2017 Volume 5 Issue 1 Pages 101-112
During the 2011 Off the Pacific Coast of Tohoku Earthquake, a great number of earth structures suffered widespread damage. It took a significantly long time to repair, restore, or reconstruct many of them, and thus the relief operation activities and the rehabilitation works to be conducted at the sites and in the nearby areas were severely obstructed owing to the loss of their function.
In this paper, a severe collapse case history of earth-fill dams for an irrigation reservoir is described as a typical and representative case. Based on the lessons learned, a proposal is made for the revision of their design policy, particularly in terms of soil compaction control and issues related to seismic design and construction of earth structures to satisfy the required seismic performance. Its application to the restoration works of the collapsed earth-fill dams is also preliminary reported.
It is shown that the old dams collapsed due mainly to their generally poor compacted state and because their top fill consisted of particularly poor compacted sandy soil. The relevant and realistic design should be based on the following procedures: a) use of Level 2 design seismic load relevant to a given earth structure; b) use of realistic soil shear strength, corresponding to the average of actual values of the degree of compaction, which is noticeably higher than the given allowable lower bound; and c) evaluation of the stability by comparing the calculated residual displacement/deformation of a given earth structure with the allowable limit specified in the design. The results of the compaction control at the early stage of the restoration work for the new dams show that the newly proposed method that controls the water content of the backfill to be compacted and the dry density and the degree of saturation of compacted soil works very well.
