# Simulation of Debris-Flow Runout Near a Construction Site in Korea
## 2 Distribution of Regolith in the Study Area  
The geology of the study area includes a Precambrian gneiss and schist complex, age unknown amphibolites, Mesozoic granites, and Quaternary colluvium. The gneiss and schist complex and the amphibolites are the major lithologies of Mt Goobong. The Mesozoic granites, with highly weathered features, are distributed in the western part of the study area (Figure 1). The amphibolites are distributed only in Mt Goobong. The intrusion direction is N10-20E on the western part of the mountain. According to the field investigation, the amphibolites are exposed as about 20m in width on the western part of Mt Goobong. The Mesozoic granites are distributed in the western part of the survey area including the construction site. Because the granites are more susceptible to weathering, it is difficult to find granites outcrops in the field. When the granites were intruded into the gneiss and schists in the area, the intrusion occurred along a fault with NE orientation. The boundary between the granites and the gneiss and schists complex shows linear feature. There is another fault with N10E orientation and higher density of fractures near the boundary. It indicates that the boundary between the granites and the gneiss and schists complex is thought to be a fault contact. Since the areas occupied by the granites have low elevation and gentle slope angle in the study area, it was easy to recognize lithological boundary showing marked difference of slope gradient and elevation in the field survey. According to the results of core logging in the construction site composed of the granites [38], depth of boundary between weathered rock and weathered soil show various distribution from 6.0 m to 18.5 m below ground surface. The difference of weathering depth in the granite area has been explained by [PERSON] [39]. The progress of weathering vertically is controlled by the pre-existing structure of rock, its mineralogy, and the discontinuities. Because granites usually have difference of weathering depth and extent,the thickness of the saprolite may be highly variable. On the other hand, the boundary between weathered rock and weathered soil is distributed from 1.0 m to 1.5 m below ground surface in the metamorphic rock area of the study area. This study undertook field mapping of regolith distribution. Regolith is unlithified material distributed over bedrock. The field mapping was performed at the western part of Mt Goobong and the construction site based on a classification of regolith suggested by [40] (Table 1). The authors classified regolith on the surface and measured areal distribution and vertical thickness of regolith in the field. The Mt Goobong area is primarily composed of outcrops, residual colluvium, and talus (Figure 2). The outcrops are gneiss and schists whose exposure is limited to near the top of the mountain (Figure 3). Although the mountain has steep slope angles, of 30-40 degrees, outcrops are only exposed in very small areas on the mountain. Figure 1: Geological map of the study area. \begin{table}
\begin{tabular}{c c} \hline \hline
**Main Phases** & **Deposits** \\ \hline \multirow{4}{*}{Recent Colluvium} & Recent landslide debris \\  & Depression colluvium \\  & Valley colluvium \\  & Rock Fall debris (Talus) \\  & Relict landslide deposits \\ Relict Colluvium & Residual colluvium \\ Saprolite formation & Saprolite \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Classification of regolith (after [40]). Figure 2: Regolith map in Mt Goobong. The residual colluvium is broadly distributed on Mt Goobong with a thickness of 1.0-1.5 m as observed in trenches and soil cross sections. They are composed of angular cobble size of rock fragments, clayey silts, and sandy silts (Figure 4). On the mountain slopes, the residual colluvium has relatively similar thickness averaging 0.5 m near the top of the mountain. This thick residual colluvium, on steep slopes, can produce high volume landslide debris flows. Talus has a limited distribution on the mountain slope (Figure 5).