic temperature boundary condition, i.e. the corresponding consolidation deformation occurs when the permafrost layer continues to thaw between August and November of each year. Comparing the calculation of subgrade settlement with the measured results, the numerical simulation of thawing and consolidation of frozen soil under the periodic temperature boundary condition established in this paper can be used to simulate the thawing and consolidation of frozen soil. Therefore, the platform can be used as a reliable tool for calculating practical engineering problems in cold regions. (5) The degree of consolidation of frozen soil roadbed increases at first and then decreases with the development of time, which indicates that the shallow soil of roadbed has completed consolidation at the beginning of operation and consolidation at the later stage. The reduction is mainly due to the thawing compression of deep frozen soil. Further calculation and analysis show that the reduction rate of thawing consolidation is proportional to the thawing consolidation rate of frozen soil roadbed. The numerical simulation platform established under the condition of thawing and consolidation law of frozen soil can better predict the consolidation and thawing law of soil; for the ideal one-dimensional state and practical engineering problems with complex boundary conditions, the thawing and consolidation characteristics of soil under the condition of periodic temperature boundary are completely different from those under the condition of normal temperature boundary; The thawing and consolidation rates of thermodynamic and mechanical properties have an important influence on the law of thawing and consolidation of frozen soils.
【学位授予单位】：兰州大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：U416.16;U213.14

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