Karst Subsidence Hazard Assessment and Ground Penetrating Radar (GPR) Survey in the Municipality of Ipil, Province of Zamboanga Sibugay

By Lawrence R. Zamoras, PhD.

The MGB-IX undertook a ground subsidence assessment from February 23 through March 16, 2017 in the Municipality of Ipil, Zamboanga Sibugay. The methods employed during the field campaign include (1) inventory of pre-delineated sinkholes interpreted from the 2013 NAMRIA Interferometric Synthetic Aperture Radar (IfSAR) satellite image, (2) ground penetrating radar (GPR) surveys of selected areas in the municipality, (3) gathering of anecdotal accounts, Information, Education and Communication (IEC) Campaign, and (4) issuance of “Threat Advisories” to all barangays in the Municipality of Ipil where sinkhole and ground subsidence can occur. The team is composed of Dr. Lawrence R. Zamoras (Supervising Geologist), Paul D. Yecyec (Supervising Geologist), Kris N. Alferez (Geologist), Ollyn T. Balignot (Geologist), and Jordan B. Gahisan (Geologist).

Photo 1. The Ground Penetrating Radar (GPR) instrument determines subsurface structures and features, and to check presence of voids or cavities along established gridline. GPR is an effective, non-destructive, non-invasive tool used for locating subsurface features and/or delineating cavities, continuity of soil layers and rock masses. It uses wide-band non-sinusoidal electromagnetic waves that produce high resolution information of the subsurface typically up to 25-30m depth. This was conducted after the geomorphological and geological assessments of the subject municipality.



Figure 1. The Municipality of Ipil is the Provincial Capital of Zamboanga Sibugay located 070 47’ N and 1220 35’ E in western Mindanao and covers an area of about 241.60 km2. Based on the 2015 population census, it has a total of 74,565 inhabitants distributed in 28 barangays.


Karst Subsidence

Subsidence is the lowering of the land surface due to sinkhole and cave collapse or during a major earthquake. Sinkholes are common and natural feature of a karst landscape, but the hazards they can cause to lives and properties are yet to be known and understood by concerned LGUs and communities. Karst subsidence is often associated with collapse of cover-subsidence type sinkholes. These are sinkholes that develop gradually and may remain undetected for over a long period of time. The covering sediments or overburden is usually permeable or sandy soil materials. Commonly, subsidence in sinkholes will start as hardly noticeable ground settlement or depression less than a meter deep. However, slow surface subsidence may be a precursor to imminent failure on a larger scale. The movements can be traced on hairline, staircase and horizontal cracks on buildings, and tension cracks as well as differential settlements on roads prior to more extensive damage. Subsidence may be very slow or very fast wherein damages can be low to catastrophic.

Limestone-underlain Barangays

Among the 28 barangays of Ipil, those that are underlain by limestone are Poblacion, Sanito, Lower Taway, Pangi, Veterans Village, Don Andres, Taway, Upper Pangi, Caparan, Tiayon, Guituan, Logan, Labi, Lumbia, Timalang, Makilas and Tenan. Most terrains of these limestone-underlain barangays have flat to rolling grounds. Typical karst geomorphology hardly appears in most portions of the municipality. Karst towers and caves only appear in a few remote barangays such as Caparan, Lumbia, Guituan and Labi. Most limestone exposures occur as scattered patches of limestone outcrops as these are mostly covered by alluvium or younger deposits.

Satellite Image-based Sinkholes

The use of IFSAR-DEM sinkhole pre-delineator identified a number of sinkholes but was unable to identify much more, which were encountered in the field. Google satellite images however provide more clues as to where the inconspicuous sinkholes occur. The barangays identified to have sinkhole complexes based on Google satellite image are Sanito, Taway, Pangi, Upper Pangi, Veterans Village, Don Andres, Caparan, Tiayon, Guituan and Logan. Their terrains show contrasting colors of ground vegetation that exhibit circular patterns of ground depressions. These are considered cover subsidence sinkholes where surface runoff seep through the subsurface. These circular topographic depressions surround slightly elevated grounds that resemble the Chocolate Hills of Bohol, only that the peaks are too low to be noticed on the ground.

