Construction of Earth Dams on Soft Ground: Principles and Examples

In areas where river sediments are thick, construction of earth dams by excavating these sediments and replacing with compacted fill causes significant cost increases. River sediments consists of, in terms of soil mechanics, either granular (sand, gravel) or fine-grained (silt, clay) materials. Granular sediments can be improved with relative ease against instability and compressibility; however, because such deposits are highly permeable, additional measures are required to prevent seepage loss through the dam foundation. Fine-grained deposits, if they do not contain extensive sand or gravel lenses, offer fewer problems relative to seepage loss and may not require seepage control measures. However, such materials typically have low strength and high compressibility and present stability and settlement concerns. The objective of this article is to present examples of earth dams founded directly over fine-grained river sediments. Available methods for improvement of highly compressible, low strength deposits are limited. Staged construction (although it is time-consuming, it can be accelerated to reasonable levels with prefabricated vertical drains) and column-supported earth fill (although more expensive it enables accelerated construction) can be cited as two main approaches for this purpose. Because it is generally more economical, the design principles of staged construction and the relevant methods of analysis are presented herein. Alibey Dam for slow staged construction and Hamzadere dam (not yet constructed) for accelerated staged construction are cited as examples of this approach. The results of analyses and observations indicate that earth dams can be constructed directly on soft fine-grained river sediments with appropriate ground improvement.