Foundation Failure!


    On June 2, 2014, the 38-span dual bridges supporting I-495 over the Christina River in Wilmington, Del., were found to be structurally unsound. The bridges were closed to traffic, an emergency declaration was prepared, and a rehabilitation project, including both short- and long-term repairs, was immediately undertaken. Since the bridges carry more than 90,000 motorists a day, time was a critical factor in implementing repairs.

    The Delaware Department of Transportation (DelDOT) retained AECOM as the lead design consultant. Its role encompassed project management, geotechnical engineering, structural and bridge engineering, civil engineering, scheduling, construction services, and construction inspection. The firm partnered with the I-495 Emergency Project Team — DelDOT, FHWA, and J.D. Eckman — to repair the bridge as quickly as possible.

    The observed bridge deformations were quickly attributed to significant lateral forces that developed in a 100-foot layer of soft compressible soil underlying the bridge site. These large forces, which were mobilized by the placement of embankment fills immediately adjacent to the bridge, overwhelmed the existing H-pile foundations of four sets of hammerhead piers. The result was gross deformation of the superstructure and insufficient load carrying capacity.

    Working around the clock, the team restored the southbound structure in 59 days and full traffic service returned in 83 days, one month ahead of schedule.

    Obstacles encountered

    Selection and construction of the new foundations was particularly challenging. Since the bridge was precariously supported on the existing damaged piles, construction of the new foundations required a delicate balance to minimize vibrations and avoid subsurface interference between old and new foundation elements. It was also recognized that the new foundations had to be robust and essentially impervious to the geotechnical conditions that afflicted the existing H-piles.

    Additionally, due to the limited available space for new foundations, as well as cost and schedule considerations, it was determined that the new foundations had to serve dual purposes as temporary and permanent supports. The complexity of the foundation construction was further increased by the need to work in the restricted headroom below the existing superstructure. The low headroom nullified many standard foundation applications. The project team recognized the benefits of the drilled shaft foundations and was able to identify innovative means to install the shafts in a timely manner.

    Modeling the deformed superstructure was required to accurately predict loads — vertical, transverse, and longitudinal — present in the bridge. It was essential to comprehend those loads in order to safely release the reactions and transition the loads from the damaged piers to the temporary shoring towers. A rigorous analysis was conducted based on field surveys of the bridge and an understanding of its deformed shape. Several iterations and model improvements produced quality results that were eventually confirmed during the jacking operations.

    Innovative techniques applied

    The project entailed application of many innovative concepts to increase the efficiency of design, hasten construction, and ensure safety in the workplace.

    Pier ties —

    To stabilize the damaged hammer-head piers and secure the adjacent areas for drilled shaft excavation, the piers were rapidly tied together to create an improvised frame. Post-tensioning bars were connected between the northbound and southbound piers to provide a tension resistance component of the improvised frame. H-pile sections were installed in the gap between the cantilevered ends of the piers to produce a compression resistance component. The frame action provided by the combination of the post-tensioning bars and the H-pile sections offered stability to the individual damaged piers.

    Shoring towers were used to correct the superstructure geometry and support bridge loads on a temporary basis. Photo: Courtesy of AECOM

    Drilled shaft installation —

    Of the 32 new drilled shafts, 24 are positioned below the existing bridge superstructure. The reduced overhead clearance at these points (less than 60 feet) would ordinarily require installation of the shaft reinforcement in segments that are spliced together while suspended above/within the excavated hole. To increase the efficiency of the operation, 6-foot square holes were cut in the bridge deck over the drilled shaft locations. The entire length of the reinforcement cage (140 to 165 feet) for each drilled shaft was fabricated adjacent to the bridge and lifted through the deck opening and directly into the excavated drilled shaft. This approach saved critical construction time — eight to 12 hours for each shaft — and improved workplace safety by fabricating complete reinforcement cages clear of the congested work area below the bridge.

    Underpinning —

    Although the foundations of Piers 11 and 14 were compromised, the measured tilt of these columns was less significant than that of Piers 12 and 13. Structural calculations indicated that the existing column shafts at Piers 11 and 14 possessed sufficient capacity to accommodate the eccentric loading as a permanent condition provided the foundation could be strengthened. Underpinning of Piers 11 and 14 allowed for preservation of four of the eight compromised hammerhead piers, thereby simplifying jacking operations and cutting in half the amount of demolition and new pier construction needed.

