基于软硬互层礁灰岩地层的嵌岩桩荷载传递规律研究

With the boosting of China's marine ambition and the ‘Belt and Road’ initiative, engineering projects built upon coral-reefs are emerging. Pile foundation, which can support large-span and large-scale structures favorably, is expected for providing satisfactory bearing capacity. Aiming at unveiling the mechanism of load transfer between rock-socketed piles and coral-reef limestone, this project is going to investigate the drilled samples of coral-reef limestone from Yongshu Reef, Meiji Reef and Weizhou Island (Guangxi Province) through in-situ and laboratory mechanical testing, physical modelling, mesoscopic testing as well as theoretical analysis. A deep insight into the mesoscopic formation of interfacial cohesion between pile and rock can thus be expected. Furthermore, in light of the interface shear test, a constitutive model describing interfacial friction between concrete and coral-reef limestone will be proposed, along with a model revealing load transfer for pile drilled into soft-hard interbedding of coral-reef limestone foundation. It is anticipated that research outcome can facilitate designing of rock-socketed pile upon coral-reef limestone, especially in terms of designing method and designing parameters. The research work would contribute to marine engineering geology and serve ocean engineering constructions.

随着我国海洋强国战略及“一带一路”倡议的逐步实施，珊瑚礁工程建设不断涌现，桩基工程作为满足大跨度和大体量建筑的基础形式，已成为提供更高承载力的考量。本项目以南海永暑礁、美济礁和广西涠洲岛的珊瑚礁灰岩为研究对象，以“软硬互层礁灰岩地层中嵌岩桩的荷载传递规律”为研究主线，综合运用现场原位测试、室内力学试验、物理模型试验、微细观试验及理论分析等研究方法，从微观角度揭示桩岩界面黏结强度的形成机制；探究桩岩界面摩擦中剪切破坏面的发展规律，建立礁灰岩与混凝土界面摩擦的本构模型；聚焦软硬互层条件对礁灰岩中嵌岩桩荷载传递规律的影响，提出适合软硬互层礁灰岩地层的桩身荷载传递模型。着力解决礁灰岩嵌岩桩的设计方法和设计参数选取问题。项目研究对完善海洋工程地质学科及推动海洋工程建设有着重要的科学意义和实用价值。

项目针对取自我国南海某岛礁的四种典型结构类型的礁灰岩钻孔岩芯，即块状结构、砾块结构、砾屑结构及砂屑结构礁灰岩，开展微相分析、物理力学性质测试、桩-岩界面剪切试验、嵌岩模型桩试验，探究礁灰岩地层的工程地质特性、礁灰岩嵌岩桩的竖向承载特性及桩身荷载传递规律。基于礁灰岩三轴压缩及无侧限压缩强度试验，获得礁灰岩在不同围压条件下的强度及变形响应规律；通过对峰后应力-应变曲线的分析，计算确定礁灰岩试样在不同围压条件下的脆性指标。开展常法向应力和常法向刚度条件下的礁灰岩-混凝土界面剪切强度试验研究，探究桩-岩界面剪应力-剪切位移关系曲线的变化规律；对比开展红砂岩-混凝土界面剪切试验，揭示两种不同沉积作用类型岩石与混凝土黏结面产生剪切强度差异的根本原因。基于礁灰岩嵌岩摩擦桩及端承桩模型试验研究，揭示围压和礁灰岩结构类型对桩身竖向承载力的影响规律；获得桩身侧摩阻力发挥的三段式简化模型；结合现场试桩结果，提出礁灰岩极限侧摩阻力标准值和极限端阻力标准值的确定方法。研究结果表明：礁灰岩的工程力学性质与其沉积组构和成岩作用密切相关；围压对礁灰岩的脆性指标影响显著，随着围压的增大，礁灰岩的脆性指标呈现趋势性降低；单轴压缩条件下，低孔隙率砂屑灰岩的脆性指标与水泥砂浆类似；砂屑灰岩和砾屑灰岩的脆性指标均与孔隙率呈负相关关系。由于水泥浆在礁灰岩中的扩散填充作用，礁灰岩-混凝土界面的黏结力和内摩擦角均高于砂岩-混凝土界面；桩岩界面剪切先后经历弹性剪切、剪应力跌落和摩擦剪切三个阶段；在桩岩界面的弹性剪切阶段，界面剪切变形以弹性变形为主，极限弹性位移呈现出随弹性模量增大而递减的趋势；在剪应力跌落阶段，应力软化急剧，并迅速过度到界面摩擦剪切。在三向受压状态下，桩端礁灰岩样的桩端应力随桩顶位移的增大呈现缓变型增长，桩端应力较单轴压缩强度提高显著，桩端应力随桩顶位移的变化曲线可归纳为三种曲线类型。