Two principles have emerged time and again in the field of construction materials and structural engineering: the first is that the new and innovative materials, such as epoxies, often have properties that are poorly understood when they are introduced or used in new applications.
In fact, they may induce failure modes that were not considered previously because they did not govern the performance of the materials used earlier. The second principle is that structural systems, when they fail, often fail at the connections.
The Boston Central Artery, also known as Big Dig, was one of the most monumental transportation efforts undertaken in the United States. It was a matter of planning and discussion that went on for decades.
The planning work for the Big Dig started in 1981 but actual construction work began in 1991. It was estimated that the tunnel construction work would be completed by 2001. However, the project was completed in 2006 and opened to the public in 2008. It became disreputable across the United States for cost overruns and construction delays.
The state transportation engineers routinely referred to the Big Dig as an excuse not to tackle due to horrific congestion problems on highway number-288 coming into Birmingham city.
It was not an easy task to build a new interstate-quality highway through the heart of a congested city, through a century’s worth of forgotten underground infrastructure. What made the project more complicated was Boston’s traffic as well as the tendency for Boston Harbor to try to work its way into any tunnel in the area.
1. Project Details
The Big Dig tunnel project was a megaproject in the city of Boston. It completely changed the route of the central artery of interstate highway number 93. The project runs through the heart of Boston through the construction of a 2.4 km tunnel, known as Thomas Tunnel. Later on, the name of the tunnel was changed to the Rose Fitzgerald Kennedy Greenway to honor the Kennedy family matriarch, Rose Fitzgerald Kennedy.
Initially, the highway authorities decided to construct a railway link for Boston city. However, the idea of a railway link was dropped due to the high population of the city. The planning for the Big Dig tunnel project started back in 1982, and the construction began in 1991. The project was completed in 2007.
The Big Dig highway tunnel project was the most expensive project in the history of the US at that time. The project was plagued due to construction delays, cost overruns, flaws in the design, poor execution, use of low-quality material, and construction accidents.
As per the earlier estimation, the project should have been completed by the end of 1998 with an estimated cost of USD 2.8 billion. However, the contractor took nine more years to complete the project, which was completed in 2007 and at a cost of USD 8 billion.
It was estimated that the project would ultimately cost around USD 22.5 billion because of the interest amount. Such a huge amount would not be paid off until 2040. Bechtel and Parsons Brinckerhoff are the organizations that oversaw this project.
2. The Tunnel Collapse
There was great relief among the engineers when the Big Dig finally opened. But during its initial days, a heavy tunnel ceiling panel fell on a car and killed a woman. Since the collapse occurred in a tunnel, the incident came under the jurisdiction of the National Transportation Safety Board (NTSB).
The failure of the tunnel ceiling panel occurred on 10th July 2006. The weight of the concrete ceiling panel was 24,000 kg, and its dimensions were 6.1 m x 12.2 m. The failure of the ceiling panel had impacted the traffic of Boston city and the Big Dig tunnel was closed for almost a year.
3. NTSB Investigation
The sudden failure of the ceiling panel of Big Dig tunnel could have been avoided if the designers had considered that the epoxy material, which was applied to secure the ceiling panels, can come out from the ceiling panels. NTSB suggested and gave the final recommendation that the collapse of the ceiling panel of the Big Dig tunnel occurred due to the use of an epoxy anchor adhesive with very poor creep resistance.
In a public meeting one year after the accident, the NTSB released a report of the highway accident. The report blamed the engineer and the contractor, with errors committed by the epoxy supplier. The safety issues identified during this investigation are as follows:
- Insufficient understanding among designers and builders of the nature of adhesive anchoring systems
- Lack of standards for the testing of adhesive anchors in sustained tensile-load applications
- Inadequate regulatory requirements for tunnel inspections
- Lack of national standards for the design of tunnel finishes
The investigation also found that there had been an incident involving the same epoxy seven years ago. Anchor displacement had been observed in the high-occupancy tunnel in 1999, but the engineers and contractors did not continue to monitor the performance of the epoxy.
By July 2006, many of the adhesive anchors supporting the tunnel portal ceilings had been displaced enough to put them in imminent danger of failure. Improper or deficient anchor performance could not, by itself, account for the anchor failure. The design calculations were consistent with the actual in-service loads.
The selection of an adhesive anchoring system to support the ceiling panels was appropriate, however, the adhesive material used for anchoring system was lacking creep resistance.
