人工挖孔嵌岩灌注桩承载性状的试验与理论分析
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摘要
随着我国交通事业的发展,对桥梁建设的要求越来越高,许多合理、经济、美观的桥梁结构形式也被不断地研究、引进和发展。高墩大跨连续刚构桥由于自身得天独厚的优点尤其被设计者所亲睐,并得到广泛应用。
高墩大跨连续刚构桥的墩、梁、基础三者固结联为一体共同受力,墩身形式、高度等对结构受力有影响。高墩大跨连续刚构桥的结构受力特点是：墩梁固结,上下部结构共同承担荷载,减小了墩顶负弯矩；墩的刚度较柔,墩允许较大变位；结构为多次超静定结构,混凝土收缩、徐变、温度变化、预应力作用、墩台不均匀沉降等引起的附加内力对结构影响很大。
由于高墩大跨连续刚构桥对地基的不均匀沉降量控制较严格,所以本文以宜万铁路渡口河特大桥群桩基础的原型观测为基础,对施工过程中桥梁群桩基础的承载性状进行了试验和理论分析,对这种桥型基础的研究提供参考。渡口河特大桥的地基为极软岩,工程性质不稳定,易受外界环境扰动。渡口河特大桥的5号墩高128m,对墩身稳定性和墩顶偏位要求严格,所以被选作原型观测的对象。渡口河特大桥采用的是人工挖孔桩群桩基础,其承载性状受施工过程影响较大,本文的研究目的是分析施工阶段桩-土-承台-桥墩共同作用机理。桩基是高墩大跨连续刚构桥常用的基础形式,其施工阶段很关键,影响贯穿桥梁施工和营运的整个过程；而且一旦出现由桩基引起的问题,补救的代价很高,所以对其研究有重要的理论和实用价值。本文主要研究软岩地基中全嵌岩单桩的荷载传递机理、群桩基础的承载性状。本文研究的方法是通过对5号墩施工过程中群桩应力应变长期的原型观测,并辅以理论分析和数值模拟,得到了施加上部荷载后桩身的荷载传递规律随施工进度的变化,以及软岩地基中群桩基础承载性状的一些特征,并利用有限元软件分析了桩侧岩石粗糙度对单桩承载力的影响,具体结论如下：
1)通过原型观测发现群桩基础桩顶和桩间岩石内力分布是随施工进度变化的,它既随着上部结构的重量和承台的抗弯刚度增大而变化,同时又受承台底与地基和桩顶与承台接触情况的影响。承台的抗弯刚度实际包含墩底实心部分贡献的刚度,它在早期上部结构较低时显著影响群桩基础的受力性状。基岩由于水的浸泡和风化而弱化,使得距承台底一定深度内的地基压缩性较大而不能承担上部荷载,所以上部结构施工初期上部荷载主要由桩承受,随着时间的增长,承台底地基被压紧,这部分桩间岩石也逐渐分担部分上部荷载。桩顶与承台的接触和地基与承台的接触情况一样也是随时间变化的,这也影响基础所受荷载的空间分布。
本文监测过程中桥梁悬臂梁的施工还未完成,由于地基的一部分是从山体开挖出来的新鲜岩石,另一部分是常年受河水侵蚀的岩石,所以存在地基不均匀对群桩内力分布的影响,就目前被监测桩的情况来看,中间桩(5-8号、5-15号)承受的荷载大于角桩(5-19号),且角桩承受荷载状况很复杂,具体表现为先受拉再受压。由于其它桩只在桩顶中心埋设了应变计,而通过监测发现桩顶平面内轴向应变的分布是极不均匀的,再考虑到桩顶与承台的实际接触情况是新老混凝土的接触,且由于桩身横截面积犬造成的桩顶显著的应力集中,使得承台顶内力分布与理论分析的差别很大,所以桩顶只埋设一个测点不能反映桩顶应变分布的全貌。实际的桩顶应力分布是不对称的,要想深入了解群桩基础桩顶应力的真实分布,还需要在桩顶增加应变计的个数。
2)通过对大直径群桩基础基桩荷载传递性状的长期监测,群桩桩顶的荷载分布复杂且随空间位置各异,随时间动态变化,基桩桩顶轴力分布与理想状态下的理论结果有较大差异,所以应该根据实际情况来调节群桩基础的设计。
软岩地基中基桩的荷载传递特性分析的结果是：侧阻力承担绝大部分上部荷载,所以可认为其是端承摩擦桩；由于人工挖孔桩的桩侧粗糙程度沿桩身变异很大,使得基桩的桩侧阻力的分布特征复杂,其沿桩身的侧阻力曲线有多个峰值；通过监测发现大直径嵌岩桩的轴向刚度大,桩顶沉降小,承载能力高,是一种适合于高、大、重结构的基础形式。
3)考虑桩侧土非线性软化的荷载传递函数法能较好的与试验曲线吻合,并能反映桩的渐进破坏特征。而且由于荷载传递函数法的灵活性,基于本方法开发的程序能够模拟多层土中桩的荷载—传递特性,但是要精确模拟还需要具体的工程数据。
