Predicting and ensuring uniform grout injection during open caisson construction

Open caisson shafts are widely used to create underground spaces. During construction, the caisson is sunk into the ground under its own weight, often aided by a bentonite-filled annulus to reduce friction with the surrounding soil. After sinking, cementitious grout is typically injected into the annulus to displace the bentonite and re-establish friction at the soil–structure interface. Non-uniform grout coverage, caused by viscous fingering (VF), poses a significant risk during this process. VF occurs when a less viscous fluid (grout) is injected into a more viscous fluid (bentonite), leading to an unstable displacement front and uneven distribution of the injected fluid. This study addresses this issue by developing a model using established VF principles and the Sisko rheological model to produce a novel closed-form equation, which is validated using data from tests conducted using a custom Hele-Shaw-type apparatus. The model relies on readily obtainable parameters, making it practical for on-site use, where grout injection is typically contractor-led. The developed model is shown to accurately predict the uniformity of final grout distribution, providing a basis for improved caisson design and construction.