4. Major Reasons Behind the Failure of the Tunnel
The major reasons behind the failure of Big Dig tunnel are summarized below:
- The engineers failed to consider that polymer adhesives are susceptible to deformation (creep) under sustained load. Testing results showed that the creep resistance of adhesive was poor. Thus, they made no provision for ensuring the long-term, safe performance of the ceiling support anchoring system. Therefore, the adhesive specification did not require creep resistance.
- The adhesive product supplied and used to support the anchors had poor creep resistance. The manufacturer’s information for the product was inadequate and misleading and left out the fact that testing had shown the epoxy to be subject to creep under sustained loading.
- The maximum load capacity of an adhesive anchor, which relates to short-term loading, does not indicate that the anchor will be able to support even lighter loads over time, and thus a larger design safety factor cannot compensate for an adhesive material that is susceptible to creep.
- After unexplained anchor displacement was found in the Interstate 90 connector tunnel in 1999 and 2001, Modern Continental Construction company should have instituted a program to monitor anchor performance to ensure that the actions taken in response to the displacement were effective.
- Had these organizations taken such action, they may have discovered the anchor creep and could have taken measures to prevent the accident.
- The authorities should have regularly inspected the area above the suspended ceilings. If they had, the anchor creep would have been detected.
- Ultimate load tests should have been conducted on the adhesive anchors before installation.
- Installing adhesive anchors in an overhead application is difficult. This type of work makes it likely that voids will be introduced into the adhesive, which will reduce holding power and reliability.
- The circumstances of this accident demonstrate a general lack of knowledge and understanding among design and construction engineers and builders of the complex nature of epoxies and similar polymer adhesives. In particular, the potential for those materials to deform (creep) under sustained tension loads.
Test protocols and standards were needed to determine the capacity and reliability of adhesive anchors in sustained tensile load applications. The NTSB determined that the probable cause was the use of an epoxy anchor adhesive with poor creep resistance, i.e., an epoxy formulation that was not capable of sustaining long-term loads.
5. Mechanism of the Creep Failure
Creep is a continued deformation under a sustained load. It is a time-dependent phenomenon. When a load is applied to a material that creeps, the resulting deformation has two components. The first deformation component is elastic, remains constant as long as the load remains the same, and is removed when the load is removed. The second component is creep, it increases gradually over time, and when the load is removed, deformation (permanent set) remains.
Structural steel does not creep, except at high temperatures. Concrete, wood, and masonry undergo creep, and the amount of creep deformation is in the range of two to three times the instantaneous elastic deformation. It is necessary to take creep into account for predicting long-term deformations, but the behavior is well understood, and engineering solutions are well established.
On the other hand, new materials, such as epoxies and plastics, may have much higher creep deformations. In a tensile anchor application, the anchor may pull partway out of the hole, reducing the bond strength of the connection. It is also possible for an anchor to pull completely out. As the anchors creep, the loads may shift to other anchors. As this failure suggests, the long-term behavior of certain epoxy materials in tension is not yet well understood.
6. Future After the Failure of the Big Dig Tunnel
The NTSB suggested the following corrective actions:
- The Federal Highway Administration (FHWA) and the American Association of State Highway and Transportation Officials (AASHTO) should develop test protocols and standards to determine the capacity and reliability of adhesive anchors in sustained tensile load applications.
- Until the standards have been developed, FHWA should prohibit the use of adhesive anchors in these applications where failure would result in risk to the public.
- Legislation should establish and implement a tunnel inspection program supported by FHWA and AASHTO’s cooperative development of design, construction, and inspection guidance for tunnel finishes.
- State departments of transportation should prohibit the use of adhesive anchors in these applications where failure would result in risk to the public and should identify existing facilities where such a risk exists. Those sites should be inspected and repaired.
- The International Code Council (ICC) should require creep testing for qualification of all anchor adhesives. Any adhesives that have not been creep tested or that have failed creep testing should be disqualified from being used in tensile loading.
- ICC building codes and evaluation reports should be revised in light of this failure.
- AASHTO should use the circumstances of the 10th July, 2006, accident to educate the public through publications, websites, and conferences, as appropriate, the risks associated with using adhesive anchors in sustained tensile-load applications where failure of the adhesive would result in a risk to the public.
- The American Concrete Institute, the American Society of Civil Engineers, and the Associated General Contractors of America were urged to undertake similar educational efforts.
Construction of the Big Dig in Boston started in 1991.
The ceiling of the Big Dig tunnel collapsed due to the poor creep resistance of the epoxy material used to secure the ceiling above the roadway of the tunnel.
It is the most expensive highway project in the history of the US. The project was completed at a cost of USD 22.5 billion.