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MWRA Metropolitan Boston Tunnel Redundancy program project update

In 1984, legislation was enacted to create the Massachusetts Water Resources Authority (MWRA). The MWRA is a public authority that provides wholesale water and sewer services to 3.1 million people and more than 5,500 large industrial users in 61 communities in eastern and central Massachusetts. The primary mission of the MWRA was to clean up Boston Harbor and modernize the area’s water and sewer systems. Other key elements have included a major capital program to repair and upgrade the systems, increase staff to improve operations and maintenance, promote water conservation and plan for the future to meet growing demand.

Boston’s water system has been governed by various water authorities over the years starting with the Cochituate Water Board (1845-1875), followed by the Boston Water Board (1875-1895), Metropolitan Water Supply Commission (1926-1946), Metropolitan District Commission (1946-1985) and finally the MWRA (1985-present).

The MWRA’s water system currently has more than 200 separate facilities, including the John J. Carroll Water Treatment Plant, with a capacity of 1.5 GL/day (405 million gpd), 11 pump stations and 14 below- or above-ground storage tanks. The water transmission system includes 167 km (105 miles) of active tunnels and aqueducts, mostly 3 to 4.2 m (10 to 14 ft) in diameter, and 63 km (39 miles) of standby aqueducts.

History of Boston’s water system

Although the MWRA’s current water system of interconnected reservoirs, tunnels, aqueducts and pipelines provides an abundant supply of clean drinking water to millions of customers, the system was not always sufficient to meet the growing needs of the city of Boston and the surrounding communities.

When the Boston area cities and towns first faced the problems of providing clean water sources in the 1600s, their methods were primitive, relying on local wells, rain barrels and spring rains on Boston Common.

By 1795, wooden pipes delivered water from a centralized water supply at Jamaica Pond to Boston proper. By the late 1840s, however, Jamaica Pond was too small and too polluted to provide water to Boston’s 50,000 residents. So, the pattern of continually moving westward in search of larger fresh water sources began.

Cochituate System: 1848-1951. In 1845, the Cochituate Water Board began construction of a new water supply and transmission system. A tributary of the Sudbury River was impounded, which created Lake Cochituate. Lake Cochituate, with its 44 km2 (17 sq miles) of watershed, 7.5 GL (2 billion gal) of storage and yield of 38 ML/day (10 million gpd), became the cornerstone of the Boston water system.

The Cochituate Aqueduct, extending 23 km (14.5 miles), was completed to transport water to the Brookline Reservoir and then to smaller distribution reservoirs in all parts of the city. This aqueduct was in service for approximately 100 years, until 1951, when water quality had declined and alternate methods of transporting water to the hub of the distribution system had been constructed.

The Sudbury Aqueduct and Chestnut Hill Reservoir: 1878. Boston grew rapidly after the Irish Potato Famine of 1843-45 and by 1870, its population exceeded 200,000 and consumed 64 ML/day (17 million gpd) of water. Planners had not anticipated this rapid growth; they thought that the Cochituate system would be adequate for many years. In order to provide the growing city with the water it needed, the process of diverting water from a western pure upland source was repeated.

In 1878, the mainstream of the Sudbury River was diverted via the Sudbury Aqueduct to the Chestnut Hill Reservoir. Between 1875 and 1898, seven major reservoirs were constructed in the Upper Sudbury River Watershed. The Sudbury and Cochituate Aqueducts were designed to operate by gravity to fill the Chestnut Hill and Brookline Reservoirs.

Wachusett Reservoir: 1897. The Boston metropolitan area continued to grow rapidly through the 1890s. Indoor plumbing became commonplace. Planners had not foreseen this development and the current water supply had become inadequate. Under the leadership of Frederick Stearns, chief engineer of the Boston Water Board, it was decided that a new water source that could be gravityoperated and would not require filtration was required.

