## 2 Brief Historical Context of Chilean Construction Regulations
### Supreme Decree 61, DS61-2011  
Following the promulgation of emergency decree 117-2011, the committee on code NCh 433, having clarified the criteria that had been temporarily defined, proceeded to a proposal that included adaptations and adjustments to the seismic site classification system and the design spectra associated with each type of ground; these changes were contained in Supreme Decree 61, approved in December 2011 and known as DS61-2011 [41]. The newly proposed classification defined six types of ground, dividing the previous ground III into two types, defining the shear-wave velocity measured in situ, requiring Vs30 as a parameter for the classification of ground types, and removing the criterion using Vs15. The detailed seismic classification method is shown in Table 8.  
DS61-2011 also specified with more detail cases that had previously not been classified and, therefore, that required special studies. These cases are described as follows: potentially liquefiable ground types such as sand, saturated silty sand or silt with (N\({}_{1}\))\({}_{60}\) < 20 blows/foot; ground types susceptible to densification by vibration; collapsible and organic soils; saturated fine soils with liquid limit (LL) >80 and thickness >20 m; fine soils with sensitivity higher than 10; and ground types with irregular topography where local amplification phenomena can occur. Even given these additions, the classification system continued to classify \"special cases\" that are now categorized as site class type F, which includes the previously indicated cases, except for topographic amplification.  
As previously indicated, the regulations adopted a minimum exploration thickness of 100 feet, that is, 30 m, for classification and incorporated it in the International Building Code 1997 [42] and in many international codes; this value was incorporated as a site seismic characterization parameter. Additionally, considering that certain structures were built mainly in the city of Santiago, which had underground levels with depths greater than 15 m, DS61-2011 required the measurement of classification properties to a maximum between 30 m and Df + 15 m, where Df is the embedment depth. Thus, in cases with several basement levels, a minimum exploration depth of 15 m below the level of the foundation system should be guaranteed.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Site Class** & **Description** & **Min (Vs30; Vs15) m/s** & **RQD** & **q\({}_{u}\) (MPa)** & \begin{tabular}{c} **(N\({}_{1}\))\({}_{60}\)** \\ **(For Sands)** \\ \end{tabular} & \begin{tabular}{c} **s\({}_{u}\) (MPa)** \\ **(For Fines Soils)** \\ \end{tabular} \\ \hline I & \
\begin{tabular}{c} Rock or cemented soil (not \\ soluble in water) \\ \end{tabular} & \(\geq\)900 & \(\geq\)50\% & \(\geq\)10 (\(\epsilon_{\text{qu}}\leq 2\%\)) & \\ \hline II &
\begin{tabular}{c} Soft rock or very dense soil \\ or very firm soil \\ \end{tabular} & \(\geq\)500 & \(\geq\)0.4 (\(\epsilon_{\text{qu}}\leq 2\%\)) & \(\geq\)50 & \\ \hline III & Medium dense or firm soils & \(\geq\)180 & & \(\geq\)30 & \(\geq\)0.05 \\ \hline IV & Loose or soft soils & \(<\)180 & & \(\geq\)20 & \\ \hline V & Specials soils & - & - & - & - \\ \hline \hline \end{tabular}
\end{table}
Table 7: Seismic site classification—DS117-2011.  
Note that the seismic classification of ground types defined from the response or characterization of the top 30 m has not been exempt from criticism; this depth can be an insufficient criterion for very stiff ground types [43] or for certain stratigraphic configurations, where two places may have the same Vs30 but different stiffness changes with stratum depth, generating a different seismic response for each case. This point will be the topic of a future ground study; thus, the analysis of information obtained from earthquakes and types of ground in the country will eventually incorporate some changes in the most recent proposal from NCh 433.