The strength of cement treated soil by the Deep Mixing Method (Dry Jet Mixing Method) has increasedwith progress of the time. However, it is reported that the strength of cement treated soil surface parts has decreased according to a rate of square root of the period. Conversely, it is lack of information a long term properties of cement treated soil due to that the Deep Mixing Method of construction has been developed less than 40 years. The authors carried out the unconfined compression test, the needle penetration test, and the pH test for cement treated soil by DJM 30 years after construction. Here we report a long-term strength properties of cement treated soil and the deterioration condition of cement treated soil surface.