In 1897, the Nashua River above the Town of Clinton in Central Massachusetts was impounded by the Wachusett Dam. An area of 16.8 km2 (6.5 sq miles) was flooded, and water was conveyed by the Wachusett/Weston Aqueduct to the Weston Reservoir and then by pipeline to the Chestnut Hill and Spot Pond Reservoirs. Work was completed in 1905 and the reservoir first filled in May 1908. The Wachusett system was built to service the 29 municipalities within a 16-km (10-mile) radius of the Massachusetts State House. At the time, the Wachusett Reservoir was the largest public water supply reservoir in the world with a capacity of 246 GL (65 billion gal).

MWRA water transmission system.

Fig.1-MWRA water transmission system.

Quabbin Reservoir, Ware River intake and Hultman Aqueduct: 1926-1946. Eventually the Wachusett System became inadequate for the increasingly industrialized city and a westward focus for a new water source resumed. The Quabbin Reservoir was Boston’s fourth westward reach for a pure upland source of water that could be delivered by gravity and not require filtration. Construction of the Quabbin Reservoir required impoundment of the Swift River and the taking of the towns of Dana, Enfield, Greenwich and Prescott.

In 1926, construction began on the Wachusett- Coldbrook Tunnel, which is now the eastern section of the Quabbin Tunnel. During the 1930s, the Wachusett- Coldbrook Tunnel was extended to the Swift River. It is a two-way tunnel. Water flows west from the Ware River to the Quabbin Reservoir during the high-water months and then east from the Quabbin Reservoir to Wachusett at other times of the year.

Construction on the Quabbin Reservoir began in 1936. Filling commenced in 1939 and was completed in 1946 when water first flowed over the spillway. At the time, the 1,560 GL (412 billion-gal) reservoir was the largest man-made reservoir in the world that was devoted solely to water supply. The existing reservoirs, located at sufficiently high elevations, could now supply an abundance of water to the metropolitan Boston area by gravity through pressurized aqueducts or tunnels.

In the 1940s, planners believed that the Quabbin Reservoir would be sufficient to supply the metropolitan area into the foreseeable future, and at the time, was the last major investment in the water system with no plans in place for upgrades to carry the system into the next century. Many of the previous expansions used gravity for supplying water instead of costly pumping. Fortunately, these crucial foundations laid by the early water engineers provide the backbone of the system run today.

Origins of the pressure aqueduct system (Hultman Aqueduct): 1937-1941. In 1937, a plan was developed for a high-service pressure aqueduct system to deliver water to the metropolitan area. A portion of the plan included two parallel aqueducts to carry water from the Wachusett Aqueduct to the new Norumbega Reservoir and the terminus of the Weston Aqueduct in the town of Weston. Work began on schedule in 1939 and by the outbreak of World War II in 1941, one of the two proposed parallel pressure aqueducts had been built. This portion of the pressure aqueduct is the Hultman Aqueduct.

Pressure aqueducts and tunnels: 1950-1978. After World War II, additional segments of the pressurized transmission system came online with the construction of the Chicopee Valley Aqueduct, metropolitan tunnel system and Cosgrove Tunnel. As these sections of the pressure transmission system have come online, the need for pumping from open reservoirs was reduced because more of the service area could be supplied by this pressurized transmission system. Older facilities that originally provided a level of redundancy to the new pressure tunnels were eventually retired from use. More reliance was placed on the newer pressurized system to the point where it is now relied upon to deliver 85 percent of the metropolitan area demand.

Redundancy in the transmission system

Transmission system overview. The current water transmission system can be divided into five major segments as shown in Fig. 1. Redundancy projects for segments 1 through 4 have been completed. The fifth segment, the metropolitan tunnels, represents the next challenge for the MWRA in improving the reliability of this great water system.

Valve chamber filled with ground water.

Fig.2-Valve chamber filled with ground water.

Metropolitan tunnel system (segment 5). The metropolitan tunnel system includes the City Tunnel (1950), the City Tunnel extension (1963), and the Dorchester Tunnel (1976). These three tunnels interconnect at shaft 7 at Chestnut Hill. Together, these tunnels carry approximately 60 percent of the total system daily demand with no redundancy.