4)嵌岩桩的桩岩界面与桩土界面不同,界面的粗糙度对基桩承载力影响显著。目前,桩岩界面的粗糙程度可用无量纲的凹凸度因子表征,本文通过对桩岩界面粗糙形状的有限元模拟,不仅证实桩顶沉降量与凹凸度因子成反比,还发现其与粗糙突起的形状因子成正比,突起高度对承载力的影响比突起半波长的影响显著。桩顶位移实际上是随桩顶荷载的增大而增大的,所以在相同的荷载下减少桩顶位移就是提高承载力。在实际工程中,要提高嵌岩桩的承载力有两种措施,其一是当桩长可以加长时,尽量利用提高嵌岩桩接触面突起高度的方法来提高桩的承载力,当然这种方法不是很经济,但提高承载力幅度较大；其二是在桩长不可调时,尽量利用制造“短粗型突起”来提高桩的承载力,这种方法比较经济。
5)根据5号墩T构整体有限元分析,可知桥身自重荷载是群桩基础力学响应的主要影响因素；在施工阶段,桥梁的风荷载在桥墩和悬臂梁上引起的混凝土拉应力不容忽视,特别是桥墩混凝土没有做预应力,容易开裂；群桩基础的受力分布是中间桩承受荷载大,周边桩承受荷载小,这与实际监测结果大体一致。根据5号墩T构整体有限元稳定性分析,可知在施工阶段的最不利荷载组合下,桥梁的失稳模态主要是沿纵桥向整体倾覆；地基局部软化降低桥梁整体失稳的特征值。
5号墩为同类型铁路桥梁中国内最高墩,通过首次对高墩群桩基础所做的原型观测以及分析,本文工作存在如下创新之处：
1)首次完成了软岩地基中嵌岩灌注桩桩群桩基础的原型观测工作,分析了单桩的荷载传递特征和群桩的承载性状随施工进度的变化规律,为相同地质情况下的研究提供了依据。
2)在考虑桩侧土非线性软化的前提下,利用荷载传递函数法分析多层土中桩的荷载—沉降曲线,得到了较好的结果。
3)利用有限元优化分析工具证实了嵌岩桩桩侧粗糙度对其承载性状的影响,并提出了用于表征单个突起形状对嵌岩桩承载力影响的突起形状因子的概念。当桩长一定时,突起形状因子可以衡量单个突起的形状对桩的承载力的影响,并通过推理发现,“短粗型突起”是比较经济的突起形式。
4)利用有限元方法定性分析了5号墩T构的整体强度和稳定性分析,群桩的受力分布与监测结果大体一致,地基的局部软化会降低桥梁的整体稳定性,为高墩大跨连续刚构桥的设计提供了参考。
As China's transport development in the building of more bridges, many reasonable, economic, aesthetic bridge structure has been constantly reserached, introducted and developed. Because of its unique advantages in particular, high-pier and long-span continuous rigid frame bridge is widely used by designer.
Pier, beam and base of high-pier and long-span continuous rigid frame bridge are consolidated as one bearing forces together, the pier's type and heigh influenc bearing behavior of structure. Bearing behavior of structure of high-pier and long-span continuous rigid frame bridge is:pier and beam consolidated as one; the lower and super structure bear force together, and which reduce negative moment in top pier; pier's stiffness is lower and larger displacement is allowed; because biridge structure is multi ultra-setting structure, Shrinkage, creep, temperature changes, prestressed of concrete caused uneven settlement of platform, which caused additional internal forces that significantly influenced the structure's bearing behavior.
