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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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Master in Engineering in Civil Engineering(MECE) majors:Structural/Geotechnical EngineeringMasteral Special Project(Thesis)
9 to 10 March 2012University of San Carlos , Cebu City
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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Master in Engineering in Civil Engineering(MECE) majors:Structural/Geotechnical EngineeringMasteral Special Project(Thesis)
9 to 10 March 2012University of San Carlos , Cebu City
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).
3
SpriSpri
Spring discharge,S ri
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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
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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Master in Engineering in Civil Engineering(MECE) majors:Structural/Geotechnical EngineeringMasteral Special Project(Thesis)
9 to 10 March 2012University of San Carlos , Cebu City
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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Master in Engineering in Civil Engineering(MECE) majors:Structural/Geotechnical EngineeringMasteral Special Project(Thesis)
9 to 10 March 2012University of San Carlos , Cebu City
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.
6
Tension
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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
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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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
geological conditions of the area, watertable effects.
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