Site Characterization of Taytay Report Paper

8/3/2019 Site Characterization of Taytay Report Paper

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Master in Engineering in Civil Engineering(MECE) majors:Structural/Geotechnical EngineeringMasteral Special Project(Thesis)

9 to 10 March 2012University of San Carlos , Cebu City

SITE CHARACTERIZATION OF TAYTAY, PALAWAN, PHILIPPINESRAINFALL TRIGGERED SHALLOW LANDSLIDE:

BASIS FOR LANDSLIDE PREVENTIONS.

Engr. Cesario A. Bacosa ,Jr, Ph.D-EM

Assistant Professor, Graduate SchoolDean, College of Engineering

OIC - College of Information and Communications TechnologyHoly Trinity University, Puerto Princesa City 5300 , Palawan , Philippines

Tel No: +63 -048-433-2161 local 245 to 246 ; Fax No: +63-048-433-2161 local 265*email:[email protected] / [email protected]

Abstract- Throughout Palawan, almost all areas near hillsides or mountain slopes are threatened by landslides

caused by heavy rainfall during rainy and typhoon season. Rainfall triggered landslides are part of a naturalprocess of hill slope erosion that can result in catastrophic loss of life and extensive property damage inmountainous, densely populated areas. This paper presents the tragic loss of 6 lives inTaytay, Palawan,Philippine landslide attracted a lot ofthe Philippines local and national media attention and with it a lot ofspeculations and rumors as to what caused the slide after 3 days heavy rain and typhoon Ondoy and Pepeng in2009. This study revealed the hydrological triggering mechanisms and rainfall thresholds of landslides inadjoining hills with permeable organic clay soil and mudstone. Site investigation and characterization wereconducted to inspect the surface structures and to obtain geotechnical properties of slope materials. In the hillslope with the impermeable mudstone, the hydraulic discontinuity beneath mudstone thin clay soil layer causes atransient positive pressure head that generates a saturated water flow. An analysis of the relationship between themagnitude of rainfall and hill slope instability provides a rainfall threshold for land sliding. The site specificcombination of rainfall intensity and duration incorporates geotechnical properties of hill slope materials andslope hydrological processes.

Keywords: Shallow Landslide ,Slope Stability, Rainfall Triggered , Site Characterization.

INTRODUCTION

Geo-hazard are events caused by geologicalfeatures and processes that present severe threats tohumans, property and the natural built environment.The need is accentuated by increased sliding inmany areas, increased concern for geo-hazardsincreased vulnerability. Climate research indicatesthat one can expect more extreme weather in the

future leading to increased flooding, landslides,erosion, scour, and rock falls [Hungre et al, 2000].

Landslides occur on any terrain given the rightconditions of soil, moisture,and the angle of slope.Integral to the natural process of the earth's surfacegeology, landslides serve to redistribute soil andsediments in a process that can be in abruptcollapses or in slow gradual [ Finn, 1987].

Soil slips is a way and a process that isassociated with the phenomenon cause by forces ofnature its cause and effects. Most cut slopes and hillslopes is prone to landslides and rock fall that will

be hazardous to human and possible cause of theloss of life as well [Coromias, and Moya, 1999].

Rainfall-triggered landslides are part of a natural

process of hill slope erosion and are affected byvarious factors such as rainfall characteristics, soilstrength and hydraulic properties that can resultcatastrophic loss of life and extensive propertydamage[ Selby, 1982].

In this research, investigate the relationshipsbetween landslides and these conditioning factors.Rainfall is the most important factor, and therelationship between rainfall characteristics andlandside has been studied intensively [Iverson, 2000;

Keefer et al, 1987].Soil hydraulic properties control rainfall

infiltration within soil layer, and soil strength refersto soil resistance against sliding force of soil mass.Considerable research has also been conducted onthese important soil physical properties [Harp et al,1990].

