This study evaluated the use of the Asaoka and hyperbolic methods to estimate the ultimate settlement of soft ground treated by vacuum preloading combined with prefabricated vertical drains. For this aim, a large-scale physical laboratory model was constructed. The model was a reinforced-tempered glass box containing a soil mass with dimensions of 2.0 × 1.0 × 1.2 m (length × width × depth). Physical models of this scale for the same purpose are rare in the literature. The soil was taken from a typical coastal region in Dinh Vu Hai Phong, Vietnam. The surface settlement near and between the two drains was measured right after the vacuum preloading started. Important properties of the soil were tested to evaluate the effectiveness of the treatment method. The measured settlement was used in the Asaoka and hyperbolic methods to predict the potential ultimate settlement. The results showed the superiority of the vacuum consolidation approach in improving fundamental engineering properties of soft soil. Furthermore, the ultimate settlement predicted by both methods showed a good agreement with the measured value, proving that the Asaoka and hyperbolic methods are suitable for the estimation of the ultimate settlement of soft soil treated with vacuum consolidation.

Lessons learned from dealing with earth dam incidents have made a significant contribution to surveying, designing, constructing, and repairing works. However, disseminating these lessons to the public must be done carefully. This study aims to analyze the causes and remedial measures for the downstream failure of the Trieu Thuong No. 2 earthen dam in Quang Tri province, Vietnam. To achieve this, the finite element software, MIDAS, was used for seepage and slope stability analysis. The simulation was based on design documents, past responses to dam incidents, and additional geological survey data. The results indicated that there were three major causes of instability in the dam: (1) the existence of a soft soil layer beneath the upstream slope foundation that was not recognized in the original design; (2) a difference in hydraulic conductivity between the original dam soil and the backfill; and (3) the absence of a drainage arrangement between the new and original layers of the dam. Therefore, proposed solutions include (1) reducing settlement by reinforcing the foundation beneath the downstream slope with soil-cement columns; (2) using soil backfill with a higher permeability coefficient than the existing dam to expand the dam crest; and (3) implementing a drainage system between the new and old layers of the dam. These solutions have been found to address the problem of downstream slope instability and can serve as a reference for future projects.