Joseph Joseph Raymu Raymund nd A. A. Sumabal Sumabal GM 206 – Introduction to Physical Sciences MS ERM, University of South Eastern Philippines March 16, 2011 Cherry Hills Landslide I.
Introduction
Cherry Hills Landslide is one of the worse tragedies in the Philippines in the year 1990. Like many landslide events, there were many factors that can contribute or trigger the landslide. This case study is produced to determine and understand the cause of the landslide and how it occurred. Also, this paper will provide how landslide mechanism can be prevented in the future. II. II.
Cher Cherry ry Hil Hills ls Subd Subdiv ivis isio ion n and and its its Dev Devel elop opm ment ent
Cherry Cherry Hills Hills Subd Subdivi ivisio sion n is ow owned ned and and develo develope ped d by Tirso Tirso Sant Santilla illan, n, presid presiden entt of Philip Philippin pine-J e-Jap apan anese ese Co Corpo rporat ratio ion n or Philja Philjas s Co Corpo rporat ration ion.. It is located in Barangay San Luis, Antipolo City. It has an area of five hectares includin including g paved paved roadway roadways. s. The residential residential units are two and single storey storey houses that are either single detached or row housing. Houses built are light roofed with concrete hollow black wall construction and reinforced concrete columns on isolated footings.
The adjacent adjacent areas areas of the Cherry Cherry Hills subdivisi subdivision on are two subdivision subdivisions s and the Sierra Madre Mountain Ranges. The two subdivisions are the Parkhills Subdivision and the Peace Village on the north and south respectively. The open pen are area of the the Cher Cherry ry Hill Hills s Subd Subdiv ivis isio ion n was exp expande anded. d. The The Philj hiljas as
Corporation made a more than 20-meter deep cut from the crest of the mountain slopes. The deepest cut was 26 meters. III.
Landslide Precursors
Human observations
Several months before the landslide event, there were cracking seen on the walls. There were also difficulties encountered in opening and closing of doors and windows. Residents said that the doors and windows somehow have lost its fit in the door and window jambs. From that, telltale of landslide were spread and some residents evacuated. Several hours before the landslide event, cracks were observed in some houses, which continued progressively. These observations were heeded by some people and evacuated. Climate
There was an El Niño phenomenon that took its height in June of 1999 and lasted a month more. The extended drought and unusually very dry and hot climate induced desiccation and shrinkage cracking of soil layers. After that, unusual wet weather and abnormally high rainfall takes place. The heavy rainfall started a month prior to the landslide event. The heaviest rainfall felt in years was two weeks prior to the said event. And there’s more…Three days before the event was the heaviest encountered rainfall. The unusual wet weather and heavy rainfall was due to the approaching typhoon Olga.
IV.
The Landslide Event
On August 03, 1999 at around 7pm, the Cherry Hills Landslide Tragedy occurred. The shape of the sliding mass was in spoon shape configuration that is typical of rotational slides in soil. There were also debris and mudflow which is referred as corollary slides. According to the surviving residents the landslide was very sudden and occurred in seconds only. There were 58 lost lives and one unaccounted. Damage to properties was estimated to run in millions as there were 379 houses that were destroyed and hundreds of families were affected.
V.
Factors causing the Landslide
Human Factors
Before the development of the Cherry Hills Subdivision, various test and investigations are made to ensure that the area is buildable just like any other development. One of these tests is the boring test which determines the characteristic and strength of the rock mass where the foundations of the building rest. A survey is also conducted to know if there are faults and seismic activities in the area. After all studies are made, the area is safe for construction and habitation. The boring test of the Cherry Hills Subdivision shows that the material strength of the rock mass was low because of highly fractured layers. These fractured layers are due to the Binangonan Fault. Therefore, the development lies near a seismic fault. Prior to the development of the subdivision, the mountain on the side was cut more than 20 meters deep from the crest of the mountain slope. The
deepest cut measured was 26 meters. The cut was to serve the quarrying operations and it was left without lateral support. The mountain slope was not properly cut and it was measured with unfavorable dip of eight degrees. An Official of the Japanese Real Estate Firm admitted that the quarrying operation may have contributed to the landslide. Climatic and Geological Factors
The results of the boring test also show that the siltstone, a soil layer, materials are highly expansive and therefore subject to shrink and swell cycles. This siltstone material shrinks when exposed to dry and hot climate, and swells when they are saturated with water The extended drought and unusually very dry and hot climate makes siltstone layers desiccate and shrink, thus causing it to weaken. Also, clay infillings on major joint system along fractured layers could also been induced to cracks. This explains the few cracks found in the road pavements of the Cherry Hills Subdivision several months before the landslide events. A month before the landslide event, the El Niño was taken place by heavy rainfall. Thus, water seeps to the ground. Extended heavy rainfall and overly wet weather increased seepage and infiltration into the joints causing widespread lubrication of the highly fractured rock mass. Water saturation increased buoyancy thus creating uplift effects. These effects are seen on increased cracks and damages on road pavements and walls of the houses. The increased water pressure induced swelling and expansion of highly expansive siltstone layers causing heaving disturbance of the intensely fractured rocks. Also, the increased water saturation decreases the strength of the slope materials. VI.
