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How Could a Pile of Dirt Cause a Major Interstate Bridge To Tilt? PDF Print E-mail
Articles - Forensics and Failures
Written by Dennis M. O'Shea   
Tuesday, 20 October 2015 07:25

On June 2, 2014, the Delaware Department of Transportation closed a bridge on I–495 that was leaning and out of plumb. The culprit? This seemingly innocuous stockpile of dirt on the east side of the bridge.

On June 2, 2014, the Delaware Department of Transportation closed a bridge on I–495 that was leaning and out of plumb. The culprit? This seemingly innocuous stockpile of dirt on the east side of the bridge.

On the evening of June 2, 2014, a major interstate that parallels I–95 and bypasses Wilmington, DE, was closed to traffic. The closure of I–495, which carries nearly 90,000 vehicles per day, caused nightmares for local commuters and out-of-state travelers.

The Delaware Department of Transportation (DelDOT) had closed I–495 after completion of a damage inspection by DelDOT bridge inspectors. The inspection took place because of a phone call from a private citizen who said the bridge appeared to be leaning.

The 40-year-old bridge was indeed leaning. Four column supports were tilted, with the worst column leaning 4 percent out of plumb toward the east. The leaning was caused by uneven settling and lateral movement of the piers.

A telltale sign was revealed by the adjacent concrete median barriers of the northbound and southbound structures. The tops of the adjacent median barriers are typically at the same elevation and about 1 inch (2.5 centimeters) apart. But in this case, the top of the southbound’s median side was 18 inches (46 centimeters) lower than the top of the northbound’s median barrier, and there was a gap of about 3 inches (8 centimeters) in between. Given these findings, DelDOT closed the bridge that very evening, because of the obvious damage to the substructure units.

Bridge 1-813 carries I–495 over the Christina River and Christiana Avenue, east of downtown Wilmington and adjacent to the Port of Wilmington. Constructed in 1974, the bridge is approximately 4,800 feet (1,463 meters) long. The steel superstructure consists of two girder/floor beam stringer main spans located over the river and 35 multigirder approach spans. Fortunately, the tilt did not affect the two girder main spans over the river.

Looking south down the line of hammerhead columns makes the problem apparent. The column in the front is clearly leaning east.

Looking south down the line of hammerhead columns makes the problem apparent. The column in the front is clearly leaning east.

The settling occurred at the approach spans at piers 11 through 14. These four reinforced concrete piers are the single-column hammerhead pier cap type and are approximately 60 feet (18 meters) tall. The affected piers are founded on steel H-piles, which have an H shape and at this location are driven to bedrock.

Prior to the June 2014 findings, DelDOT had last inspected the bridge in October 2012. The agency schedules inspections every 2 years, so the discovery of the tilt and the subsequent closure occurred about 4 to 5 months before the next inspection would have taken place.

These cracks in the soil adjacent to the stockpile of dirt suggest the extent of the differential settlement exerted by the stockpile on the existing ground. The white hardhat provides an idea of the size of the cracks.

These cracks in the soil adjacent to the stockpile of dirt suggest the extent of the differential settlement exerted by the stockpile on the existing ground. The white hardhat provides an idea of the size of the cracks.

Determining the Cause

Investigation into the cause began immediately. DelDOT engineers and their design consultants, along with bridge personnel from the Delaware Division of the Federal Highway Administration, mobilized on the night of June 2. Once onsite, the DelDOT engineers and design consultants discussed a number of possible causes of the movement, plus potential fixes. In addition to engineers who were onsite, DelDOT’s design consultants provided remote staff support from the consultant’s offices throughout the country. Together, the engineers analyzed the situation and offered their determinations and recommendations.

While reviewing the site conditions for possible reasons for the movement, the engineers noted a large stockpile of fill adjacent to and east of the bridge. In addition to the settlement at the bridge, soil cracks, settlement, and heave were observed at several locations around the perimeter of the stockpile. Although the engineers were not completely certain yet as to the cause of the bridge’s movement, they asked the owner of the stockpile to remove it immediately to eliminate any further potential impacts.

On the evening of June 2, Joey Hartmann, director of the Office of Bridges and Structures at FHWA headquarters, offered DelDOT and FHWA’s Delaware Division access to any technical resources needed. The division requested assistance with geotechnical expertise. Hartmann called on Khalid Mohamed, a geotechnical engineer at FHWA headquarters, to review the situation. Mohamed’s experience with a similar settlement problem involving the Leo Frigo Memorial Bridge in Wisconsin made him an ideal resource.

This photo shows the settling problem, as evidenced by the displacement of the concrete bridge rails in the median. At this location, one rail is 18 inches (46 centimeters) below the other.

