【作者】 于进江；

【导师】 李成辉； 程谦恭；

【作者基本信息】 西南交通大学 ， 道路与铁道工程， 2012， 博士

【摘要】 我国铁路正在实现跨越式发展,在软基上修建高速铁路,将面临地基稳定及大变形等问题,特别是对于高速铁路路基工程而言,不仅要求路堤保持稳定性,而且要严格控制工后不均匀沉降。桩-网复合地基是一种能有效控制沉降的新型复合地基,从目前的研究现状来看,前人的研究主要针对公路、铁路路堤等较小范围的软土路基作用性状,且多集中在地基表面以上,如路堤填土、加筋垫层、桩土应力等,而本文研究桩-网复合地基用于高速铁路超大面积场坪区海相沉积深厚软土地基的处理,采用现场实测与三维数值模拟相结合的方法,对超大面积场坪下桩-网复合地基承载性状与沉降变形机理进行研究,取得的主要成果有：1.根据潮汕车站超大面积深厚软土地基地层的具体分布特性、物理力学性质及指标,分析了潮汕车站超大面积深厚软土地基三个主要特性(流变特性、触变特性、有机质含量高),并分析了其对路基沉降控制的影响。分析了现场试验断面的地层分布情况及软土特性,分析潮汕车站软土的物理力学性质及软土特性的工程意义,详细介绍现场试验断面地层分布及各断面软土特性,最后提出潮汕车站软土地基沉降控制工程概况。2.结合潮汕车站软基处理段,设置监测断面,布设相关监测仪器：分层沉降管、测斜管、土压力盒、柔性位移计、布置于管桩内部的钢筋应力计以及孔隙水压力计。对管桩沉桩过程、上部路堤填土施工过程中孔隙水压力、地表沉降、深部分层沉降、深部水平位移、桩顶应力、桩间上应力等的变化进行观测,分析路堤下管桩复合地基的沉降特性、应力分布及桩土应力分担比,进而检验管桩对软弱地基的加固效果。3.通过对现场数据的整理、分析,得出以下结论：①在加载初期,桩间土和桩顶土应力都出现了一个迅速增大的过程,但是桩顶土应力要比桩间土应力增加得快,并且在填土高度达到一定高度时,桩间土应力出现了极值。桩土应力比也随着填土的增加而增加,并且在后期出现了波动,是褥垫层的调节荷载的作用。②孔隙水压力随填土高度的增加上升得并不明显,因为桩间土承担的荷载较小,大部分的荷载由管桩来承担。③管桩的轴力、摩阻力和地层情况密切相关,其中淤泥质黏土强度低,管桩受到的摩阻力小,轴力传递快,其他地层如细砂等层内,管桩受到的摩阻力较大,轴力传递较慢。④分层沉降的速率和填土速率大致呈正相关,在填土间歇期甚至出现了沉降回弹的现象；在沉降数值上,和地层深度有关,深度越深沉降量越小,越靠近地表沉降量越大。⑤加筋体拉伸位移量随着填土高度的增加而增加,并且位于桩间上位置的加筋体拉伸量大于位于桩顶处的加筋体拉伸量；此外,桩间土处的加筋体表现出滞后效应。⑥侧向位移大致呈现出随着深度的加深而减小的趋势,但是总体的水平位移不大；地层的侧向位移也与地层土类有关,淤泥质黏土中的水平位移较大,特别是靠近底部的地层分界线或附近的位置,水平位移较大。4.运用拉格朗日有限元三维计算方法FLAC3D,对试验段监测断面进行全断面数值模拟,分析研究桩体轴力、剪力、弯矩、侧摩阻力、桩顶和桩间土压力、孔隙水压力和桩土应力比的变化规律,研究了负摩阻力与填土高度、桩土沉降差、桩土应力比、桩长和下卧层的关系,中性点深度与桩间距、桩帽尺寸、褥垫层弹性模量、填土内摩擦角、桩长和下卧层的关系,论述了负摩阻力对沉降变形及桩承载能力的影响,尔后分析研究了流固耦合作用下桩间上表面沉降、桩顶沉降、桩土沉降差和侧向位移的变化规律,最后通过提取监测断面在现场工况条件下附加荷载阶段及软土流变阶段中心处桩间土沉降、桩顶沉降和路堤表面沉降数据,进行沉降预测。更多还原

