The Birmingham Bridge spanning the Monongahela River is a critical link in the Pittsburgh area transportation system. When two spans of the bridge dropped up to eight inches overnight, close cooperation between PennDOT, general contractor Trumbull Corporation, geotechnical consultant HDR Engineering Inc. and Moretrench was key to timely completion of the subsequent bridge repair.
Initial investigation revealed that the steel rocker bearings at Pier 10S had over-rotated and failed. The top of the pier was also horizontally displaced by nine inches and the bottom of the pier had visibly cracked. Bearings at Pier 10N were in a similar condition although the pier itself was not visibly damaged. Concurrent with emergency formwork by Trumbull to temporarily shore the bridge structure, an in-depth investigation of both piers was conducted by HDR Engineering. Although the underlying cause for the rocker failure proved inconclusive, the investigation indicated that the existing pile foundations terminated above top of rock elevations at both locations. The calculated theoretical Factor of Safety for service load was less than one. Moretrench was contacted by Trumbull to provide deep foundation alternatives for support of a completely new pier unit at Pier 10S and for Pier 10N retrofit. Moretrench design-build specialists developed a number of constructability-based engineering options and presented their findings at an emergency meeting with Trumbull, HDR and PennDOT Regional and Central Office personnel.
Micropiles Deemed Best Option
Several foundation options were initially considered including deep caissons, driven piles and micropiles. However, the work would have to be accomplished within a narrow space available between the temporary formwork, the bridge superstructure and the overhead girders. This precluded large caisson drilling equipment. Driven piles were also rejected due to both the overhead limitation and concern that vibrations would impact the adjacent Pier 10N. A micropile system, installed by means of low-headroom, track-mounted hydraulic drilling equipment, was therefore deemed the most feasible solution. Micropiles could be installed through the concrete footing and without affecting the existing steel H-piles. Drill cutting and spoil removal, also a concern, would be minimized. Moretrench was ultimately awarded the contract for the foundation work.
Pier 10S Replacement
The preliminary geotechnical design developed by Moretrench for Pier 10S concluded that the 200-kip pile capacity needed would be achieved through using a 7.625 OD casing socketed a minimum of 14 feet into rock, with 75 ksi threadbar reinforcement for the full length of the rock socket and the bottom 9 feet of the micropile. To verify these design assumptions, a sacrificial pile was installed and compression load-tested to 200% of design capacity. Results showed a maximum pile settlement of 1.1 inches, well within acceptable limits.
Production micropiling was accomplished using rotary percussive duplex drilling techniques to advance the micropiles through the footing and overburden and into rock for a completed micropile length of 84 feet. The centralized reinforcing bar was lowered and 4,000 psi non-shrink grout was tremied to complete pile installation. Thirty-three micropiles were installed between the existing H-piles for the new Pier 10S foundation.
Pier 10N Retrofit
Following the successful Pier 10S load test, PennDOT had elected to re-test the same sacrificial pile to 250% of a design load increased to 287 kips. Recorded settlement of 2.1 inches at maximum design load led to a minor change in the Pier 10N micropile design to increase capacity to 287 kips. This allowed retrofitting to be accomplished with just 22 micropiles. Since the pier itself was intact, micropiles were installed outside of the columns of the existing bridge crash wall to bolster the bearing capacity of the pier. Load transfer was achieved by means of coring and doweling additional reinforcing bars into the existing crash wall.