PKD Luzon (a), KRP Montalbo (a,b), JAMB Galang (a), JMJ Sabado (a, b), CMJ Escape (a, b), RP Felix (a, b), AMFA Lagmay (a,b)
(a) Nationwide Operational Assessment of Hazards, Department of Science and Technology, Philippines
(b) National Institute of Geological Sciences, University of the Philippines, Diliman, Quezon City
The 2006 Guinsaugon landslide in St. Bernard, Southern Leyte is the largest known mass movement of soil in the Philippines. It consisted of a 15 million cubic meter rockslide-debris avalanche from an approximately 700 m high escarpment produced by continuous movement of the Philippine fault at approximately 2.5 cm/year. The landslide was preceded by continuous heavy rainfall totaling 571.2 mm from February 8 to 12, 2006. The catastrophic landslide killed more than 1,000 people and displaced 19,000 residents over its 6,400 m path. To investigate the present day morphology of the scar and potential failure that may occur, analysis of a 5 m resolution IfSAR-derived Digital Elevation Model was conducted, leading to the generation of a structurally controlled hazard map of the area. Discontinuity sets that could contribute to any failure mechanism were identified using Coltop 3D software which uses unique lower Schmidt-Lambert color scheme for a given dip and dip direction. Thus, finding main morpho-structural orientations became easier. Matterocking, a software designed for structural analysis was used to generate possible areas that could fail due to the identified discontinuity sets. Conefall was then utilized to compute the extent to which the rock mass will runout. Results showed potential instabilities in the scarp area of the 2006 Guinsaugon landslide and in adjacent slopes because of the presence of steep discontinuities that range from 45◦-60◦. Apart from the 2006 Guinsaugon potential landslides, Conefall simulated farther rock mass extent in the adjacent slopes. In conclusion, there is a high probability of landslides in the municipality of St. Bernard, where the 2006 Guinsaugon Landslide occurred. Concerned agencies may use maps produced from this study for disaster preparedness and to facilitate long-term recovery planning for hazardous areas.
1.1. Guinsaugon Landslide
The largest known mass wasting in the Philippines happened in Barangay Guinsaugon, St. Bernard in the province of Southern Leyte on February 17, 2006 10:26 local time . It is characterized as a rockslide-debris avalanche which happened on the steep scarp of the Philippine Fault Zone (PFZ) . It is a part of Mt. Can-abag facing St. Bernard as shown in figure 1. The landslide’s planform area is approximately 3.3 square kilometers with a maximum width of 1.52 kilometers and a runout of 4.1 kilometers. It has also an estimated deposit volume of 15-20 million cubic meters  which buried the entire village of Guinsaugon. As of Feb 24, 2006, there are 122 confirmed deaths and 1,328 missing people.
1.2. Geotechnical Consideration
The study area is part of the Southern Leyte segment of the sinistral PFZ which has an average movement of 0.55 cm/yr  up to 3.5 cm/yr (Dequesnoy 1997, as cited by  ). This has led to formation of regional discontinuities and development of thick clay-rich gouge zones in the surface prominent in the en-tire province and other locations along the PFZ . As a result of active tectonic movement, poor rock mass quality is visibly evident . The region is represented by ophiolitic basement, Paleogene sedimentary rocks and late Pliocene to Pleistocene volcanic rocks.
The tragedy was preceded by continuous downpour of rain amounting to 674 mm as measured by the Otikon Station in Libagon, Leyte from Feb. 8-17 . The rain gauge is located 7 km west of Guinsaugon. Rainfall amount peaked from Feb. 10-12 ranging from 131 to 171 mm. These are believed to be lesser than the actual values due to orographic effect. The climate in Leyte Island is characterized with rainy season from November to January but have extended that year. It also lies in the typhoon path of those making landfall in the country. Around same time as the landslide, two earthquakes were recorded by the Philippine Institute of Volcanology and Seismology (PHIVOLCS) and the United States Geological Survey (USGS) . Each were reported 21 kilometers west and about 2 kilometers north of Guinsaugon respectively.
1.3. Failure Mechanism
The Guinsaugon landslide is categorized as rockslide-debris avalanche. The topmost contributing slope of the landslide ranged from 50 to 60 degrees. Post landslide configuration of the crown is a distinct 4 slip plane failure slope. Possibility of combined wedge, toppling and planar slide could have happened due to existence of discontinuities and immense pore pressure . Large portion of the foot slope of Mt. Can-abag are prehistoric landslide deposits which were scoured during the avalanche stage of the tragedy and then deposited as debris flow to the plane as water accumulated.
