Energy analysis of stability of twin shallow tunnels based on nonlinear failure criterion

Based on nonlinear Mohr-Coulomb failure criterion, the analytical solutions of stability number and supporting force on twin shallow tunnels were derived using upper bound theorem of limit analysis. The optimized solutions were obtained by the technique of sequential quadratic programming. When nonlinear coefficient equals 1 and internal friction angle equals 0, the nonlinear Mohr-Coulomb failure criterion degenerates into linear failure criterion. The calculated results of stability number in this work were compared with previous results, and the agreement verifies the effectiveness of the present method. Under the condition of nonlinear Mohr-Coulomb failure criterion, the results show that the supporting force on twin shallow tunnels obviously increases when the nonlinear coefficient, burial depth, ground load or pore water pressure coefficients increase. When the clear distance is 0.5to 1.0 times the diameter of tunnel, the supporting force of twin shallow tunnels reaches its maximum value, which means that the tunnels are the easiest to collapse. While the clear distance increases to 3.5 times the diameter of tunnel, the calculation for twin shallow tunnels can be carried out by the method for independent single shallow tunnel. Therefore, 3.5 times the diameter of tunnel serves as a critical value to determine whether twin shallow tunnels influence each other. In designing twin shallow tunnels,appropriate clear distance value must be selected according to its change rules and actual topographic conditions, meanwhile, the influences of nonlinear failure criterion of soil materials and pore water must be completely considered. During the excavation process, supporting system should be intensified at the positions of larger burial depth or ground load to avoid collapses.     

基于MODIS多时相的江苏启东市油菜种植面积提取

利用遥感手段,基于油菜种植生长期的MODIS-NDVI 16d合成数据,提取江苏省启东市2008~2009、2009~2010和2010~2011年3个生长季的冬季油菜种植面积。根据油菜生育期NDVI值的时相变化建立了NDVI时序曲线;利用阈值剔除非耕地数据后,对数据利用最小噪声分离(MNF)方法进行压缩处理,最后利用波谱角分类法确定油菜种植区的空间分布,计算油菜种植面积。计算结果与实际统计面积相比,精度达到90%以上,表明该方法作为一种快速油菜种植面积监测方法,具有较高的监测精度。     

非饱和浅埋隧道稳定性的上限分析

考虑土体的非饱和性质,采用极限分析上限法研究非饱和浅埋隧道环形开挖面的稳定性。基于已有的研究成果,构建非饱和浅埋隧道环形开挖面的破坏模式及相容速度场。根据外力功率等于内能耗散率建立功率方程,推导拱顶和边墙围岩压力的解析解,并通过序列二次规划算法得到围岩压力的最优上限解。将饱和与非饱和2种情况下浅埋隧道的围岩压力进行对比,相对差值非常显著,体现了在工程中考虑土体非饱和特性的必要性。此外,分析了非饱和土体参数对浅埋隧道围岩压力和破裂面的影响。