Strain softening is the mechanical behavior of the soil and rock materials and is important in understanding the soft rock foundation. To investigate the mechanical behavior of siltstone which is one of the sedimentary soft rocks, the consolidation tests using constant-strain rate loading were conducted using the consolidation ring to constrain the lateral deformation. Using the Quaternary siltstones distributed in the Boso Peninsula, central Japan as specimens, the strain-softening was confirmed in the consolidation process for some specimens by two test machines in Kyoto University and Nagoya Institute of Technology, respectively. Just before the yielding, stress decreased suddenly at increasing strain. The stress at the time of the softening were different even for specimens taken from the same formation. Furthermore, micro-focus X-ray CT images taken before and after the tests indicated that the specimen had no macro crack inside. It suggests that the strain-softening is not due to brittle failure in local areas, but due to the softening of the framework structure of the siltstone itself.