Condition of metropolitan tunnel system. Each tunnel comprising the metropolitan tunnel system consists of concrete-lined deep rock tunnel sections linked to the surface through steel and concrete vertical shafts. At the top of each shaft, cast iron or steel pipes and valves connect to the MWRA surface pipe network. These pipes and valves are accessed through subterranean vaults and chambers. The tunnels and shafts require little or no maintenance and represent a low risk of failure. However, many of the valves and piping are in poor condition.

Hydraulic valve actuators.

Fig.3-Hydraulic valve actuators.

Valve reliability for the metropolitan tunnels is a concern. As an example, the City Tunnel (1950) appurtenances are 68 years old and cannot be adequately maintained or replaced until a back-up exists. Failure of some valves can cut off a majority of the system’s capacity to supply water and, due to the physical condition, age and environment in which they were installed, have not been exercised for fear of failing in a closed position. These valves should be, but cannot be, replaced because shut down of the City Tunnel would be required.

Access to some of the valve structures and chambers is hampered by high ground water or damp conditions (Fig. 2). Original protective pipe coatings are gone, and pipes and valves are coated in thick layers of rust. Loss of metal thickness and structural strength is a concern. Bolts and fasteners have corroded and are planned to be replaced where feasible. Some chambers must be pumped down to allow access, which impedes any emergency response and aggravates further corrosion concerns.

At many of the top-of-shaft structures are piping and valves of varying diameters (ranging from less than an inch to several inches in diameter). These provide air and vacuum relief, along with drains, flushing connections, valve by-passes, and control piping for hydraulic valve actuators (Fig. 3). Some of these pipes and valves are in a similar deteriorated condition as the main pipes and valves. Failure of one of these smaller-diameter connections could require a tunnel shutdown to allow for a safe repair in some of these confined spaces. The amount of water that can flow out of a modest opening under high pressure can be significantly more than one might think: potentially more 378 GL/day (100 milion gpd).

Some of these concerns can be mitigated somewhat through replacement of corroded bolts, wrapping or coating of corroded pipeline segments, replacement of air valves, and installation of cathodic protection systems. A program is being developed to implement some of these measures to reduce the risk of certain failures that would require complete tunnel shutdown. However, all the potential failure points cannot be addressed without tunnel isolation and complete replacement or maintenance of failed or failing components at some point in the future.

Water flowing at 250 million gpd.

Fig.4-Water flowing at 250 million gpd.

Water main break of May 1, 2010. MWRA experienced a major break on a 3 m (10 ft) diameter pipe connection at shaft 5 of the City Tunnel on May 1, 2010. The break occurred at a coupling on the surface pipe interconnection between the recently constructed MetroWest Water Supply Tunnel and the City Tunnel. The MWRA had a redundant pipe (Hultman Aqueduct) at this location, but at the time of the break, the Hultman Aqueduct was being rehabilitated and was out of service.

The incident resulted in a release of approximately 98 GL/day (25 million gpd) over a period of eight hours until the break was isolated (Fig. 4). During this time, an emergency water source was activated to maintain water supply prior to shutting down the affected pipe. While the pipe was being repaired over the following two days, the Boston metropolitan area was supplied through alternate, lower-capacity mains with augmentation from an emergency raw-water reservoir with chlorination. The water service area was issued a boil-water order during that time that affected approximately two million people in 30 serviced communities.

After the water main break, the MWRA performed an economic impact analysis of a failure and forced shutdown of the metropolitan tunnel system. The analysis estimated that the economic loss to businesses and residents within the Metropolitan area would be approximately $208 million and $102 million per day, respectively, for a total estimated economic impact of approximately $310 million per day.

1937 tunnel loop plan.

Fig.5-1937 tunnel loop plan.

History of redundancy planning for the metropolitan area

1937 plan. A redundant tunnel system was proposed as early as 1937. The plan included a proposed pressure aqueduct and tunnel system with a tunnel loop beginning in Weston near the Charles River and running east into Boston, turning north to Everett, looping west to Belmont and connecting back to Weston (Fig. 5).

