【作者】 程海涛；

【导师】 谢永利；

【作者基本信息】 长安大学 ， 道路与铁道工程， 2008， 博士

【摘要】 复合地基技术在公路建设领域得到广泛应用,积累了丰富工程经验,但其理论研究相对滞后,特别是路堤荷载作用下复合地基工作机理研究更显薄弱。通过系统的土工离心模型试验、现场载荷试验及原型观测、数值分析等手段开展了路堤荷载下软黄土粉喷桩复合地基力学性状及变形性状研究,并提出了路堤荷载下复合地基承载力确定方法。基于土工离心模型试验探讨了不同工况下复合地基荷载传递及相对位移模式,分析了基础刚度、有无软弱下卧层(下卧层厚度)等因素的影响。刚性基础悬浮式桩侧出现向上的摩阻力,低荷载水平时呈倒三角形分布,高荷载水平时呈S型分布;桩与端部半球形土体一起刺入下卧层,复合地基破坏模式为桩向下刺入破坏。柔性基础悬浮式桩1/6~1/3桩长深度范围内出现向下的桩侧摩阻力,该深度以下为向上的摩阻力。向上的摩阻力在低荷载水平时呈倒三角形分布,高荷载水平时呈马鞍型分布。桩土相对位移为零的中性点出现在1/6~1/3桩长深度处。柔性基础支撑式桩侧出现向下的摩阻力,低荷载水平时呈倒三角形分布,高荷载水平时呈倒梯形分布;桩顶端发生明显向上刺入变形。通过原状土、桩间土、单桩、单桩复合至六桩复合等一系列载荷试验研究,揭示了复合地基在不同载荷板尺寸下的变形及承载性状。原状饱和黄土及桩间土载荷试验曲线为二次屈服型,桩间土极限承载力约为原状土的2倍,在复合地基设计中可以利用二次屈服承载潜能。复合地基承载力特征值随载荷板尺寸增加而减小,减小幅度受场地特性影响较大。载荷板下铺设垫层呈现出部分柔性基础特性。桩间土承载力发挥系数特征值介于1.0~1.5之间,而桩承载力发挥系数特征值介于0.3~0.8之间。桩土相对位移模式是随荷载水平发生动态变化的。路肩及坡脚处未充分利用桩间土的承载潜力。通过数值计算探讨了不同载荷板尺寸下复合地基工作性状。随载荷板宽度增加,复合地基沉降呈双曲线型减速增大趋势,中性点上移,地基沉降增长点下移;刚性基础下应力先减小后增大,柔性基础下应力先增大后减小;刚性基础下桩土应力比先增加后减小,柔性基础下桩土应力比先减小后增大;两种基础刚度下桩底处桩土应力比持续减小。基于载荷试验与路堤荷载作用下复合地基性状差异,分析了载荷试验及公式估算法确定路堤荷载下复合地基承载力的局限性;提出了适用于确定路堤荷载作用下复合地基承载力的两种修正方法。通过工程实例验证了修正方法的可行性。更多还原

【Abstract】 Composite ground technology has been widely used in expressway construction. But the theoretical study on composite ground is lagged behind engineering practice, especially the study on working mechanism of composite ground under embankment or flexible foundation. Mechanical and deformation behaviors of composite ground are studied with the means of geotechnical centrifugal model test, static loading test, prototype test and numerical analysis. At last the methods of calculating bearing capacity of composite ground are proposed.Load transfer mechanism and relative displacement mode of different kinds of composite ground and the influence of foundation stiffness and substratum thickness are studied on the base of geotechnical centrifugal model test. The distribution of upward frictional resistance of suspended pile under rigid foundation shows inverted triangle shape when the load-level is lower and S shape when the load-level is higher. Pile and hemispherical soil mass at pile tip piece into the substratum together. The failure of composite ground under rigid foundation is caused by piles piecing into substratum. There is downward frictional resistance over the depth of 1/6~1/3 times of pile length and upward frictional resistance below that depth under flexible foundation. The distribution of upward frictional resistance shows inverted triangle shape when the load-level is lower and shows saddle shape when the load-level is higher. The neutral point at which the settlement of pile and that of soil among piles is equal appears at the depth of 1/6~1/3 times of pile length. The distribution of downward frictional resistance of bearing pile under flexible foundation shows inverted triangle shape when the load-level is lower and shows inverted trapezoid shape when the load-level is higher. There is obvious upward penetration at the top of pile.Deformation and bearing characteristics of composite ground under different size of loading plate is revealed through a series of in situ tests on original soil, soil among piles, single pile, from one pile compound to six piles compound. Original saturated loess and soil among piles show obvious second yield phenomenon, which can be used in the design of composite ground. The ultimate bearing capacity of soil among piles is 2 times of that of original soil. The characteristic bearing capacity of composite decreases with the growth of size of loading plate and the properties of site have obvious effect on it. Because of the cushion under loading plate, the pile-soil stress ratio of static loading test presents the properties of pile-stress ratio under flexible foundation. The stress-bearing capacity ratio of soil among piles is about 1.0~1.5, and that of piles is 0.3~0.8. Relative displacement mode between soil and pile dynamically changes with the load-level improving. The bearing capacity of soil among piles at road shoulder and toe of slope is underutilization. Working behaviors of composite ground under different size of loading plate are investigated using numerical analysis. The relation between settlement of composite ground and width of loading plate can be described by hyperbolic curve. The neutral point at which the settlement of pile and that of soil among piles is equal moves up and the growth point of settlement downward shifts with the increasing of width of loading plate. With the growth of loading plate width, stress under rigid foundation decreases first then increases and that under flexible foundation increases first then decreases. Pile-soil stress ratio at the top of pile under rigid foundation increases first then decreases and that under flexible foundation decreases first then increases. The pile-soil stress ratio at the bottom of pile under both kinds of foundation decreases with the growth of loading plate width.The limitations of the methods, static loading test and calculation formula, to determine the bearing capacity of composite ground is pointed out. And two kinds of modified methods which are suitable to determining the bearing capacity of composite ground under embankment or flexible foundation are proposed.更多还原