When foundation installation for a new bridge across the Intracoastal Waterway in Palm Beach County, FL, caused the timber foundations of the aging bridge it would replace to settle, a stabilization program was urgently needed to avoid having to close the bridge entirely until the new structure was completed.
The Flagler Memorial Bridge on State Highway A1A in Palm Beach County, FL connects Palm Beach Island to West Palm Beach. The bridge, which was opened in 1938, carries 16,500 vehicles daily. Following a decision by the State of Florida to replace the aging structure, work began on a new, 4-lane divided bridge in 2012. Soon thereafter, vibrations from installation of the drilled shaft foundations for the new bridge caused the 1938 structure to settle. To avoid closing the bridge entirely until the new structure was built, Florida DOT elected to stabilize the bridge by the installation of a micropile auxiliary foundation system.
The extremely fast-tracked emergency project carried a number of key challenges, not the least of which was that all equipment and materials needed to be barged in. Work hours were restricted to night-time, when vehicle traffic could be suspended to raise the drawbridge. The work had to be accomplished from floating deck barges in the active river channel which experienced strong currents and large tidal fluctuations. During the day, the deck barges had to be stored safely away from the main channel, meaning nightly re-positioning and drill rig set up.
The emergency repair design consisted of the installation of a total of 67, 60-ton capacity, 10-inch diameter micropiles to underpin the four main bridge piers. Under the working conditions, a critical aspect of the project was ensuring that the micropiles were installed within a very tight horizontal tolerance. Moretrench utilized a team of divers to direct the holes into a layout template at the mud line to accomplish this.
Micropiles were installed using rotary drilling with internal flush. For the south side of the bridge, which was closer to the foundation installation for the new bridge, micropiles were drilled to a depth of 110 feet. The remaining 34 micropiles were drilled to 85 feet. Micropile installation directly underneath the fixed portion of the bridge was another challenge, since the bridge span itself presented an overhead obstruction and necessitated using short mast drills and casing in 5-foot lengths rather than the 20-foot lengths possible on the channel piles and the north side if the bridge.
Prior to production work, two full scale load tests were conducted, with the test frame and reference beam entirely in the water. Despite the considerable difficulties the crews had to overcome, Moretrench successfully accomplished the micropile installation ahead of schedule.