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波浪作用下黄河口海底滑坡研究
中文摘要

本文旨在波浪诱发黄河口海底滑坡的研究,满足滑坡预报预警服务的需要,期望减小滑坡对工程设施造成的危害,加深了解河口沉积物的运移过程和方式。本论文以黄河口埕岛海域为研究区,通过试验测试和理论计算,系统地研究了埕岛海域工程地质环境特征、滑坡破坏模式、沉积物在循环荷载作用下的破坏过程、研究区海底斜坡稳定状况、滑坡发生时间、滑坡规模与特征、滑坡发生概率等,主要开展的工作和取得的成果如下: (1)构建了黄河口埕岛海域海底滑坡的工程地质环境特征,为进一步滑坡形成分析研究准备基础参数资料。搜集各单位在埕岛海域的工程地质勘察报告,得出该区工程地质和水动力环境参数的特性及其区域变化规律;对埕岛海域工程地质环境分区,建立各亚区的典型土层结构组合。 (2)研究了黄河口海底滑坡破坏模式,为滑坡试验研究和理论计算模型的建立提供指导。分析现场勘测中发现的滑坡模式,并结合前面建立的地层结构,建立了研究区极端海况下底坡破坏的四种模式,即平面剪切破坏、圆弧振荡破坏、液化流滑和塌陷破坏;进一步分析了这四种滑坡模式的形成机制,并依据现场调查资料和试验结果进行了验证。 (3)研究了黄河口沉积物在循环荷载作用下的破坏过程,获取沉积物动力学参数变化特征,为本文后面的理论模型计算提供依据。基于现场观测试验和室内动三轴试验,测试循环荷载能量在土体中的传播、循环荷载导致土体强度的衰减、原状土体和重塑土体的孔压变化规律、不同强度粉土的抗液化性能,得出了研究区海床土现场液化判别标准和孔压比上下限值。 (4)建立了海底斜坡剪切破坏理论计算模型,据此定量分析黄河口海底滑坡形成过程与研究区海床稳定性。基于极限平衡法和极限分析法,推导出波浪作用下海床圆弧振荡剪切破坏公式,并与水槽试验结果和其它计算结果比较。基于前面建立的研究区典型土层,分析不同波浪重现期下的海底斜坡抗剪切破坏安全系数和最危险滑动面所在的深度。研究得出,极端海况下埕岛海域亚区I的典型土层结构组合A和B可发生平面剪切破坏;典型土层结构组合A还可发生圆弧振荡剪切破坏,其它亚区不会发生剪切破坏。 (5)建立了海床波致液化破坏理论计算模型,据此定量分析了黄河口海底液化滑坡形成过程与海床稳定性。利用建立的液化判别经验法,以及对累计孔压计算模型的改进法,对埕岛海域各亚区典型土层结构组合进行液化分析。发现黄河口海床液化不一定从海底面开始往下扩展,而是主要发生在淤泥质粉质粘土层内。研究得出了埕岛海域4个亚区的液化破坏土层和破坏深度,其中瞬时液化深度基本在2m以内,累计液化深度可达10m。 (6)研究了黄河口海床破坏后的宿命。黄河口沉积物液化后受地形限制,可有三种宿命—悬浮运移、坡面流滑和原地残留。研究发现对于埕岛海域土体的坡面流滑,其方向有三种:北部向北、中部先向东再向东北、东南部可能向东南。这种滑动对上覆未液化层或未液化透镜体撕裂,可造成海底孤立网绳状塌块。 (7)研究了黄河口海底滑坡发生经历的几个过程及其所需要的时间,为进一步滑坡预警提供支持。基于前面建立的理论和试验测试结果,得到不同深度处土体孔压增长和强度丧失随时间的变化过程,斜坡剪切破坏和液化破坏所需的时间下限值。 (8)分析了黄河口海底滑坡发生的概率,基于一次二阶矩法计算埕岛海域海底斜坡在不同波浪重现期下的剪切破坏和液化破坏概率。研究得出了该海域4个亚区的破坏概率,其中亚区I的剪切破坏概率在25%以上;亚区II和III的平面剪切破坏概率位于5%-39%之间,圆弧振荡剪切破坏概率位于1%-14%之间;亚区IV的剪切破坏概率在0.5%以内。 本论文主要创新点:(1)通过现场试验发现循环荷载作用下,原状土体孔压先后经历先上升、后减小、再上升和剧烈波动四个阶段,而重塑土只有上升和剧烈波动两个阶段;(2)提出了黄河口海底滑坡的四种主要破坏模式,即平面剪切破坏、圆弧振荡破坏、液化流滑和塌陷破坏,并建立了相应的理论计算模型,得到了埕岛海域不同区域的破坏程度和破坏时间;(3)基于概率分析方法,得到了埕岛海域四个亚区滑坡发生的概率。 关键词:黄河口;海床;波浪;滑坡;液化 最好记清,地球上所有最大的斜坡运动看来都发生在海底……(Varnes,1975)

