## 5 Discussion  
The research problem addressed in the paper is how to improve deformation prediction in high CFRD dams using data available at the design stage that include parameters of rockfill material to be used in construction and the results of standard laboratory tests, trying to minimize the impact of scale effect and breakage phenomena in the predictions. This approach also tries also to avoid the use of parameters taken from back analysis of similar dams, as their applicability to a particular project is always subjected to an important degree of uncertainty. The model has been applied to the Shuibuya CFRD dam, 233 m high, which is the highest dam of this type in the world. According to the literature, the maximum settlement observed was 2.5 m, while the prediction during design was 1.9 m, thus having a difference of 0.6 m. The VEPP model proposed, using data from laboratory test that were available in the design stage, predicts a maximum deformation of 2.25 m, therefore having a smaller difference of 0.25 m, which is a significant improvement in prediction capability. The VEPP model reproduced reasonably well the observed behavior of the dam through all stages of dam construction, through all stages of reservoir impoundment, and during the first two years of operation. The model addresses the scale effect problem using a plastic yield surface that incorporates actual rockfill grain size as a parameter rather than a plastic yield surface, with its parameters estimated from triaxial tests (with smaller grain size for rockfill). Another key aspect controlling the deformation in high rockfill dams is the porosity. The capability of the model to account for variations in porosity between laboratory tests and actual target dam porosities, as in Shuibuya case, provides flexibility to improve the simulations of different zones in CFRDs, with good results as observed in the case study. In Figure 6 the effect of the variation of elastic modulus with porosity for zones 3B/3D and 3C of Shuibuya Dam is shown. Curves showing the variation of elastic modulus with porosity for rockfill of different qualities found in the literature are also given for reference [8]. The predictions of the VEPP model have been compared with the predictions of an alternative model based on the critical energy breakage concept [17]. From the comparison of the predictions of both models with the observed deformations, it can be seen that the VEPP model appears as an attractive alternative to more complex methods to predict the deformation of high CFRDs in the early stages of design. In Figure 7, for three relevant milestones of the dam (construction completion, impounding at full supply level, and operation), the results obtained by the VEPP model were compared with those obtained by [17], adopting as a base for the comparison the measured settlements. In particular, the numbers inside brackets represent the difference between a measured value and a calculated one: VEPP almost always outperformed the prediction by alternative models. Moreover, in Table 4, the RMSE (Root Mean Square Error) calculated at the same three moments in time is reported: in all cases, the result obtained with the VEPP was better than that obtained with the model proposed in [17], especially for the impounding and the operation for which the differences are more relevant. These outcomes are even more valuable considering that the input parameters for the VEPP come from laboratory tests on the rockfill, and the only parameter that refers to the full scale condition is the porosity after compaction. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Phase** & **Date** & **VEPP (RMSE)** & **Zhou, Ma, Zhang 2019 (RMSE)** \\ \hline Construction & October 2006 & 0.36 & 0.37 \\ Impounding & November 2008 & 0.16 & 0.42 \\ Operation & July 2009 & 0.59 & 0.81 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Comparison between VEPP model and [17] adopting RMSE.