Tunnels in squeezing clay-rich rocks

Abstract

Squeezing ground conditions, which can lead to severe loads in tunnels, have historically been associated with the presence of clay minerals in the ground. Over the years, many methodologies have been proposed to predict squeezing in tunnels based on tunnel depth, in situ stress, ground mineralogy, and ground strength and deformation behavior. This paper presents a comprehensive review of methodologies to predict tunnel squeezing in clay-rich rocks. A new methodology is proposed where ground conditions and squeezing potential are assessed based on the Stress History and Normalized Soil Engineering Properties (SHANSEP) approach adapted to clay-rich rocks, Peck’s stability number and Hoek & Brown’s (1997) Geological Strength Index (GSI). A squeezing number S is suggested to classify ground conditions based on the level of squeezing that the ground may experience in response to tunneling. Finally, it is demonstrated that by combining the proposed classification system and an existing classification system for ground squeezing condition, an accurate estimate of tunnel strain can also be obtained. The proposed method is applied to four case studies of tunnels in squeezing ground in shale and mudstone.