As uneven settlement of foundation of high-pier and long-span continuous rigid frame bridge is strictly control, so based on the prototype observation for pile group foundation of Dukouhe Bridge of Yiwan railway, this paper conduct test and theory annlysis on it, and which offer reference to construction practice. Dukouhe River Grand Bridge located in the middle of Yi-Wan Railway, and its subgrade is made up of soft rock which engineering behavior is unsteady and vulnerability to external disturbances. Because height of No.5 pier is 128 m, and strict with stability of pier and deviation of pier top, so were selected for the prototype observation target. The aim of this paper is to study interactive mechanism of system made up of piles, soil (or rock), bearing platform and pier during the course of construction. The interaction including superstructure and substructure of bridge is a very complicate issue, and directly related to construction process; meanwhile, construction phase of piling is key and its influence run through whole process of construction and operation of bridge. Once there is flaw attributed to piles construction, the remedial cost is very high, study on interaction of bridge has important value of theory and practice. The main contents of the paper are load transfer mechanism of single pile, and loading behavior of composite pile foundation. The research methods of the paper are long years of prototype observation on pile foundation of No 5 pier, supplemented by theoretical analysis and numerical simulation. Throught methods of foregoing, some conclusions were get such as mechanism of residual strain of socketed piles and its influence on load transfer of pile before loading, rules of load trandfer during construction of superstructure, characters of load distribution of composite pile foundation in soft rock, and influence of roughness of shaft wall and inhomogeneity of subgrade to bearing capability of single pile and bearing behaviors of pile group respectively, following is details conclusions:
1) Through prototype observation it was found that pile top load distribution of the composite pile is changed with the progress of construction, that is to say, as the weight of the upper structure and the bending stiffness of the platform change. Bending stiffness of the platform actually includes the contribution of the solid part of the pier bottom, which significantly affected the bearing traits of pile group in the early. Softening by soaking and weathering, the subgrade closed to platform bottom can not bearing load, the early upper load mainly borne by pile, with the growth of time, the subgrade closed to platform bottom compressed and gradually share part of the upper load. And the contact situation between the pile caps and platform is changing with time; this also affected the load space distribution of pile foundation.
Due to construction process of the bridge has not yet been completed during this monitoring, plus possibility as a result of the uneven compress of subgrade (part of the subgrade closed to river was soften by river water, an the other was fresh rock excavated from hill), it is currently monitoring result that the middle piles (nominated as pile 5-8 and pile 5-15) bearing more load than the corner pile (nominated as pile 5-19) and the corner pile's behavior of bearing load was very complicated, for it first tension further compression. As only the top center of the other pile laid strain gauge, but it was found by monitoring that axial strain distribution of the top of the pile is very uneven, additionally considering the theoretic contact situation between old concrete of piles and new concrete of platform was more distinguishing than the practical one, so pile only planted a strain gauge on its top can not reflect the pile strain distribution of the whole picture.
2) Through long-term monitoring of load transfer characteristics of the large diameter pile of pile group foundation, it was found that the load distribution of pile top was complex and varied with spatial location and time changes, and pile top axial load distribution is significant differences with the theoretical results under ideal state, therefore, pile group foundation design should be regulating according to the actual situation.
According to analysis of load transfer characteristics of pile socketed in soft rock, it can be considered to be end-bearing friction piles, namely most upper load bearing by side resistance. As the roughness of side wall of hand-dug pile is variation, the distribution of side resistance characteristics of pile is complex, and the side resistance curve has more than one peak. It was found through monitoring that large diameter rock-socketed pile has greater axial stiffness, smaller pile settlement, higher loading capacity, so it is a suitable base type for high, big and heavy structure.
3) Method for load transfer function concerning on side resistance nonlinear softening can better fit the test curve, and reflects the gradual destruction of the pile bearing behavior. And because of the flexibility of load transfer function, based on this method, the procedures can simulate for load-settlement curve of pile in multi-storey soil, but accurately simulate needs of the engineering data.
