Objectives

A large number of bridges and structures in Colorado are supported by drilled caissons and shafts embedded in weak fine-grained rocks (e.g. Denver blue claystone shale and Pierre Shale). Use of these foundations has been common for decades for highway and bridge projects in Colorado but some critical knowledge gaps remain in this class of foundation engineering problems. One of them is in the empirical stipulation of a 4-hour maximum duration between the completion of drilling and placement of concrete for caissons founded on cohesive intermediate geomaterials (IGM) in CDOT’s Drill Caisson Specification 503 regardless of in-situ scenarios. The motivation behind the requirement is correct: to minimize the deterioration of the bedrock due to the interaction between the drilling fluid and the rock matrix. On the other hand, no systematic laboratory or field-scale studies was apparently used to arrive the time limit which can be longer or shorter depending on the rock condition. This has created practical difficulties in decision-making in the past for both the drilled shaft contractor and CDOT. Many relevant variables such as the slake durability and the degree of weathering of the bedrock, the dimensions of the borehole, and the composition of the drilling fluid that may affect the concrete placement time limit in relation to the rock’s end bearing capacity are not considered in this specification nor in the design procedures for drilled caissons in weak rocks.

This project aims to fill this knowledge gap by developing a deeper understanding of the basic material aspects and correlations between the time of fluid infiltration with the two primary design inputs for drilled caissons, namely the unconfined compressive strength of the bedrock and the shear resistance of the rock-concrete interface. The property of the rock, the type of the drilling fluid, and other geometrical factors will be considered in the proposed correlations. The goal is to provide a benchmark experimental database for determining a reasonable allowable time for concrete placement, and to develop an easy-to-implement procedure for integrating the drill fluid weakening effect in the design and construction of drilled shaft socketed in fine-grained rocks in Colorado.