【Abstract】 The construction of subgrade in high-speed railway resting on soft ground requires not only the stability of embankment, but also the strictly control for the post-construction settlement. The geogrid-reinforced and pile-supported embankment (herein after for GRPSE) is a new type of composite foundation to control the settlement, and the current researches are basically focused on normal embankments with a relatively small cross section resting on a specific commonplace soil foundation. The GRPSE used to support a station yard of a high-speed railway resting on oversize-deep-soft soil have never been studied, and no relevant research or engineering principles can be used for reference. In this dissertation, a construction section with relatively deep soft layers and a large cross section was chosen as the testing site, and the corresponding data is analyzed in detail to investigate the bearing behavior and settlement mechanism of GRPSE resting on oversize-soft-soil by using field test and three-dimensional numerical simulation methods. The main results obtained are listed below.1. Based on the specific strata distribution of oversize-deep-soft soil in the Chaoshan station and physical and mechanical properties of soil, the three corresponding main characteristics (rheological properties, thixotropic properties and high organic matter content) and their impact on the control of subgrade settlement are analyzed in detail. The stratigraphic distribution of the field test section and the soft soil characteristics, and the engineering significance are investigated specifically. Furthermore, the physical and mechanical properties of soft soil, details of sections stratigraphic distribution and cross section of field test section are introduced.2. According to the in-situ soft soil handling segment of Chaoshan station, a monitoring section are selected, and the corresponding monitoring instruments consists of layered deposition tubes, inclinometer tubes, earth pressure cells, flexible displacement sensors, stress gauges and pore water pressure gauges are arranged. The process of pore water pressure in the upper part of the embankment during construction period, the surface subsidence, the deep part of layered settlement, the horizontal displacement, the stress on the top of piles and soil among piles and so on are under observation to process the analysis of the settlement characteristics of the composite foundation, stress distribution and the pile-soil stress ratio. So that the reinforced effect of tube pile on soft ground can be estimated and verified. 3. By regulating and analyzing the monitoring data, the following conclusions can be drawn:①In the early stage of filling, the earth pressure on the soil between the piles and the top of the pile increase drastically, however the rate of change of the earth pressure on the top of the pile is larger than that in the soil between the piles. When the filling reaches a certain value, the maximum earth pressure occurs in the soil between the piles. The pile-soil stress ratio increases with time and loading, and it fluctuates under the constant adjustment of the cushion.②The pore water pressure increases slightly with the filling height because the load borne by the soil between the piles is relatively small.③Both the axial force and the skin friction of the piles increase with time and with load. In addition, the force and the friction are closely related to the properties of the soil layers. In the silt layer and the silty soil layer with low strength, the skin friction is relatively small, and the axial force can be quickly transferred.④The rate of change of the layered settlement is directly proportional to the filling speed. The settlement increases abruptly when filling is conducted in a short time; the settlement then develops gradually and decreases after a certain time period.⑤The stretching of the geogrid increases with the filling height, and the stretching in the soil between the piles is larger than that on the top of the pile. The stretch rate and tension of the geogrid located in the soil between the piles are both larger than the corresponding values on the top of the pile.⑥As the fill height and the consolidation of soil gradually increase, the lateral displacement of the embankment increases accordingly. The rate of change of the lateral displacement during the filling period is larger than that during the stable period after filling. The lateral displacement of the embankments varies in different soil layers under loading, and the lateral displacement of the embankment in soft soil layers is relatively large.4. Using the finite difference three-dimensional software of FLAC3D, the full monitoring sections are simulated to analyze the axial force, shear, bending moment, skin friction of piles as well as soil pressure on the top and the bottom of the pile body, the variation properties of pore water pressure and pile-soil stress ratio. The relationship of negative skin friction from the filling height, the pile-soil differential settlement, the pile-soil stress ratio, the length of pile and the distributions of underlying soil layers are investigated, and the connection between the location of neutral point and the pile spacing, the size of pile cap, the elastic modulus of the cushion and so on are discussed correspondingly. Furthermore, the influence of the negative skin friction on the settlement and deformation of embankment and the bearing capacity of piles are studied. In addition, under consideration fluid-solid interaction, the variation law of the surface subsidence of soil among piles, the settlement of pile top, the differential pile-soil settlement and the lateral displacement are analyzed. At last, by extracting the monitoring data from the center pile-soil subsidence, settlement of pile top and embankment surface settlement considering the additional loading stage and the rheological phase of soft soil, a relevant settlement prediction is made.更多还原