The said investigation focusedits attention on theEngineering aspects of the overall investigation. Itwas necessary to understand what caused thelandslide, and how the landslide occurred. Landslidearea was examined,each geomorphology of thelandslide, scarp, topography, soil hydraulic

properties and the rainfall condition.Information about the landslide was also

collected from residents who had witnessed the

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mailto:[email protected]%20/%[email protected]:[email protected]%20/%[email protected]:[email protected]%20/%[email protected]


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events [ Daizo and Fujita, 20011].

BACKGROUND OF TAYTAY ,PALAWAN, PHILIPPINESLANDSLIDE.

Location.

The occurred landslide is located at Ibangli,Taytay, Palawan at the rear part of the MunicipalBuilding of Taytay . The Municipality ofTaytay islocated at the Northern part of Palawan 235kilometers from the Heart of the City of PuertoPrincesa .

It is situated nearby the sea and mountainousterrain with timber land.Taytay is a First classmunicipality in the province of Palawan,Philippines. According to the 2010 census, it has a

population of 83,657 people in 10,883 households

and its administrative boundary was reduced byapproximately 500,000 hectares on 1916.Below isthe Location Map of the Site.

Fig.1 Map of Taytay, Palawan.

The Landslide area is nearby the residential areaat adjacent sides, it was situated at the back of themunicipal hall of Taytay (Fig.3) and nearby

Migrants building 10 meters about the rural healthbuilding (Fig.4).

Fig. 2 Photos of Rural Health duringconstruction.

Fig. 3 Photos with Municipal Hall.

Fig. 4 Photos of Migrants Building.

Fig. 5 Photos of Existing pavements roads

2

Taytay

Concrete Road

Present tension

Rural Health

Migrants


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Fig. 6 Photos of Residential Area.

The Rural Health Building (Fig.2) is situated ina cut slope of 2-3 meters high and 2 meters away

from the road that has been paved with plainPortland Cement Concrete(Fig.5). The Rural HealthCenter and Migrants Building were builtsometime in year 2000.

West side of the landslide is the residential areassituated 5 meters away from the landslide. The eastside of the landslide area facing the sea was

populated with residential houses that dominate thearea (Fig.6).

Based on the report of the City Environmentaland Natural Resources Office -CENRO, a landslidealso occurred in open space between the buildingand cut slope portion of the ridge with landslidedebris materials.

The landslide

The landslide ( Fig. occurred on October 7, 2009at around 12:05 p.m, had buried the one story RuralHealth Center Building and destroyed the water tankfacility of the municipal hall [Kwok,2009].

Fig.7 Landslide Site (Top view, Side view,

Front view and Far view)

The incident also claimed the lives of six socialworkers of Taytay, Palawan, who happened to beinside the Rural Health Building when landslideoccurred. The two- story 'Balay PanuluyanBuilding- Migrants Building located about 10meters away from the rural health center building isstill intact and unaffected by the landslide.

Prior to landslide incident, the area hadexperienced nearly two weeks of continuous rains

brought about by tropical storm Ondoy and typhoonPepeng. Although there were no reported rains at the

time of landslide, a heavy downpour occurred on thenight before the landslide incident.Flowing underground water is observed when

heavy rains along soil pipes, pores and cracks at thetoe of the cut slope. When landslide occurred, it wasvery sudden and according to some eyewitnesses,occurred slowly.

Fig.8 Preferential flow pathways (Spring)within soil subsurface layer.

Interviews to some Rural Health Workers andMunicipal staff and nearby residents indicated thatthere has been manifestation of slow movement ofthe cut slope in the Rural Health Center as manifest

by some cut slope deformation and debris fallencountered at the back of the rural health buildingnearby cut slope. This happened several weeks

before landslide.

DATA GATHERING AND STUDIES.

With the data already collected on theGeology of the area and specifically the orientationsof the landslide, the researcher concentrated on thefollowing objectives:

a. Gathering of the surface data byconducting soil exploration in the slide zone and theundisturbed side area.

b. Gathering of intact soil sample by

manual soil auger and classification of soil samplesusing Unified Soil Classification System (USCS).

