Selecting the TBMs for the First Tunnel Project on Bangalore Metro

The  first  large  diameter  slurry  TBM’s  in  India  -­‐  Selection  of  the  tunnelling  system  for  Bangalore  Metro    Moncrieff,  R.L.  1.  1.  Rona  Consulting  Co.,  Ltd.,  Bangkok,  Thailand    The   design   and   build   contract   for   the   first   underground   section   of   Bangalore  Metro   was  awarded  to  CEC-­‐Soma-­‐CICI   Joint  Venture   in  2010  and  tunnelling  work  commenced  in  May  2011.  Selection  of   the  of   the   tunnelling  system   for   the  5.6m   ID  bored   tunnels   through   the  varied  and  difficult  ground  conditions  found  beneath  the  congested  central  areas  of  the  city  was  one  of  the  early  decisions  that  the  contractor  had  to  make.    BMRC  Contract  UG2,  comprises  some  2,400m  of  twin-­‐bored  tunnels,  cut  and  cover  tunnels,  two  ramps  and  four  underground  stations.  Bangalore  is  situated  1,000m  above  sea  level  and  ground   conditions   along   the   tunnel   alignment   consist   of   a   shallow   layer   of   fill   or   made  ground   overlying   residual   soil,   weathered   rock   and   hard   fresh   rock   with   compressive  strengths  in  excess  of  150  mpa.    The  rock  head  varies  from  below  tunnel  level  to  just  below  surface   level.   Hard   granite,   layers   of   weathered   rock   with   differing   geophysical  characteristics  and  soft,  residual  soil  occur  at  the  tunnel  horizon.  A  full  rock  face,  soft  ground  face  and  mixed  face  conditions  with  up  to  three  types  of  material  can  be  encountered  in  the  tunnel  drives.  Ground  water  level  is  typically  just  below  the  surface.      For   the   twin-­‐bored   tunnels,   closed   face   TBM’s   were   specified   by   the   client,   however   the  contractor  was   able   to   select  what   he   considered   as   the  most   suitable   technology   for   the  prevailing   conditions.   Slurry   tunnelling   machines   were   chosen,   the   first   use   of   this  technology  on  this  scale  in  India.      

Fig  -­‐  India’s  first  large  diameter  slurry  TBM    The  east-­‐west  tunnel  alignment  on  Contract  UG2  lies  beneath  busy  roads  that  serve  areas  of  the   city  where   there   are   large   commercial   developments,   educational   establishments   and  government   buildings.   Important   buildings   close   bye   include   the   Karnataka   State  Parliament,   the  High  Court,   the   Civil   Court   and   the  General   Post  Office.  Horizontal   curves  down  to  220m  in  radius  have  to  be  negotiated  and,  unavoidably,  at  more  than  one  location,  the   tunnels   are   below   or   close   to   buildings.   Due   to   the   hilly   nature   of   the   city,   the  overburden  varies  from  2.5  tunnel  diameters  to  little  more  than  one  tunnel  diameter.  With  the  sensitivity  of  adjacent  structures  in  this  dense  urban  conurbation,  control  of  settlement  and  ground  movement  during  tunnelling  is  of  prime  importance.    Factors  that  influenced  the  choice  of  tunnelling  method  and  ultimately  the  selection  and  the  design  of   the  TBM’s  and   the   support   equipment   required   to   construct   this  difficult   tunnel  alignment  are  discussed  in  this  paper.  Problems  resulting  from  tunnelling  through  variable  mixed  ground  conditions  with  ground  water  present  are  identified  and  the  detailed  process  leading  to   the   final  selection  and  procurement  of   the   first   large  diameter  slurry  tunnelling  machines  to  be  used  in  India  is  described.