This photo shows the settling problem, as evidenced by the displacement of the concrete bridge rails in the median. At this location, one rail is 18 inches (46 centimeters) below the other.

Based on a review of as-built drawings showing subsurface soil and foundation details, Mohamed anticipated that the supporting piles might have been damaged or fractured as had happened in Wisconsin and. He recommended that DelDOT maintenance personnel bring equipment to the site to excavate the soil around and under the reinforced concrete pile cap footings of the reinforced concrete hammerhead piers to expose the piles for inspection and evaluation. The engineers assumed that the soils in the area were corrosive because hazardous materials had previously been found nearby. However, the engineers found all of the piles to be intact and not fractured as suspected, although one steel pile was buckled.

Surprisingly, the main effects were on the concrete pile cap footings, which were found to be cracked horizontally and diagonally. The footings were 5.5 feet (1.7 meters) thick, with the northbound and southbound footings tied together with a reinforced concrete strut. As a result of these findings, the engineers determined the cause to be lateral displacement of the soil due to lateral squeeze of the underlying soft soils.

“After seeing the effects that an adjacent stockpile of dirt had on the I–495 bridge, we worked with FHWA headquarters and Resource Center staff to reach out to all of our State partners through each FHWA division office to let them know of the circumstances,” says FHWA Delaware Division Administrator Mary Ridgeway. “It was also recommended that they include a visual check of the surroundings near their bridges during all of their future bridge inspections. This is an issue we are very aware of now.”

How Could a Pile Of Dirt Be the Cause?

The following perfect storm of factors caused the settling to occur:

  • The soils in the area are organic, very soft, and compressible.
  • The soft soils profile is especially deep at pier 12, where the worst damage occurred, and becomes even deeper approaching the Christina River. Below the surface, the soft soil is more than 100 feet (30 meters) deep before reaching stiffer soil and then bedrock.
  • The bridge piers were erected on top of steel H-piles that were driven to bedrock when the bridge was built in the 1970s, requiring the piles to be more than 130 feet (40 meters) in length.
  • The large stockpile of soil east of the bridge, estimated at approximately 50,000 (45,000 metric) tons, compressed the soils adjacent to the bridge and produced lateral forces on the piles, a phenomenon known as lateral squeeze of soft soil due to the occurrence of an unbalanced load at the surface forces on the piles.
  • Because the H-piles’ lateral load capacity was exceeded and the poor quality soils did not provide support, they deflected. Typically, pile capacity or resistance is developed through the strength of the pile and the soil, if it is dense enough.

Without any one of these factors, the damage likely would not have occurred.

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Early Actions

To ensure that this was an isolated condition, DelDOT inspected all of the other piers of Bridge 1-813. The engineers found no damage at any locations other than piers 11 through 14.

DelDOT’s testing consultant then installed tilt sensors on the piers to monitor the movement and determine whether the bridge was still moving. Field surveyors also came onsite to help monitor movement.

On June 3, Delaware Governor Jack Markell signed an emergency declaration, which enabled DelDOT to procure a contractor experienced in performing emergency bridge repairs. Fortunately, the agency’s on-call design consultant was onsite during the initial investigation. Within days of the incident, Division Administrator Ridgeway authorized $2 million in initial quick-release Federal emergency relief funding, allowing work to start.

Removal of the stockpile had begun immediately on the night of June 2. After observing that the work was progressing slower than expected, DelDOT brought in additional trucks and loaders to expedite the removal. Working around the clock, the agency and its contractors completed the removal by June 10.

Path Forward

With 90,000 vehicles per day displaced and a daily traffic nightmare unfolding, time was of the essence in order to reopen the bridge. A team of experts from DelDOT, FHWA, the University of Delaware, on-call consultants, and the contractor gathered on June 4 to brainstorm whether to replace the affected spans or rehabilitate them, and, depending on that decision, where materials could be acquired expediently.

Excavation of the material above and alongside the pier 13 footing shows shear cracks (seen here near the end of the tape measure).

Excavation of the material above and alongside the pier 13 footing shows shear cracks (seen here near the end of the tape measure).

Those discussions led to the decision that the most cost-effective and time-sensitive method would be to repair the structure. The critical issue for the repair alternative was the availability of fabricated materials, equipment, and manpower. Regarding the manpower issue, contractors immediately made staff available to start the drilling for new foundation shafts.

A key suggestion from the team was to contact other projects around the country, such as the Tappan Zee Bridge project in New York, to see whether any of that material was not being used and was available to be procured. In fact, DelDOT was able to procure equipment and materials from many agencies across the country.

“The response to the request for help was fantastic,” says Dan Montag, FHWA’s Delaware Division senior area engineer for the emergency project.