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SpriSpri

Spring discharge,S ri


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c. Mapping of the slide zone bytopographical survey.

Fig.9 Landslide Site Exploration.

SITE CHARACTERIZATION DATA.

The soil sample was taken both left and rightsides of the slides at the toe, middle and head of thescarp of the slide area by manual soil auger having adepth of 1 meter.

The following soil test was performed todetermine the soil types and classification; Watercontent; Unit weight; Specific gravity; Liquid limit;Plastic limit; Plasticity index; liquidity index,consistency, dispersion, shrinkage limit, shrinkageindex, shrinkage ratio, flow index, unconfinedcompressive strength , Permeability coefficients;Cohesion; angle of internal friction and other soil

properties that was suspected as triggeringmechanism of the slide.

Determination of Soil Properties.

The soil sample taken from the landslide

site was stored and preserved in the laboratory andsubjected to series of test.

Table.1 Landslide Soil Properties

Soil Properties Values/ Remarks

Soil Group Category A-7-6

Soil Types CH

Soil Classifications Inorganic Clay ofHigh Plasticity (fat

Clays)

Water Content 51.08 %

Saturation. 40.57 %

Unit Weight(wet) 17.61 kN/m

Unit weight (dry) 12.05 kN/m

Void Ratio 0.46

Porosity 0.32

Plastic Limit 19.98 %

Liquid Limit 56.0 %

Liquidity Index 1.20

Plasticity Index 36.0 %

Fineness Modulus 4.48

Activity 3.72

Flow Index 12.95

Shrinkage Ratio 1.43

Shrinkage Index 4.51

Shrinkage Limit 31.78 %

Specific Gravity 2.46

Consistency Index 1.07

Undisturbed qu 55.49 kPa

Remolded qu 33.84 kPa

Sensitivity 1.51

Permeability

Coefficient 0.001 m/sec

Discharge Velocity 0.0002 m/sec

Seepage Velocity 0.0013 m/sec

Cohesion 27.75 kPa

Angle of Internal

Friction 5.15 degrees

The table 1reveals the soil properties subjected tovarious testing. The slide materials were classifiedas CH or Inorganic Clays of high plasticity orfat clays based on Unified Soil Classification

System (USCS). The materials water content (w%)is 51.08 %, the degree of saturation ( S%)is 40.57%,

The wet and dry unit weight () of 17.61 kN/m and 12.05 respectively which classify asdense materials. Materials Specific gravity(Sp.Gr) is2.46 and its fineness modulus ( FM)of 4.48correlates to permeable soil type. Void ratio(e) and

porosity (n) of 0.46 and 0.32 respectively indicatesthat materials is potential for swelling and shrinkagethat leads to fractured and potential water flow

preference.The Potential swell and expansive test

was performed such as Plastic Limit (PL) of 19.98% indicates low to medium potential swell. TheLiquid Limit (LL) of 56 % > 50 < 60 classified as

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medium potential swell and high potential plasticity). Liquidity Index (LI) of 1.20 > 1.0indicates that materials has low strength, it deformslike a viscous fluid. Plasticity Index (PI) of 36.0 >20 < 40 Reveals that materials has high plasticity

and medium potential swell.Other materials indices were tested and

the materials Activity (A) is 3.72 > 1.2 shows thatorganic clayey soil is an active clay for swell

potential. Shrinkage test such as Shrinkage Limit(SL), Shrinkage Ratio (SR) and Shrinkage Index(SI) of 31.78%, 1.43 and 4.51 respectively is anevidence of Shrinkage potential.

Hydraulic conductivity was tested usingstandard permeability test, the permeabilityCoefficient ( k) of 0.01 m/second, Dischargevelocity ( v) of 0.0002 m/second and Seepagevelocity (vs) of 0.0013 m/second causes the water toflow along pores and cracks due to saturation and ithas a flow index of 12.95. The spring seepage wasalso measured using field seepage test , it showsthat the water spring seepage/discharge is 50 cc/sec.

