A Landslide Risk Rating System for Greater Baguio, Philippines

Artessa Niccola D. Saldivar-Sali, BSCE, M.Eng.

University of the Philippines College of Engineering

Abstract

This study formulates a LANDSLIDE RISK RATING SYSTEM for the Greater Baguio area. It is proposed as an aid to rational land-use decision-making and landslide risk minimization. This SYSTEM integrates different hazard (bedrock geology, slope gradients, vegetation) and risk (population, land use) factors.

This analysis is based on the special attributes of the Baguio area, namely: a relatively extreme topographic relief, variable bedrock geology, and near-uniform amount of rainfall during extreme precipitation events.

Bedrock geology is the basic variable used in the System because the engineering behavior of a site varies according to the geomechanical properties of its subsurface materials. The residual soils mainly involved in landsliding are directly correlated to the parent rock.

Seven geologic units are present in Greater Baguio: the oldest, the Pugo Formation, a stratified sequence of basaltic and andesitic volcanic rocks; the Zigzag Formation, a series of conglomerates, sandstone, and shale with limestone lenses; the Central Cordillera Diorite, an intrusive consisting of diorites and granodiorites; the biohermal Kennon Limestone distinguishable by its karst topography; the Klondyke Formation, a thick sequence of conglomerates, tuffaceous sandstone, volcanic and tuff breccia; siltstone, mudstone; Mirador Limestone; and the youngest, Baguio Formation, is mainly pyroclastics.

The frequency of landslides within each geologic unit (expressed in percent) are computed and classified from most to least stable, as follows: Kennon Limestone = 0% (Class I), Klondyke Formation = 0.68% (Class I), Mirador Limestone = 4.5% (Class II), Baguio Formation = 5.9% (Class II), ZigZag Formation = 6.4% (Class III), and Pugo Formation = 12.1% (Class V).

Likewise, the frequency of landslides (expressed as percent) in various classes of slope angles show that the landslides do not increase at the same rate of increasing slope for the seven geologic units. Therefore, dependence of the two factors (geology and slope) in their contribution to landslide occurrence in the Greater Baguio area is assumed. Modified Geologic Classes are assigned to take this dependence into consideration.

Inside each geologic unit, the percentage of landslides occurring in areas covered by each vegetation type was determined, with the following results (in order of increasing percentage of landslide occurrence): Broadleaf Mix or Bushes/Scrub = Class I; Grass, Crop Land/Agricultural Land = Class II; Broadleaf = Class III; No Vegetation = Class IV.

A benefit from this geotechnical tool is that it serves as a basis for improving building codes. Building regulations can be modified to take into account variations in geotechnical conditions across a given planning area. Significant savings could, in principle, be achieved by introducing a certain amount of flexibility into building codes. A site-specific preventive and remedial slope safety system for slopes in Greater Baguio area would be the ultimate goal.

 

Thesis for M.Eng. degree in Civil & Environmental Engineering
submitted to the
Massachusetts Institute of Technology, 2004