2001 Volume 54 Issue 1 Pages 48-58
Between August 1999 and January 2000, we conducted a survey on the changes in the 33 existing traditionally designed Sabo structures that were built using natural materials. They are located in the Inuyama District, which is in an Aichi Prefecture forest owned by the University of Tokyo.
These Sabo facilities were constructed between 1920 and 1950 in order to prevent the formation of sediment and its discharge from barren ground. Their foundation is comprised of earth dams, and their spillways are covered with either stones, steel wire gabions, or cncrete. Although approximately 50 to 70 years has passed since the Sabo facilities were constructed, the ones that had their spillways covered with stones or concrete have not been damaged.
On the other hand, all but one of the 19 Sabo facilities that had used steel wire gabions to cover their spillways had not been damaged. The damages to the steel wire gabions were broadly categorized into 2 conditions: one, corrosion of the steel wires which caused the stone filling to collapse; and two, deformation or destruction of the gabions, which were caused by debris flows and other external forces. No relationship was found between the cause of the damage and the degree of corrosion of the steel wire in the second condition. However, in the first condition, the corrosion of the gabions was found mostly in three places-areas close to the ground, in the joints between the gabions, and the areas that were at water level. It was assumed that all areas had constant exposure to high humidity.
The wire gabions, which were 4.2 milimeters in diameter and made from galvanized wire, had completely corroded over a period of 40 or 50 years, resulting in the gabions losing their ability to contain the stone filling. However, in some cases, the fine grain sediment between the individual stones has stayed securely in place so that the shape of the top of the spillway has not deformed even though the steel wire has corroded over time.
A study of the nearby vegetation revealed that a stratified structure had formed in the area surrounding the steel wire gabions some 70 years after the completion of the Sabo. The tree layers of the stratified structure included red pines and Quercus serrata.
The above findings have confirmed that the Sabo facilities that were constructed using steel wire gabions are not suitable for passively resisting the movement of sediment. However, they are capable of functioning as temporary structures preventing the motion of surface sediment for a certain period of time. In this manner, they help in the recovery of vegetation, and temporarily prevent sediment deposits from moving any further.