4) Interface is different when medium around pile is different; the roughness of interface is remarkably influence bearing capability of rock socketed pile. At present, roughness can be figured by convex concave factor. According to simulation by FEM, the paper not only approved that settlement of pile top are in inverse proportion to convex concave factor, but also proportional to the shape factor of asperity, and influence of height of the asperity is notable greater than that of half wavelength. Pile top settlement is actually increases with the increasing load; therefore, reduction pile displacement is to increase capacity under the same load. In engineering practice, there are two methods to enhance pile bearing capability. The first method is increasing height of asperity, when design length of pile can be lengthening, but this method is not economical; the second method is making "short-wide asperity", this method is economical.
5) According to the finite element analysis on T-shaped continuous rigid frame of No 5 pier, it was kown that selfweight of superstructure is the man influence factor on machenical response of pile group foundation; during the construction phase of the bridge piers, the tensile stress caused by wind load on the cantilever and the pier can not be ignored, especially no pre-stressed concrete on bridge pier, it easy to crack; the load distribution of pile groups is that the middle piles bear load more than the pile at Around, which is broadly consistent with the actual monitoring results. According to the finite element stability analysis on T-shaped continuous rigid frame of No 5 pier, we can see under the most unfavorable combination of load in the construction phase, the main mode instability of the bridge is collapse as a whole along the vertical direction; partial softening of the foundation can lower the overall instability eigenvalue of the bridge.
At present, the height of No 5 pier is the highest comparing to same type railway bridge in China, according to prototype observations and anlysis to it, some innovation was summarized as follows:
1) For the first time, prototype observations on rock socketed composite pile group was completed, the rules of single pile's load trandfer characters and bearing behaviors of pile group was get and it will provide reference to practice engineering.
2) In considering the non-linear soil softening soil around pile side, using load-transfer function get better result when analyze pile inthe multi-layer soil.
3) Influence of roughness of interface of rock socketed pile was approved by FEM method, and a new concept named shape factor of asperity was bring forward.
4) According to the finite element analysis on T-shaped continuous rigid frame of No 5 pier, Strength and stability of it was anlyzed, and which is broadly consistent with the actual monitoring results. Partial softening of the foundation can lower the overall instability eigenvalue of the bridge.
高墩大跨连续刚构桥的墩、梁、基础三者固结联为一体共同受力,墩身形式、高度等对结构受力有影响。高墩大跨连续刚构桥的结构受力特点是：墩梁固结,上下部结构共同承担荷载,减小了墩顶负弯矩；墩的刚度较柔,墩允许较大变位；结构为多次超静定结构,混凝土收缩、徐变、温度变化、预应力作用、墩台不均匀沉降等引起的附加内力对结构影响很大。
