FAILURE MECHANISM ANALYSIS CONSIDERS THE HYDROLOGICAL AND SEISMIC CONDITIONS AT CHASHAN SITE IN CENTRAL TAIWAN

The investigation results of large-scale landslides in the central part of Taiwan from Central Geologic Survey identified a new, largescale landslide in the Chashan Village of Alishan Township, Chiayi County (ID: D160) which has the potential to influence the safety of Chashan Village residents. In coordination with the surface geological survey, Chashan site can be divided into three potential sliding areas (zones A, B and C). The paper uses GeoStudio for evaluating the stability of different hydrological and seismic scenarios. The results of scenario simulation show that: (1) In the normal and high water level scenarios, the safety factor obtained in zone A is greater than 1.0, and the initial judgment belongs to the stable state; (2) In the earthquake scenarios, the A4 potential sliding mass in zone A will be collapsed; (3) Through the scenario simulation, when the water level in zone A rises more than 25 meters or the PGA exceeds 160 gal, it is possible to cause the collapse of A4 potential sliding mass. The result of the study will contribute that the complex phenomenon between the stability of landslides and hydrogeological conditions can be revealed and clarified through the detailed study on the hydrogeological investigation, observation and the numerical simulation. * Corresponding author


INTRODUCTION
Taiwan is an active mountain belt created by the oblique collision between the northern Luzon arc and the Asian continental margin.The inherent complexity of geological nature creates numerous discontinuities through rock masses and relatively steep hillside on this island.In recent years, the increase in the frequency and intensity of extreme natural events due to climate change brought significant large-scale landslides in the mountain area of Taiwan.Because of the extreme volume of material mobilized in a large-scale landslide, other natural disasters can be triggered by the large-scale landslide including debris flows and landslide dams.Also, transportation systems can be severed and structures buried.Therefore, large-scale landslides can severely impact and pose a significant hazard to nearby villages.This study is aimed to better understand the mechanism of triggering landslide hazards so that casualties and property damages can hopefully be reduced in the occurrence probability of natural disasters in the future through the appropriate disaster prevention planning we proposed.Through a long term project implemented by the Central Geological Survey (CGS, 2010(CGS, -2012;;2013-2015), the location of potential large-scale landslides and villages more susceptible to large-scale landslide hazards have been mapped.The CGS concluded that a potential large-scale landslide in the Chashan Village of Alishan Township, Chiayi County, (ID: D160, hereby called the Chashan site, Figure 1) may impact the safety of the Chashan Village.According to household registration office census data in January, 2018 (data source: https://chuchihro.cyhg.gov.tw),there were a total of 112 households and 297 people in the Chashan Village.

Regional geology and geological survey
According to a 1/50,000 geologic map (Central Geological Survey, Figure 2), the Chashan site is located in the Changchihkeng Formation.The Changchihkeng Formation is a thick sequence of dark gray shale and light gray fine-grained sandstone containing an assemblage of varied marine faunas.The sandstone is dense, compact, and partly calcareous subgraywacke, locally containing carbonaceous matter and broken plant remains.No white sandstone or coal beds have been found.The shale is very compact and slightly indurated (data source: https://www.moeacgs.gov.tw/english/twgeol/twgeol_western_23.jsp).On the west side of the Chashan site are the Sinvi anticline and Laitou fault, while the Laitou fault is the most important geological structure adjacent to the site.The slopes of the Chashan site are mostly 40%-55% and more than 55%; the central and west sides of the site are south-facing and south-facing slopes, while the eastern side is the west-facing and south-facing slopes.According to field survey, the joints in this site are mainly in two groups: the joint attitude (dip/dip direction) are 58/270 and 70/175 respectively.

Chashan site
Figure 2. Regional geological map of the Chashan site Through data collection and multi-period aerial photo, the study can find out the site status, topographic changes, collapse history and regional linear distribution of the survey sites.In addition, in coordination with the surface geological survey (including regional survey and site survey), the Chashan site can be divided into three potential sliding areas (zones A, B and C), the surface geological map shown in Figure 3.

Hydrogeological investigations
In order to understand the sliding mechanism and failure potential of the Chashan site, this study identifies the hydrogeological properties using several investigation methods.3), and Chiayi Earthquake (November 22, M5.5).The magnitude of latter two earthquakes larger than 5 resulted in a significant ground motion (seismic intensity > 3) at the Chashan site.Simultaneous observations of surface and subsurface observation data showed that the above events did not cause obvious coseismic surface and subsurface displacements at the Chashan site.

Hydrogeological conceptual model
The numerical geological model was established using a 6m× 6m Digital Elevation Model (DEM) of the Chashan site.This analysis profile starts from the top of the crest line and passes through boreholes AH-01 and AH-02, and then down to the bottom of the trench (Figure 8).The hydrogeological unit of this profile can be divided into five layers: which are C, SS1, SS-SH, SH, and SS2.Morphometric and geotechnical analyses were carried out to the Chashan site through a series of in-situ and laboratory tests, the results of which were used as input for the modeling process.Using the above-mentioned procedure, parameters (Table 2) and groundwater level (Figure 8) of this site were determined, and the follow-up slope stability analysis was conducted.Table 2. Hydrogeological parameters of the Chashan site

Scenario setting definition
The setting of each scenario is as follows: 1. Normal condition Steady-state water level is used as the water level and the effect of seismic force is not considered.

High water level condition
The highest water level during the monitoring period is taken as the water level and the effect of seismic force is not considered.

Earthquake condition
Steady-state water level is used as the water level and the seismic force is considered.The peak ground acceleration (PGA) adopts 0.28g (Construction and Planning Agency, 2011).

