Challenges and Uncertainties Relating to Open Caissons-Article

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The paper examines construction challenges encountered in two open caissons in Egypt and offers practical guidance for their design and sinking.<br /><br />In the first case, a 20 m diameter sewage-pumping caisson stalled in unexpectedly strong limestone and sandstone formations. Initial ground investigations had inadequately characterized the subsurface, leading to an unsuitable construction plan. After conventional excavation and sinking methods failed, contractors installed an external slurry trench and an internal trench, then used compressed-air jetting to reduce resistance and advance the caisson. The caisson eventually sank, but the underwater concrete seal failed because debris and soft deposits had not been properly removed and the concrete was poorly placed. A cutoff wall and extensive dewatering were needed to replace the seal, doubling costs and extending construction to about five times the planned duration.<br /><br />In the second case, a 10 m diameter caisson became stuck in very dense sand. Slurry piles and jetting were used to reduce wall friction, but uneven air jetting caused the caisson to tilt by 4%. Work had to stop until the tilt was corrected.<br /><br />The authors emphasize that reliable site investigation and accurate interpretation of soil conditions are essential when selecting a construction method. Sinking caissons in dense or very dense sands is risky because friction and tip resistance can prevent movement. Jetting may help, but work must be symmetrical, and any tilt should be corrected before sinking resumes. The excavation bed must also be cleaned thoroughly, and underwater concrete placed correctly to ensure an effective seal. Careful monitoring and a flexible, observational approach can help identify problems early and reduce risks to nearby structures, project schedules, and budgets.

Keywords

open caisson

construction challenges

Egypt

sinking methods

limestone

sandstone

compressed-air jetting

underwater concrete seal

dense sand

site investigation