Paul B. Fossier, Louisiana Department of Transportation and Development
IIn the early morning hours of August 29, 2005, Hurricane Katrina made landfall south of Buras, Louisiana, as a Category 4 hurricane. By 9:00 a.m., the eye of the hurricane was just east of New Orleans with wind speeds reaching 130 mph (210 km/h) and high storm surges reaching into Lake Borgne and Lake Pontchartrain.
The I-10 twin span bridge crosses Lake Pontchartrain northeast of New Orleans near Slidell, Louisiana. The I-10 bridge is 5.4 miles (8.7 km) long and is one of the few major routes in and out of the New Orleans metropolitan area. The twin bridges were constructed in 1963. Each bridge consists of 433 low-level simple spans and three high-level spans. The low-level 65-ft (19.8-m) long spans consist of AASHTO Type III precast, prestressed concrete girders that were connected with a 6.5-in. (165-mm) thick concrete deck before erection. The highlevel portion of the bridge consists of two 100-ft (30.5-m) long and one 200-ft (70-m) long composite steel plate girder spans.
The substructure for the entire bridge consists of a cast-in-place reinforced concrete cap supported by either a single or double row of 54-in. (1.37-m) diameter precast, prestressed concrete cylinder piles. The finished grade elevation for the lowlevel spans was constructed approximately 12 ft (3.7 m) above mean high water elevation.
Emergency Repairs
The Louisiana Department of Transportation and Development (LA DOTD) staff inspected both bridges the day after the hurricane and found major damage that led to immediate closure of the structure. Storm surges from Hurricane Katrina damaged both east and west low-level bridges by misaligning 170 eastbound spans and 303 westbound spans. Thirty-eight eastbound spans and 26 westbound spans were displaced into Lake Pontchartrain. Due to the storm surge on the back side of the reinforced concrete bridge rails, 13,910 ft ((4.24 km) of railing failed on the westbound structure and 130 ft (40 m) on the eastbound structure. Approximately nine prestressed concrete cylinder piles were damaged when the superstructure was displaced into the lake and struck the cylinder piles. Most of the high-level approaches and the main navigational span were not damaged due to the increased bridge elevation above the storm surge.
The LA DOTD initiated an accelerated repair plan development so that competitive bids could be accepted to repair the twin bridges in a short time frame. An in-depth structural inspection confirmed that all the misaligned spans were in good condition and could be reused. However, all displaced spans that were located in the lake were required to be replaced. Since the eastbound bridge had fewer displaced spans, it was decided, during Phase I of the repairs, to realign all of the 170 eastbound spans and to replace the damaged eastbound spans by taking spans from the westbound bridge. The eastbound bridge then could be opened to two-way, single lane traffic in a short time period to reestablish traffic flow to New Orleans. Phase 2 of the repair contract consisted of realignment of the remaining westbound spans and replacement of the missing westbound spans with temporary steel truss bridges. Both phases also required the replacement of damaged bearing pads, repair of damaged cylinder piles, and repair of barrier rails on the eastbound bridge. Due to the amount of damage on the existing barrier rails on the westbound bridge, temporary precast concrete barrier rails stored in the LA DOTD District maintenance facilities were used as a temporary solution for the damaged bridge rails.
The spans that were realigned on both the eastbound and westbound bridges used a combination of barge and self-propelled modular transporters to perform the span alignment work. The same system was also used to remove, transport, and erect the selected westbound spans to replace the displaced eastbound spans.
Accelerated Replacement Plans
The LA DOTD has also completed the design and planning to replace both bridges because of concerns with future hurricane storm surges. The new twin three-lane bridges will be built on an offset parallel alignment to the existing repaired bridge and will provide a long service life and better ability to withstand storm surge and vessel collision loads.
A bridge design using a cast-in-place concrete deck on BT-78 precast, prestressed high performance concrete (HPC) beams was prepared in-house by the LA DOTD. A precast concrete segmental design alternate using span-by-span erection was prepared by the LA DOTD’s consultant to encourage competition among contractors and suppliers. To meet longer service life and structural efficiency requirements for the bridge structure, HPC was specified for both alternates. For permeability, a value no greater than 1000 coulombs was required. A minimum concrete strength of 8,500 psi (58.6 MPa) was also required for the BT-78 precast, prestressed concrete girders for span efficiency. Precast, prestressed concrete driven piles with either cast-in-place or precast caps will be used for the substructure of both alternates. Two bids were received on April 12, 2006 for the bulb-tee alternate with an apparent low bid of $379 million for the first phase. No bids were received for the precast segmental alternate.