PHIVOLCS
 
Foreword
The July 16 1990 Luzon Earthquake Rupture
Inventory and Characterization of Landslides induced by the 16 July 1990 Luzon Earthquake
Mapping of Areas Affected by Liquefaction during the 16 July 1990 Earthquake
The 16 July 1990 Luzon Earthquake and its Aftershock Activity
Soil Study of Area Damage due to Liquefaction during the 16 July 1990 Philippine Earthquake
Vital Engineering Lessons from the Earthquake of July 16, 1990
Quantifying Spatial and Temporal Dimensions of Premonitory Animal Behavior of the July 16, 1990 Luzon Earthquake
Households and Communities in a Post-Earthquake Situation: Lessons on Survival and Self-Reliance
Organizational Response to the July 1990 Luzon Earthquake Disaster
Psychosocial Issues in Disasters
Management Strategies for Earthquake-Related Psychosocial Problems Community-Based Interventions
Some Implications of the July 16, 1990 Earthquake on Urban and Regional Planning in the Philippines

 

 

 

 

 

 

Mapping of Areas Affected by Liquefaction during the 16 July 1990 Earthquake
Ronnie C. Torres*, Ma. Lynn O. Paladio*
Raymundo S. Punongbayan*, and Rosalito A. Alonso**
* Philippine Institute of Volcanology and Seismology - DOST
** National Institute of Geological Sciences - UP

 

DAGUPAN CITY : A Case Study

Physical Development of Dagupan City

Spanish writers described Dagupan as an extensive marshland with rich alluvial soil. It was thickly covered with mangrove and nipa palm trees which served as habitat to many marshland wildlife species. Early settlers lived in small clusters of houses along the shoreline and river banks of Calmay, Pantal, and Bonuan. Later migrants moved inland occupying the agricultural lands of Malaued, Lasip, Pogo, and Bacayao. Pantal and Bonuan became the fishing, salt-making, and "bangus" (milkfish)raising centers and Malaued, the agricultural settlement area. Travel was mainly by "bancas" (dugout canoes) and sailboats through the river channels.

In 1590, the house clusters were resettled into compact communities and converted into a town named initially as Bacnotan and renamed later in 1720 as Dagupan. A site for the town plaza was constructed along Pantal River surrounded by the town hall in the east public market in the north, and the Catholic Church in the west (Pangasinan Folio, 1970 In 1780, Pantal, originally named Pantalan (port), became a trading center and docking station for merchant ships. At about the same time, bangus industry thrived and more mangrove swamps were converted into fishponds. The development of Dagupan as a commercial center was firmly established in 1891 when the Manila-Dagupan railway was completed. Up to the 1900's the site of the present public market was still a swamp with waist deep water. Much of the present downtown area along A. B. Fernandez Ave. (formerly Torres Bugallon Avenue) was a marsh. Travel by boat along the Agno River was still the most practical means of transport. In 1908, the commercial center extended beyond the Quintos Bridge on the other side of Pantal River. The continuous growth of the city necessitated the construction of Perez Blvd. and Magsaysay Bridge in 1948 to create more space for commercial activities through the usual practice of reclaiming and construction on swamplands and less productive fishponds.

Fluvial Sedimentation and Artificial Channel Cut-offs

The active shifting of Pantal River left a belt of abandoned meanders alongside it. The positions of the old meanders are indicated by the looping configuration of swampy areas and other natural drainages (Fig. 16). These are also prominent and, therefore, easily identifiable in aerial photographs. The age relationship of an abandoned meander to the active channel could be inferred from its depth, distance from the present active channel and, where several meanders intertwined, cross-cutting relationships.

Most of the meander scars exhibit well-developed point bar deposits and scroll bar ridges. However, some point bars are already obliterated by cross-cutting meander loops and are covered by levee deposits. A peculiar characteristic of very young active meanders, particularly those within the vicinity of the city proper, is the absence of distinct levees. Based on this, it is very likely that some of these channels have been artificially made, probably during the development of the city.

Channel abandonment along Pantal River also resulted from construction of artificial cut-offs. Slow sedimentation of suspended materials and episodic influx of flood-borne sediments eventually filled up the abandoned channels. These natural reclamation materials are largely uncompacted and water-saturated sediments and, as such, highly susceptible to amplified ground shaking and liquefaction. At some later stages of natural reclamation, the abandoned channels are transformed from oxbow lakes into swamps or marshlands.

