SubTerra Engineering were appointed by Costain Civil Engineering to carry out high quality geotechnical mapping of all underground excavations during construction of the Plymouth Sewage Treatment Scheme.  The scheme comprised a new treatment works, and 4,500m of 2.4m ID deep level interceptor sewer, up to 30m below the streets of Plymouth.

Geological map along the 4.5 km tunnel alignment (black line).

Ground conditions along the tunnel alignment comprised slate (purple), meta-mudstones (orange & yellow) and very strong limestone (pale & dark blue).  Excavation was by drill and blast using hand held drills on air-legs, from within an open face shield.  Permanent support comprised a 2.4m ID Charcon one-pass segmental lining installed utilising the shield mounted segment arm. 5 shafts of 4m to 9m in diameter were excavated by drill and blast with a one-pass segment lining installed top down.  The bottom 5m of the shafts adjacent to the tunnel eyes were supported by shotcrete and rock bolts.

Site Services
At the peak of construction seven faces were being excavated simultaneously, with up to 3 blasts per day, between 7am and 10pm.  Each face was mapped daily to record lithology, discontinuity data, water inflows and geologically controlled overbreak.  Samples were also obtained for Point Load Index and UCS testing to determine the intact rock strength.

Additional site services included the temporary works design for a 350m tunnel in slate using rockbolts, weld mesh and steel arches, supervision of support installation, probe drilling, and face grouting to control water inflow.

Claims Evaluation
A complete review of the information provided at tender was carried out, and compared to the conditions encountered.  Data acquired from face mapping, laboratory testing, and blast compliance monitoring was compiled on to a database for detailed analysis.

Joint orientation data at Tender indicated a relatively benign tunnelling environment, with a stable excavation and only 2% of geologically controlled overbreak to be expected.  However excessive overbreak (15% to 20%) was encountered during construction.  A kinematic analysis of the potential wedge failures in the crown and sidewalls of the tunnel was performed based on the discontinuity data acquired  during face mapping.  This indicated that the joint orientations were different to those envisaged at tender, resulting in larger wedge failures and consequently a higher incidence of geologically controlled overbreak.

Tunnel lining prior to finishing

The specified method of excavation was by drill and blast, however no trial blast was carried out at tender to enable the Contractor to accurately determine an acceptable charge weight. SubTerra Engineering used published data to predict the levels of vibration that could reasonably have been expected at tender.  However, during excavation, excessive vibrations were recorded resulting in the imposition of reduced charge weights and reduced round lengths, slowing production.

SubTerra Engineering carried out an extensive review of blasting practices, and analysed the blast vibration data.  Comparison of the encountered and predicted data demonstrated that the local geology had an adverse affect on the  transmission of vibration that could not have been envisaged at tender.

Claim reports were prepared on behalf of the Contractor for submission to the Client.

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