Effect of Water–Rock Interaction on the Axial Capacity of Drilled Caissons Socketed in Claystone Bedrock

A large number of bridges and structures in Colorado are supported by drilled caissons and shafts socketed in weak clay-bearing rocks (e.g., Denver blue claystone and Pierre shale). During the construction of caissons using the wet drilling technique, drilling fluid gets in direct contact with the bedrock, potentially causing their slaking and degradation and, thus, undermining the foundation capacity. This paper evaluates the effect of water infiltration on the axial capacity of caissons embedded in claystone. The methodology of this research combines laboratory testing and coupled finite-element analysis for unsaturated porous materials. A series of experiments were conducted to determine the hydraulic characteristics and the strength-suction relations of the claystone samples obtained from three sites near Denver, Colorado. These parameters were then used in numerical studies of wet drilled caissons subjected to different wetting times (i.e., the time interval between the completion of drilling and the placement of concrete). Adopting the coupled unsaturated flow and mechanical analysis in ABAQUS, a workflow was developed for modeling the full process of drilling, fluid infiltration, caisson construction, and in situ axial loadings. The results showed that among the three sites, the immediate caisson capacity reduction was the highest ($\approx 19\%$) for the most permeable and water-sensitive formation. On the other hand, the capacity reduction was found to be negligible ($\approx 0\%$) if the rock was relatively intact with low permeability, regardless of its strength-suction characteristics. Parametric studies also showed that bedrocks with a lower initial degree of saturation experience more rapid and significant strength reduction upon wetting. These results provide the basis to establish practical guidance to evaluate and minimize the impact of wetting time on caisson constructions.