Annular Monoblock Bridge Foundations-Paper

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The paper presents annular monoblock foundations as an alternative to conventional pile-and-cap foundations for bridge piers in riverbeds. The system is formed by intersecting drilled piles arranged in a circle around the pier axis, creating a large, unified annular foundation. A precast, self-sinking caisson protects the foundation through the depth vulnerable to erosion.<br /><br />Unlike conventional construction, which may require deep excavations, temporary riverbed supports and work below water level, the annular method is built from ground level. The caisson also guides pile placement, protects the structural piles from water and debris, and helps preserve the riverbed’s natural surface and protective soil layer. The continuous pier-to-foundation profile reduces changes in hydraulic obstruction as local scour develops. Structurally, the intersecting piles act together as a monolithic element, limiting group effects and carrying vertical, horizontal and bending loads.<br /><br />The approach was used for the new 665-metre Torre River bridge in Italy. Each pier foundation comprised 12 intersecting piles, 0.75 metres in diameter and nearly 30 metres long. Its estimated capacity was about 25,000 kN in compression and 15,000 kN in tension, with a breaking bending moment of about 28,000 kNm.<br /><br />For this project, the annular solution cost approximately 25% less than the tender design using pile caps and reduced the expected construction time to about one-third. The authors conclude that the method can provide safe, efficient and economical foundations for bridges over waterways, including in challenging ground conditions. Its pile arrangement and bearing-capacity assessment can be tailored to project requirements.

Keywords

annular monoblock foundations

bridge piers

riverbed foundations

intersecting drilled piles

precast caisson

scour protection

pile-and-cap alternative

Torre River bridge

bearing capacity

economical bridge foundation