8/12/2019 TRIGRS Poster

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Landslide Susceptibility of the Madrid Quadrangle Using TRIGRS A Fortran Program for Grid-Based Regional Slope Stability Analysis

CUSTODIO, Lawrence T., DELA PENA, Federico, DELA RESMA, Marvee B., ECO, Rodrigo Narod C.,

FERRER, Peter Khallil F., MENDOZA, John Paul A., QUIMORA, Patricia Nicole L.,SATURAY , Ricarido Jr. and

VILORIA, Leander Francis C.

National Institute of Geological Sciences, University of the Philippines, Diliman, Quezon City, Philippines

 jmendoza@nigs.upd.edu.ph

SE05 A012

 ABSTRACT

e study aims to illustrate the effect of factors, i.e. slope, topography and elevation, on the factor of 

ety and consequentially, landslide susceptibility of a topography mantled with lateritic soil. Using a

M, flow direction map and slope map of the Madrid Quadrangle (Surigao Del Sur, Philippines),GRS (Transient Rainfall Infiltration and Grid Based Simulation), a Fortran based program, was used to

ermine the values for the factors of safety of the slopes in the area. Overlays with a slope map, a

dslide inventory and a topographic map are used to determine relationships of the mentioned

tors and the factors of safety. A slope in a topography mantled with lateritic soil will fail at a slope

gle higher than 20 degrees. Generally, the factor of safety decreases as the slope angle increases; this

reases the probability and risk of slope failure. Elevation has no bearing on the computation for the

tor of safety. The factor of safety is heavily dependent on the slope angle.

METHODOLOGY 

RESULTS AND DISCUSSION

CONCLUSION

Using the factor of safety as a quantitative measure of slope stability¸ topography mantled with

ateritic soil will fail at slopes with slope angles greater than 20 degrees. This usually happens, but is

not limited to, slopes of ridges or generally mountainous terrain.

lopes become more unstable as the slope angle increases. This is reflected by a decrease in the factor

of safety. Elevation is irrelevant as a factor to be considered in shallow landslide processes. Hence,

hallow landslides are heavily dependent on the slope angle and the geotechnical properties of the

oil cover.

Figure 1. Output map from TRIGRS simulation. Each point represents a factor of safety associated with its

assigned slope. Red represents a factor of safety that is less than 1, yellow for values between 1and 1.5, a nd blue

for values greater than 1.5.

Figure 2. (a) slopes with slope ang les greater than 20 (b) slopes with factors of safety less than 1.

Figure 3. Digital elevation model of study area. Red dots indicate factors of safety less than 1.

Figure 4. Comparison of published Geohazard map from the Mines and Geosciences Bureau (a) and the factor ofsafety output of TRIGRS

allow landslides are the most commonly observed failure involving soil-mantled slopes and are

nsidered major geohazards, often causing property damage and other economic loss. Hence

merous studies on landslide susceptibility including numerical models used infinite slope equation

carabelli, 2007) in order to identify slopes prone to occurrences of shallow landslides.

e study used the software Transient Rainfall Infiltration and Grid Based Simulation which is a

rtran program that computes transient pore-pressure changes and its consequential changes to the

ctor of safety due to rainfall infiltration (Baum et al, 2002). It is a time-dependent raster-based

dromechanical slope stability assessment tool (Chen, 2010) that follows   Iverson’s (2000) model for

ope stability and his definition for the factor of safety (Baum et al, 2002).

INTRODUCTION

Equation 1. Factor of Safety from Iverson (2000). ϕ’ is the soil friction angle for

effective stress, Z is depth of soil, c’ is the soil cohesion for effective stress, ψ

(Z,t) is the pressure head,  γw is the unit weight of groundwater and γs is soil

unit weight and δ is the angle with respect to the horizontal.

00x600 gridded Digitalelevation map fromographic map (20 sq. mper grid resolution)

HydrologicalCorrections

(ILWIS)

Topoindex (TRIGS companionprogram) prepares input map

files (DEM, flow direction,slope)

Input

Parameters

(see table 1)

TRIGRS

Computation

(factor of

safety, run-off

and

infiltration

Table 1. Input for Laterite Soil Parameters.

Values taken from published material. Soil

thickness and water table depth areassigned values.

All cells that yielded values of less than 1 for its factor of safety coincide with slopes greater than 20

ee Figure 2)which are mostly distributed generally on rugged terrain with steep slopes, usually found

ong ridges. The ridges themselves, however, are stable (see Figure 1,3). No direct correlation between

evation and the factor of safety is observed. Red values occur on the slopes of ridges, low elevation

lues and slopes near streams. Values for the planar features of the ridges are stable.

Analysis of the slope subsets (Table 2) shows that majority of the values for failure come from slopesnging from 20-30 degrees. No critical cells were observed for slope angles less than 20. First

pearance of critical cells occurs at slopes greater than 20 degrees.

The percentage of cells predicted to fail, as compared to the total number of cells with the same slope

gle, increases as the slope angle increases (see Table 2, Figure 5). Though it seems that slope stability

heavily dependent on the slope angle, the properties of the soil also have a weight on the failure of a

ope. This is explained by the equation provided by Iverson (2000) (see Equation 1).

Figure 5. Factor of Safety percentage bar graph per slope subset.

Table 2. Percentages of failed slopes

slopes (FS<1)per slope subset.