## 3 Geospatial Informatics of the Geotechnical Database
### Optimization of the Geotechnical Database  
In geotechnical engineering practice, various borehole layout schemes are used to design a one-dimensional geo-layer profile and its characteristic coefficient. Since borehole log information is interspersed in time and place, there are inherent instabilities in the integrity of geotechnical DBs. These arise from differences in the purposes of the projects, ideas and techniques of the investigators and analyzers, and equipment performance. When experiments are translated into soil design properties with empirical or theoretical models, transformation uncertainty is added. Such uncertainties make geotechnical designs less reliable [33]. In measurements with appropriate statistical methods applied, it is important to minimize or eliminate \"errors\". Thus, for reliable post-processing, the blended definition of the attribute or biased value from a borehole log should be checked for abnormalities in the geotechnical DB. Accordingly, quality assurance procedures were established first (Figure 3) to verify and exclude errors based on the standardized schemes used to develop the geotechnical DB. Figure 2: Schematic showing the geospatial characterization procedures for regional seismic site classification. The abbreviation in geospatial interpolation procedure: IDW (inverse distance weighting); SK (simple kriging); OK (ordinary kriging); EBK (empirical Bayesian kriging). The abbreviation in site response parameters: \(V_{S30}\) (average shear wave velocity up to 30 m below ground); \(V_{S,Sol}\) (average shear wave velocity of the overlying soil); \(T_{G}\) (natural site period); \(H\) (bedrock depth). Coordinates, engineering geo-layers, and SPT results are required to evaluate seismic site effects. The engineering geo-layers consist of four types: FL, AS, WS, WR, and bedrock (rock below soft rock). In a partial borehole log or when unreliable measurements are excluded, missing values are introduced. If the composition and thickness of a geo-layer or other required information is missing, it will be deleted. Input errors such as typographic errors in the geo-layer definition, thickness (e.g., 2505 m), and SPT-\(N\) (e.g., 12.5) were corrected manually. In 2007, the Korean government officially chose to follow the global geodetic reference system (ITRF and GRS80). The coordinates of boreholes investigated before 2007 were transformed. The standard transformation protocol adopted by the National Geographical Information Institute, South Korea, was applied [34]. To determine the geo-layer boundary and the correlated index property of soil, the geo-layer was classified according to the Unified Soil Classification System (USCS) [35]. The USCS classifies the geomaterial composition, measured in core samples from a borehole. In the same borehole, if the geomaterial did not match USCS criteria it was excluded from the geotechnical DB. The ground elevation of the borehole log was also shifted to match the elevation of the DEM (Figure 1b) at the borehole location. Even though screening and calibration of errors was carried out, the inherent soil variability induced by geologic processes and uncertainty is introduced in transformations into empirical or theoretical models [17]. Usually, irregular measurements are numerically far away (usually in 10% confidence interval) from the remainder of the results, or may vary substantially from other nearby observations [17; 36; 37]. The spatial uncertainty and bias of geotechnical parametric values undermine the prediction accuracy of geospatial interpolation. Accordingly, we introduced geotechnical outliers that are numerically distant from the rest of the data or may deviate significantly from nearby observations [7; 17; 38]. In this study, a hybrid model blended with a cross-validation-based method [17] and [PERSON]'s I statistic-based method [39] was proposed and applied to the screened bedrock depth in the geotechnical DB of Daejeon city (Figure 4). Engineering bedrock was defined as rock strata below soft rock.