由于高墩大跨连续刚构桥对地基的不均匀沉降量控制较严格,所以本文以宜万铁路渡口河特大桥群桩基础的原型观测为基础,对施工过程中桥梁群桩基础的承载性状进行了试验和理论分析,对这种桥型基础的研究提供参考。渡口河特大桥的地基为极软岩,工程性质不稳定,易受外界环境扰动。渡口河特大桥的5号墩高128m,对墩身稳定性和墩顶偏位要求严格,所以被选作原型观测的对象。渡口河特大桥采用的是人工挖孔桩群桩基础,其承载性状受施工过程影响较大,本文的研究目的是分析施工阶段桩-土-承台-桥墩共同作用机理。桩基是高墩大跨连续刚构桥常用的基础形式,其施工阶段很关键,影响贯穿桥梁施工和营运的整个过程；而且一旦出现由桩基引起的问题,补救的代价很高,所以对其研究有重要的理论和实用价值。本文主要研究软岩地基中全嵌岩单桩的荷载传递机理、群桩基础的承载性状。本文研究的方法是通过对5号墩施工过程中群桩应力应变长期的原型观测,并辅以理论分析和数值模拟,得到了施加上部荷载后桩身的荷载传递规律随施工进度的变化,以及软岩地基中群桩基础承载性状的一些特征,并利用有限元软件分析了桩侧岩石粗糙度对单桩承载力的影响,具体结论如下：
1)通过原型观测发现群桩基础桩顶和桩间岩石内力分布是随施工进度变化的,它既随着上部结构的重量和承台的抗弯刚度增大而变化,同时又受承台底与地基和桩顶与承台接触情况的影响。承台的抗弯刚度实际包含墩底实心部分贡献的刚度,它在早期上部结构较低时显著影响群桩基础的受力性状。基岩由于水的浸泡和风化而弱化,使得距承台底一定深度内的地基压缩性较大而不能承担上部荷载,所以上部结构施工初期上部荷载主要由桩承受,随着时间的增长,承台底地基被压紧,这部分桩间岩石也逐渐分担部分上部荷载。桩顶与承台的接触和地基与承台的接触情况一样也是随时间变化的,这也影响基础所受荷载的空间分布。
本文监测过程中桥梁悬臂梁的施工还未完成,由于地基的一部分是从山体开挖出来的新鲜岩石,另一部分是常年受河水侵蚀的岩石,所以存在地基不均匀对群桩内力分布的影响,就目前被监测桩的情况来看,中间桩(5-8号、5-15号)承受的荷载大于角桩(5-19号),且角桩承受荷载状况很复杂,具体表现为先受拉再受压。由于其它桩只在桩顶中心埋设了应变计,而通过监测发现桩顶平面内轴向应变的分布是极不均匀的,再考虑到桩顶与承台的实际接触情况是新老混凝土的接触,且由于桩身横截面积犬造成的桩顶显著的应力集中,使得承台顶内力分布与理论分析的差别很大,所以桩顶只埋设一个测点不能反映桩顶应变分布的全貌。实际的桩顶应力分布是不对称的,要想深入了解群桩基础桩顶应力的真实分布,还需要在桩顶增加应变计的个数。
2)通过对大直径群桩基础基桩荷载传递性状的长期监测,群桩桩顶的荷载分布复杂且随空间位置各异,随时间动态变化,基桩桩顶轴力分布与理想状态下的理论结果有较大差异,所以应该根据实际情况来调节群桩基础的设计。
软岩地基中基桩的荷载传递特性分析的结果是：侧阻力承担绝大部分上部荷载,所以可认为其是端承摩擦桩；由于人工挖孔桩的桩侧粗糙程度沿桩身变异很大,使得基桩的桩侧阻力的分布特征复杂,其沿桩身的侧阻力曲线有多个峰值；通过监测发现大直径嵌岩桩的轴向刚度大,桩顶沉降小,承载能力高,是一种适合于高、大、重结构的基础形式。
3)考虑桩侧土非线性软化的荷载传递函数法能较好的与试验曲线吻合,并能反映桩的渐进破坏特征。而且由于荷载传递函数法的灵活性,基于本方法开发的程序能够模拟多层土中桩的荷载—传递特性,但是要精确模拟还需要具体的工程数据。
4)嵌岩桩的桩岩界面与桩土界面不同,界面的粗糙度对基桩承载力影响显著。目前,桩岩界面的粗糙程度可用无量纲的凹凸度因子表征,本文通过对桩岩界面粗糙形状的有限元模拟,不仅证实桩顶沉降量与凹凸度因子成反比,还发现其与粗糙突起的形状因子成正比,突起高度对承载力的影响比突起半波长的影响显著。桩顶位移实际上是随桩顶荷载的增大而增大的,所以在相同的荷载下减少桩顶位移就是提高承载力。在实际工程中,要提高嵌岩桩的承载力有两种措施,其一是当桩长可以加长时,尽量利用提高嵌岩桩接触面突起高度的方法来提高桩的承载力,当然这种方法不是很经济,但提高承载力幅度较大；其二是在桩长不可调时,尽量利用制造“短粗型突起”来提高桩的承载力,这种方法比较经济。
5)根据5号墩T构整体有限元分析,可知桥身自重荷载是群桩基础力学响应的主要影响因素；在施工阶段,桥梁的风荷载在桥墩和悬臂梁上引起的混凝土拉应力不容忽视,特别是桥墩混凝土没有做预应力,容易开裂；群桩基础的受力分布是中间桩承受荷载大,周边桩承受荷载小,这与实际监测结果大体一致。根据5号墩T构整体有限元稳定性分析,可知在施工阶段的最不利荷载组合下,桥梁的失稳模态主要是沿纵桥向整体倾覆；地基局部软化降低桥梁整体失稳的特征值。
5号墩为同类型铁路桥梁中国内最高墩,通过首次对高墩群桩基础所做的原型观测以及分析,本文工作存在如下创新之处：
1)首次完成了软岩地基中嵌岩灌注桩桩群桩基础的原型观测工作,分析了单桩的荷载传递特征和群桩的承载性状随施工进度的变化规律,为相同地质情况下的研究提供了依据。
2)在考虑桩侧土非线性软化的前提下,利用荷载传递函数法分析多层土中桩的荷载—沉降曲线,得到了较好的结果。
3)利用有限元优化分析工具证实了嵌岩桩桩侧粗糙度对其承载性状的影响,并提出了用于表征单个突起形状对嵌岩桩承载力影响的突起形状因子的概念。当桩长一定时,突起形状因子可以衡量单个突起的形状对桩的承载力的影响,并通过推理发现,“短粗型突起”是比较经济的突起形式。
4)利用有限元方法定性分析了5号墩T构的整体强度和稳定性分析,群桩的受力分布与监测结果大体一致,地基的局部软化会降低桥梁的整体稳定性,为高墩大跨连续刚构桥的设计提供了参考。
As China's transport development in the building of more bridges, many reasonable, economic, aesthetic bridge structure has been constantly reserached, introducted and developed. Because of its unique advantages in particular, high-pier and long-span continuous rigid frame bridge is widely used by designer.
Pier, beam and base of high-pier and long-span continuous rigid frame bridge are consolidated as one bearing forces together, the pier's type and heigh influenc bearing behavior of structure. Bearing behavior of structure of high-pier and long-span continuous rigid frame bridge is:pier and beam consolidated as one; the lower and super structure bear force together, and which reduce negative moment in top pier; pier's stiffness is lower and larger displacement is allowed; because biridge structure is multi ultra-setting structure, Shrinkage, creep, temperature changes, prestressed of concrete caused uneven settlement of platform, which caused additional internal forces that significantly influenced the structure's bearing behavior.
