硬质粘土疏浚过程中快速移动射流切削机理研究

The water-jet cutting is an effective means to improve the dredging efficiency of underwater stiff clay. However, the great increase of the energy consumption does not always obviously improve the efficiency in dredging practices. The barrier to analyze and solve this problem is that the mechanism of cutting stiff clay by fast moving water-jet is unclear. This project will take the stiff clay (shear strength＞50kPa as defined in the dredging specifications) as the research subject, and focus on the soil failure patterns, such as surface scouring, slop slipping and hydraulic fracturing, under the cutting by fast moving water-jet. To this end, we will firstly investigate the flow shear stress on the soil surface, the suction resistance on the sliding surface, and the coupling between crack seepage and crack propagation of the stiff clay. Then the models of the surface erosion rate, the quick slip and the crack expansion of the hydraulic fracturing will be respectively built to describe the characteristics of the fast failure of underwater stiff clay. Secondly, the cutting of the stiff clay by fast moving water-jet will be simulated and tested with the change of soil strength, jet pressure and moving speeds. Consequently, with the analysis of the soil disappearance pattern, the critical jet pressure of soil failure and the final cutting shape, the failure mechanism of the stiff clay will be identified according to different soil parameters and different jet-cutting conditions. The achievement of this project will provide a theoretical foundation for optimizing the dredging water-jet system and improving the construction efficiency in dredging engineering.

射流切削是提高水下硬质粘土疏浚效率的有效手段，但实际施工中常常能耗大幅增加而功效并不显著，分析和处理该问题的主要障碍在于，对快速移动射流切削硬质粘土的机理不明。为此，本项目将以疏浚行业规范中的硬质粘土（抗剪强度＞50kPa）为研究对象，针对其在快速移动射流切削下的表面冲刷、边坡滑移和水力劈裂等破坏形式，开展下列工作：首先，探索硬质粘土的表面流动切应力特性、滑移面负压阻力衰减规律和裂缝渗流-扩展耦合关系，分别建立反映水下硬质粘土快速破坏特点的表面刷蚀速率模型、快速滑移模型和水力劈裂裂缝扩展模型；然后，改变土体强度、射流压力和移动速度等参数，对快速移动射流切削硬质粘土过程进行数值模拟和实验研究；最后，通过对土体消失形式、临界破土压力和最终切削形貌等方面的分析，揭示不同土质参数和射流切削条件下硬质粘土的破坏机理。研究成果将为优化设计疏浚射流系统、提高疏浚施工效率提供理论依据。

粘土是疏浚工程面对的典型土质之一，在疏浚粘土时，利用射流切削的方式可以产生良好的挖掘效果。然而，快速移动射流条件下，粘土切削机理不明，临界射流压力与切削深度的确定缺乏理论依据。这导致疏浚射流系统的设计与使用缺乏理论指导，应用效果无法得到保证，大大制约了水射流技术在疏浚工程中的发展与推广。. 针对上述问题，本项目分别建立冲击载荷下粘土的剪切失效与拉裂失效模型，并采用这两个模型进行数值模拟分析。在一系列数值方法的探索过程中，有效克服了切削区域网格大变形的难点，成功模拟了粘土单元失效及分离的过程。获得了射流切削过程中的沟槽形貌、土体应力场和射流流场等信息，结合移动射流切削试验成果，分析水力拉裂、剪切滑移和表面刷蚀三种破土模式的作用，最终确定剪切滑移是粘土破坏的主要模式。针对临界射流压力与切削深度的确定，研究提出采用逼近的方法，首次获得了移动射流切削粘土的临界射流压力。研制移动射流切削实验台与大尺度粘土制备装置，首次提出移动射流的特征切削时间概念，进行不同粘土强度、射流压力、喷嘴直径与移动速度等多组参数下的系列试验，构建了临界射流压力模型与切削深度预测模型。. 本项目研究并展示了射流切削冲蚀粘土过程的详细信息，揭示了移动射流切削粘土的机理，提出了粘土切削的临界射流压力模型和切削深度预测模型，研究成果可为疏浚工程中射流系统的优化设计和疏浚施工工艺改进提供理论基础。