## 4 SPI Prediction and Application
### Application for the Date of the SPI  
The following method was used to predict the SPI values in the near future. Based on the PSO-LSSVM model, the time series of four influencing factors and the SPI from March 2016 to March 2019 are chosen as the training samples. The SPI from September 2019 is predicted by taking the cumulative precipitation, maximum daily precipitation, and rainfall days from March to September 2019 as input parameters. The predicted results are given in Table 5 for the different test sites. The relevant national standards of China can be referenced to meet the actual requirements of subgrade maintenance [33]. Four subgrade classification levels (excellent, good, medium, poor) are collected in the SPI test sites. The values range from 0 (poor) to 100 (excellent). For a SPI between 90 and 100, the subgrade is classified as excellent; a SPI of 80-89 is classified as good; a SPI of 70-79 is classified as medium; a SPI less than 60 is considered poor. As can be seen from Table 5, the SPI of the K95-K96 section of S223 is the smallest. S223 was opened to traffic in May 2007. After twelve years of operation, a large number of subsidence, cracks, and other distresses have appeared in the subgrade. In addition, by predicting the subgrade performances of other sections of this highway, most of the SPI values will be in the middle or below the classification value in September 2019, which poses a great risk to driving safety. Therefore, it is necessary to take corresponding maintenance measures based on the different SPI values. Figure 9 shows the impact of reasonable treatment measures taken on subgrades with various SPI values. The preventive maintenance treatment is applied before the SPI drops below 70. Preventive maintenance is a proactive approach that systematically looks for opportunities to eliminate small problems before they become serious. This concept was first utilized for pavement, and has been widely used [55; 56; 57]. Although preventive maintenance treatments do not significantly improve the load-carrying capacity of subgrade, they do improve the level of service and extend the service life [58]. When the SPI is less than 70 and greater than or equal to 60, the subgrade requires maintenance and rehabilitation. When the SPI is less than 60, there is a large-scale disease or landslide on the subgrade. In this situation, overhaul or \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Parameters** & **K2452–K2453 of G105** & **K95–K96 of S223** & **K103–K104 of S270** & **K9–K10 of S356** \\ \hline \(\gamma\) & 14.743 & 0.1 & \(1.0\times 10^{6}\) & 11.455 \\ \(\sigma\) & 1.922 & 7.672 & 57.009 & 1.313 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Optimized parameters of the PSO–LSSVM model for the four test sites. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Sites** & **Cumulative** & **Maximum Daily** & **Predictive Values** \\
**Sites** & **Precipitation** & **Rainfall Days** & **Precipitation** & **for September** \\  & **(mm)** & & **(mm)** & **2019** \\ \hline K2452–K2453 of G105 & 1720 & 114 & 108 & 76.82 \\ K95–K96 of S223 & 1333 & 109 & 76 & 70.16 \\ K103–K104 of S270 & 1914 & 116 & 123 & 81.70 \\ K9–K10 of S356 & 1363 & 90 & 63 & 78.96 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Predicted SPI results for September 2019.  
emergency disposal measures should be taken. In order to guide and verify the applicability of this definition, in the first half of 2019, technical personnel within the research institute of the Ministry of Highway Transport carried out a comprehensive inspection on the road section, and formulated a comprehensive maintenance plan for the whole section. The other three highways were newly built or reconstructed between 2008 and 2010, and as such, they have not yet reached the critical point for preventive maintenance. However, the life-cycle decision-making for preventive maintenance of a subgrade needs to be carried out and included in the annual management cycle.