Unconfined Compression Test wasconducted of the undisturbed sample shows that theundisturbed and remolded Unconfined CompressiveStrength ( qu) of 55.49 kPa and 33.84 kParespectively .The Cohesion(c) of 27.75 kPa with aninternal angle of friction () of 5.15 degrees is anevident of triggering landside after heavy rains .

Strength potential of the slide materialswas significantly affected due to layer movements,changes in volume, swelling, crackings and

expansion due to prolonged water saturation.However, the presence of water at

saturation levels clearly caused lubrication andreduction of effective stresses at the joints andweakening of the Inorganic clayey materials andmudstones as to have contribution to the overallweakening of the slide supported by Morales et al.,[ 2001].

Likewise soil indices affects anddirectly contributed to the potential swelling,expansion, crackings and disintegration of soil

particles due to water saturation that easily water toflow freely along the pores and cracks that leads to

slide.

Potential Expansion Test .

The organic clayey soil and mudstonematerials was pulverized and tested based on theUnified Soil Classification System (USCS). Allorganic clayey soil and mudstone materialsclassified as CH or Inorganic Clays of highplasticity or fat clays.

The Liquid limit (LL) values ( > 50%)and the plasticity indices (PI = LL- PL = 36 > 20)indicated that the materials is highly expansive soilthat will be subjected to shrink and swell. This

contributed to the disturbance and movements in theupper layer joints from shrinkage and expansion

prior to slide after saturation[ Morales et al, 2001].

Slope Geometry Determination

Slope geometry and topographical data

was obtained using the Theodolite transit. The actualsurvey was taken at the site area of the failure planeand the original hill slope. The next figure is

presented.

Fig. 10 Topographical condition of the landslide.

Fig. 11 Topographical condition of the landslide atthe

(left side).

Fig. 12 Topographical (center)condition of thelandslide

Fig. 12 Topographical (right side )condition of thelandslide

The landslide highest elevation from the sealevel is 138 meters. The total height of the landslidefrom the toe to the peak of the landslide is 42.82meters. The scarp height is approximately 20 meters.

Table 2. Natural Hill slope and slope of theFailure plane.

5

5 5.13

6.01

11.13

14.41

17.96

21.28

25.84

29.60

33.3

38.42

42.82

23.50 M

80.00 M

Natur

algro

undb

eforethe

land

slide

groundp

rofile

afte

rthe

land

slide

38.00 M

92.00 M

AVERAGE NAT. GROUND SLOPE =38/92 =0.41

PROFILE AT SECTION A-ASCALE :

VERTICAL 2:1

HORIZONTAL 1: 1

0.00

5.00

10.00

15.00

20.00

25.00

30.00

35.00

40.00

45.00

50.00

9.5000

9.500.00 21.0 28.61 39.53 50.79 59.05 68.84 77.6 86.86 96.39 101.9

ROAD

CENTER

LINE

At present

At present

At present


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STA

O C CO B S B S A Z IM U T H U P P E R L O W E RM E A N V E R T IC A LS L O P E D S LO P E T A P E V E R T IC A LH O R V E RT I C A LELEV

H I R E A D IN GREA DINGREAIN GIN TERCEPTIN TERCEPTDISTAN CESLO PE A NGLEDISTAN CEDISTAN CE

0 1.6 353.00 6.600 D ISTA N C E 5. 00

1 2 5 9 .2 5 6 . 6 00 1 . 5 15 1 . 42 0 1 .4 6 8 0 . 0 95 0 . 0 9 5 9 . 5 0 0 1 0 . 00 0 0 . 0 0 9 . 5 00 0 . 00 0 5 . 13

