Engineering Analysis of Swelling Claystone in Tunnel Design

ABSTRACT: This paper focuses on the engineering analyses of the mechanisms for swelling claystone and the impacts to design and construction of underground structures. The paper examines stress and time dependency, based on available laboratory testing and in situ monitoring data. This study also provides design recommendations for the underground structures that are constructed in swelling claystone. Laboratory tests were conducted to evaluate the swelling potential and swelling pressure based on modified free swell test, semi-confined swell test, and null swell test as well as ASTM D4546 methods. The laboratory testing results provide critical swelling parameters that include swelling strain at varying pressures, null swell pressures, and related swell potential parameters, which are essential for tunnel support design in swelling rock. Rock support design in swelling rock requires consideration of construction implications and appropriate sequencing as well as an understanding of the time and stress dependency of swelling, to manage swelling-induced stress and deformation in underground construction. A swelling model, based on Grob's law, was implemented in numerical analysis with staged construction to estimate swelling-induced design loads for underground structures. Design considerations and recommendations are provided from a practical perspective for underground construction in swelling rock based on the analysis results that were used in the design.