Geological and geotechnical investigation in a metro project in Nanjing, China

Currently, there are six cities constructing their metro systems in China, among which, the Nanjing metro project is characterized by its complicated and unfavorable geological conditions, such as varied topography, rugged bedrocks, many faults passing through, rapid variation in lithology and geotechnical properties, soft ground, high groundwater table and multiple groundwater types. Therefore, the geological and geotechnical investigations play a vital role in the project. This paper describes and discusses the investigations taken in the project. From a seismological point of view, the project is in a relatively stable tectonic setting, the major regional active faults are a little far. Among many faults found in the city area, the NWW Dinghuaimen-Gulou fault and the NNW Nanjing-Hushu fault merit attention to their influences on the foundation stability. High groundwater table and multiple groundwater types feature the hydrogeological conditions. The paper describes the hydrogeological characteristics and parameters on the basis of groundwater type division. The inflow prevention of abundant pore water in ancient riverbed, fault water in Gulou and karst water in Xiaohongshan are difficult. Along the metro axis, the local morphology can be divided into five sections: three sections of tectonic denudated low-rise hills and two sections of ancient channel alluvial plains. As many as 50 of Quaternary soil formations have been encountered. They are too many for architects to conceive and establish the engineering geological model. Therefore, generalization and division of engineering geological strata groups is necessary. The Quaternary deposits at the Metro site were divided according to their origin and relevant age into four engineering geological strata groups and subdivided on the basis of their composition, physical state and geotechnical properties into distinct subgroups. Data from this study indicate that the division is feasible and the subgroup II2(CS) and III2(CS), which are soft cohesive soils of Holocene and late Pleistocene respectively, represent the most sensitive and weak geotechnical unit, and the Holocene sandy soils subgroup II4(SL) is of the most potential danger of liquefaction. None of the current rock mass classification systems has been found to originate from and be adequate for the shallow metro tunnel. The CGIMC, Vp and BQ systems have been combined in this study. The study indicates that the combination of multiple systems, qualitative and quantitative, is helpful to ensure the reliability of rock masses evaluation. To overcome the deficiency of the rock mass intactness index Kv and the BQ system in the national standard, their determination methods were modified. The paper describes in details the modified Kv and BQ procedure taking account of the characteristics of the rock masses at the metro site. The rock masses are categorized into very poor, poor and fair quality. The poor conditions of the rock masses are largely due to intensive weathering and dense fracturing. Along the metro axis, eight geotechinal zones were divided according to their identical characters of topographical, engineering geological and hydrogeological conditions and construction methods. The paper highlights the different geotechnical problems and challenges that will be encountered in different geotechnical zones. (A). "Reprinted with permission from Elsevier". For the covering abstract see ITRD E124500.

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  • Authors:
    • LI, X -
    • LUO, G -
    • LIU, Y -
    • ZHANG, D
    • YAN, S -
  • Publication Date: 2004-7

Language

  • English

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  • Accession Number: 01011668
  • Record Type: Publication
  • Source Agency: Transport Research Laboratory
  • Files: ITRD
  • Created Date: Dec 19 2005 3:24PM