汶川震区狭陡型泥石流典型特征与防治

汶川MS 8.0强震发生6年来,震区暴发了数量众多、类型多样的泥石流灾害,其中一类可总结为"狭陡型泥石流",其特征指标及防治对策与震前常规泥石流存在显著差异,给灾后重建和灾害防治技术提出了新的命题。在对(极)重灾区泥石流活跃的五大片区(汶川、都江堰、绵竹、北川、安县)进行大量现场调查和资料分析的基础上,提出了强震区狭陡型泥石流的概念并给出定义,提炼了其在形成、活动及危害等方面的典型特征;通过工程实践,分析了其独特的治理难点及相应的防治对策,介绍了几项极具适应性和有效性的防治新技术,可为强震扰动区泥石流治理研究和工程设计提供一类新的思路和借鉴。     

移动荷载作用下土岩组合基坑吊脚桩变形分析

以青岛地区特有的土岩组合地质条件为背景,采用现场监测和Plaxis有限元模拟相结合的方法,研究了土岩基坑中吊脚桩在龙门吊移动荷载作用下的变形规律及动力响应。结果表明：桩身变形模拟结果与实测值吻合较好,基坑的变形主要发生在基坑上部软弱土层,吊脚桩嵌岩处产生应力集中;在龙门吊移动荷载作用下,桩顶水平位移较大,但其动力响应最小,而嵌岩处水平位移较小,但其动力响应最大;嵌岩处、桩身最大正弯矩处及最大负弯矩处的土压力动力响应较大,且移动荷载刚经过时刻影响最大。研究成果可为类似土岩结合地区深基坑支护设计提供参考。     

Effects of scour-hole dimensions and soil stress history on the behavior of laterally loaded piles in soft clay under scour conditions

Bridge pile foundations in the midst of water current are often subjected to scour, which induces the loss of soil support around the piles and thus results in a significant decrease of foundation capacities or even the failure of bridges. In the current practice, when the scour-affected behavior of the pile foundations is analyzed, either the whole scour-hole geometry or the possible changes in the stress history of the remaining soils is often ignored. In reality, however, scouring creates scour holes with certain dimensions around the pile foundations and the remaining soils that are not scoured away undergo an unloading process at the same time, which will increase the over-consolidation ratios of the remaining soils and accordingly changes of their mechanical properties. This paper examines the behavior of laterally loaded piles in soft clay under scour conditions by using the p-y method. The conventional p-y curves have been modified appropriately to reasonably reflect the effects of three-dimensional scour-hole geometry as well as the stress history of the soils, with the aid of integration of Mindlin’s fundamental solutions. A field test is used to serve as a reference case and to investigate the effects of stress history, scour depth, scour width, and scour-hole slope angle on the responses of laterally loaded piles in soft clay. The results indicate that neglecting the stress history effect can be unconservative for scour-affected pile foundations in soft clay, whereas neglect of the scour-hole dimensions and geometry would lead to over-conservative predictions/design of the laterally loaded piles under scour condition.     

内撑式排桩支护结构的设计优化研究

在分析内撑式排桩支护结构受力特性和设计优化空间的基础上,基于满应力设计基本思想,建立了支护桩－内支撑（节点）部位的设计优化数学模型,提出了用于求解该（节点）部位局部优化解的新的目标函数 和相应的设计优化判据,推导并比较了钢筋混凝土内支撑等效轴向抗压容许应力 和支护桩等效抗弯容许应力 的3种取值方法,并以杭州某基坑支护应用为例作了介绍。可供同类基坑支护设计优化时借鉴。     

层状地基静压桩贯入过程机理试验

通过在多层软粘土地基中静力压入单桩的室内模型试验,对模型桩在整个沉桩过程中压桩力、桩动端阻力及桩动侧阻力随桩贯入深度的变化情况进行了研究,获得了桩在贯入不同土层分界面时阻力的变化规律以及桩周土体应力的分布特征。并对开口管桩和闭口管桩贯入试验情况进行了比较分析,揭示了不同桩端形式桩在贯入过程中桩动侧摩阻力的发展规律,以及分层土体中开口管桩贯入过程中土塞的变化情况。试验结果表明：在粘性土中沉桩时,压桩阻力主要来自桩端向下穿越土体产生的端阻力,而侧摩阻力较小;由于桩侧水平应力的释放使得同一深度点上的动摩阻力随着桩的下沉表现出不同程度的降低,出现摩擦疲劳。     

