桩承式路堤的理论与数值研究
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摘要
软土由于其压缩性高和抗剪强度低,一般不宜作为建筑物的地基。建造于软土上的路堤由于其荷载较大,会产生一系列的问题,诸如承载力不足,差异沉降过大,侧向压力过大等导致的施工期延长或过早发生破坏等。当在软土上建造路堤时,应采用特殊的施工方法以保证路堤安全。桩承式平台为在软土上建造路堤、挡土墙和储油罐等提供了经济有效的解决办法,尤其是施工工期要求紧且变形要求严格的条件下更显示出其优势。本文对桩承式路堤进行了理论分析和数值模拟研究。
近十年来,复合地基尤其是多桩型复合地基已经得到了广泛的应用。多桩型复合地基就是采用多种不同的桩所形成的复合地基,其桩长和桩径可能都不一样。长桩通常比短桩的强度高,与常规的刚性的混凝土类桩的用法相似,即桩端尽量置于硬土层,充分调动深部土层的承载力。另一方面,柔性桩加固浅层的地基土。这样,就可以达到同时加固浅层地基土和充分利用深层地基土的承载力的目的。
首先,本文对于建造于软土地基上的粒状材料填筑的路堤提出了理论分析方法,软基采用正方形布置的桩和土工合成材料进行联合加固。该方法与Low提出的方法比较相似,但是,作了以下改进:假设路堤填土承受了均布的超载,每个桩顶均有一个方形的承台,考虑土与土工合成材料的界面摩阻力。与目前已有的方法相比,本文所提出的方法更简单,而且不需要对未知参数进行试算来求解。该方法的分析结果表明,土工合成材料的存在,可以增加桩所承担的荷载,也就是提高了桩的效率。而且桩所分担的荷载随着桩顶承台的面积比的增加而增加。对于某一个固定的面积比,当路堤填土的高度与桩承台的距离之比较大时,桩分担荷载的效率达到最大值。与现有方法的计算比较表明,结果与Low提出的方法的结果比较一致。英国规范BS8006和Guido方法在二维情形下高估了作用在土工合成材料上的竖向应力,且低估了桩分担荷载效率和应力集中比。
其次,为了研究多桩型复合地基在不同的荷载分布下的承载及变形特性,本文采用有限元分析软件PLAXIS 3D FOUNDATION对多桩型复合地基进行了载荷试验模拟。数值模拟中,通过改变桩长、桩径和垫层厚度等参数,详细探讨了多桩型复合地基在不同荷载作用下的变形规律。同时还探讨了垫层对荷载沉降曲线特性的影响。研究分析结果总结成为一系列图表,可应用于工程实践。
最后,采用有限元软件PLAXIS对多桩型支承的路堤的固结特性进行了二维有限元分析。对参数的分析研究表明,多桩型的组合显示出了非常显著的效果。详细地探讨了孔隙水压力的上升和消散规律,沉降、水平位移、差异沉降以及桩的轴向力变化规律。
本文的研究成果对桩承式路堤的理论研究以及工程应用均有较重要的指导意义。
Very soft soils are often avoided in construction due to their high compressibility and low shear strength. Embankment constructed on soft soils, where the structure impose a large load onto the ground raises several concerns on factors like bearing capacity failure, differential settlements, lateral pressures and instability which results in long construction delays and/or premature failure. Special construction methods are adopted when embankments are constructed on very soft soils. Geosynthetic-reinforced and column-supported (also known as pile-supported) earth platforms provide an economic and effective solution for embankments, retaining walls, and storage tanks, etc. constructed on soft soils; especially when rapid construction and/or strict deformation of the structure are required. In this dissertation, the theoretical analysis and numerical simulation of column-supported embankment have been performed.
