## 5 Results and Discussion
### Application for Site-Specific Characterization  
One of the main applications of these empirical correlations is the estimation of the average shear wave velocity of the upper 30 m of a site (V\({}_{\rm s30}\)). This metric is generally used for the classification of sites into the general characteristic of the subsurface, for earthquake engineering design. In the Philippines, V\({}_{\rm s30}\) is the most important parameter needed for the site classification scheme stipulated by the National Structural Code of the Philippines (2015) [44]. This scheme classifies a site into six categories used for earthquake engineering design (Table 4). The magnitude of V\({}_{\rm s30}\) can be computed in accordance with the expression  
\[\rm V_{s30}=\frac{30}{\sum_{i=1}^{n}\frac{h_{i}}{v_{i}}} \tag{18}\]  
where \(h_{i}\) and \(v_{i}\) are the layer thickness (m) and shear wave velocity (m/s) of the \(i\)th layer, respectively, in a total of n layers existing in the top 30-m of a site. The magnitude of the V\({}_{\rm s}\) is estimated from SPT-N values using the proposed empirical relations applicable for all soil types. Sample boreholes with penetration depths reaching 30 m were examined, and the proposed empirical correlation was applied to the datasets (Table 5). The calculated V\({}_{\rm s30s}\) highlight the site classifications obtained from SPT data. Site Class D classification is dominant in Metro Manila, particularly on the western sector of the region along the coast. This suggests that the sites are generally underlain by stiff soil deposits. This is further supported by the Quaternary Alluvium deposits mapped along the western coast, proximal to Manila Bay. \begin{table}
\begin{tabular}{c c c c} \hline \multirow{2}{*}{**Soil Profile Type**} & **Soil Profile** & **Shear Wave Velocity,** & **SPT-N** \\  & **Description** & **V\({}_{\rm s}\) (m/s)** & **(Blows/30 cm)** \\ \hline S\({}_{\rm A}\) & Hard Rock & \(>\)1500 & - \\ S\({}_{\rm B}\) & Rock & 760–1500 & - \\ S\({}_{\rm C}\) & Very Dense Soil and Soft Rock & 360–760 & \(>\)50 \\ S\({}_{\rm P}\) & Stiff Soil & 180–360 & 15–50 \\ S\({}_{\rm F}\)1 & Soft Soil & \(<\)180 & \(<\)15 \\ S\({}_{\rm F}\) & & Soil requiring site-specific evaluation & \\ \hline \end{tabular} \({}^{1}\) Soil profile type S\({}_{\rm E}\) also includes any soil profile with more than 3.0 m of soft clay defined as soil with plasticity index \(>\) 20, moisture content \(\geq\) 40\(\%\), and undrained shear strength \(<\) 24 kPa.  
\end{table} 
Table 4: Site classification scheme according to the National Structural Code of the Philippines, employing the V\({}_{\rm s30}\) metric to determine the general soil description and its typical corresponding N-values. Figure 8: Measured versus estimated V\({}_{\rm s}\) for (**A**) all soils, (**B**) sandy soils, and (**C**) clayey soils with lines of 1:1.5, 1:1 and 1:0.65. An additional utility of these correlations is in the refinement of the corresponding N-values for a specific range of shear wave velocities. Based on the outcomes of this study, a simplified correlation can be established for basic soil strength classification (i.e., soft soils, stiff soils, and very dense soils) (Table 6). This demonstrates the utility of the proposed empirical correlations for geotechnical description at site-specific scales, as well as seismic microzonation efforts in the study area. Specific ranges of V\({}_{\rm s}\) have a corresponding range of N-values, where geophysical measurements of V\({}_{\rm s}\) can complement point borehole profiles and establish the lateral continuity of subsurface material between drilling points. In situations where SPT boreholes might not be feasible, due to economic or logistic reasons, refraction microtremor or other geophysical tests that can reliably measure shear wave velocities and can serve as a proxy to determine the relative strength of soils, especially for smaller construction projects.