2 2 5 9 .2 5 6 . 6 00 0 . 6 9 0 0. 4 8 0 0. 5 8 5 0 . 2 10 0 . 2 1 0 2 1. 0 0 0 20 . 0 00 0 . 0 0 2 1 . 0 0 0 0 . 0 00 6 . 01

3 2 5 9 .2 5 6. 6 0 0 0 . 66 0 0 . 3 65 0 . 5 1 3 0 . 2 95 0 . 2 9 1 2 9 . 05 2 3 0 . 00 0 10 . 0 0 2 8 . 6 11 5 .0 4 5 1 1 . 1

4 2 5 9 .2 5 6. 6 0 0 0 . 72 5 0 . 3 15 0 . 5 2 0 0 . 4 10 0 . 4 0 1 4 0 . 10 7 4 0 . 00 0 11 . 9 8 3 9 . 2 33 8 .3 2 7 1 4 . 4

5 2 5 9 .2 5 6. 6 0 0 0 . 6 10 0 . 0 7 5 0 . 3 43 0 . 5 35 0 . 5 2 1 5 2 . 12 9 5 0 . 00 0 13 . 0 0 5 0 . 7 93 1 1 . 72 6 1 7 . 9

6 2 5 9 .2 5 6. 6 0 0 0 . 9 10 0 . 2 8 0 0 . 5 95 0 . 6 30 0 . 6 1 0 6 0 . 99 3 6 0 . 00 0 14 . 5 0 5 9 . 0 51 1 5 . 27 1 2 1 . 2

7 2 5 9 .2 5 6. 6 0 0 0 . 8 70 0 . 1 2 5 0 . 4 98 0 . 7 45 0 . 7 1 6 7 1 . 61 4 7 0 . 00 0 16 . 0 0 6 8 . 8 40 1 9 . 73 9 2 5 . 8

8 2 5 9 .2 5 6. 6 0 0 1 . 4 00 0 . 5 5 0 0 . 9 75 0 . 8 50 0 . 8 1 2 8 1 . 21 3 8 0 . 00 0 17 . 1 7 7 7 . 5 96 2 3 . 97 0 2 9 . 5

9 2 5 9 .2 5 6. 6 0 0 1 . 1 65 0 . 2 1 0 0 . 6 88 0 . 9 55 0 . 9 1 1 9 1 . 08 0 9 0 . 00 0 17 . 5 0 8 6 . 8 65 2 7 . 38 8 3 3 . 3

1 0 2 5 9 .2 5 6 . 60 0 1 . 4 3 0 0 . 3 5 5 0 . 8 9 3 1 . 0 75 1 . 0 1 8 1 0 1 .7 9 5 1 00 . 0 0 0 1 8 . 75 9 6 . 39 3 3 2 . 72 0 3 8 . 4

1 1 2 5 9 .2 5 6 . 60 0 1 . 5 8 0 0 . 4 2 5 1 . 0 0 3 1 . 1 55 1 . 0 8 5 1 0 8 .4 8 9 1 10 . 0 0 0 2 0 . 07 1 0 1. 9 0 33 7 . 22 3 4 2 . 8

1 2 2 3 6 .4 3 6 .6 0 0 1 . 8 3 0 1 . 2 70 1 . 5 5 0 0 . 5 60 0 . 5 2 2 5 2 . 19 3 5 2 . 0 00 2 1 . 25 4 8 . 64 4 1 8 . 91 6 2 3 . 9

1 3 2 2 6 .4 6 . 6 0 0 0 . 8 3 0 0 . 39 0 0 . 6 1 0 0 . 4 40 0 . 4 1 3 4 1 . 34 7 4 1 . 00 0 20 . 0 0 3 8 . 8 53 1 4 . 14 1 2 0 . 1

The table above reveals that themaximum slope of the landslide is 21.35 from thehorizontal toe up to the inclined plane failures, thusthis slope triggered the landslide.

Fig. 7 Original Topography before slide.

Fig. landslide Topography after slide.

Effects of Water Saturation on Strength.