As uneven settlement of foundation of high-pier and long-span continuous rigid frame bridge is strictly control, so based on the prototype observation for pile group foundation of Dukouhe Bridge of Yiwan railway, this paper conduct test and theory annlysis on it, and which offer reference to construction practice. Dukouhe River Grand Bridge located in the middle of Yi-Wan Railway, and its subgrade is made up of soft rock which engineering behavior is unsteady and vulnerability to external disturbances. Because height of No.5 pier is 128 m, and strict with stability of pier and deviation of pier top, so were selected for the prototype observation target. The aim of this paper is to study interactive mechanism of system made up of piles, soil (or rock), bearing platform and pier during the course of construction. The interaction including superstructure and substructure of bridge is a very complicate issue, and directly related to construction process; meanwhile, construction phase of piling is key and its influence run through whole process of construction and operation of bridge. Once there is flaw attributed to piles construction, the remedial cost is very high, study on interaction of bridge has important value of theory and practice. The main contents of the paper are load transfer mechanism of single pile, and loading behavior of composite pile foundation. The research methods of the paper are long years of prototype observation on pile foundation of No 5 pier, supplemented by theoretical analysis and numerical simulation. Throught methods of foregoing, some conclusions were get such as mechanism of residual strain of socketed piles and its influence on load transfer of pile before loading, rules of load trandfer during construction of superstructure, characters of load distribution of composite pile foundation in soft rock, and influence of roughness of shaft wall and inhomogeneity of subgrade to bearing capability of single pile and bearing behaviors of pile group respectively, following is details conclusions:
1) Through prototype observation it was found that pile top load distribution of the composite pile is changed with the progress of construction, that is to say, as the weight of the upper structure and the bending stiffness of the platform change. Bending stiffness of the platform actually includes the contribution of the solid part of the pier bottom, which significantly affected the bearing traits of pile group in the early. Softening by soaking and weathering, the subgrade closed to platform bottom can not bearing load, the early upper load mainly borne by pile, with the growth of time, the subgrade closed to platform bottom compressed and gradually share part of the upper load. And the contact situation between the pile caps and platform is changing with time; this also affected the load space distribution of pile foundation.
