Micropiles and Ground Anchors Increase the Capacity of Existing Bridge Abutments-Paper

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The Route 145 (Winthrop Avenue) bridge in Revere, Massachusetts, required replacement because increased traffic loads, deterioration, and updated design standards exceeded the capacity of its existing granite-block abutments. The abutments were retained and reinforced after investigations found they were 26.5–30 feet deep and 12–14 feet wide, with a 2.5-foot void beneath the eastern abutment. Bedrock was 95–120 feet below the roadway.<br /><br />Reinforcement comprised 20 micropiles and 41 tiebacks, drilled through the granite masonry. The micropiles were designed to carry the replacement bridge’s full load independently of the abutments. Verification tests demonstrated sufficient rock-socket capacity, allowing the specified socket length to be reduced from 20 to 10 feet. The two test piles performed successfully at loads up to 530 kips, and the transportation agency waived production-pile proof tests because of the test results and the difficulty of applying test-frame reactions without affecting the existing bridge.<br /><br />The tiebacks were installed at shallow angles and bonded in the granular fill behind the walls, a practical solution given site constraints and the need to limit added vertical loads. Each used two corrosion-protected prestressing strands and was secured to the granite wall through a steel load-transfer unit. Performance and proof tests met acceptance criteria; measured creep was well below the allowable limit.<br /><br />Construction involved challenging logistics, including nighttime micropile installation near traffic and tieback work beneath the bridge beside active commuter rail tracks. Drilling through loose, shifting granite blocks also caused difficulties. Despite these constraints, the micropiles and tiebacks performed successfully, and installation was completed on schedule.

Keywords

Route 145 bridge

Winthrop Avenue

Revere Massachusetts

granite-block abutments

micropiles

tiebacks

bridge replacement

rock-socket capacity

prestressing strands

traffic loads