## 4 Conclusions  
The seismic classification of foundation ground types has direct implications for the seismic design of a structure, impacting its construction costs and conditioning its feasibility. Therefore, a clear knowledge of the genesis of regulatory criteria allows engineers to better understand, interpret and apply professional criteria for classifying ground types and following the methodologies in a design code, which defines guidelines that are ultimately intended to safeguard the lives of those who will use the structure. Great seismic events to which Chilean territory has been subjected have generated important information and experience in the seismic design of structures, which have allowed Chilean regulations to identify and include variations in seismic demand according to the type of ground since the 1930s, prior to most design codes worldwide. The main conclusions have been scattered among the different design codes proposed for Chile, generating seismic classification proposals that could be considered in the vanguard for each period. Along these lines, NCh 433 has recently proposed the replacement of properties associated with ground resistance that are usually applied in different design codes (ground density, q\({}_{\text{u}}\), or N-value, SPT) by parameters that represent the dynamic stiffness properties of a specific location, which correlate more consistently with the ground amplification phenomenon. Thus, in addition to the commonly employed Vs30, the code proposes incorporating the fundamental period of ground vibration measured in situ, T\({}_{\text{g}}\), as a parameter for seismic classification. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Site Class** & **Description** & **Vs30, m/s** & **T\({}_{\text{g}}\) s \({}^{\star 1}\)** \\ \hline A & \begin{tabular}{c} Rock or cemented soil (not \\ soluble in water) \\ \end{tabular} & \(\geq\)900 & \begin{tabular}{c} \textless{}0.15 \\ (or plane HVR) \({}^{\star 2}\) \\ \end{tabular} \\ \hline B & \begin{tabular}{c} Soft or fractured rock; very \\ dense soil or very firm soil \\ \end{tabular} & \(\geq\)500 & \begin{tabular}{c} \textless{}0.30 \\ (or plane HVR) \({}^{\star 2}\) \\ \end{tabular} \\ \hline C & \begin{tabular}{c} Dense or firm soils \\ \end{tabular} & \(\geq\)350 & \textless{}0.40(or plane HVR) \({}^{\star 2}\) \\ \hline D & \
\begin{tabular}{c} Medium dense or firm soils \\ \end{tabular} & \(\geq\)180 & \textless{}1.00 \\ \hline E &
\begin{tabular}{c} Medium compactness or \\ medium consistence soil \\ \end{tabular} & \textless{}180 & - \\ \hline \hline \end{tabular}  
* Soil period, T\({}_{\text{g}}\), measured in situ using the HVR method. \({}^{\star 2}\) A flat measurement is considered when the HVR is less than 2.0.  
\end{table} 
Table 9: Seismic site classification—NCh 433 proposed for public consultation in 2021. Regarding the predecessor Chilean design codes and homologous regulations at the international level, the most recent proposed version of NCh 433 presents an important advance regarding the selection of seismic site classification parameters in the process of performing a practical characterization that can better correlate with the phenomenon of ground amplification. Likewise, it is recognized that the Vs30 parameter, which is widely employed internationally, should not be substituted but rather should be complemented with dynamic parameters, such as the proposed T\({}_{\text{g}}\), which, according to the observations of the last earthquakes, was considered with suitable parameters. Seismic engineering in its different disciplines has made important advances that have allowed the adjustment of seismic design codes. This review highlighted the clear need to undertake a continuous evaluation of the classification criteria, supported by records of new earthquakes, as well as physical and non-linear numerical models that allow incorporating variables that condition the response of the terrain, such as topography, lateral heterogeneities, and basic effects.