1.4. Digital Elevation Model
Data used in the study is an airborne Interferometric Synthetic Aperture Radar (IfSAR)-derived Digital Elevation Model with a spatial resolution of 5 meters. Acquisition date of these images is between March to July 2013.
The purpose of this study is to identify areas highly susceptible to structurally controlled landslide in Southern Leyte. Out-put of this study includes lineaments in the area, the prominent regional discontinuities, present slope that could fail due to structural instability and the runout zone of the identified rockslide zones. Ongoing simulations on shallow landslide and debris flow shall complete the landslide susceptibility map on the area.
2. Review of Related Literature
2.1. Mass Wasting
Mass wasting (also called mass movement) is a term used to collectively call the downslope transport of surface materials in response directly to gravity. Gravitational force is represented by the weight of such surface materials. This movement operates in a variety of ways and scales. Even a single rock rolling down a hillslope is a form of mass wasting as well as the transport of an entire hillside sliding downhill which could cover hundreds to thousands of property at the base of the slope. Mass wasting can occur slowly or at very rapid rates. Rock strength and friction are factors that could resist the gravitational force pulling down the surface materials. Rock strength is defined by the physical and chemical property of a rock related to any kind of break or gap in a rock while friction is the force resisting the relative movement of materials. Friction increases with roughness or angularity of a rock as well as the surface roughness. Slope angle also contributes to the determination of mass wasting occurrence. The closer a slope to being parallel to the downward direction of the material, the easier it is for gravity to overcome friction and rock strength resistance. Other factors that contribute to mass wasting include water, earthquakes as well as human activities. Fractures, joints, faults and other structural features weaken rock strength. Having these gaps favor water seepage which contributes to bond weakening through the weathering process. Materials on a slope that do not have the ability to resist gravitational force will respond by sliding, falling, flowing or creeping until they reach a more stable place wherein they are able to resist further movement .
Landslides are mass wasting events of rocks or debris asso-ciated with other major natural disasters such as earthquakes, floods and volcanic eruptions . It has become a general term used for rapid mass movement. Scientists however commonly apply such term to refer to large, rapid mass wasting events that are dicult to assign to a specific term since they contain several types of motion and materials in just one massive slide . Landslides are one of the deadly natural hazards. They usually occur without warning and it is hard to predict where a landslide will occur next. Terrain with steep slopes are often the areas where landslides occur. There are no disasters when landslides happen in remote areas but when they happen in urban zones, landslide can become an extreme hazard .
Falls (or topples) are mass wasting events that consist of Earth materials fall freely through the air and land at the bottom of the slope. Rockfalls are the most common type of fall. Rocks fall from the steep bedrock clis which could occur one by one as weathering weakens the bonds of individual clasts from the clis; or it can happen as large rock masses that fall from a cliface or an overhang.
2.2. Slope Failure Mechanisms
2.2.1. Planar Failure
A planar failure is a rare occurence in rock slopes due to the fact that all the geometric conditions required to produce it only happens occasionally on an actual slope . In planar failure, the rock mass only slides on a single surface and for it to continue, certain geometric conditions need to be satisfied .
- The sliding plane must strike parallel or nearly parallel to the slope face;
- The sliding plane must have its plane dip less than the dip of the slope face.
- The dip of the sliding plane should be greater than the angle of friction of the sliding plane.
- The upper end of the sliding plane should intersect the upper slope or where the tension crack terminates.
- Lateral boundaries of the slide should be defined by the negligible resistance to sliding provided by the release surfaces.
2.2.2. Wedge Failure
Wedge failures occur when a mass of discontinuous rock slides on two intersecting planes . They occur over a much wider range of conditions as well as geologic factors which is why it is more dicult to study this component of rock slope engineering. Wedge failure has besic mechanis analyzed in terms of its geometry and conditions defined by :
- The two sliding planes will always intersect in a line. The line of intersection can be represented on a stereonet by the point where the two great circles of the two sliding planes intersect. The orientation of the line is then defined by the trend and plunge of the line.
- The line of intersection plunge should be less steep than the dip of the slope face and steeper than the friction angle of the two sliding planes. The slope face inclination is determined by the view at right angles to the intersection line. The true dip of the slope face would only be determined if the dip direction of the intersection line is the same as the dip direction of the slope face.
- The intersection line must dip in a direction out of the slope face for sliding to be possible.
Stereonets can show a wedge failure is kinematically possi-ble. However, the actual factor of safety of the wedge cannot be determined by it. This factor also depends on the geometry of the wedge as well as the plane shear strength and water pressure.
