By using aluminum rods for simulating an granular medium in plain strain, a series of model twin trap-door tests has been performed with the main objective of studying the local yielding problem in granular medium. Especially, in order to observe the interference phenomenon between two local yielding, two trap-doors installed side by side was studied. During the tests, the pressure on the trap-doors was measured, and the surface settlement distribution of the medium were photographically recorded. From the results of various tests, the pressure on the trap-doors was found to be expressed as pressure range curves similar to that of Janssen's. Some trends on the volume change characteristics of the medium including the interference between two local yieldings are also clarified.
