SubTerra Engineering were  appointed  by  the main Contractor  Balfour Beatty Civil Engineering to review the site investigation data,  rock support requirements, and trial blast data and advise  on  excavation methods  for  the two tunnels, the cavern, and associated shafts and adits which formed the scheme.

The cavern is located  in the  Fort Regent  Granite which forms a prominent outcrop in the centre of St Helier. The scheme was required to reduce the risk of flooding in St Helier and prevent the discharge of untreated sewage into the sea during periods of high flow.  During high flows, waste water is stored in the 28,000 cubic metre cavern for later treatment and discharge.

Engineering Geology
A comprehensive review of the geology, structure and geotechnical parameters  using  the  site  investigation   data,   published  information and field mapping was carried by out by SubTerra Engineering post tender. 

The  regional   structure   is   dominated by an abundance of sub-vertical dykes between 1m and 10m thick, trending ENE-WSW.  Most  dykes  occupy  minor  faults or fissures and there is evidence of post dyke reactivation. No evidence of major faulting was found at the site, however minor faulting was indicated by zones of no recovery in the three cavern boreholes, one of which was orientated enabling structural data to be correlated with the surface outcrops. 

As the surface outcrops were some 80m from the  cavern  location  probabilistic  analysis methods were used to project the structure into the cavern and  predict  the  likely zones of influence.  This was particularly important as a minor fault was predicted to intersect the corner of the cavern where the end wall was at its maximum height of 25m.

Construction mapping  data was  analysed  and  fed  back  into the  geological  model  to revise the projected  locations  of  planes of  weakness  on  the  sidewalls so that the contractor could adjust the excavation sequence and plan support design changes with minimum disruption.   A  comprehensive  analysis  of  the  site  investigation geotechnical data  was carried out and compared with the results from contract testing.  This  analysis  demonstrated  that the granite was considerably stronger and  more  abrasive  than  anticipated. SubTerra Engineering developed a probabilistic model for drill and blast excavation to demonstrate the effect of increased rock strength on the drilling time and  overall  cycle  time.  This model  enabled  the  contractor to demonstrate the effect of rock strength on his program duration.

Construction
The cavern was constructed by drill and blast with a central top heading excavated as a  continuation  of  the  main access tunnel, followed by two side slashes to the full 20m span.  The  70m  long cavern  was  then excavated in a series of 3.5m deep benches to its full height of 15 to 25m.  Rock support comprising 5m long resin grouted rockbolts and fibre reinforced shotcrete was installed in line with excavation on  a  1.5m  or 1.0m grid depending on the ground conditions.  The final lining of the cavern crown was also shotcrete with a reinforced concrete lining for the walls and floor.

Blast vibration was an important aspect of the construction with sensitive properties within 50m of the cavern.  Blast vibration monitoring data was analysed and the MIC constantly reviewed to ensure the maximum allowable instantaneous charge to keep the vibrations within specified limits.

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