The undisturbed and remolded sampleswere grouped and tested for immersion for threetime settings of 24 hours, 48 hours and 72 hours

Fig.6 Soil disintegration and effects of immersion.

The figure 7 of the UnconfinedCompressive Strength qu( kg/cm) and its watercontent (w%) is presented in figure 6 for soil andmudstones.

Fig. 7 Unconfined Compressive strength ofmudstone

Fig. 8 Unconfined Compressive strength of Soil.

The trend lines in the figure aboveindicate the decrease in unconfined compressivestrength while the water content is increasing clearlyevidence establishing that water saturation couldhave an effect on the strength of soil( Morales etal,2001].

GEOTECHNICAL STUDIES, ANALYSIS

AND ASSESSMENT

The Engineering studies and analysestook into account the following information in orderto determine the triggering mechanisms. Informationabout the landslide was also collected from residentswho had witnessed the events.

All of the information was synthesizedand confirmed that several factors are vital toclarifying the mechanism of landslide occurrence:

1. Preferential flow pathways withinthe subsurface layer;

2. Soil properties , includinghydraulics properties and soilstrength ; and

3. Rainfall characteristics.

This study used a physical experimentto clarify the mechanisms by which these factorscontribute to the occurrence of landslides.

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Tension


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Fig.9 Landslide Site(Scarp, Tension crack andspring level).

Landslide Hazard Assessment.

Based on Geohazard mappingconducted in the affected area and its immediatevicinity together with anecdotal account of the local

people, hereunder are the researcher assessment;

1.Landslide in the area shows characteristics ofslumping. It occurred at the cut slope on thenorthwestern side of a moderate to steeply slopingridge. 2. The highest peak of the ridge reaches 138meters above sea level based on the available

NAMRIA topographic map of Taytay.3. The toe of the slope was cut to provide

adequate space for the said Rural Health Centerbuilding.

3 .The landslide materials are composed oforganic clayey soil, mudstone, and highly weatheredand clay altered rock. Highly weathered rock

boulders were also noted in some landslidematerials.

4. The crown of the main slump is arcuate, andthe scarp height is roughly 20 meters with landslideextending up to 100 meters. The general direction ofthe slump is S 45 to 50 W.

3. Ground tension crack are still visible on thenorthwestern side of the main slump near the headscarp. The cracks are about 1 to 2 meters from the

edge of the landslide/ failed slope4. Prior to the landslide incident around 8:00 am

of October 7, 2009, people in the area noted asudden occurrence of water/ spring flowing at the

base of the slope. Some employees of the municipalhall actually dug a canal to let the surface water flowfreely. This situation was already an indication ofhighly unstable slope condition of the area.Furthermore, the eventual destruction of rural healthcenter building and water tank had blocked andstopped the landslide run out to move fartherdownslope.

5. The triggering factor of the landslide is rain

and causative factors are combination of thick soilcover and highly weathered bedrock, organic clayeynature of soil and the alteration of the originaltopography with removal of toe support at the baseof the slope.

6. The Present slope face of the landslide is stillhighly prone to slope failures/ landslide. Based onthe present cracks on the ground, the magnitude ofimminent landslide is generally lesser in volumecompared to the recent landslide.

7. It was observed that landslide is progressive,meaning it becomes larger as the height of thevertical cliff created by previous slope failureincreases. Basically when rocks are exposed by

landslide, they are more susceptible to weatheringprocesses which weaken the inherent strength of the

rock. This scenario had already been proven sincethe landslide started as shallow failure along theunstable cut slope. ( show a picture of progressiveslides)

Landslide Risk Assessment.

1. High risk areas include;(a) the directly affected sites of the slope

failures;(b) the northwestern side of the landslide area

where ground tension cracks were observed, and(c) the upper slopes of the landslide area.

