Washington DC is about to issue RFQs for construction of the fourth and final Anacostia River CSO storage and diversion tunnel. Once completed, the 7,925m-long North East Boundary tunnel (NEBT, or J-Division Tunnel) will form the final link in the US capital’s ambitious program to reduce sewer overflows into the Anacostia River.
Chris Allen, Clean Rivers Project Construction Manager and Deputy Program Director for owner DC Water, told TunnelTalk that RFQs will be issued next month for excavation of the 7.1m i.d. storage and conveyance tunnel. The new infrastructure will link the First Street (823m x 7.1m i.d.) and Anacostia (3,810 x 7.1m i.d.) tunnels – both of which have EPBMs already in the ground – via the recently excavated Blue Plains tunnel and all the way through to the Blue Plains wastewater treatment facility for completion of a continuous 18,600m-long CSO connection (Fig 1). DC Water is expecting the final contract price for the NEBT, which includes excavation of six dropshafts and adits, to come in well below the initial estimate of US$550 million.
Early utility relocations under a separate US$25 million contract are already under way, said Allen, ahead of the start of procurement for construction of the NEBT.
“It’s going to be a modified design-build contract, and we are doing the utility relocation package early. These relocations have been the bane of my existence for underground work in urban areas, so by stripping them out of the main contract all we are going to be asking the main contractor to do is supportive design, excavation of the tunnel, shaft connections and production and design of the segmental lining.
“The owner has designed all the hydraulic structures to 100%, and the power transformers for the EPBM at the site where they are going to be driving from are already in place. What all of this means is that this is going to be a great job for somebody, a tunnellers’ dream contract,” said Allen.
Excavation is complete for the first of the four Anacostia River CSO tunnels – the 7,193m x 7.1m i.d. Blue Plains Tunnel. Breakthrough is achieved by the Traylor/Skanska/Jay Dee JV following a drive of 23 months from the launch shaft near the wastewater treatment plant at the southern end of the alignment.
DC Water’s Clean Rivers Deputy Program Director Chris Allen told TunnelTalk: “We’ve taken the cutterhead and the main bearing cartridge out. We hit to within 0.75in (19mm) for line and 0.5in (12.5mm) for grade, through four-and-a-half miles, various curves and with very little opportunity for GPS correction, so we are very happy with the way it went. Ground loss was minimal, 0.25% or less. We had planned for ground loss of up to 1% for settlement to adjacent structures in our calculations so we couldn’t be happier with the 8m diameter Herrenknecht EPBM machine that the contractors used.” Final breakthrough was achieved in June (2015).
“We had a novel carousel system down in the mucking shaft that took a little tweaking but overall this was one of those rare jobs where there wasn’t a whole lot of excitement and everything went pretty much as planned – which is nice for a change."
As with all four of the tunnels that together will reduce combined sewage overflows into the Anacostia River by 98%, geology was predominantly through Potomac Clay. “It’s very stiff, not extremely sticky and it proved well suited for EPBM excavation,” said Allen. “We expected to have to use a lot of conditioning material but in the event water was the main agent used. Water ingress was almost non-existent, it was a very tight drive.”
Design was completed by CH2M-Hill with construction management by EPC. Best 24-hour performance was 45.7m, including erection of the precast segmental lining, with the best weekly progress recorded at 201.2m, for an overall average during the 23-month drive period of 16.5m/day including intervention stops. Peak hydrostatic pressure reached 3 bar, well within the threshold of the machine and in line with what had been expected. “All stops were planned, and cutterhead wear was really good until we got into a bit of abrasive material as we approached the Anacostia River undercrossing,” said Allen. “But overall the Potomac Clay was very good to mine and for most of the drive there were ideal mining conditions.”
There will not even be a need to excavate a launch shaft for the 8m diameter EPBM that will eventually be procured for the drive northwards. A “double-barrelled” shaft was completed as part of contract scope for the Division H (Anacostia) tunnel. To facilitate the future tunnelling requirements of the Division J (NEBT) tunnel – which will launch northwards from the same location – the Impregilo/Healy/Parsons JV excavated both a north and south shaft at the start of the Anacostia alignment, connected by a starter tunnel excavated using SEM. The north shaft has since been sealed off, ready for assembly of the NEBT TBM, probably in late 2017 or early 2018.
“You can see from all this that we are doing everything we can to streamline this J-Project [NEBT] to be a tunnellers’ dream – all that the contractor will have to do is just get in and tunnel,” said Allen. “It’s a kind of hybrid design-build procurement that we call ‘expedited design-build’. With regard to the dropshafts in particular it makes for a quicker start to construction if the contractor does not have to do the design for the hydraulic structures or deal with aspects such as air pressure relief. All of these are part of a very complex design process in themselves, but we have gone ahead and done that for them. All the contractor has to do is decide if they want to use secant walls or slurry walls, or even ground freezing to construct the shafts – in fact for the First Street tunnel (District P) we had drop shafts literally outside people’s front doors and to keep noise to a minimum during construction the contractor has quietly undertaken a ground freeze and excavated out.”