Reinforcing steel cages for the drilled shafts came from the Tappan Zee Bridge project, saving an estimated 10 to 12 weeks off the schedule. Steel casings used for the drilled shafts arrived from Oklahoma and Washington State. Drilling rigs, along with an efficient contractor for the drilled shafts, became available on short notice from New Jersey and Texas. These rigs were especially important because of the low working clearance under the existing bridge superstructure and the need to drill shafts up to 162 feet (50 meters) deep.

The work plan had two main phases. First, crews would make temporary repairs to stabilize the bridge by building new foundations for piers 12 and 13 and underpinning the foundations of piers 11 and 14, all of which would allow the opening of the bridge. Second, crews would complete the permanent solutions by demolishing and replacing piers 12 and 13, and reconstructing the girder bearings for the other piers. The goal was to open the bridge to traffic in one direction at a time: first southbound and then northbound.

Officials with New York’s Tappan Zee Bridge project provided DelDOT with these reinforcing steel cages for the drilled shafts.

Officials with New York’s Tappan Zee Bridge project provided DelDOT with these reinforcing steel cages for the drilled shafts.

Due to the urgency to reopen I–495, all work took place around the clock. That included borings, corings, survey, design, materials delivery, and drilling. Two DelDOT assistant directors, Barry Benton and Javier Torrijos, led the efforts of the team and made sure that coordination ran smoothly.

A first order of work was to provide additional safety and stability by tying the existing concrete pier hammerheads together, allowing them to act as a single unit. The team installed strain gauges on these ties to monitor any movement. The engineers also installed inclinometers and piezometers to monitor soil movements and pressures.

Coordination between DelDOT’s design consultant and contractor was similar to a construction manager/general contractor method of project delivery. The consultant worked directly with the contractor to develop the design solutions.

Temporary Phase Schedule

Activity

Location

Date Complete

Drilled shafts began on 6/13/14

All

7/16/2014

Underpinning

Southbound

7/26/2014

Northbound

8/5/2014

Concrete grade beams

Southbound

7/8/2014

Northbound

7/25/2014

Temporary jacking towers erected

Southbound

7/22/2014

Northbound

8/5/2014

Jacking operations

Southbound

7/29/2014

Northbound

8/20/2014

Open to traffic

Southbound

7/31/2014

Northbound

8/23/2014

One of the early safety measures was to tie together the hammerhead piers to make the two piers act as one.

One of the early safety measures was to tie together the hammerhead piers to make the two piers act as one.

The Temporary Repairs

Because the bridge did not tilt or settle uniformly, it needed to be brought back to its original alignment and cross section. Therefore, the first focus for the temporary repairs phase was to stabilize the bridge by supporting and realigning its 400-foot (122-meter) section. To accomplish this work, DelDOT’s contractor completed the operation in phases. The first phase was to concentrate on the bridge’s southbound section.

Proposed Temporary Repairs

Proposed Temporary Repairs

To provide stable support for the repairs to each of the four piers, DelDOT’s contractors constructed new concrete-filled drilled shafts down to bedrock.

Piers 11 and 14 were underpinned by encasing the existing foundations with the new shafts, relying on those new drilled shafts for support. Underpinning restored the load-carrying capacity of these piers.

For piers 12 and 13, the contractor constructed a reinforced concrete grade beam supported by the concrete drilled shafts, which extended to the outer reaches of the bridge.

Then, the contractor erected temporary jacking towers on these grade beams, which supported the bridge until completion of the permanent phase.

As mentioned, the contractor restored the bridge’s southbound superstructure to its original position through several intricate jacking operations vertically and laterally, including leveling. DelDOT completed a load test by running loaded dump trucks across the bridge and braking over the repaired area. Upon verification that the test was successful, the bridge was reopened. Once the southbound lanes were open, the focus shifted to the northbound section, and the same activities were completed.

Grade beams shown here were cast on top of drilled shafts.

Grade beams shown here were cast on top of drilled shafts.

As a load test, these six (two rows of three) loaded dump trucks are stopped on the completed, leveled superstructure, which is supported solely on the temporary towers.

As a load test, these six (two rows of three) loaded dump trucks are stopped on the completed, leveled superstructure, which is supported solely on the temporary towers.

To the delight of many motorists, I–495 southbound reopened to traffic on July 31, more than a month earlier than the original estimate of Labor Day. On August 23, I–495 northbound reopened. After opening the bridge to traffic, the contractor and design consultant were able to throttle down from the 24/7 schedule to more regular shifts. They completed the work for the removal and replacement of piers 12 and 13 under the bridge with little impact on the motoring public.