Nearby houses are situated on the northwestern sideof the landslide area while the Migrants building isvery much proximal to the area of landslide run out.2. Moderate risk areas are farther downslopes fromthe northwestern slope of the ridge where themunicipal building is located and along the slopeson the other side of the landslide area where thenearby houses is situated.

3. Landslide is normally induced by the effectsof groundwater or thin layer of surface waterinfiltrating the cracks on the ground during rains.Water induces hydraulics pressure on the slope.

Strength Degradation of Soil.

The results of the laboratory tests on the

organic clayey soil and mudstone in the slide areasevidently and potentially expansive nature of the

mudstone as evidenced by:

1. Very high mean Liquid Limit (LL) andPlasticity indices (PI) 16.26% and 51.25%respectively classifying as OH (Organicclays of medium to high plasticity).

2. Disintegration of the core samples whensubjected to immersion after cooling fromoven drying indicates susceptibility todegradation due to drying and saturationeffects

Fig. Immersion of soil (24,48 and 72days).

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Significant strength degradation due toswelling and volumes changes of mudstone samplessubjected to varying periods of water immersion for24 hours, 48 hours and 72 hours respectively

Shrinkage, cracking and expansion due to prolongedwater saturation could have affected the in placestrength characteristics and also have a contributionto the initiation of the landslide.

DIFFERENT INFLUENCES ON THELANDSLIDE.

Climatic Effects and Rainfall intensity

The unusually wet weather andunpredictable abnormal high rainfall intensityoccurred in the months before the landslide. This

was preceded by the tropical storm Ondoy andtyphoon Pepeng.

Fig. 10 Satellite Rainfall mapping oftheTropical

Storm Ondoy.

On September 26, 2009, Tropical StormOndoy dropped a months worth of rain in amatter of hours on the Philippines in 2009dumped a total of 455 millimeters of rain in 24hours. Ondoys rainfall turned out to be of a flashflood type and was very unanticipated andunprepared for, which led to many deaths andextensive destruction of property. Sliding

Maximum Rainfall Depth: 6 hours - 381.5 mm ;9 hours - 418.0 ; 12 hours - 448.5.

Fig 11 Rainfall Intensity mapping of Typhoon

Pepeng and Tropical stromOndoy.

The highest rainfall amounts more than600 millimeters (23.6 inches) appear in blue. Thelightest amounts appear in pale green. Grayshading indicates topography throughout thePhilippines.Rainfall occurs over the entire region

shown in this image. The heaviest pocket of rainappears off the west coast of southern Luzon,over the South China Sea.

Fig.12Rainfall mapping of the Typhoon Pepeng.

The two weeks preceding the landslidehave seen the heaviest rainfall in the area in the last3 days immediately before the landslide eventencountered.

October 6 7 , 2009 : 6 hours: 347.5 mm ; 9 hours:413.0mm; 12 hours: 448.5mm and SlidingMaximum Rainfall Depth: 6 hours: 381.5 mm ; 9hours: 418.0 mm; 12 hours: 448.5mm;24 hours

587.3mm.

Fig.13 Rainfall Chart 2 weeks preceding theLandslide

The Figure 13 above reveals that therainfall intensity of more or less 600 mm a day thattriggered the landslide due to massive and prolongedwater saturation.

Consideration of Preferential Flow Pathwayswithin Subsurface Layers

Preferential flow pathways within soillayer generally hasten rainwater infiltration andmake a slope more unstable. However, researcherhave consistently shown that the clogging of

preferential flow pathways decreases slope stabilityand may triggered landslide [ Pierson, 1983;Unchida et al., 1995].

Other studies have confirmed the effectsof preferential flow within the soil layer onrainwater infiltration and slope stability usingexperimental data.[Tsutsumi et al., 2005].

Preferential flow through fractures inbedrock has similar function to that in a soil layer. In

this situation, it is possible that a rapid rainwatertransportation through the fractures in the weatheredclayey -altered rock quickly produced pore water

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pressure at the bottom of the weathered rock layer,causing landslide without any delay after the time

peak rainfall [ Sidle and Chigira, 2004].

