【作者】 马洪林；

【导师】 朱浮声；

【作者基本信息】 东北大学 ， 结构工程， 2009， 博士

【摘要】 由于沿黄冲湖积平原地区地质条件的特殊性,在高速公路沿线路基下,经常存在不同厚度的软弱土层,极大地影响了道路的长期稳定性和安全使用。因此,冲湖积平原软弱地基的处治及沉降问题的研究对沿黄地区道路的建设意义重大。本文在对银川绕城高速公路沿线软弱地基进行地质、水文调查和室内土工试验的基础上,总结了冲湖积平原软弱土的结构性状及工程特性。该区软土是由于第四系以来区内黄河河道不断由西向东迁移,迁移过程中形成许多牛轭湖,使二级阶地上部湖沼发育,形成了流塑～软塑的淤泥、淤泥质土。软土粘粒和粉粒含量较高,砂粒含量非常低。淤泥质软土中含有大量腐殖质及植物根系,土性略偏碱性。与南方沿海地区软土相比,该区软土的物理指标很不典型,含水率平均28.33%,孔隙比平均0.81,但在力学指标上呈现出中高压缩性和低承载力,给工程建设带来了很大的危害,是必须处理的软弱土层。采用土工离心模型试验方法,对采用换填砂砾垫层、袋装砂井、挤密砾石桩等三种加固软弱土地基方法的效果进行了研究,模拟了路堤分期填筑的过程和工后软弱土地基的逐步固结过程,得出了路堤沉降随时间的变化曲线,并对最终沉降和工后沉降做出了初步预测。结果表明,三种软基处治方法的沉降主要发生在路堤的分期填筑过程中,沉降和时间呈现良好的线性关系,工期沉降量占工后总沉降量的70%左右,竣工后,沉降速度大大减缓,其增加速度以对数曲线形式上升;换填砂砾垫层的工后沉降和最终沉降最小,挤密砾石桩次之,袋装砂井最大;换填砂砾垫层地基的垫层是否能形成骨架结构是影响换填砂砾垫层沉降量大小的关键因素;在相同井径和井深的条件下,袋装砂井复合地基的沉降随砂井间距的增大而增大,但当砂井间距减小到一定程度后,地基固结度增长幅度并不明显。砂井地基在排水固结过程中可能导致地基不均匀沉降;在断面在桩径和桩间距不变的条件下,如果挤密砾石桩复合地基的桩长从中间向两边适当减小,整个断面的沉降均匀,路堤整体稳定,同时还可降低工程造价。在软土地基路堤稳定性影响因素研究的基础上,利用大型土工数值分析软件Geostudio,对软弱土地基之上路基内部结构受力特征及边坡稳定性进行了数值计算,选取K39+900和K40+160两个断面进行建模分析,数值计算结果表明了软土地基路堤有较好的稳定性。通过对天然软弱土和换填砂砾垫层地基两种离心试验模型的数值计算,表明换填砂砾垫层地基的安全系数基本增大一倍,抗剪切强度参数也有很大提高。结合工程实际,对换填砂砾垫层、袋装砂井、挤密砾石桩三种处治措施的加固机理、适用范围、设计方法以及施工工艺进行了深入的分析和研究,确定了软弱地基路堤施工沉降监测的方案,通过对监测结果的分析,总结了三种处治措施地基在填土施工过程中的沉降规律。结果表明,断面最大沉降点并不在路堤中心线上,而是随着观测断面所处路段地质情况的不同而不同;经三种技术处治后地基的水平位移相对于竖直沉降特别小,所以在填土施工时应以垂直方向的沉降变形为主要控制指标;处治地基在填土施工过程中始终处于非常稳定的状态,处治效果良好。通过施工后现场监测的沉降、地基土体水平位移、孔隙水压力等资料,对银川绕城高速公路软弱地基的处治效果进行了经济性分析。研究结果表明:采用换填砂砾垫层、袋装砂井、挤密砾石桩等三种地基处治技术,能够有效地加固冲湖积平原的软弱地基,减小沉降量,是合理有效的地基处理措施。本文对冲湖积平原软弱地基处治的研究成果和建议,为今后该地区的高速公路建设积累了经验。更多还原