While much of the 1937 plan for pressure aqueducts and tunnels was implemented from 1937 to present day, the proposed tunnel loop was never completed.

Redundancy planning 1990 through 2016. In 1990, a plan was proposed to construct a tunnel from Marlborough to Weston (the MetroWest Water Supply Tunnel) to provide redundancy for the Hultman Aqueduct and a future northern tunnel loop from Weston to Stoneham and Malden (Fig. 6). The MetroWest Water Supply Tunnel was approved for construction and was completed in 2003. However, the proposed northern tunnel loop was not constructed.

In 2011, the MWRA completed a new evaluation of alternatives for redundancy within the metropolitan Fig.4 Water flowing at 250 million gpd. Fig.5 1937 tunnel loop plan. Boston area. This evaluation included surface pipe alternatives in addition to tunnel alternatives with an objective of incorporating redundancy planning into the existing pipeline asset management program (that is, allocating funds already budgeted for rehabilitation of existing pipelines toward replacing the existing pipelines with larger pipelines). The result of that evaluation was a plan of constructing primarily large-diameter surface pipes to provide redundancy (Fig. 7). However, as the planning for this program progressed, it became apparent that the construction of large-diameter pipelines through dense urban areas would cause unacceptable community disruption and had serious implementation challenges.

Finally, in 2016 MWRA revisited the all-tunnel approach to providing redundancy to the metropolitan area. More than 30 alternatives were screened based on the level of redundancy, constructability, cost and operation and maintenance. Based on this evaluation, an all-tunnel alternative was recommended for redundancy.

1990 northern tunnel loop plan.

Fig.6-1990 northern tunnel loop plan.

Proposed plan

Given the difficulties associated with the construction and significant community impacts associated with large-diameter surface pipe together with operational reliability concerns, MWRA staff are pursuing a preferred all-tunnel redundancy alternative. The preliminary alignment, which will be subject to more detailed review and alternatives analysis during the public review period, is shown in Fig. 8.

This alternative consists of two deep-rock tunnels beginning at the same location in Weston near the Massachusetts Turnpike/Route 128 interchange. The Northern Tunnel generally follows the route of MWRA’s existing Weston Aqueduct Supply Main (WASM) 3 transmission main to a point about midway along the pipeline near the Waltham/Belmont border, which will allow flow in WASM 3 in both directions. The length of the Northern Tunnel would be approximately 7.2 km (4.5 miles) and the tunnel would have a finished inside diameter of approximately 3 m (10 ft). It would include one connection shaft to provide a redundant supply to MWRA’s Lexington Street Pump Station and to allow isolation of the WASM 3 line in segments for repair and maintenance. The Northern Tunnel has an estimated midpoint of construction cost of $472 million.

The Southern Tunnel would run east to southeast to tie into the surface connections at shaft 7C of the Dorchester Tunnel and about midway down the southern surface mains allowing flow in both directions. The length of the Southern Tunnel would be approximately 15.2 km (9.5 miles) and it would have a finished inside diameter of 3 m (10 ft). The estimated midpoint of construction cost of the Southern Tunnel is approximately $1.003 billion.

2011 surface pipe plan.

Fig.7-2011 surface pipe plan.

The proposed plan limits community disruptions and construction impacts to the locations of the tunnel construction and connection shaft sites. The all-tunnel alternative meets the strategic objective of being able to make a seamless transition to a backup supply, allowing maintenance to be scheduled for the metropolitan tunnels without use of a boil-water order, without impacting the ability to provide for local fire protection, and without noticeable changes in customers’ water quality, flow or pressure. It has the ability to meet high-demand conditions that extend the potential timeframe for future maintenance and rehabilitation activities.

Proposed tunnel redundancy plan.

Fig.8-Proposed tunnel redundancy plan.