CONCLUSIONS

In the long week intensive heavyrainfall from September 26 to October 6immediately preceding the landslide of October 7,2009 at 12;05 p.m, record rainfall intensity levelswere experienced with the highest level of 587.3 mmoccurring on October 6, 2009. With this data,development of hydrostatic pore pressure was

possible.It was reported that water are not

coming out from the weep holes during heavyrainfall. The weep holes helps in lowering the watertable of the slope. The higher the water table, thehigher the hydraulic pressures on the slope.

The spring was intermittently observedand continuous surface stream by the landslidemovement can add water to the ground water in theslide area. Prolonged water saturation andsubsequent pore pressure buildup was the finalcaused that triggered landslide.

Moderate risk areas are those areaswhich could be affected by progressing landslide orreached by landslide run out or mudflow in case

bigger landslide occurred during heavy andcontinuous downpour. Slumping is sometimeassociated with mudflow when the landslidematerials are heavily soaked with water.

The shrinkage caused by extremely dryweather induced cracking on the organic clayinfilling of the joints, thus, further weakening thestrength of the intensely jointed mudstone causingsoil and rock layered movement.

The extremely heavy rainfall in turninduced the injection of water into the cracks and

joints causing increase seepage and infiltration;increased water pressure induced swelling andexpansion of mudstone and organic clayey soil; andincreased saturation induced weakening of themudstone.

The heavy rainfall in the area

immediately preceding the Ibangli Landslide eventhad contributed a critical part in the initiation of theslide. In fact, many other minor soil slip weretriggered days before the Ibangli landslide becauseof the heavy rainfall. Thus, this clearly indicates thecritical role of water saturation and buildup of pore

pressures on the slide ( Morales et al, 2001).The soil properties was obtained in the

laboratory test the following findings comprised thefollowing;

Based on field evidences gathered,landslide in the areas is in the form of slumping.Thick soil cover and highly weathered bedrock,Inorganic clayey nature of the soil and the alterationof the topography with the removal of toe support onthe base of the slope are all primary factors that

caused landslide in the area. With the presentcondition of the landslide, the failed slope is stillhighly prone to slope failures.

RECOMMENDATIONS

1. The actual site of the landslide needs to befurther verified in addition to the limited

boring test.

2. Seismic Refraction isrecommended to determine thedepth analysis of the futurepossible slide at the back of theMunicipal hall.

3. Further exploration of the nearby areas ofthe landslide is needed to characterize and

prevent future slope failure specially alongthe eastern part whereby the residential

areas is located.4. As part of the disaster risk reduction and

preparedness in the area, the houses withinthe high risk areas should be relocated.

5. Continuous monitoring of the landslideareas should be conducted to observe allindications of landslide such as theoccurrence of crack on the upper slopes ofthe landslide areas and the presence ofspring or water on the base of the slope. Incase all indications of landslide areobserved, all the people staying within themoderate risk areas should leave the place

or prepare for possible evacuation.6. Drainage canals / ditch above the landslide

area is recommended to divert or minimizethe flow or rain water to the unstable slope.

7. The weep holes on the retaining wall at the back of the municipal hall have to bechecked in its effectiveness in drainingwater from the slope.

8. People living with moderate risk areasalways are alerted especially when there istyphoon or continuous rainfall.

9. 7. The local residents should religiouslyobserve the conditions of the affectedground and should be alert on the presenceof major cracks and slumps/landslideelsewhere. In case cracks appear on higherground near the community settlement,residents should temporarily leave the areaand report the situation to the Municipaldisaster coordinating council.

10.For the future , Come up theComputerized model analysis of the slopeto further analysis and predict the nearbyareas prone to landslide.

11. The researcher recommends the futureresearchers to study the parameters notincluded in this research such as depth

possible slide, seismic refraction, effects ofvegetation, effects of soil suction,

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geological conditions of the area, watertable effects.

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