Study On Mechanism Of Wave-induced Submarine Landslide At The Yellow River Estuary, China

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).