In the 18th century Rhode Islanders deposited their wastewater from sinks, washtubs and outhouses directly into local rivers believing that "dilution will render the waste harmless."
This is part of a fairly graphic recount of the history of the Providence region’s wastewater system as provided by the Narragansett Bay Commission (NBC), the entity in charge of wastewater collection and treatment. NBC’s service area encompasses the metropolitan Providence and Blackstone Valley areas. The actual Narragansett Bay is an estuary, a semi-enclosed body of water with a free connection to the open sea, and within it, fresh water from land drainage dilutes seawater.
The city of Providence is one of the oldest communities in the US and has a long history of attempting to protect the health of its rivers and citizens, while keeping abreast of new developments in sewer systems in both the US and Europe over the last two centuries.
Like many of the cities and municipalities in the northeast working to meet consent decrees with the Environmental Protection Agency (EPA) for CSOs, Providence is tackling a threephase underground construction project.
Starting in 2001, the NBC broke ground on the _ rst phase for its CSO plan. The main component, the 3-mile-long (4.8km) Main Spine Tunnel, 26ft (7.9m) in diameter and 250ft (76.2m) below ground, was put into operation in 2008. This was the largest civil works project in the history of Rhode Island.
The NBC started construction on Phase Two of the CSO Program in 2011, which includes two near surface interceptors to bring additional flow to the Phase I tunnel. While the work requires comparatively small diameters in shallower ground, the projects are not without challenges.
"In Phase One, for the most part, our work was 250-300ft below the ground," says NBC’s executive director Ray Marshall. "Because of the nature of the tunnelling work, we only had very localised areas of surface disruption. In Phase Two, most of our work is near surface, and this means that we have to rip up streets, disturb neighbours and re-route traffic. Providence is an old city; many of its streets are very narrow, and often the existing utilities are not where the maps indicate."
The Phase One facilities included the tunnel, tunnel pump station and seven drop shafts. Through the end of 2012, approximately 4.6 billion gallons (17.4 billion litres) of combined water and wastewater that would have gone straight into Narragansett Bay were treated at a wastewater treatment facility instead.
Phase Two comprises two interceptors, two sewer separation projects and a constructed wetlands facility that will reduce the discharge from 17 CSOs. The interceptors will be located along the Seekonk and Woonasquatucket Rivers and will convey flows to the Main Spine Tunnel constructed as part of Phase I. The Woonasquatucket Interceptor will be 16,400ft (5km) long with a 1,800ft (549m) long tunnel adit, and the Seekonk Interceptor will be 7,200ft long (2.2km).
As part of Phase Two there are a number of shallow underground structures including manhole, diversion, regulator, and gate and screening structures, connected by approximately 23,000ft (7,010m) of conduits with finished inside diameters ranging from 12 to 72in (305 to 1,830mm). The total estimated cost for Phase Two is USD 233M. There are a total of 14 contracts for the second phase. All have been awarded, or have gone through the bidding process and the low bids have been approved by the NBC board of commissioners.
Excavation methods will primarly be microtunnelling and pipejacking with diameters ranging from 48 to 72in (1,219 to 1,830mm). To connect to the phase I storage tunnel, a 27ft (8.3m) diameter, 220ft (67m) deep shaft will be constructed from where the 1,800ft-long adit, with an 8ft (3m) diameter will be mined by drill and blast to the foundry shaft.
Marshall says the most challenging soil condition for the project is a substantial amount of glaciolacustrine soils deposited in layers ranging from coarse sands and silts to fine silts and sands.
On the west section of the Woonasquatucket CSO Interceptor (WCSOI), contractor DiGregorio will construct approximately some 2,325ft (709m) of pipeline will installed by open-cut methods, as well eight structures. A trench box system for excavation support was used on this section of pipe. However, because of the measured settlement of the pipe, dewatering issues and heaving silts at the base of the excavation, sheeting was used to install the remaining pipeline.
"When located beneath the water table as these are, certain layers weaken when subjected to stress and or vibration and in extreme cases can liquefy and lose strength," Marshall says. "While the design phase boring logs identified the silty and sandy layer, it is difficult to accurately predict how those layers will perform. In addition, differences in subsurface conditions can occur within short distances, so even the borings may not tell the whole story.
"We have chosen to implement jet grouting at these sites because it provides the greatest chance for success and enables us to continue the pace of construction."
For the WCSOI Main section, some 18,200ft (5.5km) long, the contractor joint venture of Barletta Heavy/Shank Balfour Beatty is undertaking a ground freezing program to support the shaft with Moretrench developing the freezewall.
"We are freezing one main shaft 31.5ft in diameter through the entire soil column of 165ft [50m]. Then we’ll excavate bedrock from that point to a depth of 220ft [67m]. This shaft accesses the adit that will connect the WCSOI to the Main Spine Tunnel constructed in Phase I," Marshall says.
All things considered, when asked what is challenging about constructing phase II, he looks to logistics. "Coordination of 14 separate construction contracts, traffic control, access to businesses and residences and communication with all of the affected parties are both challenges we face every day," he explains.
Northeast Remsco holds the contract for the Seekonk CSO Interceptor, which includes construction of approximately 4,775ft (1,455.5m) of 48in (1,219mm) diameter and 940ft (286.5m) of 60in (1,524mm)diameter CSO interceptor along the Seekonk River in Providence. Other work for the project includes the WCSOI’s north section, and contractor Cardi Corporation is in the process of installing 1,800ft (549m) of pipeline by open-cut methods. John Rocchio and CB Utility each have contracts for sewer separation work.
The Louis Berger Group is the lead design manager for Phase Two with construction management by BETA Engineering and Gilbane Building Company. Construction for the second phase is expected to be complete in December 2014. Phase Three, which is slated to be another deep rock tunnel approximately 13,000ft (4km) long, has just begun preliminary design.
Not gone fishing Each year, the 64 csos in the nBc district release an estimated 2.2bn gallons (8.33bn litres) of untreated combined sewage into the Bay and its tributaries, namely the Blackstone, Moshassuck, Providence, seekonk, Woonasquatucket, and West rivers. to protect human health in rhode island, certain areas of the bay are permanently closed to shellfishing, and over 11,000 acres are temporarily closed for harvesting when there is more than one-half inch of rainfall.
