It is generally known that cement treated clay (CTC) shows improved shear strength in comparison with clay but becomes ductile during shear. Recent research has demonstrated that the toughness of CTC is improved by adding shredded tire chips (cement treated clay with tire chips: CTCT). In the present research, several types of shear tests were conducted on CTCT, including the unconfined compression test, beam bending test, and punching shear test. This paper discusses the relationship between shear modes and the effect on the failure mechanism when tire chips are added to CTC. The test results showed that addition of tire chips improves the toughness of CTC in the unconfined compression shear mode and punching shear mode, but less improvement was seen in the bending shear mode, in which extensive cracks were observed. The difference in these results means that compressive deformation of tire chips is responsible for improvement in the toughness of CTC.