To the north, the all-tunnel alternative will provide redundancy for the critical WASM 3 pipeline. To the south, it will eliminate the need for the Chestnut Hill Emergency Pump Station during metropolitan tunnel shutdowns, thereby reducing operational risks associated with extended use of the emergency pump Station at higher system pressures. The estimated total midpoint of construction cost for both the recommended north and south tunnels is $1.475 billion with an estimated time to completion of 17 years. This estimate includes 30 percent contingency and 4 percent annual construction cost escalation.

Geologic conditions

The new redundancy tunnels will be hard-rock pressure tunnels, similar to the seven existing tunnels that currently make up the main MWRA water distribution system. These existing tunnels ranging in size from 3 to 4.2 m (10 to 14 ft) in finished diameter are primarily concrete lined with reliance on the overlying rock for confinement. Existing tunnel depths range from approximately 15 m (50 ft) to approximately 200 m (660 ft) below ground surface. Tunnels within the existing metropolitan tunnel system (City Tunnel, City Tunnel Extension and Dorchester Tunnel) are approximately 45 to 106 m (150 to 350 ft) below grade. The seven existing tunnels, constructed from the 1930s to the early 2000s, were mined using methods that progressed from drill and blast, drill jumbos, to modern tunnel-boring machines (TBMs). It is anticipated that the new redundancy tunnels will be mined using TBMs with only short segments (tail and/or starter tunnels) constructed using alternate methods.

The alignments, both horizontal and vertical, of the new redundancy tunnels are not finalized at this time; however, it is possible that both tunnels could cross the Northern Boundary Fault and extend into the Boston- Avalon Terrace and the Boston Basin. Within the Boston- Avalon Terrace, the primary rock type is anticipated to be the Dedham Granite and within the Boston Basin, the primary rock types are anticipated to be the Cambridge Argillite and the Roxbury Conglomerate. The existing MetroWest Tunnel, City Tunnel, City Tunnel extension, and Dorchester Tunnel were mined in these same bedrock formations (Fig. 9).

The current proposed alignment of the North Tunnel generally follows the Northern Boundary Fault that could pose challenges for tunneling. The existing MetroWest Tunnel intersects this fault near its eastern limit. Extensive geotechnical investigations were conducted to determine the location of this fault as well as provide information on the anticipated behavior of the bedrock during mining. This level of investigation is largely credited for the preparedness that occurred during construction when the fault was actually encountered. It is worth noting that the only location along the MetroWest Tunnel where a steel liner was installed is along approximately 610 m (2,000 ft) of tunnel where it intersects this fault.

The current alignment of the South Tunnel places this tunnel primarily within the Roxbury Conglomerate formation; crossing the Stoney Brook Fault but not extending sufficiently south to cross the Mount Hope Fault. Both the existing City Tunnel and Dorchester Tunnel are located primarily within the Roxbury Conglomerate formation with only the southern limit of the Dorchester Tunnel extending past the Mount Hope Fault and/or into the Cambridge Argillite. The geology of these tunnels is well documented (Tierney et. al., 1968; Richardson, 1977).

The quality of rock encountered along the City Tunnel is noted by Tierney as being excellent for tunneling. Just 5 m (16 ft) of the 7.7-km (4.78-mile) tunnel required structural steel support with the remaining approximately 7.7 km (4.7 miles) of tunnel mined with no need for temporary supports. The bedrock through which the City Tunnel extends was considered unusually good as compared to that encountered along other tunnels previously mined through similar geology in the greater Boston area.

Geologic conditions.

Fig.9-Geologic conditions.

The 10.2-km (6.4-mile) long Dorchester Tunnel was mined through bedrock consisting of Roxbury Conglomerate and Cambridge Argillite. This tunnel was excavated using drill and blast for approximately 90 percent of the tunnel length. A “mole” (rotary boring machine) was used to mine the remaining 10 percent of the tunnel length. Tunnel excavation using the mole was considered experimental at the time because the contractor had not used similar equipment in the past and it had not previously been used in the Boston area prior to this project. Steel supports were required over a fraction of the total tunnel length — primarily where the tunnel crosses the Stoney Brook Fault.

