By Lawrence R. Zamoras, PhD.
The MGB-IX conducted a ground subsidence assessment from March to April 2018 in the Municipality of Liloy, Zamboanga del Norte. 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 Liloy where sinkhole and ground subsidence can occur. The team is composed of Dr. Lawrence R. Zamoras (Supervising Geologist), and geologists Kris N. Alferez, Daryl Mae M. Gerodiaz, Glen B. Gamelo and Ian T. Demetillo.
The Municipality of Liloy stands on the Liloy Limestone that is underlain by Tungawan Schist (Dansalan Metamorphics), and Tampilisan Ultramafics. The Liloy Limestone sequence underlying Liloy Municipality formed from Pliocene-Pleistocene carbonate reef that underwent tectonic uplift and formed a several reef terraces observable from Baybay to Fatima. Since it is a geologically young coralline limestone, it is generally characterized by high porosity thus having the tendency to develop a complex karst landscape forming extensive and bigger caves and subterranean river system. In the northern and central barangays of Liloy, bigger caves have been detected or previously reported such as in barangays Santa Cruz, Punta, Lamao, San Miguel, Villa M Tejero, Villa Calixto Sudiacal, Patawag, Fatima, Baybay, Comonal and Santo Nino. The rest of the subsidence susceptible barangays are generally underlain by calcarenitic limestone, which usually develop fissure-type cavities described as vertical, narrow and deep.
GROUND PENETRATION RADAR SURVEYS
One of the GPR Survey Lines is at the Gayam Elementary School situated on a flat to rolling terrain in northern Liloy. It is a coastal area composed of uplifted carbonate reef terraces. This limestone sequence is characterized by massive coral fossils. This barangay has a known extensive underground cave. A 60-meter long GPR Survey line was laid inside the Gayam Elementary School on the school ground (Fig. 6). Its GPR radargram reflects a spacious and extensive underground cave system throughout the survey line (Fig. 7). The floor-to-ceiling height is mostly around 5 meters wherein the ceiling can be encountered at 2 meters depth and the floor at 7 meters depth. On the surface can be observed ground subsidence features, cracked and tilted garden boxes, and cracked walls and floors of school buildings (Fig. 8). However, several sections along the survey line show subcrops of the coralline limestone, which may be part of the sturdy cave ceiling as reflected in the radargram. As per anecdotal accounts, the classroom floors are shaky when children play inside.
Liloy National High School
Liloy National High School is situated on an uplifted carbonate reef terrace, which is of the same level as that of the Liloy Municipal Hall, about 55 meters above sea level. The school campus appears flat in the main section, but at the back side is a roughly 30-meter deep ravine, which separates the rear section which has a rolling terrain. Three newly built 4-storey buildings have been erected: one at the main section and two in the rear section of the campus. Three GPR Survey lines were laid within the school ground (Fig. 9): Line A in the main section, Line B along the road that crosses the ravine, Line C in the rear section, making a total length of 294 meters.
Line A was laid across the main open ground of the school campus which appears to be a locally flat ground (Fig. 10). Its GPR radargram indicates multi-level cavity systems (Fig. 11). Between intervals 0 to 40 meters, or shallower cavities were indicated between depths 2 to 5 meters. Right above this subsurface cavity can be observed a slight cover subsidence feature. A deeper cavity system is indicated between intervals 38 to 85 meters of the survey line occurring at depths 5 to 8 meters below the surface. This deeper cavity is distinctly indicated in the radargram, which laterally interconnect with much deeper cavity system.
Line B is 100 meters long, laid along the saddle road across the deep gulley, which serves to connect the main (front) and rear sections of the campus. The road steeply descends toward the bottom of the gulley, after which ascends toward the rear section of the campus (Fig. 12). The radargram image indicates very distinct cavity system from interval 0 up to 50 meters and then from interval 55 to 85 meters (encircled yellow in Fig. 13). In this survey line the interval 0 to 15 meters is the flat portion at the back of the old buildings front section of the campus; the portion that slopes downward is from interval 15 to 65 meters; the gulley bottom installed with culverts across the creek, and then ascends from interval 75 to 100 meters.
