Discrete element modelling of geogrid-reinforced railway ballast and track transition zones
Discrete element modelling of geogrid-reinforced railway ballast and track transition zones
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发表时间：
2013-07
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通讯作者：
Cheng Chen
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作者：
Cheng Chen
轨道恶化严重影响列车运行的安全性和效率（限速）。由于沉降问题，通常需要进行许多昂贵的、破坏性的和频繁的维修操作来维持镇流器特性。正因为如此，土工格栅解决方案已被证明是一种简单且经济的加固轨道道碴的方法，得到了广泛的应用。该项目对土工格栅加固铁路道碴的性能进行了评估。进行了大型箱体拉拔试验，以研究影响道碴与土工格栅之间相互作用的关键参数。试验结果表明，三角形孔径的三轴土工格栅的性能优于方形孔径的双轴土工格栅，且土工格栅孔径尺寸的影响大于肋剖面和连接剖面。然后，通过模拟大型箱体拉拔测试并与实验结果进行比较，使用离散元方法（DEM）对道碴与土工格栅之间的相互作用进行建模。 DEM 模拟结果已被证明可以很好地预测拉拔阻力，并揭示土工格栅加筋道碴系统中接触力的分布。离散元方法也被用来模拟土工格栅加筋道碴在有约束和无约束条件下的循环荷载。针对约束条件，对不同土工格栅位置和土工格栅孔径的未加筋样品和加筋样品进行了箱体试验模拟。土工格栅加筋道碴层在重复荷载作用下的响应与试验结果一致。结果发现，土工格栅的最佳位置是距离基层100 mm深度，并且三轴土工格栅的性能优于双轴土工格栅。在无侧限条件下，还模拟了道碴槽的循环荷载，土工格栅距次道碴层50mm处的样件表现最佳。研究还发现，在距离子道碴 50 毫米和 150 毫米处使用两块土工格栅比使用单层土工格栅或无筋道碴产生的沉降更小。土工格栅加筋限制了加筋区的横向位移，土工格栅上下约50mm。先前的研究表明，轨道支撑的刚度突然变化通常与轨道几何形状恶化速度加快、维护需求高和行驶质量差有关。然而，目前对于过渡区轨道几何恶化的机制还没有详细的了解。这项工作深入了解了可能导致或加速过渡区轨道退化的因素，以便识别和评估适当的缓解设计。使用 PFC3D 模拟了尺寸为 2.1m x 0.3m x 0.45m 的简单轨道过渡模型。为了确定和评估适当的缓解方法，对路基刚度分布的两种过渡模式（包括单步变化和多步变化）进行了测试。还研究了列车行驶方向和速度对过渡的影响。此外，在道碴层中采用土工格栅，检验土工格栅的加筋效果。
Track deterioration has a serious influence on the safety and efficiency (speed restriction) of train operations. Many expensive, disruptive and frequent repair operations are often required to maintain the ballast characteristics due to the problem of settlement. Because of this, a geogrid solution that has proved to be a simple and economical method of reinforcing track ballast is widely used. This project presents an evaluation of the behaviour of geogrid-reinforced railway ballast. Experimental large box pull-out tests were conducted to examine the key parameters influencing the interaction between ballast and the geogrid. The experimental results demonstrated that the triaxial geogrid with triangular apertures outperforms the biaxial geogrid with square apertures and the geogrid aperture size is more influential than rib profile and junction profile. The discrete element method (DEM) has then been used to model the interaction between ballast and geogrid by simulating large box pull-out tests and comparing with experimental results. The DEM simulation results have been shown to provide good predictions of the pull-out resistance and reveal the distribution of contact forces in the geogrid-reinforced ballast system. The discrete element method has also been used to simulate cyclic loading of geogrid-reinforced ballast under confined and unconfined conditions. For the confined condition, box tests have been simulated on unreinforced samples and reinforced samples with different geogrid positions and geogrid apertures. The response of the ballast layer reinforced with geogrid under repeated loading agrees with experimental results. It was found that the optimum location of geogrid is 100 mm depth from base, and the triaxial geogrid outperforms biaxial geogrid. For the unconfined condition, cyclic loading of a trough of ballast has also been simulated, and the sample with the geogrid at 50mm from the sub-ballast layer performs best. It was also found that the used of two geogrids at both 50mm and 150mm from the sub-ballast gave a smaller settlement than using a single layer geogrid, or the unreinforced ballast. The geogrid reinforcement limits the lateral displacement in reinforced zone, which is approximately 50mm above and below the geogrid. Previous investigations have shown that the abrupt stiffness change in track support is often associated with accelerated rates of deterioration of track geometry, high maintenance demand, and poor ride quality. However, at present, there is no detailed understanding of the mechanisms of track geometry deterioration at transition zones. This work provides insight into the factors that can cause or accelerate track degradation at the transition zones, in order to identify and evaluate appropriate mitigation design. A simple track transition model with dimensions 2.1m x 0.3m x 0.45m was simulated by using PFC3D. In order to identify and evaluate appropriate mitigation methods, two kinds of transition patterns, including a single step change and a multi step-by-step change for subgrade stiffness distribution were tested. The influence of the train direction of travel and speed on the transition were also investigated. In addition, geogrid was used in the ballast layer to examine the effects of geogrid reinforcement.