The Washington Metropolitan Area Subway System was identified by the Directors of the American Underground Construction Association as the "Most Distinguished United States Underground Project, 1975-2000, in commemoration of the 25th anniversary of the founding of the International Tunnelling Association.
The Washington Metro System, which includes subway, surface rail and connecting buses, is a total of 103 miles long and has 83 stations. The subway portion of the system is 51 miles (82 km) and has 47 stations. The system serves a population of 3.2 million in Washington, D.C., and portions of the states of Maryland and Virginia, a service area of approximately 1500 square miles. The subway portion represents a $5.8 billion, 30 year effort, with completion planned for 2001. The cost of the entire system is $9.6 billion.
The system, which opened in 1976, is extremely efficient, clean and attractive. Ridership in 1996 was estimated to be 731,000 daily weekday trips, with 508,000 of those at least partially by rail.
The Washington Metro was designated the most distinguished U.S. underground project for the following reasons:
- The overall management has demonstrated a competent, persistent and public-service oriented approach to complex public-works planning, design and construction
- Contracting practices used for the Metro were innovative and changed for the better contracting for underground projects in the United States. These practices include the full disclosure of geotechnical information through geotechnical data and design reports.
- A "team" approach was in effect from the early days of the project, unified by a consistent design theme.
- The project involves all types of underground conditions from hard rock to soft water-bearing material and river crossings.
- Construction included conventional and NATM hard rock tunnels and stations, soft ground tunnels of several types, stations, major underpinning of monumental buildings and, sunken tube river crossings.
- A quantified approach to estimating settlements associated with underground construction was initiated on the project; this method has subsequently been used on projects worldwide.
- The project saw the use of innovative construction techniques and the first U.S. use of products such as composite linings, shotcrete and resin-anchored rock bolts; it also saw the first large-scale use of grouting as a means of protecting existing structures against the effects of settlements.
Tunneling Program
The tunneling program encompassed a variety of innovative design and construction techniques. The first large diameter Earth Pressure Balanced Tunnel Boring Machine in North America was utilized on the Washington Metro for the Anacostia River Crossing, preceding the use of EPBM technology in Western Europe.
Soft ground tunnels have included cast iron, steel, cast-in-place concrete and more recently the single-pass concrete segmental construction as well as the two-pass concrete lining systems. Hard rock tunnel methodology has included TBM, road header and drill-and-blast, including NATM.
The subway system has established an international standard for creating large, column-free underground passenger stations, complimenting the surface architecture of Washington, D.C. Station construction methods have included, in addition to mining methods, various cut-and-cover and top-down techniques, utilizing soldier piles and lagging and temporary and permanent structural slurry walls.
The underground system waterproofing techniques have been perfected to produce completely dry tunnels in the more recently-constructed sections of the subway.
Geological Approach
The Washington, D.C. Metropolitan area has a diverse and challenging geology ranging from shallow bedrock on the west, to deep soil sediments on the south and east. An important reason for the success of the Washington Metro's tunneling program is that the challenges posed by the geology were correctly defined up-front on a segment-by-segment basis, and effective solutions were then devised and implemented to overcome potential difficulties.
The following approach led to successfully meeting the geologic challenges:
- The performance of detailed subsurface investigation programs at the route planning phase, followed by additional investigations at the preliminary and final design phases. Sufficient resources were allocated for the subsurface investigation effort throughout the life of the tunneling program, recognizing that this investment would more than pay for itself by minimizing construction delays and differing conditions later.
- An analysis of the subsurface investigation data to accurately establish design parameters and to formulate construction specifications.
- Full use of the observational approach to determine actual ground conditions during the construction phase, based on the findings of the instrumentation program. Lessons learned from previous contracts were systematically applied to the subsequent construction, throughout the life of the tunneling program, advancing the state-of-the-art.
Advancement of the State-of-the-Art
Washington Mertro's Tunneling program owes it success to the many creative and innovative features that it embodies.
- Englightened Approach. From its very inception, the program has been based on an enlightened approach to tunneling in that all subsurface information was openly and explicitly shared with the bidding contractors. This was not a common approach in the U.S. at the time. Thus the Washington Metro took a leadership role in equitably sharing potential risks posed by subsurface conditions with its construction contractors. This changed forever the way tunneling contracts were bid in the U.S.