Scenario simulation
This study uses the GeoStudio software to perform slope stability analysis (Chung et al., 2017) on a two dimensional model of the Chashan site.In GeoStudio software, the limit equilibrium method module SLOPE/W was used.The factor of safety (F.S.) was assessed for various scenarios by entering groundwater level and PGA into the model.Therefore, the stability of each sliding mass under normal, high water level, and earthquake conditions is evaluated in the analysis.
After completing an analysis of the Chashan site along crosssection AA', four areas of potential sliding masses (A1~A4) were identified (Figure 9).In this study, in addition to analyzing the above four potential sliding masses, the function of "Auto Locate" built in GeoStudio is also used to automatically search for the sliding mass with the lowest F.S.
The stability analyses of each of the masses are assessed for normal, high water level, and earthquake conditions.Results are shown in Table 3 and Figure 10-12.The results show that: (1) Analysis results reveal that of the five potential sliding masses, regardless of the scenario, the F.S. of Auto is lowest for the Zone A; (2) In the normal and high water level scenarios, the safety factor obtained in zone A is greater than 1.0, and the initial judgment belongs to the stable state; (3) In the earthquake scenarios, the A4 potential sliding mass in zone A will be collapsed.
The results in Table 3 show that zone A may collapse in the earthquake condition.In order to evaluate the potential collapse volume reasonably, the potential sliding mass obtained from Figure 12 is used in this study.With the aid of landslide microtopography, as shown in Figure 13, the calculated results show that the amount of collapse is 917,729 m 3 .
Figure 9. Geological profiles on the Chashan site (along AA' line in Figure 3)  With the water level under normal condition as a benchmark, the water level in each scenario is raised 5 meters upwards and the water level is no more than the ground surface.Figure 15 shows the relationship between the water level rise and F.S. at AH-02 borehole.The results show that when the water level rises more than 10 meters, the F.S. will start to decrease.The F.S. will be reduced to 1.0 when the water level is raised over 25 meters, which shows that the A4 potential sliding mass may collapse at this time.Observation data show that the change of water level at AH-02 is about 0.3 meters (marked as ★ in the Figure 15), which is not enough to cause the A4 potential sliding mass to collapse.

PGA threshold
Based on the above analysis results, this study sets different conditions by PGA, shown in Figure 16, in order to explore the mechanism of seismic-induced landslide.The PGA in each scenario is from 0 to 560 gal. Figure 17 shows the relationship between PGA and F.S. in zone A. The results show that the F.S. of A4 potential sliding mass decreases with increasing PGA.The F.S. will be reduced to 1.0 when the PGA is over 160 gal, which shows that the A4 potential sliding mass may collapse at this time.Observation data show that the PGA of zone A at Chiayi Earthquake (November 22, M5.5) is between 26 and 48 gal (marked as ★ in the Figure 17), which is not enough to cause the A4 potential sliding mass to collapse.

CONCLUSIONS
A list of conclusions and suggestions is summarized below as a reference for the reader: 1.According to the results of geological survey, there are two to four potential sliding masses in zone A, B and C, respectively.It is recommended to assess the different conditions of the potential sliding mass, and pay attention to its activity.2. The results of scenario simulation show that: (1) In the normal and high water level scenarios, the safety factor obtained in each zone is greater than 1.0, and the initial judgment belongs to the relatively stable state; (2), In the earthquake scenario, the A4 and Auto masses in zone A may collapse, of which the volume of A4 collapse is larger, about 917,729 m 3 .3. Through the deterministic threshold assessment, when the water level in zone A rises more than 25 meters or the PGA exceeds 160 gal, it is possible to cause the collapse of A4 potential sliding mass.4. The result of the study will contribute that the complex phenomenon between the stability of landslides and hydrogeological conditions can be revealed and clarified through the detailed study on the hydrogeological investigation, observation and the numerical simulation.

Figure 1 .
Figure 1.Location of the Chashan site Figure 3. RIP-AA and RIP-BB are located in the center of the zone A and zone B, which runs parallel to the slope direction.RIP-AA and RIP-BB are 270 m and 660 m long and are used to understand the hydrogeological conditions along the profile.RIP-AB is established running perpendicular to the slope of the site.It is 275 m long and is used to understand the hydrogeological conditions along the profile.Geological drilling is established along the potential failure direction of the site.A total of 6-hole geological drilling (total depth of 320 meters, Table1) was carried out in zones A, B and C in this study.The formation judged by drilling core can be divided into six layers: which are C, SS1, SS-SH, SH, SS2 and SS/SH.The borehole columnar section is shown in Figure4.The results of the borehole data are paired with geophysical observations to accurately interpret the subsurface conditions along RIP-AA, RIP-BB and RIP-AB.Figure5shows the results of the geophysical survey and borehole interpretation along RIP-AA.

Figure 3 .
Figure 3. Surface geological map of the Chashan siteAccording to the above results, it can be applied to establish the geological profile shown as Figure6and the hydrogeological parameters of each formation.This paper will focus on zone A, there are four potential sliding masses.It is recommended to assess the different conditions of the potential sliding mass, and pay attention to its activity.

Figure 7 .
Figure 7. Layout of monitoring system at the Chashan site

Figure 8 .
Figure 8. Hydrogeological conceptual model of the Chashan site

Figure 15 .
Figure 14.Groundwater level setting under different conditions

Figure 17 .
Figure 16.PGA setting under different conditions

Table 1 .
Borehole drilling date of the Chashan site The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XLII-3/W4, 2018GeoInformation For Disaster Management (Gi4DM), 18-21 March 2018, Istanbul, Turkey

Table 3 .
Analysis results of zone A in the Chashan site