Xe Nammov dam failure in Laos was caused by piping and percolation

On July 2018, the Xe Nammoy saddle dam (Laos) collapsed and devastatedseveral villages. Initially, local authorities blamed an extreme precipitation event and climate change for the collapsed, arguing that the saddle dam (under construction) was overtopped by the extreme event. However, a recent study based on site visit, gran size analysis and numerical simulations show that was not the case. 

Analysing the dam failure

The failure was structural, involving additional phenomemom such as piping and rotational slumping. In this post we present a summary.

A first analysis based on satellite altimetry and cumulative precipitation suggests that at the failure time the water level was about 17 m below the dam crest.

A site visit, and an analysis of photos taken after the failure show that portions of the dam on either side of the breach exhibit evidence of rotational slumping through the dam’s core. This suggests that the pore pressure within the dam might have been high enough to initiate the slumping and implies the dam’s core was not dry.

An analysis of grain size samples shows the material and soils are dominantly heterogeneous, porous, friable and massive. Thus, it is likely the gran-size analysis might have been skewed towards higher percentages of clay; hence, underestimating the permeability. Such assumption was supported by performing two different analysis based on two different methods. The analysis provided different results.

The dam did not fail because of an extreme precipitation and overtopping. The failure involved percolation of the water into the earthen dam and slumping.

More details are provided in the article.