The expedited design-build construction procurement strategy is in line with DC Water’s own preferred method, which has worked well on all three of the previously let Anacostia River CSO tunnel contracts.
Allen explained: “This is a best value process – we take any number of qualification statements from anybody in the world and we will reduce that down to about three or four based on the team. Here I should say that we place a big emphasis on the personnel – not so much the company, as the personnel that company are supplying.
“On J [NEBT] it looks like we are going to shortlist four teams, and we will then enter a collaboration period for an extended period of time [about six months] that gives the contractors time to get familiar with the project so that we don’t get any crazy bids when it gets down to giving us a number. The contractors are paid during this period, usually about half a million dollars apiece.
“Then we take technical proposals, and this is where the contractors can innovate. We keep the results of our collaboration with each contractor team a secret from the others, and this process has worked well so far. This then ends with a technical proposal, which is weighted at about 35%. Later, we take a sealed price proposal, but we open up the technical proposal first – without knowing the price – so that we are not gravitating to the low bidder necessarily. We then combine the scores to come out with the best value for the authority.
“For two of the three major tunnel projects that we have let in this way, the highest technical proposal also had the lowest price, so we think that is interesting. The extra time given, pre-bid, during the collaboration phase, seems to give a better result.”
The exception to this was the contract awarded to the Impregilo/Healy/Parsons JV in May 2013 – at a low bid of US$253.9 million – for excavation of the 3,810m x 7.1m i.d Anacostia tunnel (Division H). “Here, the superior technical proposal was high, and the price overshadowed the fact that the best value did not have the best proposal, but [the lowest bidder] did have a heck of a price so we had no option but to take it, and it’s worked out well so far.” The TBM that will excavate this tunnel (an 8m diameter Herrenknecht EPBM, TBM Nannie) is currently being assembled in the south shaft at the northern end of the alignment ahead of testing and launch later in the summer for scheduled connection with the already-completed Blue Plains tunnel by 2017.
Meanwhile, the 6.1m diameter EPBM for the Skanska/Jay Dee JV on the US$157 million First Street tunnel (Division P), has made progress of approximately 106m while still on its umbilical system. This shorter tunnel contract (823m), which feeds into the NBBT alignment, was broken out of the original scope of the NBBT tunnel to facilitate earlier completion. The contractor is using a manufacturer-refurbished Herrenknecht EPBM (TBM Lucy) that worked on the now-completed Abu Dhabi deep sewerage STEP tunnel project. “It’s such a short drive that we agreed to a refurbished machine for this contract,” said Allen. “It’s a 20ft (6.1m) diameter machine, but we actually would have let them go as low as 18ft (5.5m) if it had been necessary.”
The entire Anacostia tunnel system is now to be gravity-fed – a change to the original design specification that included provision for connection to an existing (but enlarged) pumping station near the Kennedy Centre. However, according to Allen, “the restrictions on the site posed a great number of layout problems and so we decided to do ourselves a favour in terms of long-term maintenance and run it instead all the way over to the Blue Plains treatment plant site on a gravity system.”
The decision to deviate from the original design was taken earlier this year, and coincided with a modification to the original Consent Decree from the EPA that may allow the owner to reduce tunnel project scope during the future Potomac River CSO phase of the Clean Rivers project. DC Water will be allowed to drop the 800m long Rock Creek tunnel and significantly reduce the length of the planned Potomac tunnel if can prove by way of US$50 million worth of demonstration projects that green infrastructure alternatives do actually work. ”This may enable us to eliminate one tunnel altogether, but the jury is still out on that,” said Allen.
Two conceptual designs are currently being investigated in relation to the Potomac tunnel – a much shorter “land tunnel” between CSO-024 and CSO-020 (Fig 2), that would end near the White House; or a much longer 4-mile gravity tunnel that would travel from CSO-024 all the way south along the Potomac for a connection with the existing Blue Plains tunnel serving the Anacostia River (Fig 2). Successful demonstration of green infrastructure will allow DC Water to dispense with the need to tunnel between CSO-029 and CSO-025.
DC Water is now moving ahead with early conceptual design and Environmental Impact permissions. Under the terms of its Consent Order the city needs to cut its combined sewage overflows into the Potomac by 96%. Geology along the likely alignments of these tunnels includes some areas of decomposed rock, and may call for use of multi-mode TBM technology.