In past ten years, composite foundations, especially a multi-column composite foundation has been extensively used. A multi-column composite foundation is a method of ground improvement that involves using different column types with varying lengths and diameters. The long columns are generally much stiffer than short columns and they are used in a manner similar to that used for conventional columns; that is, to mobilize bearing capacity from the deeper soil strata. On the other hand, the short and flexible columns strengthen the shallower soil strata. Consequently, the strength and stiffness of the shallow soil strata can be improved and utilized together with contribution from the deeper soil strata.
Firstly, a new theoretical analysis similar to the analysis proposed by Low was presented for the analysis of an embankment of granular fill on soft ground supported by a rectangular grid of columns and geosynthetic. The main refinements were: the inclusion of a uniform surcharge loads on the embankment fill, individual square caps have been used, and taking into account the skin friction mechanism, which contributes to soil-geosynthetic interface resistance. In particular, the proposed method has the advantage over existing method in that it is simple to get the solution of the developed equations and there is no need for using trial values to get the solution of unknown parameters. The method showed that inclusion of geosynthetic reinforcement can increase the fill load carried by columns, as indicated by efficiency. The method also showed that the portion of the fill load carried by columns increases with the area ratio of column caps. For a given area ratio, the efficiency reached a maximum value when the ratio of the thickness of the fill to the spacing of the column caps is large. Comparison of results of the present method to that of current design methods showed a good agreement with the results of Low method. It seems that the methods for 2D situation in BS8006 and Guido overpredicted the vertical stress acting on the geosynthetic. As a result, BS8006 and Guido methods underestimated the efficiency and stress concentration ratio.
Secondly, a full scale load test on multi-column composite foundation was provided using the finite element software PLAXIS 3D FOUNDATION in order to investigate the behavior of the composite foundation under various load distributions. The parameters investigated include the length and diameter of columns and the thickness of the cushion. The influence of the cushion on load-settlement behavior was also studied. The results of these analyses were summarized into a series of design charts, which can be used in engineering practice.
Finally, a series of two-dimensional (2D) finite element analyses on the mechanism of consolidation behavior of multi-column supported embankment using the finite element software PLAXIS. Parametric studies were presented that shed light on effective combinations of columns. Development and dissipation of excess pore pressure, settlement, horizontal displacement, differential settlement, and axial force of columns were also presented and discussed in details.
The achievements in this dissertation can be valuable for theoretical study and practical application of column-supported embankments.
近十年来,复合地基尤其是多桩型复合地基已经得到了广泛的应用。多桩型复合地基就是采用多种不同的桩所形成的复合地基,其桩长和桩径可能都不一样。长桩通常比短桩的强度高,与常规的刚性的混凝土类桩的用法相似,即桩端尽量置于硬土层,充分调动深部土层的承载力。另一方面,柔性桩加固浅层的地基土。这样,就可以达到同时加固浅层地基土和充分利用深层地基土的承载力的目的。
首先,本文对于建造于软土地基上的粒状材料填筑的路堤提出了理论分析方法,软基采用正方形布置的桩和土工合成材料进行联合加固。该方法与Low提出的方法比较相似,但是,作了以下改进:假设路堤填土承受了均布的超载,每个桩顶均有一个方形的承台,考虑土与土工合成材料的界面摩阻力。与目前已有的方法相比,本文所提出的方法更简单,而且不需要对未知参数进行试算来求解。该方法的分析结果表明,土工合成材料的存在,可以增加桩所承担的荷载,也就是提高了桩的效率。而且桩所分担的荷载随着桩顶承台的面积比的增加而增加。对于某一个固定的面积比,当路堤填土的高度与桩承台的距离之比较大时,桩分担荷载的效率达到最大值。与现有方法的计算比较表明,结果与Low提出的方法的结果比较一致。英国规范BS8006和Guido方法在二维情形下高估了作用在土工合成材料上的竖向应力,且低估了桩分担荷载效率和应力集中比。
其次,为了研究多桩型复合地基在不同的荷载分布下的承载及变形特性,本文采用有限元分析软件PLAXIS 3D FOUNDATION对多桩型复合地基进行了载荷试验模拟。数值模拟中,通过改变桩长、桩径和垫层厚度等参数,详细探讨了多桩型复合地基在不同荷载作用下的变形规律。同时还探讨了垫层对荷载沉降曲线特性的影响。研究分析结果总结成为一系列图表,可应用于工程实践。
最后,采用有限元软件PLAXIS对多桩型支承的路堤的固结特性进行了二维有限元分析。对参数的分析研究表明,多桩型的组合显示出了非常显著的效果。详细地探讨了孔隙水压力的上升和消散规律,沉降、水平位移、差异沉降以及桩的轴向力变化规律。
本文的研究成果对桩承式路堤的理论研究以及工程应用均有较重要的指导意义。