- The Washington Metro also introduced the concept of the Geotechnical Baseline Report to the tunneling contracting industry. The concept was based on providing an explanation in plain English of the construction specifications to prospective bidders so that they could understand why certain construction methods were chosen and others rejected. Initially the GBR was an informational document, but later became part of the bid package, promoting competitive bidding, and establishing a fair basis to resolve differing site condition issues.
- Peer Review. All tunneling designs were subjected to the Peer Review process through various stages of evolution. This group, known as the Board of Engineering Consultants, functioned on an as-needed basis through the 30 year life of the program. The process resulted in economical and safe designs as well as the timely and efficient resolution of critical problems during construction.
- Observational Approach. Use of the observational approach has been a cornerstone of the Washington Metro tunneling program from the beginning. It was used successfully to create large underground station caverns in less than competent rock and under shallow ground cover on the Red Line. Instrumentation designs have always represented an integral element of the tunneling design as well as of the construction monitoring procedure, to facilitate the application of the observational approach.
- Use of Shotcrete. Washington Metro has used shotcrete on an extensive basis from the beginning of the tunneling program, predating the introduction of the New Austrian Tunneling Method (NATM) in the U.S., which occurred in the 1980's.
- Computer based analytical techniques have been used for the design of underground facilities since the early days of the tunneling program. Advanced finite element (FEM) tools have been used on several segments of the underground system since the early 80s. Both two and thee dimensional FEM analyses have been used. One of the largest computer models ever used in the U.S. for a transportation project was developed for the deep rock station at Wheaton, on the red line, in the mid eighties. The model was used to analyze a complex configuration of the station vault, escalatorways, shafts and running tunnels to predict ground behavior during sequential excavation and initial support installation processes.
- Tunnel Boring Technology. Washington Metro's tunneling program led the way in the utilization of advanced shield and TBM technology.
- Washington Metro pioneered the use of chemical and cementitious grouting for ground modification on a comprehensive basis.
- Recent advances in tunnel lining technology on the Washington Metro include the single-pass bolted and gasketed as well as the two-pass concrete lining systems. The first major river crossing in North America utilizing the single-pass concrete segmental lining was performed on the Anacostia River crossing in the mid-eighties.
Construction Economy
The Washington Metro Tunneling program, considering the diversity and extent of construction involved and the high quality of the end product, is very cost effective when compared with other similar systems around the world. Cost analyses indicate that subway construction cost increases over the years have been below the escalated costs due to inflation. There are many factors responsible for the economic success of the tunneling program:
- Sound Planning and Design. Design criteria and construction standards were updated on a contract-by-contract basis to ensure the cost competitiveness of the Metro tunneling program. Route planning was based to the maximum extent possible on the efficiency of the mining process.
- Inclusion of alternative designs has been a hallmark of the Metro tunneling program.
- Value engineering proposals from contractors were encouraged during the construction phase to permit the contractor to exercise creativity and innovation to the fullest extent possible.
- A conscious effort was made to understand the capabilities of those contractors who typically bid on Washington Metro's tunneling projects and to reflect their preferences in the bid designs.
- Metro's organizational structure permitted the tunneling program to proceed in an environment of checks and balances, which permitted the inclusion of only the best design and construction procedures in the bid documents.
- The Washington Metro Authority ensured that it had the necessary, high calibre talent available to it throughout the life of the program in the engineering, construction management and administrative areas. This organizational structure was complimented, on an as needed basis, by eminent experts in several fields to address the specific challenges of the program.
The Washington Metro subway construction program is nearly complete. Currently 73 km of the system is operational, and by the year 2001, the remaining 9 km of the subway will also become part of the system. Washington Metro's $5.8 billion subway construction program represents an uncommon success story in recent times in the field of urban tunnel engineering. This achievement has been made possible by advancing the state-of-the-art in all areas of tunneling technology. The lessons learned on the Washington subway are directly applicable to subway construction programs envisioned for other major cities around the world.
This article was based substantially on "Washington Metro - The Modern-Generation Subway Project of the Century", by Mohammad Irshad, Parsons Transportation Group, Washington, D.C., and N. Tahir, Washington Metropolitan Area Transit Authority, Washington, D.C. The paper was presented at the 25th General Assembly of the International Tunneling Association and World Tunnel Congress '99, May 29 - June 3, 1999, Oslo, Norway
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