海洋工程中大直径超长钢管桩动力沉桩基础理论研究

With the ocean resource exploitation rapidly expanding to the deep sea areas, super-long and large diameter steel piles are being increasingly used in offshore platform constructions. Large pile dimensions significantly increase the difficulty and complexity of offshore pile driving. Therefore, it is essential to conduct accurate and rigorous pile driveability analyses based on which comprehensive construction schemes, procedures, and correlative management systems can be established. Accurate prediction of pile-driving response is also important for the estimation of pile capacity, the selection of a suitable hammer system, and for saving project cost. However, pile driving causes complex changes to the state of soil surrounding the pile and involves the interaction among the hammer, pile and soil, and thus, analysis of pile driving is a challenging problem. In this research, based on the structure characteristic of large dimension steel pipe piles, the drivability analysis considering the interaction among hammer-pile-soil are performed. The controlling the plugged effect and the driving refusal and pile-run are analyzed. The main topics include: (1) Based on the neural network model, develop a database with quite amount of data and references to instruct the relative pile drivability analysis. (2) Deduce the partial safety factor of pile freestanding analyzing with the reliability theory. (3) Study on the mechanism of the soil plugged effect during a large dimension steel pipe pile is driven into the subsoil. A prediction method will be established to estimate the soil plugged effect. (4) Establish the laws for the development and dissipation of the accumulated pore pressure during pile driving. (5) Investigate the phenomena of the driving refusal and pile-run, a revised prediction method will be proposed to analyzing these phenomena. (6) Find out the characteristic of the reflection wave in the large-strain test on offshore platform and put forward a reliable analyzing method for monitoring the driving processes and determining the pile bearing capacity. This research project will provide fundamental and methodological achievements for the large dimension steel pipe pile driving analysis on offshore engineering, which would contribute to scientific and technical innovation of Chinese offshore engineering.

动力沉桩分析不仅是工程中亟待解决的关键问题，更涉及波动理论、动力学、土力学和材料损伤机制的基础研究。为突破海上打桩施工精确可控的技术瓶颈，本项研究以实现对大直径、超长钢管桩可打入性的准确预判为目标，以揭示海上打桩施工问题的理论根源为创新导向，针对大尺度钢管桩的结构特点开展锤－桩－土系统耦合动力作用机理、土塞闭塞效应机理、土体水力劈裂机理、拒锤、溜桩机理等方面的基础理论研究。主要包括：开发基于神经网络模型具有相当规模和辨识力的打桩分析数字化平台；针对置锤瞬间荷载特点，基于可靠度理论推导桩自由站立稳定性的分项系数；揭示大直径、超长钢管桩土塞闭塞效应产生机理，建立判断方法；揭示拒锤与溜桩现象的理论根源，提出预测方法体系；研究海洋平台桩基动测的反射波特征，建立测试结果的科学分析方法。通过研究，为海洋工程大直径、超长钢管桩可打入性的准确预判提出理论和方法的支持，提升我国海上桩基施工的科技创新水平。

本项目开展了以下几方面的研究工作：（1）基于神经网络模型开发了动力沉桩分析数字化系统。以神经网络模型为基础，收集和整理了现有海上打桩施工记录，建立了包括打桩工程相关信息的数字化系统，为同类打桩工程确定桩段长度、选择锤型、停锤位置等技术参数提供指导。（2）开展了大直径钢管桩自由站立屈曲失稳理论研究。针对大直径管桩的自由站立失稳模态和失稳发生机制构建了系统的理论分析方法，基于可靠度理论考虑荷载作用的瞬时特点，确定了工程设计中应采用的安全系数。（3）开展了土塞形成机制及闭塞效应判断方法研究。结合大直径、超长钢管桩土塞的形成机制和特点，提出了宜采用太沙基或迈耶霍夫深基础公式计算桩端阻力，进而建立了判断大直径钢管桩土塞效应的改进静力平衡方法和拟静力方法。（4）开展了动力沉桩过程中溜桩现象产生机理研究。基于桩锤—桩—土相互作用原理，揭示了打桩过程中土体性状的变化以及溜桩现象的发生机理，建立波动方程数值模拟方法，结合实际工程验证，实现了对溜桩现象及溜桩区间的准确预测。（5）开展了拒锤现象研究并提出了桩身结构优化方案。揭示了拒锤现象发生的机理，提出根据停锤后重新启动所需要的锤击数来判断土体强度随时间的恢复程度，从而预测后继打桩拒锤现象的发生。进而基于理论分析成果，优化桩身结构设计，提出了指导打桩停锤位置和停锤时间的理论方法，避免拒锤现象的发生。（6）开展了海洋平台桩基动测研究。结合海洋平台的桩基施工开展了海洋平台的大应变桩基动测实验，获得了打桩过程中桩身最大应力、桩身完整性、打桩锤工作状态以及桩基承载力的实测数据，为桩周土体强度增长规律的研究奠定了基础。通过以上研究解决了海洋大直径超长钢管桩沉桩全过程精准控制的前沿难题。