## 4 Discussion  
From the results of the tests in this paper, the methods of improving the loess can be generally divided into the following: the first is through the addition of other soils to improve their own grain size and gradation characteristics, and it will often be used in filled ground, landfill capping layers, and the upper layers of an artificial water body. The second method is achieved by means of additives such as cement and lime and is often used in situations where very low permeability coefficients are required. The third method combines both the first two methods by adding admixtures to the loess initially by improving the grain size and gradation characteristics. In this way, the requirements for impermeability can be met, and the requirements for green, low-carbon building materials can be better achieved at a cost saving.  
In the absence of admixtures, the permeability coefficient of the loess material [26] decreases with permeation time in the presence of backfiltration protection, and the permeability coefficient is a power function of time. In the absence of backfiltration protection, the material undergoes a continuous seepage deterioration process of particle transport, the leaching of binder, and particle re-transport under the effect of long-term infiltration. The deterioration process leads to a slow decay of the permeability coefficient due to the loss of calcium carbonate and other binders in the later stages of infiltration, which may result in a continuous loss of intergranular cohesion and a continuous separation of small particles from larger ones, further causing slow particle transport and the blocking of the pores.  
The effect of age on the improved loess material was insignificant when lime was the admixture. In the tests of [PERSON] [27], the permeability coefficients of the three ratios of lime/loess at different ages did not generally vary significantly, fluctuating by less than two at the same order of magnitude. Except for the 3:7 lime/loess ratio, which decreases nearly linearly with age, the trend in the remaining two ratios of lime/loess is not apparent. When the cement was the admixture, the effect of age was relatively significant, with the coefficient of permeability decreasing at a greater rate up to the age of 14 d and then at a slower rate after that. When improved loess materials are used as impermeable layers in landfills and artificial water-impermeable layers, the part of the permeability coefficient that is reduced by the deterioration mechanism or by the increase in the age of curing is usually used as a safety reserve in order to prevent leakage and further engineering accidents. Therefore, this paper does not consider the effect of age in the test design, and the permeability test is carried out after the sample has been prepared for one day of curing. Further experimental research is therefore required to rationalize the effect of age and to reflect it in the prediction model.  
\begin{table}
\begin{tabular}{c c c c} \hline
**Prediction Models** & **SVM Regression** & \
\begin{tabular}{c} **Multiple Linear** \\ **Regression** \\ \end{tabular} &
\begin{tabular}{c} **Neural Network** \\ **Regression** \\ \end{tabular} \\ \hline RMSE & \(1.30\times 10^{-7}\) & \(2.11\times 10^{-6}\) & \(1.07\times 10^{-6}\) \\ R\({}^{2}\) & 0.494 & 0.898 & 0.982 \\ \hline \end{tabular}
\end{table}
Table 4: Comparison of error in calculation results of different models.