【作者】 雷鸣；

【导师】 王星华；

【作者基本信息】 中南大学 ， 交通运输工程， 2012， 博士

【摘要】 我国东部沿海地区广泛分布有软土地基,在软土地基上修筑高速铁路,必须严格控制路基沉降。真空预压法是一种行之有效的软基处理方法,但高速铁路对软基沉降要求更加严格,相应工程处理经验偏少,尚待进一步研究。当前,我国的高速铁路建设已进入大发展时期,探究真空预压处理高速铁路软土地基的实际效果,具有重要的理论意义和应用价值。本文依托我国京沪高速铁路真空预压软基处理工程,采用现场监测、室内模型试验、理论分析和数值模拟相结合的方法,对真空预压法在修建高速铁路中的地基加固机理、土体性状、沉降计算和数值分析方法,以及加固效果等进行了探索。主要工作如下：(1)通过现场取样,开展各种室内实验,搜集区域性土的特性,获取了丰富的数据资料；采用参与研制的专利仪器进行堆载和真空作用下的土体对比固结试验,着重进行了真空作用下的土体渗透试验。结果表明,真空作用下土样的含水率和孔隙比比堆载作用下略偏小,证明真空作用在这两个方面优于堆载作用；真空初始阶段,渗透系数先出现波动,之后渗透系数逐渐下降,最终趋于平稳。试验表明,两种加载方式下土样固结效果总体是一致的。(2)通过现场监测,获得了真空预压下地基沉降速率、沉降及孔隙水压力等参数的分布和变化规律,结合地质条件和工况分析出其原因。通过采用固结概念分析了真空预压法和堆载预压法加固软土的机理,得到如下共识：两者都是由于形成压差,使土中孔隙水排出,孔压降低,有效应力增大,土体固结。(3)通过对超孔隙水压力实测资料的分析,首次提出将真空预压下土体固结过程在时间上分成三个阶段的观点：①真空吸力导致浅层土体渗透固结,并对深层土体产生“堆载”效应；②浅层土体固结稳定的同时,深层土体由主要受“堆载”效应向主要受“负压”效应过渡；③深层土体在“负压”效应下产生渗透固结,并达到稳定平衡。首次提出加固土体分为浅层土体和深层土体的概念,并提出：浅层土体主要产生渗透固结,深层土体主要是由水位下降产生的“堆载”效应和“负压”效应共同产生的。利用这个理念能很好的解释加固土体中某些测点的超孔隙水压,在抽真空初始阶段提升至正值和基本维持不变的现象。(4)提出真空预压在土体固结第一阶段所能影响的最大深度lmax(即浅层土体厚度)和地下水位下降造成的“堆载”效应p概念。lmax可通过实测土体超孔隙水压力的变化曲线得到,p可通过监测水位下降的深度计算得到。真空预压法的作用机理根源并不是单独的流体渗流场产生的负压差,而是不同时间段“正压”和“负压”共同作用以及两种“压力”互相叠加发生的。“正压”现象会削弱真空渗流场的强度。(5)建立了一维“双弹簧”土体固结模型和模型方程,并通过界定了初始边界条件,得到解析解。此模型着重从有效应力路径、应变路径角度研究了真空及堆载预压法的固结机理和特点。考虑砂井径向固结,进行了超孔隙水压力解析解研究。探讨了基于大变形固结理论和非饱和土理论的真空预压机理。(6)采用基于比奥固结理论的有限单元法,进行现场情况数值模拟分析。结果显示：①数值计算中不能将真空作用等效为堆载。②建议在进行沉降计算时,可将竖向排水通道作为渗透性很强的介质,而在进行坡脚处水平位移计算时,宜将之作为负压边界。③数值计算中竖向排水通道负压分布模式界定很重要。(7)建立了灰色理论及遗传算法地基沉降预测模型,进行了真空预压下软基全时段沉降预测。基于真空预压下士体单向固结机理,得到了真空预压下的主固结沉降计算方法。根据太沙基一维土体固结理论,推导出适合真空堆载联合预压法沉降计算公式,并通过计算结果与实测值的对比分析,提出了兼顾了真空和堆载作用、考虑了瞬时沉降和侧向变形等因素影响的修正参数值。(8)讨论了抽真空强度、地下水位下降对加固效果的影响。增加射流泵会使得加固效果变好,但费用会增加；地下水位下降越大会使效果更好。用常规指标评价了软基处理效果。更多还原

