Prediction of driven pile resistances in shales considering weathering and time effects

Abstract

The difficulty with shale sampling and testing and the lack of pile load tests in shales have created challenges with shale classification, pile resistance prediction, and understanding time-dependent pile responses. Treating shale as soil can yield a conservative pile design. Shales were classified into soil-based and rock-based depending on their weathering conditions, mechanical properties, and measured pile resistances. Failure behaviors of soil- and rock-based shales are discussed. Prediction equations were developed to account for predicting shale properties. Properties of rock-based shales decrease with the increase in weathering. New static analysis (SA) methods were proposed to predict unit shaft resistance ( qs) and unit end bearing ( qb) of piles in shales and validated. Our study yields higher resistance and efficiency factors for the proposed SA methods and piles in shales than existing SA methods developed for piles in soils. The qs in the soil-based shale exhibits pile setup, while the qs in rock-based shales experience both setup and relaxation. The qb in both soil-based and highly weathered shales is likely to experience pile setup. However, the qb in the moderately and slightly weathered shales experience both setup and relaxation.