【作者】 刘俊飞；

【导师】 赵国堂；

【作者基本信息】 西南交通大学 ， 岩土工程， 2011， 博士

【摘要】 本论文以铁道部重点科研项目“高速铁路CFG桩复合地基综合技术研究”(编号2007G046)为依托,在对京沪高速铁路李窑、凤阳试验段CFG桩筏复合地基和桩网复合地基各部分应力、变形测试结果进行综合分析的基础上,结合数学解析和数值模拟分析,较为系统地提出了铁路路堤下CFG桩复合地基的荷载传递机理、附加应力计算模型和沉降计算方法。1.在路堤下地基压力数值模拟的基础上,结合试验段剖面管沉降测试、路堤填土下土压力的现场测试结果,建立了铁路路堤下地基压力的扩散角法计算模型。2.试验段现场测试结果显示出两种复合地基形式桩顶/桩帽顶处土体以及路堤填土中的应力与荷载大小有关,同一荷载下应力随时间变化不大；桩筏复合地基LTP主要通过刚性板约束下的垫层来调节桩顶平面的桩、土应力,使桩承担更多的荷载；桩网复合地基LTP主要通过桩顶上拱效应,并通过面积扩大的桩帽“收集”作用,使更多的荷载转移到桩上。李窑、凤阳桩网复合地基LTP加筋垫层中的土工格栅变形较小并未发挥明显的拉膜效应。就桩顶/桩帽顶平面的桩土应力比而言,一般情况下桩筏复合地基的桩土应力比明显大于桩网复合地基。但就桩的荷载分担比而言,桩网复合地基并不“逊色”于桩筏复合地基。李窑、凤阳两个试验段中,桩网复合地基的桩土应力比仅仅约为桩筏复合地基的1/10,但由于桩帽增加了受力面积,最终使桩的荷载分担比差别不大,各分区的CFG桩均以不足7%的面积置换率获得了65-79%的荷载分担。3.凤阳试验段桩身轴力现场测试和分析结果显示,桩筏、桩网复合地基在桩的荷载传递方面有共同的特征。即,桩顶段都存在负摩阻力,在其作用下,部分桩间土所承受的荷载又重新传递到桩上,使桩轴力增加,桩间土附加应力减小。在某一深度,桩身轴力达到最大,附加应力降低到到最小值。在该深度以下,桩身轴力减小。计算分析表明,在该深度以下的一定深度范围内桩间土竖向应力等于或接近于路堤填筑前地基土中竖向应力。桩顶、桩端处桩侧摩阻力发挥较为充分；中性点(等沉面)两侧一定范围内,由于桩、土相对位移较小,摩阻力发挥受到限制,摩阻力较小；负摩阻段相对较短,中性点出现在桩身偏上的位置。4.基于一维压缩,桩侧摩阻力充分发挥,并近似认为中性点处桩间土附加应力减小到一个较低水平,可以忽略不计,提出了桩筏复合地基LTP的弹簧组模型,对“碎石垫层+钢筋混凝土板”型LTP的荷载传递效果进行模拟和分析。采用该模型,可以对桩顶平面桩、土应力,桩顶刺入变形等进行计算。5.指出桩顶土拱作用是桩网复合地基“桩帽+土工格栅+碎石垫层”型LTP的主要荷载传递方式。通过对桩网复合地基桩顶土拱的现场试验研究和数学解析,指出桩顶土拱的拱形接近于抛物线。建立了桩顶平面桩、土应力计算的球孔扩张法和抛物线拱法。6.以Mindlin解法为基础,提出了CFG桩复合地基加固区—下卧层并联模型。加固区与下卧层土体的模量比将影响到这一并联效应,使其应力比约等于模量比。由于加固区具有一定的整体性,加固区宽深比也将对并联效应产生影响。加固区宽深比越大,下卧层受力越大。综合加固区与下卧层土体模量比和加固区宽深比的影响,可得下卧层附加应力系数ψ,即下卧层中附加应力与等代深基础Boussinesq解附加应力之比。7.系统地提出了非疏桩条件下铁路CFG桩复合地基土中附加应力的简化计算方法：加固区附加应力根据侧限假定下的桩土相互作用简化模型进行计算；下卧层附加应力通过加固区与下卧层的并联效应分析,得出下卧层附加应力系数ψ,采用条形荷载下Boussinesq解计算,其计算结果乘以ψ即为下卧层附加应力。8.在地基中附加应力计算方法的基础上,提出了地基总沉降(最终沉降)和固结计算方法：地基总沉降计算采用分层总和法。在计算沉降时,应对室内试验和原位测试指标进行综合分析,结合当地经验确定地基土变形参数。铁路路堤下CFG桩复合地基的固结以下卧层固结为主,可以采用Terzaghi一维固结理论进行计算。9.实测的沉降—时间曲线可以分为快速发展、发展和稳定三个阶段。对拟合残差在沉降—时间曲线进入稳定阶段后的变化趋势进行分析,可以得出预测沉降值的偏离方向,从而提高沉降预测的可靠性。更多还原

