青藏高原多年冻土区斜坡稳定性研究

【作者】 靳德武；

【导师】 刘玉海； 陈志新； 倪万魁；

【作者基本信息】 长安大学 ， 地质工程， 2004， 博士

【摘要】 青藏高原地区冻土分布面积达1.5×10~6 km~2，占我国多年冻土分布面积的70％，在全球气候转暖的背景下，多年冻土有着退化的趋势。而青藏高原地区的各类工程活动则进一步加剧了多年冻土退化的趋势，由此引发的冻融灾害不仅威胁着公路、铁路、桥梁、输油管道等线性工程的建设，而且导致寒区生态环境恶化，使多年冻土丧失恢复能力，热融滑塌就是最为典型的斜坡冻融灾害。 本文从青藏高原多年冻土区冻土斜坡失稳类型分析、典型冻土斜坡地质结构分析入手，通过对青藏公路K3035里程典型斜坡热融滑塌模型试验研究、稳定性分析、现场监测、温度场和变形场数值分析，开展了青藏高原多年冻土区斜坡稳定性研究，取得了如下成果： ① 在全面总结青藏公路沿线有关斜坡研究成果的基础上，通过调查对青藏高原多年冻土区冻土斜坡失稳类型进行了划分，根据其形成原因和冻土赋存环境，正融土滑坡可以划分为两种类型：融冻泥流型和热融滑塌型；正冻土滑坡可划分为两种类型，即蠕变型滑坡和崩塌型滑坡。 ② 在青藏高原多年冻土区斜坡地带活动层土体沿确定的滑动面(冻融界面、厚层地下冰面)在融化季节滑动形成融冻泥流和热融滑塌两类典型的滑坡灾害。热融滑塌的失稳因素可以归结为：a．厚层地下冰的存在且埋藏深度较浅易受地表气温变化的影响，形成冰土分界面相对隔水且抗剪强度极低的滑坡条件，这是热融滑塌形成的内在因素；b．不合理取土或自然营力侵蚀引起的厚层地下冰的暴露且使坡体形成临空面而使其失去支承力，这是热融滑塌形成的外在因素(诱发因素)。 ③ 青藏公路K3035热融滑塌的显著特点是滑塌体不是整体一次性滑动，目前的滑坡范围扩展是从后缘原坡体融化、开裂、坍塌，塌落的土体沿滑面滑动。扩展发育时期为每年气温正值期，且主要集中在0～2m深度土体范围内，处于融化状态的7-10月。综合2000～2002年3年内滑坡范围扩展情况，估算现今滑坡范围向后扩展的速度为5m／a。 ④ 任何滑坡的产生，水分条件的变化是其重要的影响因素。热融滑塌更是如此，现场监测结果表明：融化季节在青藏公路K3035滑体2m以上范围的土体含水量高于冰层之上的粉质粘土液限值；在青藏公路K3057处大于同期对未滑动活动层土体的含水量的测量结果，即滑体水分显著高于原土体。冰面水分的聚集主要是由地下冰的融化所引起，正是由于大量融化水的存在，使得土体强度显著降低，并沿地下冰面产生了滑动。 ⑤ 根据相似理论第一定律，应用积分类比法推导并建立了冻土斜坡模型试验的相似指标和相似判据，得出以原状土作为模型介质时，6个相似常数可以减少为2个相似常数，即c_τ和c_ι。模型与原型的时间比例尺是由几何比例尺决定的，即c_τ=c_ι~2，仅有一个相似参数为自变量，另一个为因变量。应用自行设计的变形测量装置对斜坡土体表面冻胀和融沉进行了计算和分析，得出冻土斜坡的融沉变形远远大于冻胀变形的结论，与青藏公路沿线多年冻土地区公路路基变形观测结果一致。通过模型试验，得出变形场受温度场控制的结论，并在多个冻融循环中，存在着一个斜坡后继变形的“温度门限”，控制着斜坡变形的进一步发展。这些成果填补了多年冻土区滑坡研究的空白，为青藏高原多年冻土区道路工程建设中滑坡整治提供了依据。 ⑥本文推导了相变热传导问题微分控制方程的一般形式，应用有限元法对青藏公路K3035斜坡地温场和变形场进行了模拟，建立了冻土斜坡温度场和变形场数值分析模型。模拟了滑坎后缘拉张区5年内位移变化情况，得出冻土滑坡体每一年中位移由两个阶段即稳定阶段和非稳定阶段组成的结论，稳定阶段对应台阶面，非稳定阶段对应台阶壁;每一个台阶面均向右微微“上翘”，可以得出土体在冻结过程中还发生上坡位移，而不单单仅存在下坡位移，这一计算结果与风火山冻土蠕变监测结果取得了很好的一致。从模拟坡面内不同深度对应的下坡位移可以得出冻土斜坡的位移主要发生在浅部，深部基本不发生位移的结论，与青藏公路K3035斜坡现场测斜监测资料一致。采用基于灰色系统理论的GM(1，1)模型，对滑塌距离动态变化进行预测，计算滑塌距离为5.76m，误差为12.94%，说明所建立的GM(1，1)模型可以作为青藏公路K3035滑塌距离年内月平均变化的预测模型。 ⑦应用有效应力原理推导了不同渗流条件下无限斜坡稳定性分析的计算公式，对各种不同表达形式的安全系数进行了讨论和对比，得出在流线平行于斜坡条件下，两类特殊条件(干土坡、饱水土坡)下的分析结果与相同情况下安全系数统一式结果相一致的结论;当已=O及0(m(l时，无限斜坡安全系数的通式为尸=[(1一m)+m上]竺旦竺ysat tana流线为相互平行的水平线、流线与坡向一致两种情况分别为孔隙压力和安全系数的两种特例。流线与水平线的夹角为。‘刀‘a，二，.、二二/。、__刀_____:__:____翔。二。，，、。卜、、、。孔隙水压力u(刀)=mzy*一二mzr，sin，a，刀一。和刀=a分别对应上述两种极 口值情况(流线为相互平行的水平线、流线与坡向一致)。 ⑧绘制出了不同坡度、不同含水条更多还原