英文摘要

This dissertation studies the mechanism of wave-induced submarine landslide at the Yellow River estuary, to satisfy the need of landslide forecast, to reduce the harm to submarine engineering facilities caused by landslide and better understanding the transport process of sediment. The Chengdao Sea at the Yellow River estuary is taken as the study area in this dissertation. Through the experiment measuring and theoretical calculation, several topics, including the landslide geological environmental characteristics, landslide failure modes and sediment failure process under dynamic load, submarine slope stability, landslide failure time, scale, characteristic and probability were studied. The mainly studies and results achieved are following: (1)The landslide geological environmental characteristics of the Chengdao Sea at the Yellow River estuary were established, for preparing the basic soil and hydrodynamic parameters in submarine landslide analysis. Through the collection of geotechnical reports at the Chengdao Sea from universities, academy agencies and enterprises, the soil properties, soil property regional variation regular pattern and hydrodynamic environmental characteristics of the Chengdao Sea were established. Then, four geological sub-areas of the Chengdao Sea were divided, and their typical stratum sections were established. (2)The submarine landslide failure modes at the Yellow River estuary were studied, for guiding the implementation of the after-going test design and theoretical calculation model establishment. The landslide model investigated In-Situ were analyzed, and then combined with the established strata, some possible failure modes under extreme sea conditions were put forward. Four main failure modes, namely, liquefaction flow sliding, plane shear failure, arc oscillation failure and collapse failure were put forward, and their formation processes were analyzed, then failure modes were verified according to the information investigated In-Situ and from experiment. (3)The failure process of sediment under dynamic loads was measured through experiment, some data and conclusions taken as the parameters in the theoretical model latter. Based on the experiment In-Situ and cyclic tri-axial experiment indoor, some sediment dynamic parameter variation under dynamic loading including the dynamic loading energy dissipation in seabed, soil strength reduction, excess pore water variation pressure of undisturbed sediment and remolded sediment, the liquefaction characteristics of soil with different strength were measured. Some new insights, such as liquefaction standards, pore pressure ratio upper and lower limits were gotten. (4)The submarine slope shear failure theoretical models were established for analyzing the slope failure mechanism and slope stability. Based on the limit equivalence theory and limit analysis theory, the arc oscillatory shear failure models were established, and the results were compared with those of flume test and other theoretical methods. The anti-shear failure safety factor and slide depth were calculated based on the established typical stratum under waves with different return periods. The typical stratum A and B of sub-area I at the Chengdao Sea could fail in mode of plane shear failure under extreme sea conditions, typical stratum A could fail in mode of arc oscillation shear failure, however other sub-areas would not occur in shear failure mode . (5)The wave-induced seabed liquefaction models were established, for analyzing the liquefaction landslide formation process and slope stability. One liquefaction experience evaluation method was established, accumulated pore pressure calculation model was improved, and liquefaction degree of typical stratum was calculated based on those established models above. The liquefaction of stratum does not always start from the surface to expand down, but mainly occurs at the weak layers, such as silt clay mud layer. The liquefaction layers and depth of strata of four sub-areas at the Chengdao Sea were calculated, in which at the instantaneous liquefaction depth is within 2m, and accumulated liquefaction depth could reach 10m. (6)The failed sediment fate at the Yellow River estuary was also studied. Restricted by sub-terrain, the liquefied sediment has three kinds of fates, namely, suspend to migration, flow-slide along slope and residual In-Situ. As for the sediment flow-slide at the Chengdao Sea, its direction has three types: the northern part flows to north, central part firstly flows to east, and then to northeast, and the southeast part probably flows to southeast. The flow of liquefied layers could tear out the non-liquefied upper layer or non-liquefied lens to form collapsed blocks found in investigation In-Situ. (7)The submarine landslide processes and their time required were also studied, for early harm warning. Based on the established theoretical models and experiment results, the soil pore pressure building up and strength loss with time were calculated, and time of lower limit of the shear failure and liquefaction failure were also gotten. (8)The probability of submarine landslide at the Yellow River estuary was also studied. Based on the First Order Second Moment method, the probability of shear failure and liquefaction failure of the four sub-areas at the Chengdao Sea under waves with different return periods was calculated. The probability of shear failure at sub-area I is above 25%, and the probability of plane shear failure at sub-area II and III lies between 5%-39%, the probability of arc oscillation failure at sub-area II and III lies between 1 %-14%, however, the probability of shear failure at sub-area IV is below 0.5%. The main innovations in this dissertation include: (1) The In-Situ experiment reveals the pore pressure of undisturbed soil include four stages, increase, decrease, re-increase and severe vibration under cyclic loading, however, the pore pressure of remolded soil just include two stages, increase and severe vibration. (2) The main four failure modes at the Chengdao Sea were put forward, and the calculation models of three of them were established. Based on the calculation models, the failure depth, stability safety factor, and failure time of different sub-areas at Chengdao Sea were gotten. (3) The probability of slope slide at the four sub-areas of Chengdao Sea was calculated based on the Fist Order Second Moment (FOSM). Keywords: the Yellow River estuary; seabed; wave; submarine slide; liquefaction.

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