长江三角洲北翼第一硬土层理化特征及其地质成因

Physicochemical characteristics and geological formation of the first hard soil layer of the north wing of the Yangtze River delta

在长江河口两翼广泛分布第一硬土层(FHSL),研究其形成机制及工程地质特性对工程建设具有很好的指导意义。根据调查资料(钻孔935个,累计进尺42128 m)和试验资料,首次精确确认了长江河口北翼第一硬土层分布界线,研究了第一硬土层的形成年代、粒度特征、地球化学特征、工程地质特性等。研究表明:第一硬土层形成年龄为20~11 ka B.P.(多个光释光和14C测年资料);硬土层含水率随深度的增加有增大的趋势,表明气候自下向上逐渐变凉和变干;第一硬土层的颗粒级配、粒度分布频率曲线、C-M沉积图等特征显示,第一硬土层主要由粉砂、极细砂和粘土粒级组成,样品的粒度频率曲线主要呈单峰分布,反映出物质沉积前所受搬运营力性质单一,土体颗粒沉积以均匀悬浮占绝对优势,沉积环境是一种相对稳定的低能环境。第一硬土层的发育受气候控制,大致可以分为3个阶段:第1阶段(20~15 ka B.P.)为沉积与成土交替作用时期,且以沉积作用为主,硬土层剖面厚度主要受该阶段控制,至末次盛冰期结束;第2阶段(15~11 ka B.P.)为暴露成土期,这时洪水不能形成越岸沉积,加积作用基本停止,硬土层厚度不再明显增加,已形成的第一硬土区域受到频繁变迁的分合河网的侵蚀切割,形成多条不规则古河道和台地,硬土层逐渐脱水成陆,经历了风化成壤的过程;第3阶段(11 ka B.P.至今)为淹埋期,随着全新世的到来,气候变暖,海平面不断上升,硬土层被其上覆的海相沉积层掩埋,成岩作用开始直到现今。土体易溶盐含量较高,为典型氯盐渍土类型,自下而上具有从低变高的趋势,为海相层覆盖硬土层以后成岩过程造成的。

The first hard soil layer(FHSL)is widely distributed in the two wings of the Yangtze River estuary,and the study of its formation mechanism and engineering geological characteristics is of good guidance for engineering construction.Based on the survey data(935 boreholes with a total depth of 42128 m)and experimental data,the distribution boundary of the FHSL in the northern flank of the Yangtze River estuary was accurately confirmed for the first time,and the formation age,grain size characteristics,geochemical characteristics and engineering geological properties of the FHSL were studied.The study shows that the formation age of the FHSL was about 20~11 ka B.P.(OSL and 14C dating data).The water content of the FHSL tends to increase with depth,indicating that the climate was gradually cooler and drier from bottom to top.The particle size gradation,frequency curve of particle size distribution and C-M sedimentation diagram of the FHSL show that the FHSL mainly consists of fine sand,very fine sand and clay.The grain size frequency curves of the FHSL mainly show a single peak distribution, reflecting that the transport camp force was single before thematerial deposition. The deposition of soil particles were absolutely dominated by uniform suspension, and the deposition environmentwas a relatively stable low-energy environment. The development of the FHSL was controlled by climate and can be roughly dividedinto three stages: the first stage (about 20 ~ 15 ka B.P.) was the period of alternate deposition and soil formation, and the deposition wasthe main effect, and the thickness of FHSL was mainly controlled by this stage until the end of the last bloom ice period. The secondstage (about 15 ~ 11 ka B.P.) was the period of exposed soil formation, at which time floods cannot form transgressive deposits,accretion basically stops, the thickness of FHSL no longer obviously increased. The formed area of the FHSL was erosion cutting byfrequently shifting divergent river networks, forming many irregular ancient river channels and terraces. The FHSL graduallydehydrated into land and underwent the process of weathering and pedogenesis. The third stage (about 11 ka B.P. to present) was theinundation period. With the arrival of the Holocene, the climate warmed, and the sea level kept rising. The FHSL was buried by itsoverlying marine sedimentary layer, and the rock formation began until the present day. The soils have a high content of soluble saltsand are typical chlorosaline soils. The soluble salt content has a trend of increasing from low to high, which was caused by thediagenesis process after the marine layer was covered with the FHSL.