【Abstract】 Permafrost area of Qinghai-Tibet Platean is 1. 5x106 km2,70 percent of permafrost area in China. On the basis of global warming, there is a degradation tendency for permafrost. The degradation tendency of permafrost will become worse caused by all kinds of human engineering activities. Geological hazards from freeze-thaw action will not only threat linear engineering construction for example, highway, railway, bridge, oil pipeline, etc et, but result in deterioration for ecological environment of cold region to lose recovery ability for permafrost. Thermal-thaw sliding collapse is a typical example.The following researches have been carried out in this paper, including slope instability patterns analysis in permafrost region of Qinghai-Tibet Platean, geological structure analysis for typical permafrost slope, model experiment of thermal-thaw sliding collapse of Qinghai-Tibet milestone K3035, stability analysis method, in-situ monitoring and numerical simulation of temperature field, displacement field. The following conclusions have been put forward.(1)On the basis of concluding research results of permafrost slope along Qinghai-Tibet highway, permafrost instability patterns are divided into two categories, freezing soil sliding and thawing soil sliding.Gelifluction & thaw slumping are concluded in thawing soil sliding. Creep sliding & collapse sliding are concluded in freezing soil sliding, It is precondition that soils are saturated by water and thick ice takes as sliding face for gelifluction and thaw slumping.(2)Factors caused by thaw slumping are as follows, a. thick layer ground ice and shallow embedding depth, b. thick layer ground ice uncovered(3)Thaw slumping in milestone K3035 of Qinghai-Tibet highway is not sliding at a time. At present, movement of slope scale begin with melting, crack, collapse. During positive temperature every year, sliding collapse begin to develop with depth in 2m,between July and October, in the period of 3 years between 2000 and 2002 ,it is estimated velocity of thaw slumping is 5m/a.(4)Water conditions are important for any slop sliding. It show that during thaw season, for collapse sliding of milestone K3035 of Qinghai-Tibet highway , water content of soil above 2m in depth is more than 30%, it is higher than liquid limit value situated on thick layer ice. Water content is more than 19% for collapse sliding of milestone K3057 of Qinghai-Tibet highway. At the same time, water content of no sliding soil is less than 18%, water content of sliding soil is obviously higher than that of no sliding soil. Water Congregation at ice surface is mainly caused by ground ice melting. It is melting water that soil strength is obviously decrease, and sliding along thick ice surface. 〢ccording to the first law of simulation theory, simulation index and criterion for permafrost slope model have be concluded and built by applying integration comparison method. It is concluded that taking in-situ soil as similar material, six similar parameters willbe decreased two parameters, this is ct and cl .Time scale factor between prototype andscale model is determined by geometry scale factor, this is ,ct =cl2 . Only one similar factoris self-variable, another is dependency variable. The following conclusions have been concluded, deformation field is controlled by temperature field. During a series of temperature cycles, there are critical temperature value successive displacement. Frost heaving and thaw settlement for slope soil surface are calculated and analyzed by applying self-design model triangle frame. It is concluded that frost heaving is much more than thaw settlement, it is in accordance with observation results along Qinghai-Tibet highway.(6)Differential equation of heat transfer with phase change is derived in this paper. Simulation of temperature displacement field of permafrost slope at milestone K3035 of Qinghai-Tibet highway have been carried out by Marc finite element software. Displacement of 5 years is simulated behind sliding step. It is drawn out that displacemen更多还原