SubTerra Engineering Ltd were appointed as sub-consultant to Travers Morgan to undertake a feasibility study for a 76,000m3 underground cavern to house a proposed sewage treatment works (STW) on the Gower Peninsular near Swansea in South Wales.

Engineering Geology
The Gower Peninsular is an area of outstanding natural beauty formed from  a  plateau  of  Carboniferous Limestone which rises rapidly to approximately 75m above sea level.  Field mapping and a desk study were carried out to determine the lithology, regional structure and discontinuity data, for input to the preliminary design and cost estimates required for the feasibility study.

The proposed cavern site is in the Hunts Bay Oolite, a strong to very strong (75 to 150 Mpa) grey  to dark  grey  fine  grained,  slightly  weathered,  partially  dolomitised  oolitic  LIMESTONE.   During  mapping  four  discontinuity sets were identified and analysed using the stereographic projection package DIPS.

Cavern Design
The preliminary design assessment involved the application of empirical design methods, experience  and  engineering judgement to estimate  rock  support  requirements.   The  approach used was to develop parameters for input to the Norwegian Geotechnical Institute (NGI)  'Q' system.  This system was used to produce a worst case and best case classification for the rockmass for input into rock support design and cost comparisons.

From a preliminary review of the geotechnical data acquired,  it  was  considered  that  excavation spans of up to 20m, with wall heights of 10m were feasible.  Applying an arch span : rise ratio of 3.5, the required volume of 76,000m3  could be  accommodated in two parallel 190m long caverns.

Review of the stereographic projection data indicated that the smallest possible kinematically feasible rock wedge failures that could fall from the unsupported roof of  the  caverns  occurred when the caverns were orientated between 200 and 300 degrees.  In addition to rock support requirements, sufficient rock cover had to be maintained, consequently a preferred orientation  of  285  degrees was selected.

Excavation Assessment
Due to the strength of the rock and potential programme constraints excavation by drill and blast was considered as the most economical method for the caverns, with possibly roadheaders on the access tunnel for lithologies less than 120 Mpa.  The study proposed working either both caverns simultaneously from two faces or from six faces via a central access tunnel dependant on allowable construction time.

Excavation was proposed to be carried out in a  sequence  of heading and benches with a central top heading followed by two side slashes to be excavated and supported prior to full width bench excavation. Primary support was to be by 5.0m long rockbolts at 1.0 to 2.0m centres with between 50 and 100mm of fibre reinforced shotcrete depending on the rockmass quality with  fibre reinforced shotcrete also proposed as the final lining, using a float finish.

Cost Estimation
Based on the best and worst case rockmass classifications minimum and maximum rock support quantities and shotcrete volumes were calculated.  Based on previous experience, cycle times for drill and blast construction were also  calculated  resulting  in a fully costed programme for the Client

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