The aim of this study was to investigate the arching phenomenon in granular assembly. A series of two-dimensional trapdoor tests were conducted using model particles coated with a thin photoelastic sheet. Digital photo images of the model particle assembly were continuously taken during the tests, and the force of each model particle was quantitatively evaluated using a digital RGB photoelastic analysis. The test results showed that two arch structures were initially generated with trapdoor yielding, and the inner arch gradually expanded to the outer arch, while the outer arch became more solid. Given that the force per particle in the arch structure did not change significantly during the arch formation process, we concluded that the arch formation process progressed through the transmission of the force chains rather than by a change in the magnitude of the particle force. (C) 2020 Elsevier B.V. All rights reserved.