2.2.3. Toppling Failure
Toppling failure is the forward rotation out of the rock slope about an axis below the center of gravity of the unstable rock. It is one of the basic landslide mechanisms that leads to serious and hazardous rock slope instability. This failure can be classified into four types: flexural, blocky, blocky-flexural and secondary type. Two mechanisms causes for toppling to proceed, one of which is the growth of erosion notches that is produced due to dierential weathering. The support area of the rock will decrease and in consequence, the center of gravity of the unstable rock will oset outward the slope. Secondly, the development of tension cracks due to tensile stress concentration leads to rotational toppling failure of unstable rocks because of momentum unbalance. They will inevitably occur under self weight and fissure water pressure water as well. To summarize, the total toppling failure occur due to erosion notches and tension cracks and continue on to gravitational transport and accumulation .
2.2.4. Circular Failure
Circular failure occurs under conditions where individual particles in a rock mass or soil are smaller compared to the size of the slope and when such particles are not interlocked as a result of their shape. For example, crushed rock will tend to behave as soil and failure will occur in circular mode.Highly altered and highly weathered rocks tend to fail in this manner .
3.1. Lineament Pattern Analysis
The discontinuity sets and their slopes were identified by manually delineating it from the airborne IfSAR-derived Digital Elevation Map (DEM). IfSAR or Interferometric Synthetic Aperture Radar DEM has a 5 meter spatial resolution. Dis-continuity sets were traced and overlayed on the shaded relief images. Shaded relief images were generated using ENVI 4.8, this so far, provides the most defined shaded relief image.
3.2. Coltop 3D Analysis
Coltop3D is a pseudo 3-Dimensional topographic analysis software. It can represent not only slope aspect, but also slope angle at the same time. Slope aspect maps are active tools for locating target sites that warrant structural investigation, and have particularly significant implications for the stability of slopes (Tengonciang, 2008). With the ability of Coltop 3D to combine slope aspect with slope angle, there would be an added capacity to delimit targets further to those significant in the study. In this case, slope angles greater than 45 would be of significant interest.
By using the selection tool and keying in the discontinuity sets with their dip angles in Coltop 3D, recurring linear patterns could be isolated with a particular tolerance and viewed separately. Viewing linear trends in Coltop 3D would then enable to combine slope aspect with slope angle, there would be an added capacity to delimit targets further to those significant in the study. In this case, slope angles greater than 45 would be of significant interest.
By using the selection tool and keying in the discontinuity sets with their dip angles in Coltop 3D, recurring linear patterns could be isolated with a particular tolerance and viewed separately. Viewing linear trends in Coltop 3D would then enable the identification of the dip directions of the discontinuity sets. Afterwards, multiple subsets are carefully selected to be the representative areas for generating rose diagrams in GeOrient. Finally, the dominant discontinuity sets, along with their derived dip directions, were used for Matterocking.
3.3. GeOrient Rose Diagrams
Georient is a software that can generate rose diagrams for up to 70,000 measurements. Using the representative areas from Coltop allows the selection of a large number of dip/dip direction measurements from an already specified set. This lifted measurements from Coltop were keyed in GeOrient to generate Rose Diagrams.
3.4. Rockslide Zone Identification Process
Structurally-controlled landslide zones were mapped with the software Matterocking. Matterocking computes and estimates the locations where rock instabilities can occur according to the identified discontinuity sets that allow sliding. The software require binary or ASCII format files for import. Digital Elevation Maps (DEM) taken via IfSAR method were converted to binary and ASCII format files. Matterocking was used to identify planar rock slide zones from a single discontinuity set, and wedge failure zones from paired discontinuity sets. For planar rock slide zones, the Intersection (yes/no) treatment was used, while the Number of Wedges treatment is used for the wedge failures. This creates a file containing 1 and -1 which corresponds to the zones where the discontinuity can produce rock slope failures. Wedge results to a wide array of values that should also be reclassified into 1 and -1 in ArcMap. Prior to the reclassification of wedges, the wide array of values undergo a statistical minimization through the fuzzy membership tool; transforming the input values into a 0 to 1 scale. The membership type used is MSLarge which calculates membership based on the mean standard deviation of the input data where large values have high membership. The resulting value set is reclassified in such a way where all values less than 0.5 are -1, while all values greater or equal to 0.5 will be 1. The output is an ASCII file. All the files created by Matterocking are then overlain in a basemap in ArcMap, showing the different areas prone to sliding.
3.5. Simulating Rock Mass Propagation Extent
Conefall software is used to estimate the potential rockslide extent using a DEM and a grid file having the plane failure source as input data. Using the generated rockslide zones ASCII file by matterocking as input data, a rock mass propagation extent ASCII file is generated.The cone slope angle used is 20.
The parameter used in this method of detecting zones of potential instabilities is based on topographic slope. It is assumed that the steeper the slope is, the more it contain instabilities. Based on the rock type, a critical slope can be established wherein rock instabilities could occur. The 45 dip angle is considered to be the critical angle for failure based on a general rock type.
