Abstract: Cobblestone-soil is a natural geological situation consisting of cobblestones surrounded by soils. The mechanical performance of cobblestone-soil is between soil and rock mass and significant different from any component. Through the investigation in Chengdu Metro Line 1, the geological situation can result in a few non-normal damages (such as cutter wear, jam of roller cutterhead) and affects heavily the TBM performance. It is a typical mixed-face ground, which has an adverse influence on TBM tunnelling. This paper is intended to address the deformation problem of cobblestone-soil. Based on strain uniform assumption, analytic solutions were derived from the theoretical approach for equivalent elastic modulus while the cobblestone is assumed perfect circle and ellipse. The analytic solutions were validated by numerical modelling method. The results indicate that the equivalent elastic modulus derived from theoretical model and numerical is rather closely. The analytic solutions are helpful for improving the understanding of deformation for such ground condition and further to enhance TBM’s performance

Key words: tunnel boring machine (TBM), mixed-face ground, cobblestone-soil matrix, equivalent elastic modulus