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Summary of Findings
Water saturation of the slope triggered the main slide, although creep was occurring over time prior to the main slide The lateral and vertical deformations of the roadways of the subdivision were caused by slumping of mass. The slide was a compound slide made up of slump and rotational components. The main slide mechanism was the rotational component towards the east to west direction. Debris and mudflow occurred in the north and south gullies.
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Creep contributed to the weakening of the subdivision platform causing it to fold and bend. Not one, but several factors or influences coming together caused the landslide.
VII.
Prevention and Monitoring Measures
In every development, the necessary field test and survey must be conducted to ensure construction and habitation. Every developer must seek advices from the technical experts and professionals and secure clearances and permits from the local government so that risks will be minimized. Building along or near a fault line, constructing in a highly unstable soil, and deeply cutting the crest of mountain slopes are very dangerous works and may cause disasters. But technology nowadays provides solutions to these problems. A structurally sound building design can withstand a certain magnitude of earthquake or any seismic movements. Also, cutting the crest of mountain slope can be approved. However, precautionary measure should be added to guarantee the stability of the slope. This can be achieved by making the cut stable, providing supports such as retaining walls and buttresses, and providing percolation holes to enable groundwater percolation. Professionals and experts must conduct every necessary study that will be required in such development and to present the data with utmost honesty. By doing that, all potential risks are evaluated and taken into considerations. Thus, if the development is hazardous to such area, preventive measures and safety precautions will be taken. Also if the area is identified to be very hazardous for any development, any developments will be blocked. It may be a loss for economic opportunity, however many lives are spared because of this action. VIII. Conclusion
High levels of rainfalls was preceded by extended droughts and unusually very dry and hot climate. Then in the three days, August 1 to 3 of 1999, the record rainfall levels were experienced with the highest level of 500mm ON August 2, 1999. With the record of the rainfall, there was a possible development of full hydrostatic pore pressure. This means that pressure from the groundwater, due to water saturation, is greater than the bearing capacity of the soil. All these combined with the large cuts for quarrying operations and intensely fractured rock layers, which moved during the water saturation and expanding of siltstone materials triggered the landslide. The convergence of these seemingly unrelated and unfortunate events, more like a twin disaster of climate twist and geological line of weakness, was therefore responsible for the landslide. Prolonged water saturation and subsequent pore pressure buildup was the final blow that triggered the landslide. As borne out by foregoing studies and findings, no single factor
could have exclusively caused this landslide and it had to take several events and factors (geologic, environmental, and human) to meet, in the slopes of Cherry Hills, to cause the slide.
IX.
References
1. Willie D. (1992) “Foundation on Rock”. First Edition. Chapman Hall. 2. 3.
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London. Page 68. Goodman, R.E. (1989) “Introduction to Rock Mechanics” Second Edition, John Wiley and Sons, New York. Pp 43 to 46. "Landslide on Cherry Hills Subdivision (August 02, 1999)". Philippine Atmospheric, Geophysical and Astronomical Services Administration. Kidlat. 2000. Lopez, Celeste (September 1999). "Why 60 people died in the Cherry Hills housing estate". World Socialist Website. Morales, Emilio; Marissa Camaclang, Gilberto Reyes (2001). "The Cherry Hills Landslide Tragedy". The 2nd Civil Engineering Conference in the Asian Region, (Tokyo, Japan). Rufo, Aries (11 January 2000). "Quarrying caused Cherry Hills Landslide". Manila Standard Today. Catane, Sandra; Mark Albert Zarco, Ricarido Saturay, Jr. (2006). Landslide-Risk Reduction Strategies and Practices in the Philippines. Quezon City: University of the Philippines
