In July T&TI learnt that one of the 8.23m diameter Herrenknecht hard rock open TBMs being used by the Obayashi MassAna JV in Atlanta, Georgia, US on the West Area CSO Tunnel (T&TNA, June 2005, p35) had suffered a failure of the ring gear and some main drive pinions bringing tunnelling to a halt.
T&TI contacted Herrenknecht who confirmed that on 11 July, approximately 2,750m into the excavation damage was sustained to one of the pinions and subsequent ring gear of the main drive unit of its TBM S-289. The TBM was set to drive 6.6km on the contract, but Herrenknecht said that currently works had been halted to allow its personnel and those of the JV to get started on an “inspection, repair and refurbishment program”. A representative of Herrenknecht told T&TI that the cause of the damage had not yet been determined. This was confirmed by Obayashi’s project manager on the scheme who told T&TI that investigations were underway, but it was not possible to comment at this stage on the possible findings. Nevertheless, Herrenknecht said the machine is expected to recommence excavating at the end of August, which equates to nearly two months of stoppage.
The city of Atlanta is the scheme owner. It commissioned CH2M Hill to carry out 30% of the design, the balance of the design was done by the JDH JV comprising Jordon, Jones & Goulding Inc., Delon Hampton & Associates and Hatch Mott MacDonald. The construction of the 13.4km long tunnel has been costed at approximately US$210M. The Obayashi MassAna JV was awarded the contract as Atlanta CSO Constructors (ACC).
Herrenknecht has two TBMs working for ACC on the scheme. In addition to S-289, a similarly specified sister machine, S-288 is also at work. At the time of going to press, this machine had excavated 2,700m without any problems along the tunnel alignment that is predominantly gneiss and granite. In fact, for contract week 23 of 2006 this TBM clocked up an impressive 187m of advance.
Once complete the scheme should stop combined sewer overflows (CSO) from contaminating the Chattahoochee River during times of high rainfall when the CSOs handle the surplus water via the wastewater system.
