3 August 1999
On 3 August 1999 a devastating landslide occurred in Cherry Hills subdivision, Antipolo City, Rizal Province, which killed 60 people and displaced hundreds of families. Heavy rains induced by Typhoon Ising (international name, Olga) triggered the landslide. Ising developed from an active low-pressure area on 28 July and intensified into a Typhoon. It did not directly hit the Philippines but continuous monsoon rains were experienced over the western sections of Luzon displacing more than 73,000 people. As many as 109 people died in the Philippines due to Typhoon Ising with Manila, Pampanga, Rizal and Pangasinan as the worst hit areas (Lopez, 1999).
The Cherry Hills landslide in Rizal is attributed to many compounding factors. These are: 1) the effects of 3 consecutive days of continuous rainfall (Aurelio, 2004) that led to water saturation and buildup of pore pressure on the slide surface; 2) strength degradation of the sedimentary layers underlying the mountain due to shrinkage cracking and expansion from prolonged water saturation; 3) intense fracturing of the rock mass due to its proximity to the Binangonan Fault; 4) unfavorable 8° dip angle of the rock layers to the southwest and; 5) destablization of slopes due to large cuts made by quarrying operations prior to the development of the Cherry Hills subdivision. The combined effects of natural and man-made conditions lead to instability of the slopes where the subdivision was built, which was triggered to collapse by continuous heavy monsoon rains following an El-Niño year.
Aurelio, M.A. (2004). Engineering Geological and Geohazard Assessment (EGGA) system for sustainable infrastructure development: the Philippine experience. Engineering Geology for Sustainable Development in Mountainous Areas. Free & Aydin (eds) ©2004 Geological Society of Hong Kong. ISBN 962 7320 07 2.
Lopez, C. (1999). Why 60 people died in the Cherry Hills housing estate. World Socialist Website. International Committee of the Fourth International (ICFI).