A Study On The Engineering Sequence Stratigraphical Pattern Of Chengbei Area, The Yellow River Subaqueous Delta

In1855, the yellow river rechanneled to Tongwaxiang where it flows into theBohai Sea thereafter. Since then, many lobes have been formed in south part of theBohai Sea, which was the beginning of the evolution of modern Yellow River Delta. Inthe northern part of modern yellow river delta located Chengbei area, where YellowRiver formed the subaqueous delta during1964-1976when it flowed into Bohai Seathrough Diaokou path. Yellow River is known for its high content of water and sand,which is also the cause for its unstable flow path. The sediments are complex bothtopographically and geotechniquely which has caused many calculation mistakes inbearing capacity of foundation soil, resulting in great economic loss concerningoffshore platform foundation and pipe line paving. Sequence stratigraphy uses datafrom seismology, well drilling and outcrop survey, combined bysedimentology, lithology and paleogeography; it can give accurate explanation of thestructure of sequence stratigraphy. Since the geotechnique features of soil from inone same sedimentogical environment are similar, it should have the same reflectionfeatures in sub-bottom seismic profile data. Therefore, it is possible to achieveaccurate strata classification and acquire a regional engineering sequencestratigraphy pattern by combine both geotechnique data and geological data ofregional soil body, which is of guiding significance for marine engineering geologyand for accurately assessing complex engineering data.This paper did micropaleontology analysis and sedimentary environmentclassification based on full coring engineering borehole sample of Yellow Riversubaqueous Delta. The sequence surfaces and system tract are identified byanalyzing high-resolution sub-bottom profile. Using the fluctuation of sedimentssupply and accommodation space as reference, the response pattern of all systemtracts by regional soil deposition process and soil geotechnique characteristics isanalyzed. The conclusions are that the research area has one complete system tract sincelast glacial period, which is so called the low sea level system tract of last glacialperiod, Huanghua transgression system tract and high sea level system tract. Thegeological characteristics and engineering characteristics of each system tract areintroduced as follows:The top surface of low sea level system tract is a disconformity on whichdevelops rivers and lakes, part of it developed into marsh. Engineering characteristicof the soil body shows that water content ranges from17.6%to35.4%, densityranges from1.94to2.15g/cm³, void ratio ranges from0.482to1.03, internal frictionangle ranges from11to38.4degree, and compressibility ranges from0.06to0.31Mpa^-1.The accommodation space enlarged as the sea level grew up inthe transgression system tract, sediments overlap anti-sementary slope bed by bed,filled the channels and lakes on surface. For the input of sediments is insufficient,sedimentary rate is low. The sediments comprise taupe silty clay with high moisturecontent, low density, good plasticity and compressibility. Engineering characteristic ofthe soil body shows that water content ranges from18.9%to69%, density rangesfrom1.44to2.1g/cm³, void ratio ranges from0.5to2.2, internal friction angle rangesfrom2.6to31.4degree, compressibility ranges from0.11to0.81Mpa^-1.The high sea level system tract is a subaqueous delta facies developed by YellowRiver Delta downlaps the transgression system tract that correspond to theengineering soft soil bed. Engineering characteristic of the soil body shows thatwater content ranges from21%to54.1%, density ranges from1.7to2.11g/cm³, voidratio ranges from0.55to1.4, internal friction angle ranges from9.3to42.1degree,compressibility factor ranges from0.04to0.81Mpa^-1.In summary, the engineering sequence stratigraphic model can be summarizedas follows: the bottom interface of the lowstand systems tract is generally aengineering suitable strata, but with an uneven lateral change; transgressive systemstract corresponds to the normal consolidated strata, with normal variation ingeotechnical parameters; highstand systems tracts corresponds to the soft soil, i.e. the poor engineering soil.