GPR-surveyed Areas

Because the areas underlain by limestone have generally flat to rolling terrain, these become the center of urbanization and concentration of infrastructures and development. However, through the use of the GPR instrument, it was discovered that cavernous subsurface has widespread occurrence in most areas of Ipil. The schools that lie over cavernous grounds are Pangi Elementary School, Zamboanga Sibugay National High School Campus A and B, Upper Pangi Elementary School, Tiayon Elementary School, Caparan Elementary School, Sanito Elementary School, Taway National High School, Luis Ruiz Sr Elementary School, Marcelo Spinola School, Guituan Elementary School, and probably Marian College. Other building institutions that stand over cavernous grounds are the Municipal hall of Ipil, Barangay Hall Lower Taway, Integrated Bus Terminal of Ipil.

GPR Survey in Pangi Elementary School, Brgy Pangi

Figure 2. The GPR survey lines at the Pangi Elementary School in Brgy Pangi, Ipil, Zamboanga Sibugay; Line A (150 m long) was laid between the DepEd Building and the low of school buildings; Line B (150 m long) was laid diagonal across the open ground.



Figure 3. A 150-meter long GPR line was laid inside the Pangi Elementary School. The survey was conducted on a weekend in order to minimize the effects of noise interferences. The site is located on a flat ground surrounded by school buildings. It is underlain by limestone although no limestone exposure has cropped out of the ground. The GPR result shows presence of a complex of cave system underground at 5 to 15 meters depth. The cave system appears to consist of interconnected voids that are positioned at varying levels. The ground surface shows slight depressions with fractured concrete grounds/pathways.



Figure 4. This is a 100-meter long GPR line diagonally laid across the school playground. The resulting GPR radar gram indicates occurrence of a roughly 50-meter long subsurface cavity about 5 to 12-meter deep. The grass-covered ground above shows very little indication of any presence of subsurface cavern below.



Photo 2. Pangi Elementary School: [A] Highly cracked concrete pathway indicative of its subsurface condition characterized by cavities; [B] very slight depression on the playground above a 50-meter long underground cave.



Figure 5. Karst Subsidence Susceptibility Map of Ipil, Zamboanga Sibugay.

Limitations of this Ground Subsidence Survey

Due to the limited period of this ground subsidence survey, only selected schools and a few public areas underlain by limestone were covered in the GPR survey. Other public places that are crowded, or roads where vehicles frequently use have been excluded in the survey as vehicular movements generate noise interferences and create false GPR results. Thus most of the commercial areas as well as residential areas, which occupy a larger proportion of urban Ipil have not been GPR-surveyed although these are also underlain by limestone. The urban Ipil is also potentially underlain by sinkhole complexes characterized by subsurface caverns, thus, susceptible to subsidence hazard.

Nature of the Underlying Limestone

This limestone sequence, which is part of the Sibugay Formation, had been thermally metamorphosed causing its marbleization and making it more indurated and nonporous. It is therefore stronger and has higher capacity to support infrastructures compared to typical limestone deposits. But since its chemical composition remains soluble to acidic water, cavities still develop along fractures. These cavities may widen in considerable period of geologic time especially during the lowering of the water table. Such process prevails in the limestone underlying Ipil to result in complex subsurface cavern system. Therefore, depending on the size or shape of the cavity and the weight of the overburden, the cavernous limestone ground may still have certain competence in supporting infrastructure loads. Signs of subsidence will be manifested through the occurrence of tension cracks in the concrete walls and floors. When these tension cracks further proliferate, it is time to assess the infrastructure condition of the building whether it is for immediate abandonment or not.

Role of Water Table

The dissolution of limestone occurs when it lies above the water table. Natural rainwater that contains dissolved carbon dioxide forms carbonic acid which brings chemical reaction with the calcium carbonate, the composition of limestone. The dissolution of limestone takes place when it is not submerged in ground water. Its dissolution rate is very gradual but given a long period of time, this can transform a crack into a fissure and eventually into a cavern. Thus, it is best to maintain the water table in its current level. Ipil area has two aquifers, the shallow and the deep aquifer. The shallow aquifer in Ipil which protects the limestone has its water table at about 5 meters below the surface. To keep this level of water table, pumping of groundwater from the shallow aquifer should be monitored or regulated such that it will not cause lowering of the water table.