Artificial cut-off and reclamation have long been employed along Agno River for two primary reasons: (1) to shorten travel time around meander loop and (2) to diffuse floodwater especially at constricted portions. One exceptional case was the famed Limahong Channel, named after the Chinese pirate who established his colony in Lingayen during the 1600s. According to historical accounts, the combined Filipino and Spanish forces laid siege on his fortress by blocking the river outlets. Limahong broke through the siege by sects digging a channel from the Agno River to Lingayen Gulf (Callanta, 1989)

An indication of man-made alteration on the fluvial environment of Dagupan City isle abrupt changes in the sinuosity of Pantal River. Based on the ratio of channel length and meander wavelength, the sinuosity of the active and abandoned channels of Pantal Rid, lying south of Dagupan City proper has an average ratio value of 2.31 and 4.19, respectively. In contrast, the meander character around the city proper was calculated at 1.40 formula} active channel and 2.14 for the abandoned channels. These sinuosity measurements distinguish straight and meandering channels at boundary ratio of 1.5. Therefore, the channel character of Pantal as it approaches the City proper transforms from a meandering channel to a straight channel. Furthermore, the similarity of the sinuosity of the abandoned channels passing through the city and the active meanders at the southern continuity of the river suggests that these abandoned meander were not yet primed for natural cut-off but were artificially severed from the main channel.

The effects of the 16 July 1990 Earthquake

Sand boils were the most extensive effects of liquefaction in Dagupan City. During the earthquake, sand boils were erupted through cracks at the sides of buildings, ruptured pavements, and covered concrete roads with dark gray fine sands and muddy waters. Drainage systems were clogged by the accumulated sand causing temporary flooding of the main thorough fares.

Sand boil distribution in Dagupan City, as delineated from aerial photographs taken three days after the earthquake and from field investigation, is shown in Fig. 13. Based on standard penetration test data of the Department of Public Works and Highways, the liquefiable layers are up to about 5m below the ground surface. In some places, however, these are shallow enough to be directly observed through open fissures. For example, the deposit found inside the Divine Word Academy adjacent to Nazareth Hospital along Rizal Extension (Fig 10) appears to have originated from a layer less than a meter below the ground surface.

Lateral spreading accounted for most of the destruction of swampy areas in Dagupan City. Buildings and other manmade structures were damaged when river banks slid into Pantal River and dry lands into the swampy areas during the earthquake.

Lateral spreading along Pantal River appears to be confined to constricted portions of the stream channel. Magsaysay Bridge is located across one of these constrictions and its collapse was brought about by lateral spreading on both sides of Pantal River (Plate 1). During active lateral spreading of opposing banks of Pantal River where Magsaysay Bridge abutted, the local compression generated was directed towards the bridge, and it swept away the piers at the same time when the riverbed was liquefying. As a consequence, the bridge , was broken into several segments like an accordion, although the portion close to the west abutment collapsed horizontally onto the riverbed. The segment abutting against the bank was thrust westward under the adjacent segment and the middle pier underwent subsidence and tilting towards the east. A similar compressional phenomenon was observed where a portion of Galvan St. overlying an open canal was thrust over a resisting abutment.

Ground subsidence is a direct consequence of liquefaction. However, the amount of subsidence in areas affected by liquefaction can only be determined relatively due to the absence of an undisturbed or fixed reference point. In general, places where buildings stood subsided relative to the streets, and understandably so. Subsidence phenomena are notable within the commercial district where some buildings sank by as much as 2 meters (Fig.14).In general, most of the affected buildings in Dagupan City subsided by less than a meter.Adjacent concrete pavements dip towards the subsided structures and ground cracks are disposed perpendicular to the direction of subsidence. The resulting subsidence in Dagupan City becomes more evident when the affected areas got flooded by rainwater, high tide in cursion and, jetted-up groundwater. Flooding was aggravated by the disruption and clogging of both the natural and man-made drainage systems. Some houses remained underwater by 30-50 cms. for several months. The whole stretch of Don Jose Calimlim St. and swampy areas and fishpond communities, such as Bgy. Lasip Grande, remained underwater even during low tide conditions.

