Open Well Caissons of A 1917 Railroad Bridge – How 19th Century Construction Can Influence 21st Century Construction-PDF

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The 2005–06 replacement of Amtrak’s Thames River bascule bridge in New London reused the existing foundations. During work, Pier 2 began tilting south, prompting an investigation into its 1917 construction and the early engineering methods used.<br /><br />Pier 2 rested on a deep open-well caisson: a timber structure with eight dredging wells and a concrete cutting shoe. The caisson was intended to reach elevation -140, but construction records indicate it stopped about ten feet higher. Workers excavated beneath it to lower it through soft sediments and into sand and gravel, while adding stone and concrete around the wells. They then filled the wells with underwater concrete placed through a tremie pipe.<br /><br />Historical reports describe difficulties keeping the caisson aligned, including a significant tilt during sinking and boulders obstructing excavation. After dewatering the first wells, builders found about eight feet of weak, cement-washed material, or laitance, atop the tremie concrete. They removed the laitance, but records do not indicate whether the concrete below was repaired. Later borings found that some wells contained mostly aggregate, while others had sound tremie concrete, suggesting the builders eventually improved their placement technique.<br /><br />The author concludes that incomplete or inconsistent concrete may have left the caisson’s cutting shoe as the principal load path. In 2006, drilled-pile work near Pier 2 apparently disturbed material beneath the shoe, restarting movement that had developed historically; Pier 2 had already settled and tilted before a major rehabilitation in 1980.<br /><br />The case shows how early construction decisions and emerging techniques can affect modern projects. It also emphasizes the value of historical records and careful investigation when working with century-old infrastructure.

Keywords

Amtrak Thames River bridge

New London

Pier 2

open-well caisson

timber caisson

tremie concrete

bascule bridge replacement

foundation reuse

caisson tilt

historic engineering