    Four new drilled shafts were placed around the compromised foundation of each hammer-head pier and a new pile cap placed above the drilled shafts and the existing pile cap. The new, 8-foot-thick pile cap engaged the existing pier footings through the installation of vertical posttensioning bars. The moment connection to the existing pier was further enhanced through shear bond and keying between the existing column shafts and the new pile cap. Multiple-stage concrete placement of the new pile caps was employed to avoid surcharging the compromised pile foundations and to minimize the need for mass concrete temperature control. The underpinning approach minimized the time required for excavation, temporary excavation support, and de-watering needs when compared with typical underpinning methods.

    Temporary shoring towers atop grade beams were used to jack and support the bridge until new piers could be constructed. Photo: Courtesy of AECOM

    Superstructure modeling —

    Three-dimensional finite-element modeling was employed to predict the structural interaction between the continuous girder spans and the eight compromised hammerhead piers. Tilting and displacement magnitudes varied significantly from pier to pier, causing redistribution of moments and shears within the four-span continuous girders. Span continuity and the positive connections between girders and piers produced a complex structural system of force interactions. Survey data of the distorted spans was utilized to quantify principal axis reactions at each girder bearing.

    While the existing steel girders were constructed to be non-composite with the concrete deck, the superstructure was analyzed as both noncomposite and composite to predict actual behavior and produce an “envelope” (i.e., maximum and minimum range) of jacking forces.

    Cross frames were then analyzed and those cross frames with substandard capacities were strengthened to prevent buckling during jacking. The results of the analysis provided the basis for the sizing and configuration of jacking and temporary support equipment, as well as the sequencing of work to release forces between spans and piers in a safe, controlled manner.

    The new piers are constructed atop the grade beams and drilled shafts. Temporary falsework supports the formwork for the new pier cap during concrete placement.Photo: Courtesy of AECOM

    Bridge jacking —

    Piers 12 and 13 displayed the greatest deformation and, as a result, the superstructure at these locations possessed the largest deflections and cross section rotation. To raise the superstructure and return the bridge cross section to its original geometry, J.D. Eckman employed an innovative two-level jacking approach at Piers 12 and 13. For this jacking approach, a hydraulic jack was placed atop a transverse header beam and directly under each of the six bridge girders. These jacks were used during the initial jacking operation to lift the bridge from the existing bearings and transfer the load to the temporary shoring towers. A second pair of hydraulic jacks was positioned below the header beam and atop the temporary shoring towers. The location of these “tower” jacks allowed for rotation of the header beam and therefore restoration of the superstructure geometry.

    Resolving traffic and economic hardships

    The closure of I-495 imposed significant social and economic hardships on the greater Wilmington area. Traffic congestion, particularly on I-95 in Wilmington, increased substantially, slowing the flow of traffic and adversely affecting safety. Accidents in the area resulted in gridlock as alternative routes already exceeded capacity.

    Economically, the interstate closure affected shipments to and from the nearby Port of Wilmington. The delivery of goods and materials was delayed due to the extended detours required to circumvent the damaged bridge.

    The closure also had a serious impact on tourism throughout the state. During the summer months, I-495 supports the travel of beachgoers to the coastal areas of southern Delaware. Reopening of the bridge before the end of summer was a benefit to the beach towns and the state economy.

    The emergency repair project was of sufficient magnitude to attract the attention of President Obama, who visited the bridge and Port of Wilmington area in mid-July, when the initial phases of the temporary repairs were nearing completion. The president used the scene as a backdrop while calling for increased investment in the nation’s infrastructure and creation of new public and private partnerships to support that goal.

    The structural failure, by lateral deformation, of the existing pile groups had been attributed to significant strain induced in the surrounding compressible soils. This phenomenon, commonly referred to as “lateral squeeze,” has chiefly been an academic issue with little actual case history to draw upon. The I-495 Emergency Project, including the investigations conducted, the geotechnical data collected, and the repairs undertaken, provides invaluable information for future projects and serves notice of the catastrophic effects that can be produced in those soil conditions.

    Neil A. Shemo, P.E., is an associate vice president in the Mechanicsburg, Pa., office of AECOM.

    Harry R. Roecker, P.E., is a vice president in the company’s Philadelphia office.