基于现场实测的缓倾硬性结构面连通率统计

通过对缓倾角硬性结构面数据的分析、判别,现场实测结构面几何特征的各基本要素,如结构面坐标、结构面产状、结构面间距、结构面的连续性、结构面迹长等,运用三维空间投影图解,最终求解结构面连通率,取代以往现场用线连通率经验判断的传统模式。本方法已应用于水电工程施工过程,效果良好。     

锚索双排桩与刚架双排桩的对比研究

在大型滑坡治理工程中常常采用双排抗滑桩,锚索双排桩和刚架双排桩是常见的两种型式。采用有限元程序ANSYS分别建立锚索双排桩和刚架双排桩的有限元分析模型,对比分析这两种型式双排桩的内力分布,找出能更好发挥抗滑效果的结构型式。通过对比分析可知,锚索双排桩的前后两排桩的内力分布差距十分明显,刚架双排桩的前后两排桩的内力分布差距较小,所以,刚架双排桩能够更好地发挥抗滑效果。选取锚索双排桩和刚架双排桩2个工程实例,监测分析发现锚索双排桩的前排桩侧向位移和弯矩均远小于后排桩,刚架双排桩前后两排桩的位移和弯矩比较接近,这与有限元分析结果相吻合,因此,双排抗滑桩的设计中建议采用刚架双排桩     

砂土中单桩基础冲刷的修正应变楔计算

由冲刷引起的深水结构物桩基础周围土体损失致使基础水平承载性能下降的问题越发受到重视。应变楔方法假设桩前土体抵抗为三维楔形体,其尺寸发展与楔形体区域土体发挥的内摩擦角有关,从而得到水平受荷桩的p-y曲线。本文对应变楔方法进行修正和拓展,建立非线性位移假设以考虑桩前楔形体区域土体应变沿深度的非均匀分布,将冲刷坑底以上土体的有效自重作用等效为竖向荷载,对楔形体的深度进行修正,以解决楔形体方法只适用于地表水平的情况,得到砂土中单桩基础冲刷的修正应变楔计算方法;并通过与模型试验及三维有限元分析的对比来验证该方法的合理性。分析结果表明:冲刷深度增加会显著降低桩基水平承载性能,冲刷深度3.2D和6.4D情况下的桩顶位移比平均值分别趋近于1.8和3.0;相比有限元方法,本文修正SW方法计算的p-y曲线结果与实测结果更为接近。     

Centrifuge modeling of batter pile foundations under earthquake excitation

Although batter pile foundations are widely used in civil engineering structures, their behavior under seismic loadings is not yet thoroughly understood. This paper provides insights about the differences in the behavior of batter and vertical piles under seismic soil-pile-superstructure interaction. An experimental dynamic centrifuge program is presented, where the influences of the base shaking signal and the height of the gravity center of the superstructure are investigated. Various seismic responses are analyzed (displacement and rotation of the pile cap, total shear force at the pile cap level, overturning moment, residual bending moment, total bending moment and axial forces in piles). It is found that in certain cases batter piles play a beneficial role on the seismic behavior of the pile foundation system. The performance of batter piles depends not only on the characteristics of the earthquakes (frequency content and amplitude) but also on the type of superstructures they support. This novel experimental work provides a new experimental database to better understand the behavior of batter pile foundations in seismic regions.     

真空联合堆载预压下混凝土芯砂石桩复合地基固结特性理论分析

真空联合堆载预压与混凝土芯砂石桩复合地基相结合是处理深厚软土地基的一种新方法。该方法既能利用预制混凝土芯桩提高地基承载力,又能利用砂石外壳缩短排水路径、传递真空负压,加快固结。根据真空联合堆载预压下混凝土芯砂石桩复合地基的固结特点,将堆载预压和真空预压的固结效应分开考虑,然后再进行叠加,推导出了真空联合堆载预压下混凝土芯砂石桩复合地基的平均固结度解析解,并利用三维数值模拟对平均固结度解析解进行了验证。通过对解析解的参数分析,探讨了置换率、涂抹区大小及渗透系数、混凝土芯砂石桩长径比和芯桩率对混凝土芯砂石桩复合地基固结特性的影响。结果表明,混凝土芯砂石桩复合地基的固结速率随着置换率和芯桩率的增大而增大,随涂抹区和砂石外壳直径之比、未扰动区和涂抹区渗透系数之比、长径比的增大而减小。