【Abstract】 In this dissertation, the measurement results of stress and deformation acquired at the CFG pile-raft and pile-net composite foundation test sections at Liyao and Fengyang in Beijing-Shanghai high-speed railway were analyzed comprehensively based on the key research project of MOR, "Integrated technical study of CFG piled high speed railway embankment" (No.2007G046). Combining with mathematical analysis and numerical simulation, the load transference mechanism, computation model of additional stress and the settlement calculation method for CFG piled railway embankments were proposed systemically.1. Based on numerical simulation for foundation stress, combined with test result of settlement tubes and soil stress under the embankments, a flare angle model for calculating foundation stress under railway embankment was created.2. The measured result of test sections shows that, for the two kind of composite foundations, the soil stress at the top of pile or pile cap and the soil stress in fillings is relevant to load, and the soil stress under the same load has little change with time. LTP of pile-raft composite foundation regulates pile and soil stress at pile top mainly according to the cushion constrained by stiff plate to make sure the piles support more loads. LTP of pile-raft composite foundation transfers more loads to piles by soil arch effect on pile top and pile-caps collection effect with their extending area. The deformation of geogrid among LTP of pile-net composite foundations in Liyao and Fengyang test sections is little, it doesn’t exert obvious membrane effect.As far as the stress ratio of pile to soil at pile top or pile cap top is concerned, usually, the stress ratio of pile-raft composite foundations is obviously bigger than that of pile-net composite foundations. But as far as the pile efficacy is concerned, pile-net composite foundations are not inferior to pile-raft composite foundations. At Liyao and Fengyang test sections, the stress ratio of pile-net composite foundations is about 1/10 times as likely as that of pile-raft composite foundations, while the pile efficacy of both of the two kind of composite foundations are similar. At every section, the CFG piles is shared 65～79% of all load with the area replacement ratio less than 7%.3. In-situ test results of the axial force of piles at Fengyang show that, the pile-raft and pile-net composite foundations have the same character at the respect of load transfer. That is there is negative friction at the upside of pile. In the effect of negative friction, part of the load supported by piles is transferred to soil, so the axial force of pile is increased, and the additional stress of soil is reduced. At a depth where the axial force of pile grows to its maximum, the additional stress of soil decreases to its minimum. Below this depth, the axial force of pile began to reduce. The calculated results indicate that below the depth the vertical stress of soil between the piles is approximately equal to the vertical stress of soil foundation before filling.Lateral friction at pile top and pile bottom is brought into full play. Within a limited range of both sides of neutral point (equi-settlement plane), the relative displacement between pile and soil is small, so the lateral friction is restricted to a small value. The segment of negative friction is short comparatively, and neutral point is at the upper place of pile.4. Based on the simplification, which includes one-dimensional compression, lateral friction of pile is brought into full play and the additional stress of soil at neutral point is reduced to about zero, it proposed a spring-set model for LTP of pile-raft composite foundation to simulate and analyze the load transfer effect of LTP consisted of crushed-stone +steel reinforced concrete slab. Pile and soil stress, penetration deformation at pile top and so on can be calculated with this model.5. It pointed out that the soil arch effect was the mainly load transfer method for LTP of pile-net composite foundation, which consisted of pile cap+geogrid+crushed-stone layer. By research on in-situ test and mathematical analysis of soil arch at pile top of pile-net composite foundations, it also pointed out that the soil arch of top-pile was close to parabola, and then built a spherical cavity expansion method and a parabola arch method to calculate the stress of pile and soil at pile top.6. Based on Mindlin’s solution, it proposed the parallel model between the reinforced body and the substratum of CFG pile composite foundation. The parallel effect is affected by modulus ratio of reinforced body to substratum. The stress ratio of reinforced body to substratum is almost equal to modulus ratio. Because reinforced body has some characters of integrality, the ratio of its width to depth also affects the parallel effect. The bigger the ratio of width to depth is, the greater the stress of substratum has. Combining the two influences produced by modulus ratio of reinforced body to substratum and the ratio of width to depth of reinforced body, the additional stress coefficientΨof substratum can be solved.Ψis the ratio between additional stress in substratum and equivalent deep foundation Boussinesq’s solution.7. It proposed a simplified calculation method systematically for additional stress of CFG pile composite foundation under the condition of non-sparse piles. That is, the additional stress of reinforced body is calculated according to simplified model of interaction of pile-soil under confined compression. By analyzing the additional stress in substratum using parallel effect between reinforced body and substratum, the additional stress coefficientΨof substratum can be solved. And then, we can get the additional stress of substratum by multiplying Boussinesq’s solution under strip load withΨ.8. It proposed a calculation method for total settlement (final settlement) and consolidation of foundation based on above calculation method for additional stress. It is the final settlement of foundation can be computed by layer-wise summation method. During settlement calculation, the laboratory test indexes and in-situ test indicators should be analyzed comprehensivly. Deformation parameter of foundation soil should also be established combined with local experience. Consolidation of CFG pile composite foundation is mainly consisted of the consolidation of substratum. It can be calculated based on Terzaghi’s one-dimensional consolidation theory.9. The measured settlement-time curve could be divided into three stages as follows: rapid developing stage, developing stage and stable stage. By analyzing the change trend of fitting residual in stable stage of settlement-time curve, the deviated direction of predicted settlement values can be obtained. Thereby, the reliability of predicted settlement can be improved.更多还原