SubTerra Engineering Ltd were appointed by a specialist  tunnelling contractor  to review  the  anticipated  and encountered ground conditions for 13 crossings for a 18.6 km long,  900mm  diameter  gas transmission pipeline in the Irish Republic.

All  crossings   were  specified   as  1.2m diameter  to  accommodate  the  900mm   diameter steel pipeline.  Ground conditions comprised   very   strong   carboniferous   limestone,  overlain by boulder clay and recent deposits including a rubbish tip, with rockhead located between 0.5m and 4m below ground level.  Site   Investigation   information  was  limited to trial pits and the occasional borehole, typically one per crossing. 
 
Based  on   the   tender  information   the  Contractor selected auger boring for the short  drives  in  overburden,  and  microtunnelling machines on the hard rock drives, and the Liffey crossing.

The  main microtunnel drives comprised a 70m tunnel below the N7 Naas Road, a 70m  tunnel  below   the   Galway Railway and Royal Canal, and a 60m tunnel below the River Liffey. The two 70m   drives  were successfully driven using a 1.2m diameter Herrenknecht hard rock microtunnelling  machine through the 200 MPa limestone. 

The proposed 1.2m diameter River Liffey tunnel was increased to a 1.5m diameter tunnel after additional boreholes revealed that 90% of the tunnel would be in wet overburden comprising sand, gravel and boulders rather than rock as indicated at tender.

One of the 60m Augerbores indicated to be in overburden for its entire length based on the tender information encountered 150 MPa Llimestone 40m into the drive and had to be completed by drill & blast.

SubTerra Engineering reviewed the borehole and laboratory testing information available at tender and used this information to produce an independent assessment of the anticipated ground conditions, and an independent assessment of the appropriate tunnelling method. .

Where  the  encountered  ground  conditions appeared to differ from those anticipated, SubTerra Engineering collected additional data including  geological and geotechnical field mapping of the excavated launch and reception pits, and logging bulk samples recovered from known locations within the tunnels.

Discontinuity data from the excavations was analysed using a stereographic projection software package, and rock samples were obtained for strength testing.  A number of additional boreholes were also drilled on either bank of the River Liffey to determine rockhead prior to commencing shaft excavation.

Based on  the  additional   information   acquired,  SubTerra Engineering reviewed the encountered ground conditions for a number of the crossings and reviewed the selection of tunnelling methods in light of the additional data.

A report was prepared  for  the  Contractor  detailing the effect of the changed conditions on the efficiency of the selected methods and subsequent production rates

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