In a more severe form of liquefaction, differential subsidence is usually associated will the tilting of heavy structures. Severely tilted buildings are concentrated along Perez Blvd. One is tilted by as much as 19 degrees (Plate II), but generally, the magnitude of tilt is within 2-5 degrees (Fig. 15).

Buried buoyant structures such as gasoline storage tanks (Plate III), septic tanks, and drainage pipes exerted upward pressure resulting in the upheaval of the ground and breaking of pavements, thus affecting the operation of some gasoline stations. Liquefaction also caused water supply problems, especially the availability of potable water many weeks after the earthquake.

Ground undulation is another feature associated with liquefaction. An eyewitness observed that the ground appeared to roll during the earthquake. He described a jeep parked along the road appeared and vanished from his line of sight. Evidence of rolling ground seems to have been preserved in the deformation of some fences and roads (Plate IV).Along the portion of Dagupan-Lingayen road enclosed by Tapuao-Malued diversion road, cracks developed in a concrete fence coincide with the crests and troughs of ground undulation wavelength. The cracks which formed at the crests are characteristically open and tapering downwards - an extensional feature - and the ones that formed at the trough exhibit shortening feature in the form of tight fractures. Using this observation as basis for determining the wavelength of ground undulation, the amount of separation between the extensional and the shortening cracks was measured along a deformed residential fence and a ground undulation wavelength of about 25 meters was obtained. Observable effects of ground undulation did not extend beyond the junction of the Tapuac-Milued and the Dagupan-Lingayen roads.

Correlation of Fluvial Sedimentation and Distribution of Damage

Liquefaction damage and the distribution of affected areas in Dagupan City appear to have strong geological controls. The extensive earthquake-induced secondary ground a failure in Dagupan City is confined mostly to built-up areas on recently abandoned river meanders, banks of active channels, and relatively young point bars. Structures constructed outside of these young geologic features suffered little or no damage during the earthquake. Conversely, it is also to be expected that areas with extensive occurrences of a sand boils and those with heavily tilted and subsided buildings should be where these geologic features are to be found. The margins of abandoned river channels thus serve to demarcate the boundaries between areas with totally damaged buildings and those with partially to undamaged ones. The liquefaction-prone layer would most probably correspond to the deposits in the abandoned channels and point-bars and would have a thickness of about 5-6 meters, which is approximately equal to the depth of Pantal River.

Liquefied deposits tend to flow towards open and recently reclaimed river channels. When this happened in Dagupan City during the earthquake, multi-storey buildings and residential houses on the liquefied point-bar deposits tilted away from the open and recently reclaimed river channel and in a direction opposite to that of lateral spreading.

Varying degrees of damage were sustained in the different affected areas of Dagupan City. For instance, the area around Perez Blvd., which is located in a readily recognized meander scar and well-developed point-bar, sustained more damages than the area around Tapuac-Malued road, which is an older meander belt. Similar liquefaction effects in the vicinity of Perez Blvd. can be observed in Pogo Grande, but these effects are at a subdued level because only residential houses are found here. Pogo Grande is also situated in a young meander scar of about the same age as that around Perez Blvd. All these observations clearly indicate that susceptibility to liquefaction and the attending degree of damage to structures within the different parts of Dagupan City can be related to the relative ages of the geologic features they occupy and therefore, by the relative ages of deposits underlying them. In general, sandy deposits in relatively old meander scars are less prone to liquefy than those found in recently abandoned channels and when they do liquefy, damage to structures built on them would only be of limited extent.

The degree of destruction along A.B. Fernandez Ave., which largely lies on a reclaimed swampland, is generally less than that in the Perez Blvd. area except near its intersection with Rizal St. where there occurred pronounced relative subsidence and structural tilt. The heightened effect of liquefaction along a 100m stretch of A.B. Fernandez Ave. can be traced to modification of the channel of Pantal River. Prior to north eastward expansion of Dagupan City, Pantal River used to meander around the area now partly occupied by A.B. Fernandez Ave. and Rizal St. before running parallel to Pantal Road (Figs 16). Thus, the segment of A.B. Fernandez exhibiting severe liquefaction related damages coincides with the crossed-over area of the old Pantal River. It is here underlain by young deposits of similar age as those in the Perez Blvd. area. The rest of A.B. Fernandez Ave. that was built on a reclaimed swampland suffered less damage through liquefaction because it is basically underlain by relatively older and better compacted deposits.

>> Summary and Conclusion

 

 


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