Due to construction process of the bridge has not yet been completed during this monitoring, plus possibility as a result of the uneven compress of subgrade (part of the subgrade closed to river was soften by river water, an the other was fresh rock excavated from hill), it is currently monitoring result that the middle piles (nominated as pile 5-8 and pile 5-15) bearing more load than the corner pile (nominated as pile 5-19) and the corner pile's behavior of bearing load was very complicated, for it first tension further compression. As only the top center of the other pile laid strain gauge, but it was found by monitoring that axial strain distribution of the top of the pile is very uneven, additionally considering the theoretic contact situation between old concrete of piles and new concrete of platform was more distinguishing than the practical one, so pile only planted a strain gauge on its top can not reflect the pile strain distribution of the whole picture.
2) Through long-term monitoring of load transfer characteristics of the large diameter pile of pile group foundation, it was found that the load distribution of pile top was complex and varied with spatial location and time changes, and pile top axial load distribution is significant differences with the theoretical results under ideal state, therefore, pile group foundation design should be regulating according to the actual situation.
According to analysis of load transfer characteristics of pile socketed in soft rock, it can be considered to be end-bearing friction piles, namely most upper load bearing by side resistance. As the roughness of side wall of hand-dug pile is variation, the distribution of side resistance characteristics of pile is complex, and the side resistance curve has more than one peak. It was found through monitoring that large diameter rock-socketed pile has greater axial stiffness, smaller pile settlement, higher loading capacity, so it is a suitable base type for high, big and heavy structure.
3) Method for load transfer function concerning on side resistance nonlinear softening can better fit the test curve, and reflects the gradual destruction of the pile bearing behavior. And because of the flexibility of load transfer function, based on this method, the procedures can simulate for load-settlement curve of pile in multi-storey soil, but accurately simulate needs of the engineering data.
4) Interface is different when medium around pile is different; the roughness of interface is remarkably influence bearing capability of rock socketed pile. At present, roughness can be figured by convex concave factor. According to simulation by FEM, the paper not only approved that settlement of pile top are in inverse proportion to convex concave factor, but also proportional to the shape factor of asperity, and influence of height of the asperity is notable greater than that of half wavelength. Pile top settlement is actually increases with the increasing load; therefore, reduction pile displacement is to increase capacity under the same load. In engineering practice, there are two methods to enhance pile bearing capability. The first method is increasing height of asperity, when design length of pile can be lengthening, but this method is not economical; the second method is making "short-wide asperity", this method is economical.
5) According to the finite element analysis on T-shaped continuous rigid frame of No 5 pier, it was kown that selfweight of superstructure is the man influence factor on machenical response of pile group foundation; during the construction phase of the bridge piers, the tensile stress caused by wind load on the cantilever and the pier can not be ignored, especially no pre-stressed concrete on bridge pier, it easy to crack; the load distribution of pile groups is that the middle piles bear load more than the pile at Around, which is broadly consistent with the actual monitoring results. According to the finite element stability analysis on T-shaped continuous rigid frame of No 5 pier, we can see under the most unfavorable combination of load in the construction phase, the main mode instability of the bridge is collapse as a whole along the vertical direction; partial softening of the foundation can lower the overall instability eigenvalue of the bridge.
At present, the height of No 5 pier is the highest comparing to same type railway bridge in China, according to prototype observations and anlysis to it, some innovation was summarized as follows:
1) For the first time, prototype observations on rock socketed composite pile group was completed, the rules of single pile's load trandfer characters and bearing behaviors of pile group was get and it will provide reference to practice engineering.
2) In considering the non-linear soil softening soil around pile side, using load-transfer function get better result when analyze pile inthe multi-layer soil.
3) Influence of roughness of interface of rock socketed pile was approved by FEM method, and a new concept named shape factor of asperity was bring forward.
4) According to the finite element analysis on T-shaped continuous rigid frame of No 5 pier, Strength and stability of it was anlyzed, and which is broadly consistent with the actual monitoring results. Partial softening of the foundation can lower the overall instability eigenvalue of the bridge.
引文
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