Subsidence Susceptibility Map

The subsidence susceptibility map (Figure 5) prepared by the team will assist the LGU in monitoring the extensive subsidence-susceptible areas of Ipil. It will also be used by the development planners in locating sites that are less susceptible to subsidence, or institute appropriate mitigation measures, create a subsidence proof design or innovate engineering interventions appropriate for infrastructure development in the municipality. The subsidence susceptibility map was generated covering the assessed areas based on the 1) subsurface configuration made by the GPR radargram and 2) occurrences of caves and sinkholes and 3) geologic structures (joints, faults lineaments). Other parameters considered are the signs of subsidence such as differential settlements, progressing tension cracks and subsiding road surfaces, staircase cracks and horizontal cracks in concrete structures. Highly susceptible areas correspond to the red zones translated on the map. The proceeding matrices and discussions are the results of the subsidence hazard assessments conducted in every puroks of the municipality.

CONCLUSIONS

Foregoing considered, the following are the conclusions of the team:

1) About 50% of the total land area of the Municipality of Ipil is highly susceptible to subsidence hazard because of three (3) important factors, namely: 1.) the area of concern is underlain predominantly by soluble carbonate sedimentary rock formations, 2.) presence of lineaments and faults, and 3.) climatic conditions that favor caves and sinkholes formations. The barangays with high exposure to subsidence hazard include Poblacion, Sanito, Don Andres, Veterans Village, Lower Taway, Taway, Pangi, Upper Pangi, Tiayon, Caparan, Makilas and Tenan; Those with partial exposure to subsidence hazard are barangays Lumbia, Timalang and Labi. Most of these barangays have flat to rolling terrain, thus, becoming vital areas for development. Infrastructural developments in Ipil are concentrated in barangays Poblacion, Ipil Heights, Sanito, Don Andres, Veterans Village, Lower Taway and Pangi. Among these, only Brgy Ipil Heights, which is underlain by volcanic rock sequence, is outside subsidence susceptible areas.

2) This study adopted the lithological classification and nomenclature assignments of the Geology and Mineral Resources of the Philippines (2002). The rock units encountered are Sibugay Formation, Ipil Volcanics (Lumbog Formation), Zamboanga Volcanic Complex and Quaternary Alluvium. The limestone sequence underlying Ipil is part of the Sibugay Formation which has an age of Oligocene to Lower Miocene. This limestone is marbleized due to thermal metamorphism making it sturdier. It is this rock that underlies most areas with low-lying and flat or rolling terrains. Towards the lower section of the unit, it interlayers with mudstone-sandstone-conglomerate sequence. This unit is overlain by the Ipil Volcanics (Lumbog Formation), which is composed of andesite flows, agglomerate and tuff. Younger volcanic deposits of age Pliocene-Pleistocene are grouped as the Zamboanga Volcanic Complex. Those barangays that lie outside the high ground susceptibility such as Barangay Ipil Heights, are underlain by these volcanic rock units.

3) The limestone terrains of Ipil exhibit a moderately developed karst landscape. These form low relief karsts and karst plain traversed by networks of narrow to moderately spaced cave systems and sinkholes. Generally, these features are observed in plains near sea level up to elevation around 200 meters above sea level. The plains and rolling terrains of Ipil have been identified as widely underlain by sinkhole systems based on the circular patterns visible on the Google Satellite Image.

4) Ground verification confirms about 80% of pre-delineated sinkholes by IFSAR-DEM of 2013 and identified 12 cave openings in the Municipality of Ipil. Sinkholes and caverns are continuously forming through time in the study area because of gravity, the nature of the underlying rocks, presence of faults and lineaments, actions of rainwater, and the rise and fall of water table.

5) Among the schools in Ipil, the following schools were identified as highly susceptible to subsidence: Pangi Elementary School (Brgy Pangi), Zamboanga Sibugay National High School Campus A and B (Brgy Pangi), Upper Pangi Elementary School (Brgy Upper Pangi), Tiayon Elementary School (Brgy Tiayon), Caparan Elementary School (Brgy Caparan), Sanito Elementary School (Brgy Sanito), Taway National High School (Brgy Taway), Luis Ruiz Sr Elementary School (Brgy Veterans Village), Marcelo Spinola School (Brgy Don Andres), Guituan Elementary School (Brgy Guituan), and probably Marian College (Brgy Poblacion).

(This excerpt was taken from the MGB-IX Technical Report entitled “Karst Subsidence Hazard Assessment and Ground Penetrating Radar (GPR) Survey in the Municipality of Ipil, Province of Zamboanga Sibugay”.)

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