Very soft soils are often avoided in construction due to their high compressibility and low shear strength. Embankment constructed on soft soils, where the structure impose a large load onto the ground raises several concerns on factors like bearing capacity failure, differential settlements, lateral pressures and instability which results in long construction delays and/or premature failure. Special construction methods are adopted when embankments are constructed on very soft soils. Geosynthetic-reinforced and column-supported (also known as pile-supported) earth platforms provide an economic and effective solution for embankments, retaining walls, and storage tanks, etc. constructed on soft soils; especially when rapid construction and/or strict deformation of the structure are required. In this dissertation, the theoretical analysis and numerical simulation of column-supported embankment have been performed.
In past ten years, composite foundations, especially a multi-column composite foundation has been extensively used. A multi-column composite foundation is a method of ground improvement that involves using different column types with varying lengths and diameters. The long columns are generally much stiffer than short columns and they are used in a manner similar to that used for conventional columns; that is, to mobilize bearing capacity from the deeper soil strata. On the other hand, the short and flexible columns strengthen the shallower soil strata. Consequently, the strength and stiffness of the shallow soil strata can be improved and utilized together with contribution from the deeper soil strata.
Firstly, a new theoretical analysis similar to the analysis proposed by Low was presented for the analysis of an embankment of granular fill on soft ground supported by a rectangular grid of columns and geosynthetic. The main refinements were: the inclusion of a uniform surcharge loads on the embankment fill, individual square caps have been used, and taking into account the skin friction mechanism, which contributes to soil-geosynthetic interface resistance. In particular, the proposed method has the advantage over existing method in that it is simple to get the solution of the developed equations and there is no need for using trial values to get the solution of unknown parameters. The method showed that inclusion of geosynthetic reinforcement can increase the fill load carried by columns, as indicated by efficiency. The method also showed that the portion of the fill load carried by columns increases with the area ratio of column caps. For a given area ratio, the efficiency reached a maximum value when the ratio of the thickness of the fill to the spacing of the column caps is large. Comparison of results of the present method to that of current design methods showed a good agreement with the results of Low method. It seems that the methods for 2D situation in BS8006 and Guido overpredicted the vertical stress acting on the geosynthetic. As a result, BS8006 and Guido methods underestimated the efficiency and stress concentration ratio.
Secondly, a full scale load test on multi-column composite foundation was provided using the finite element software PLAXIS 3D FOUNDATION in order to investigate the behavior of the composite foundation under various load distributions. The parameters investigated include the length and diameter of columns and the thickness of the cushion. The influence of the cushion on load-settlement behavior was also studied. The results of these analyses were summarized into a series of design charts, which can be used in engineering practice.
Finally, a series of two-dimensional (2D) finite element analyses on the mechanism of consolidation behavior of multi-column supported embankment using the finite element software PLAXIS. Parametric studies were presented that shed light on effective combinations of columns. Development and dissipation of excess pore pressure, settlement, horizontal displacement, differential settlement, and axial force of columns were also presented and discussed in details.
The achievements in this dissertation can be valuable for theoretical study and practical application of column-supported embankments.
引文
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