长江马湖大堤崩岸环境地质背景分析

马湖大堤位于江西省彭泽县境长江东南岸,是崩岸多发之险段,1996年1月曾发生大规模的崩岸灾难性事件。对马湖大堤崩岸的成因,各有关部门的专家、学者进行了较为深入的研究。仅从地学的角度进行分析,初步认为,马湖大堤堤基为全新世河流和湖泊相沉积,相变明显,为不良工程地质体。下伏地层为二叠系灰岩,岩溶发育。马湖江段受北东向和北北东向两组断裂控制,为追踪北北东向裂发育的江段。造成马湖崩岸的原因：（1）长江河道主泓的持续南迁,是造成崩岸的主要地质因素(2)小姑山－彭郎矾卡口的“瓶颈”作用有上游主流的顶冲,加大了水流对长江东南岸的掏蚀和侧向侵蚀;（3）堤岸岩性及护岸条件表明,马湖堤段抗侵蚀能力弱;（4）堤基不良工程体的存在及突加荷载或震动,导致了大堤的失稳;（5）地下水产生侧压力作用,降低了大堤的抗剪强度。

ENVIRONMENTAL GEOLOGIC BACKGROUND OF BANK COLLAPSE OF MAHU DYKE OF THE YANGTZE RIVER

Mahu dyke,which is the place of bank collapse occurring frequently(e.g. the disastrous incident of large scale bank collapse once occurred here in January of 1996) is located in the south east bank of the Yangtze River in Pengze county, Jiangxi province.The thorough investigations and studies on the genesis of bank collapse of Mahu dyke have been carried out by experts of different discipline. The authors analyze the bank collapse in the paper only from the point of earth science and consider preliminarily that the base of Mahu dyke which is Holocene epoch river facies and lakes facies deposits with abvious facies change is not a good engineering geologic body. The under lying stratum is permiain system limestone and karst develops. Mahu river section,which is controlled by two groups of NE and NNE striking rifts,is the developed section of NNE striking rifts.The causes of bank collapse of Mahu dyke are:(1) The trunk stream of the Yangtze River moves continually towards south,this is the main geological factor causing the bank collapses.(2) The "bottle neck" function of the narrow pass of Xiao Gu Shan(hill) Peng Lang Ji(a rock projecting over the water)and the river worn action in the upper reaches have intensified the sapping and side erosion of south east bank of Yangtze River by the river water.(3) The lithological characters and protection condition of the embankment show that anti sapping and anti eroding capacities in Mahu dyke is weak. (4) The existing no good engineering body of the dyke base and sudden load or vibration cause the dyke unsteady.(5) The side pressure function caused by the ground water reduces the dykeis shearing strength.