【Abstract】 Owing to the special geological property of alluvial-lacustrine plain along the Yellow River, there are often soft soil layers of different thickness under the roadbeds along the expressways, which exerts a great effect on the highway’s long-term stability and safety usage. Thus, the study on the treatment and settlement problem of soft soil embankment on alluvial-lacustrine plain is significant to the road construction in the area along the Yellow River.On the basis of geological, hydrological survey and indoor soil experiment on the soft foundation along the Yinchuan Ring Expressway, the paper summarizes the structure property and engineering property of alluvial - lacustrine plain’s soft soil. The soft soil in this area is formed because, since the Quaternary, the Yellow River channel in this area continued moving from the west to the east. In this moving process, lots of oxbow lakes were formed, which made the lacustrine bog on second terrace develop and then form current-formed and soft flowing silt and silt soil. Soft soil has high clay particle and clay powder contents, while the sand grain content is very low. Soft mucky soil contains lots of humus and plant roots, which makes soil a little alkalescent. Compared with soft soil in the southern littoral area, the physical indexes of soft soil in this area are non-typical, with 28.33% average moisture content, and 0.81 void ratio. However, referring to mechanical index, it presents medium, high compressibility and low bearing capacity, which brings a serious damage to project construction. So, the soft soil layer has to be treated.Resorting to the geotechnical centrifuge model tests, this paper conducts researches on the effect of such three consolidating soft foundation methods as Replacement of Sand-Gravel Cushion, Sand-Wick and Gravel Compaction Pile, simulates the process of road embankment filling by stages and the gradual consolidation process of soft base after construction, arrives at the embankment settlement curve according to time, and makes a preliminary forecast on the final settlements and settlement after construction. The outcome reveals that settlements in these three soft foundation treatments mainly happen during the process of road embankment filling by stages, the settlement related with time shows a good linear relationship, and the settlements during construction account for 70% of the gross after construction. After completion of the construction, the settling speed would greatly slow down, and the sedimentation increasing rate raises up in form of logarithmic curve; as to the amount of settlements after construction and the final, Replacement of Sand-Gravel Cushion is smallest, Gravel Compaction Pile takes the second place, Sand-Wick is the greatest; whether the bedding course of Replacement of Sand-Gravel Cushion basement can form skeletal structure is critical to the settlements in Replacement of Sand-Gravel Cushion; with the same drain radius and depth, the settlements of sand-wick composite basement increase along with the increase of spaces between drains. However, when the spaces between drains decrease to a certain degree, the increase extent of basement concretion is not obvious. Sand drain basements may result in uneven basement settlement in process of drainage consolidation; under the condition of unchanged radius and depth of the cross section, if the length of gravel compound foundation pile reduces properly from the center to both sides, the settlement of cross section is even, the embankment is entirely stable, and at the same time, the project cost can be reduced.On the basis of the study on the affecting factors of the stability of soft soil embankment, through large-scale soil engineering numerical analysis software Geostudio, the paper makes numerical calculation of internal structural characteristic under force and slope stability of the embankment, selects K39 +900 and K40 +160 section to make analysis. The numerical results show that soft ground has better stability. The numerical analysis of the two centrifugal models of natural soft soil ground and replacement sand-gravel cushion shows that safety factor of replacement sand-gravel cushion doubles, and the shear strength parameters are also greatly improved.Combining the engineering practice, the paper makes an in-depth analysis and research on the reinforcement mechanism, application scope, design method and construction technique of such three treatment methods as Replacement of Sand-Gravel Cushion, Sand-Wick and Gravel Compaction Pile, and determinates the settlement monitoring program of embankment construction. With the analysis on the monitoring results, it summarizes the settlement rules of the foundation in the filling construction process treated by the three treatment methods. The results show that the maximum section sedimentation does not lie in the center-line, but differs with different geological conditions of the survey section. Compared with the vertical settlement, the horizontal displacement of the ground after treated by these three techniques is extremely small. Therefore, the main control indexes should lie in the vertical sedimentation and deformation in the filling construction process. The treated foundation in the filling construction process remains very stable all the time and the treatment effect is quite sound. With the materials such as field monitoring settlement, soil horizontal displacement, pore water pressure after construction and so on, the paper makes analysis of economy on the treatment effect of the Yinchuan Ring Expressway’s soft foundation.The results show that such three treatment methods as Replacement of Sand-Gravel Cushion, Sand-Wick, and Gravel Compaction Pile, can effectively reinforce the soft soil embankment on alluvial-lacustrine plain, and reduce settlement, which are sound and effective methods to deal with foundation. The research results of and suggestions on the treatment of soft soil embankment on alluvial-lacustrine plain put forward in this paper accumulate experiences for the later expressway’s construction in this are.更多还原