Previous large projects

The MWRA has planned, designed and constructed a number of large projects, including mega projects, in the past. The largest and most notable project in recent years is the Boston Harbor Project (BHP), which spanned from the mid-1980s to the early 2000s. This nearly 20- year program focused on the clean-up of a much polluted Boston Harbor and involved numerous significant program elements including construction of a new Deer Island wastewater treatment plant, a 15.3 km (9.5 mile) by 7.3 m (24 ft)-diameter outfall tunnel, and a 8 km (5 mile)-long and 3.5 m (11.5 ft)-diameter inter-island tunnel. The overall BHP cost was $3.8 billion.

Overlapping the BHP was the integrated water supply improvement program, which occurred between 1995 and 2005 and cost approximately $1.7 billion. This 10-year program included construction of the 28.3 km (17.6 mile)-long and 3.6- to 4.2-m (12- to 14-ft)-diameter MetroWest Water Supply Tunnel, seven covered storage tanks, and the new state-of-the-art John J. Carrol Water Treatment Plant.

Following the completion of the BHP and integrated water program, the MWRA moved on to other significant wastewater projects including the planning, design and construction of the Braintree-Weymouth relief facilities project. This $200 million project, executed between 2002 and 2010, included an intermediate pump station, 4.3 km (2.7 mile) long and 3.6 m (12 ft)-diameter deep-rock tunnel and shafts.

Alongside the Braintree-Weymouth project is the South Boston CSO storage tunnel and related facilities. This $260 million project, executed between 2006 and 2011, included a 3.4 km (2.1 mile) long and 5.2 m (17 ft)- diameter soft-ground CSO storage tunnel, shafts, pump station, sewer and storm drains, and ventilation building.

The MWRA currently executes approximately $100 million in capital programs each year to add redundancy to improve and maintain its current water and waste water assets.

Approval of a tunnel redundancy plan by MWRA Board of Directors

After the May 2010 water main break and during the mid-2010s, it became apparent that executing another large water tunnel program would be needed in the near future. On Oct. 6, 2016, the MWRA Board of Directors held a special meeting where MWRA staff provided a briefing on the status of the existing MWRA water transmission system and the lack of redundancy for the Metropolitan Tunnel System. The preferred alternative of constructing two tunnels, one to the north and one to the south, was recommended.

At the conclusion of the special meeting, staff members were directed to brief member communities and state and local officials on the Metropolitan Tunnel Redundancy initiative in order to build consensus and support for the preferred project approach. On Dec. 8, 2016, a Long-Term Water Redundancy Forum hosted by the MWRA Advisory Board for the customer communities was held at Boston College. MWRA staff presented the history of the MWRA waterworks system, the need for metropolitan tunnel redundancy and the challenges, both implementation and financial, of building redundancy.

Projected debt service as of 2017.

Fig.10-Projected debt service as of 2017.

On Jan. 19, 2017, the MWRA Advisory Board met and voted to support moving forward with a deep-rock, two-tunnel project. They voted also to recommend: a program management division approach to manage the program similar to the model used for the BHP; concurrent construction of both tunnels rather than a phased approach; and allocation of any revenue from nontypical or one-time water users (for example, emergency drought connections) toward the cost of the program.

On Feb. 15, 2017, the MWRA Board of Directors approved the preferred alternative of construction of northern and southern deep-rock tunnels from the Hultman Aqueduct and MetroWest Water Supply Tunnel to the Weston Aqueduct Supply Main 3 (WASM3) and to the Southern Spine water mains for the purpose of providing redundancy for the metropolitan tunnel system (City Tunnel, City Tunnel Extension and Dorchester Tunnel), and directed staff to proceed with preliminary design, geotechnical investigations and Massachusetts Environmental Policy Act (MEPA) review of the project.

