Geological formation and geo-hazards during subway construction in Guangzhou

This paper summarises the stratigraphy, tectonic structure, geomorphology, hydrogeology and engineering geology in Guangzhou, China, and analyses the relevant geo-hazards and countermeasures during subway construction. The ages of the geological materials found in Guangzhou range from the Proterozoic era to the Cenozoic era. The bedrock is generally made up of sedimentary rock and weathered granite strata covered by soil with a thickness ranging from 2 to 20 m. The presence of underground karst caves, active faults and fissure water together with high piezometric levels leads to difficulties in shield tunnelling in Guangzhou. A case history, where two tunnels and one subway station on the Guangzhou Metro Line 7 were constructed, is referred to in order to analyse the mechanism between geological formations and geo-hazards. Three potential geo-hazards are listed: (1) water ingress and strata erosion, (2) ground surface settlement and (3) sinkholes; and their implications for shield tunnelling in Guangzhou are summarised and discussed. To mitigate the risk of these geo-hazards, a series of countermeasures, comprising (1) optimising TBM cutting tool design, and tunnelling parameters, (2) grouting to enhance watertightness and stiffness of strata and (3) injecting bentonite slurry and foam into the head chamber, are recommended in this paper.