Conefall is implemented using the geometric rule known as energy line method based on a simple Coulomb frictional model. An analogy is made that with a rock mass moving along a slope dissipates energy by friction. This friction can be associated to an apparent friction angle equivalent to the angle between the line connecting the source clitop and the end of the deposit. Applying this principle to rockslides without volume dependency, a use of predefined angle of the line connecting the source clitop and the end of the deposit. This angle was determined to range from 22 to 37 according to field evidence. Since there are still no field data for this type of landslide, we considered an angle of 20 according to the 2006 Guinsaugon Landslide extent.
4. Results and Discussion
4.1. Lineament Analysis of Southern Leyte
The Philippine fault is classified as a left lateral strike slip fault. Segment of this 1200 km long geologic feature cuts Southern Leyte on its mountainous regions. Along this lining are conjugate shears and corresponding splays scattered in the whole province. General trends of these lines are identified as shown in figure2. The study took into consideration four sets of regional discontinuities. We used four position of light source; 315, 225, 135 and 45 degrees all at an altitude of 45 degrees.
4.2. Identification of Discontinuities Using Coltop3D
Guided by the lineaments produced in the earlier step, slopes to represent each trend are selected. Dip and dip direction of each slopes measured using COLTOP 3d are shown in table 1.These sets of discontinuities are considered for the stability analysis of the slopes in the study area. Measurements are of+/- 10 on both criteria as limited by the software. First is the obvious plane intersecting the ridge of the mountain trending southwest with dip and dip direction 41/061 degrees. This actually is of the same direction of one of the slip planes of the Guinsaugon landslide. Next is the dip and dip direction of the slopes following the trend of the Philippine fault. It measures 32/250 degrees. Third is the lineament trending northwest cutting the point of landslide with 37/204 degrees. Forth is the measured discontinuity of the southwest trending lineament with 36/307 and 32/111 degrees. This is of the same direction of another slip plane of the landslide. Last is the discontinuity set trending east with 31/185 degrees. We can verify these measured dip and dip direction to the actual field measurements considered in a post disaster report by  as shown in table 2.4.3. Verifying Rose Diagrams of Discontinuity Sets Selections of discontinuities along the Philippine fault are also presented in rose diagrams. Shown in figure 3 are different segments of the lineament compared to the usual diagram of a left lateral strike slip fault. In a study of 44 established sinistral strike slip fault, usual rose diagram is as shown in figure 4 .
4.3. Verifying Rose Diagrams of Discontinuity Sets
Selections of discontinuities along the Philippine fault are also presented in rose diagrams. Shown in figure 3 are different segments of the lineament compared to the usual diagram of a left lateral strike slip fault. In a study of 44 established sinistral strike slip fault, usual rose diagram is as shown in figure 4. 
4.4. Potential Rock Slide Zone Generated by Matterocking
Failure zones generated by Matterocking are shown in figure 5. The province of Southern Leyte is highly susceptible to structurally controlled landslide as obviously presented. This includes the analysis for a planar and wedge failure that may occur due to discontinuities considered. Generally steep slopes in the region and presence of discontinuities due to the segment of PF caused this high susceptibility. Closer look on the Guinsaugon Landslide area, we can expect more failure as for the presence of the scarp before the mass movement happened. It is the case of wedge failure that could trigger most rock slide. This is due to the persistence of intersecting discontinuity sets in the area.
Kinematic analysis was performed for several slopes of the Mt. Can-abag east facing facade as shown in figure6. Slopes covered in red are those that were identified as rockslide source. These were chosen just to verify and further analyze the mode of failure that could occur as shown in table3. Present discontinuity sets in the area were used in the analysis. Results have shown the probability of wedge and planar failure in almost all cases. Friction angle used is the minimum angle parameter of 23 degrees . Intersection of Discontinuity sets 41/61 and 41/185 most probably would cause wedge failure on this side of the mountain.Toppling failure couldnt be taken into account in this case due to lack of very steep recorded discontinuity. We considered the limitation of DEM’s resolution that din angles would always be lesser than the actual values on the field. Planar slide in S1 follows the configuration slide in one slip plane of the Guinasugon landslide.
4.5. Potential Runout Zones Using Conefall
Since the sources of rockfall/slide were already identified, zones where it will propagate was also mapped using Conefall. The study utilized a runout angle of 20 degrees as it showed the representative event of the Guinsaugon landslide in terms of runout distance by using the software. Ratio of “safe” zones from structurally controlled landslide to high susceptibility area is (Ratio). Shown in figure 7 is the combined map for the source and propagation extent of Southern Leyte.
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