In June 2018, the MWRA Board of Directors approved the fiscal year 2019 capital improvement program (CIP), which includes $1.4 billion (2019 dollars) for the tunnel redundancy program.

Project goals

The Metropolitan Tunnel Redundancy Program was conceived to address several outstanding challenges, most notably the fact that the existing Metropolitan Tunnel System cannot be maintained or repaired nor can an emergency be readily addressed because shut down of the system is not currently possible without imposing a boilwater order.

The first and foremost goal of the program is an operational goal: to protect public health, provide sanitation and provide fire protection. The MWRA exists to provide these services. In support of this overall goal, the tunnel redundancy program is intended to:

  • Provide full redundancy for the metropolitan tunnel system.
  • Provide normal water service and fire protection when the existing tunnel system is out of service
  • Provide the ability to perform maintenance on existing tunnels year-round.
  • Provide uninterrupted service in the event of an emergency shutdown.
  • Meet high day demand flow with no seasonal restrictions.
  • Avoid activation of emergency reservoirs.
  • Meet customer expectations for excellent water quality.
  • Preserve sustainable and predictable rates at the water utility level.
  • Minimize cost of borrowing.
  • Be constructible.
  • Result in no future boil-water orders.

The selected tunnel alternative is expected to meet all of these goals.

Project costs and financing

The cost of the Metropolitan Tunnel Redundancy Program is being allocated in the MWRA’s CIP with the goals of: preserving sustainable and predictable rates, ensuring adequate capital is available when necessary and minimizing the cost of borrowing. Since 1985, MWRA has spent approximately $8.4 billion to upgrade the waste water and water systems. The majority of these improvements were funded through the issuance of taxexempt bonds. The MWRA is projected to reach the peak of its debt service payments in fiscal 2022 (Fig. 10), which provides an opportunity to mitigate water rate impacts of financing the proposed tunnel program.

MWRA uses a multi-year rate management strategy to provide sustainable and predictable assessments to its communities. The impact on the CIP and the debt service on the current expense budget (CEB) were evaluated for a variety of options for the Metropolitan Tunnel Redundancy Program. The options evaluated ranged from “do nothing” to the most expensive tunnel option.

MWRA communities are either combined water and sewer users, only sewer users or only water users. The projected average annual increase on the combined water and sewer assessment of the preferred alternative is 1.3 percent. The projected average annual increase on the water-only assessment of the preferred alternative is 4 percent.

The rate impacts of the preferred option on both the combined and water-only assessments are within the MWRA’s long-term rates management strategy. The preferred option is both consistent with the authority’s core mission of providing reliable, cost-effective and highquality water, and its goal of providing sustainable and predictable assessments.

Project outlook

The Metropolitan Tunnel Redundancy Program is currently at the early stages of planning and design. The organizational framework to manage the program within the MWRA is in place in the form of the Tunnel Redundancy Department. Procurement of initial consultant contracts for program support services and preliminary engineering are underway.

It is expected that the next several years will include a number of program-wide activities including risk management planning, quality management planning, health and safety planning, design criteria and standardization, document management and project controls, work breakdown planning, procurement planning, construction package planning, field investigation procedures, rock core storage, critical path scheduling, and budget planning and management.

The preliminary design phase of the program will involve significant efforts on geotechnical investigations, preliminary route and shaft site alternative evaluations, preliminary design, an assessment of environmental permits needed and preparation of the Massachusetts Environmental Policy Act review for the project. This phase of the project will initiate actual design. Preliminary design is anticipated to be complete by 2023. It is envisioned that final design(s) will follow on the heels of preliminary design with the first tunnel construction package issued in or around 2027.


Tierney, F.L., Billings, M.P. and Cassidy, M.M. (1968). Geology of the City Tunnel, Greater Boston, Massachusetts, Jour. Boston Soc. Civil Eng., V. 55, pp. 60-96.

Richardson, S. M. 1977. Geology of the Dorchester Tunnel, Greater Boston, Massachusetts, Jour. Boston Soc. Civil Eng., V.63. pp. 247-269.

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