【作者】 刘雨；

【导师】 龚维明；

【作者基本信息】 东南大学 ， 结构工程， 2005， 硕士

【摘要】 本文在分析多年冻土地区桩基工作机理的基础上,结合冻土的物理力学性质及热学性质,着重研究了青藏高原多年冻土地区钻孔灌注桩设计中基础冻结力的形成过程、回冻过程中基桩承载力的变化规律、施工后桩周冻土的回冻规律等几个主要力学问题。对灌注桩基础在多年冻土条件下的力学特性和工作机理进行了阐述,表明了其力学问题的差异性和特征性,提出多年冻土中桩基的设计准则是控制桩的蠕变速度,使桩侧剪应力不超过冻土与桩之间的极限长期冻结力,是承载力控制与变形控制的统一。采用自平衡静载荷测试方法对回冻过程中不同地温条件下的单桩承载力进行了测试,探讨了基桩的承载力变化规律与荷载传递规律。测试结果表明,随着冻土温度的降低,桩侧冻结力和单桩承载力有明显提高。由于施工加载及稳定性需要,桩侧冻土融化后能否及时回冻就成为影响工程进度和施工期间基础稳定的关键。本文基于现场实测资料,分析了多年冻土区地温对桩基础施工过程的响应,及其在工程结束后的恢复过程,探讨了在多年冻土区灌注桩施工后,桩周冻土的回冻规律。研究表明,在低温冻土区灌注桩的回冻较快,20天左右桩体温度就降到0℃以下,静载试验显示此时冻结力即已产生,桩基已具备相当承载力;混凝土单桩施工的热扰动范围一般为3倍桩径左右,要使近桩地温完全恢复到施工以前的水平,则可能需要很长时间。最后对桩周冻土的回冻进行了数值模拟分析。结合地质资料、冻土类别,建立了热力学计算模型,模拟桩周冻土融化圈的形状和大小,得出近桩土层温度随时间的变化规律和不同时期桩周土层温度的空间分布规律。结果表明,用有限元分析桩周冻土的温度变化趋势是有效的,可以满足工程上的精度,从而为设计施工提供一定的依据。更多还原

【Abstract】 Based on the analysis of physical and mechanical characteristics of frozen soil, the working mechanism of cast-in-place pile in permafrost areas on the Tibetan Plateau is studied in this paper. The main factors affecting the bearing capacity and the stability of pile foundation in permafrost are presented. Some key technical problems of pile engineering in permafrost were discussed. These problems comprise the formation process of freezing strength between the pile and frozen soil, the variations of the pile’s bearing capacity during the freezing back period and the law for distribution of ground temperature field at different ages of pile formation, etc.During the construction of concrete filling pile in permafrost, the heat provided by hydration leads to increase of the ground temperature and thawing of the frozen soil around the pile. Then, with the decrease of the hydration heat, the soil freezes back slowly. In this period, a self-balanced testing method was adopted to test the bearing capacity of the pile for two times respectively. The results indicate that the bearing capacity increases as the soil temperature decreases. From the Q-S curve of single pile, conclusion can be drawn that after about one month of soil freezing back, the pile can provide enough bearing capacity for the next step of construction.The heat of cement hydration brought into the frozen earth during concrete engineering should be taken into account. The response of soil temperature profile to the construction of engineering piles in permafrost is studied by analyzing the in-situ data. For the specific situation observed, it is found the temperature in the pile decreases to below 0℃in 20 days after reaching its maximum value; however, it may be a long process for the soil temperature around the pile to recover its original value.Based on the theory of heat conduction, a heat analysis model of the temperature field of ground around a single cast-in-place pile in permafrost was established. The variations of temperature field of ground are calculated by using the finite element method with consideration of actual boundary conditions and initial conditions of temperature. The FEM solutions and it’s comparison with the measured data indicate that numerical analysis by program can simulate the recovery of the soil temperature effectively.The conclusions drawn out from this paper can provide reference for working out construction plans of cast-in-place pile in permafrost regions.更多还原