【Abstract】 Building high speed railway in the soft soil, especially in the east coastal area of China, high requirements are put forward to foundation treatment for controlling settlement rigidly. Vacuum preloading is an effective method for soft foundation improvement. However, the engineering experience in design using such treatment is relative poor, it must be studied further. Nowadays, as the construction of high speed railways has stepped into great development period, further understanding of the vacuum preloading have theoretical and practical significances. In this paper, with the case of the Beijing-Shanghai high speed railway in China, the soil deformation behavior, numerical simulation and consolidation mechanism of the vacuum preloading for soil treatment were completely studied by applying the field and laboratory tests, theoretical and numerical analysis. Central works were as followed:(1) The regional soil characteristics of the undisturbed soil samples taken from field were obtained by various laboratory tests. The consolidation simulataneous tests between vacuum preloading and surcharge preloading were carried out with patent apparatus. Permeability tests under vacuum preloading were also carried out. The tests showed the vacuum preloading had better effects to decrease the water ratio and void ratio of the soil as comparison to surcharge method. Conductivity coefficient has some fluctuation in initial stage afer vacuum preloading. Later it declined slowly and declining velocity became slower and slower. However, the consolidation effects under the two different loads were almost accordant.(2) Through field tests, the soil behavior, settlement velocity and pore water pressure distributing, etc. were statistically analyzed and the mechanisms of vacuum preloading and surcharge preloading were also studied. Both of them are aimed to cause pore pressure differences for draining and then increase the effective stresses with the pore pressure dissipation to reach soil consolidation.(3) Through the analysis of measurements of the excess pore water pressure, consolidation process of soil in vacuum preloading was compartmentalized three phases for the first time. The three phases were:①seepage consolidation in shallow soil and " surcharge loading" effect in deep soil by vacuum suction;②consolidation stabilization in shallow soil and transformation of "surcharge loading" effect to "negative pressure" effect in deep soil;③seepage consolidation in deep soil under "negative pressure" effect and to balance. The soil in vacuum preloading was divided into shallow soil and deep soil. Seepage consolidation happened in shallow soil and the superposition of "surcharge loading" and "negative pressure" caused deep soil consolidation. Such proposed theory could explain the phenomenon that the excess pore water pressure in some survey points became positive or maintained constant in first several days after picking up the suction.(4) Maximum deepness (depth of shallow soil) lmax by vacuum preloading in the first phase was brought forward. And momentary "surcharge loading" P resulted in by groundwater decline was also developed. lmax could be obtained from variation curves of excess pore water pressure of all measurement points. p could be approximately calculated through decline depth of groundwater. The essential cause of soil behavior by vacuum preloading was not negative pressure difference solely caused by seepage field, but rather the working together of "positive pressure" and "negative pressure" during different phases as well as the especial phenomenon:groundwater decline. Meanwhile,"positive pressure" would weaken intensity of seepage field.(5) A one-dimension vacuum consolidation model including "two springs" was established and applied to analyze soil consolidation. The excess pore water pressure was obtained by solving the consolidation equation under the given initial and boundary conditions, and a basically reasonable agreement existed with the measurements.Mechanisms of vacuum preloading and surcharge loading were analyzed in effective stress path and strain. Considered radial consolidation of sand drain, analytical solutions of excess pore water pressure were analyzed. Based on large deformation consolidation theory and unsaturated soil theory, mechanisms of vacuum preloading were discussed. (6) The model of the vacuum preloading considering Biot consolidation theory was established using finite element method (FEM). The results showed that:①In numerical calculation, surcharge preloading was not identical with vacuum preloading.②Vertical drainage channels were applicable to be treated as the medium of strong permeation during the settlement calculation of roadbed and applicable to be treated as negative pressure boundary conditions for calculating horizontal displacement at the slope toe of roadbed.③It was very important to define negative pressure distribution mode of vertical drainage channels.(7) Based on the grey theory and Genetic Algorithms, settlement forecast model were established and applied to forecasting roadbed settlements under vacuum preloading. Based on one-dimension consolidation theory, the consolidation settlement calculation formulas method of both vacuum preloading and that combined surcharge preloading were deduced under some simplifications. By comparing the theory solution and the measurements, some correction parameters taken account into the instantaneous settlement and lateral deformation were proposed for both vacuum and surcharge preloading.(8) The influences of vacuum intensity and groundwater decline on soil consolidation effect were discussed. Increasing the quantity of jet pumps or declining the groundwater would make better reinforcement effect, but the cost would rise simultaneously. At last, the treatment effect was appraised by conventional criterions.更多还原