The Permanent Phase

After DelDOT stabilized the bridge with the temporary towers and reopened it, the crews demolished the existing pier 12 and 13 columns and hammerheads, and erected permanent new concrete columns on the grade beams. The drilled shafts constructed for the temporary towers were also used for the support of the new piers.

The designers altered the configuration for the reconstructed piers. Instead of two hammerhead piers supporting each direction, a single pier supported by three columns would carry both directions of I–495.

The crews performed additional work to reconstruct the girder bearings at piers 11 and 14. Fortunately, there was no damage to the superstructures due to the movement of the bridge.

The agency completed the permanent piers and bearing work in April 2015.

The Costs

Total cost for all of the repairs was close to $40 million.

As mentioned, Federal emergency relief funding was authorized within days of the incident. As with all emergency relief funds, the temporary repairs during the first 180 days were 100 percent reimbursable. Federal funds for permanent repairs were provided at the normal 90/10 Federal/State interstate share.

Traffic Mitigation

DelDOT mitigated the traffic situation by providing signed detour routes with adjusted signal timing. I–495 was closed between Terminal Avenue and 12th Street, which are the closest exits from each end of the bridge. The detour routed vehicles through Wilmington. DelDOT hired Wilmington police officers to direct traffic during peak hours. The agency also installed temporary signals at the exit to Terminal Avenue. For through traffic, DelDOT re-striped I–95 to create three lanes southbound from Wilmington. Lastly, the agency suspended all construction projects that impacted the detour routes.

Shown here is the jacking and leveling operation, taking place at night.

Shown here is the jacking and leveling operation, taking place at night.

Outreach to Other FHWA Division Offices And the Public

FHWA emailed an event report on June 16 to all of its divisions notifying them of the issue and recommending that they alert their State transportation counterparts to review their bridges for any similar issues. A few States found similar situations and requested the owners move the soil stockpiles.

To provide updates to the public, DelDOT held several press conferences, both onsite and offsite when conditions required. The first was the day after the closure to explain what was known at that time.

Review of Other DelDOT Bridge Sites

DelDOT reviewed the agency’s entire bridge inventory, searching for similar bridge designs in similar soil conditions and similar stockpiling uses. Field inspectors checked 29 bridges over 500 feet (152 meters) in length and 34 bridges 200 feet (61 meters) to 500 feet (152 meters) in length.

The inspectors found only one structure with a stockpile of soil located adjacent to the bridge. The owner moved the stockpile expeditiously.

As a result of this experience, DelDOT updated its bridge inspection protocols and also increased its level of comfort in the use of monitoring technology.

Lessons Learned from a Tilting Bridge

  • Monitor any stockpile of materials on adjacent properties, even if not on the State’s right-of-way.
  • React immediately to any notice of concern from the public.
  • Develop an emergency relief skeleton contract for use during future events.
  • Remember that normal contracting and contract administration methods do not always work during an emergency event.
  • Realize that construction manager/general contractor methods of delivery might be especially useful, as this arrangement is a partnership working toward an end goal.
  • Do not hesitate to ask other States for help.
  • Direct all media requests through one outlet to ensure a consistent message.
  • In emergency situations, everyone works 24/7.
  • Deliver daily reports to leadership to resolve any issues before they become critical path items.

Visitors to the Site

Being located along a major north/south interstate route, the project attracted a great deal of attention and support, including the following:

  • Then Delaware Secretary of Transportation Shailen Bhatt was on site immediately, continued to visit throughout the entire process, and attended planning meetings.
  • Governor Markell toured the site on June 5.
  • U.S. Secretary of Transportation Anthony Foxx showed his support and visited the site on June 13, along with Delaware’s U.S. Senators Tom Carper and Chris Coons and U.S. Congressman John Carney.
  • President Barack Obama visited the site on July 17, along with members of the Cabinet, Secretary Foxx, and U.S. Treasury Secretary Jacob Lew.

During the ordeal, DelDOT’s Benton noted that one of the youngsters on his baseball team had heard a saying that truly applied to the bridge situation: “Everything will be okay in the end. If it’s not okay, it’s not the end.”


Dennis M. O’Shea is the bridge engineer in the FHWA Delaware Division Office. He joined FHWA in 2010, and until 2015 was responsible for the bridge programs for Delaware and Maryland in the former DelMar Division. Prior to joining FHWA, O’Shea worked for 27 years for DelDOT in many capacities, from bridge designer to assistant director responsible for several areas, including bridge design. He holds a B.S. in civil engineering from the University of South Alabama and is a professional engineer in Delaware and Pennsylvania.

For more information, contact Dennis M. O’Shea at 302–734–3609 or dennis.o’shea@dot.gov.

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