The third GPR survey was conducted in the rear section of the campus wherein two 4-storey school buildings were constructed and more of the same building projects have been approved and budgeted for construction (Fig.14). Line C was laid on the open ground fronting the two newly constructed 4-storey buildings. During the GPR Survey, noise interference from nearby operating machines had been continuous throughout the survey. The GPR radargram indicated presence of relatively large cavities from intervals 0 to 25 meters and from 73 to 100 meters (encircled yellow in Fig. 15). The noise interferences between 25 to 73 meters may have seriously obscured the radargram image such that any cavity occurrence is not clearly reflected. However, there is a possibility that the spacious cavities indicated at both sides of the radargram are laterally interconnected.
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.
A. Nature of the Underlying Limestone
The Liloy Limestone is composed of relatively young carbonate reef, which got uplifted through tectonic activities. Being a carbonate reef, it is mostly composed of fossil corals, bivalves and other reefal organisms. Because of its geologically young age, it is not yet thoroughly consolidated having spaces in between fossils filled with soft muddy materials. Such condition makes it highly porous for water to pass through and eventually develop subterranean rivers. Therefore, depending on the size or shape of the caverns 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 along the roads or in the concrete walls and floors buildings. 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 such as in most parts of Liloy. Its dissolution rate is very gradual but given a long period of time, this can transform a crack into a fissure and eventually a cavern. It is best to maintain the water table in its current level. Thus, pumping of ground water from the shallow aquifer should be regulated in order to keep the water table protecting the limestone in its current level. Furthermore, extreme weather conditions due to climate change may hasten degradation of sinkholes that will result to subsidence because of flooding or lowering of water table.
B. Distribution of Limestone
Among the thirty-seven (37) barangays of Liloy, thirty-two (32) are underlain by limestone. These are Santa Cruz, Punta, Lamao, Villa M. Tejero, Villa Calixto Sudiacal, Fatima, Baybay, San Miguel, Banigan. Patawag, Ganase, Tapican, Comonal, Sto. Nino, Dela Paz, Timan, Kayok, Canaan, La Libertad, Candelaria, Cabangkalan, Causwagon, Mauswagon, San Francisco, San Isidro, Panabang, Mabuhay, Maigang, Silucap, New Bethlehem, Goaw and Goin. Most terrains of these limestone-underlain barangays are characterized by rolling to hilly topography. Typical karst geomorphology is not yet fully developed on the surface as its present stage is still on the subsurface level. Common Karst features in Liloy are cave openings, sinkholes and subterranean rivers.
Subsidence Susceptibility Map
The subsidence susceptibility map In Figure 78 prepared by the team will assist the LGU in monitoring the extensive subsidence-susceptible areas of Liloy. 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 (1) the subsurface configuration made by the GPR radargram, (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.
Since Liloy is strategically located at the crossroad of national highways it becomes a center of urbanization. Despite being underlain by limestone, its generally rolling terrain attracts infrastructures and further developments. However, through the use of the GPR instrument, it was discovered that cavernous subsurface has widespread occurrence in most areas of Liloy.
C. GPR-Based Assessments of Surveyed Schools
Due to the limited period of this Ground subsidence survey, not all the forty-four (44) schools of Liloy Municipality were surveyed using the GPR. This Ground subsidence survey has only one target municipality/city per year for each region in the country, in which Liloy was the target for 2018. The limitation of the GPR survey is that the obtained subsurface data were taken from the open ground and not on the site of the building itself. However, it is assumed that the homogeneous characteristic of the limestone rock creates similar subsurface conditions. The subsurface condition along a line is the same as the adjacent areas as long as the same rock unit is present.
Exposure to Subsidence Susceptibility
The subsurface images generated by the GPR radargram indicate the approximate location, depth, size and shape of the underground cavity. Thirty-one (31) schools have been GRP-surveyed; nine (9) schools were not covered due to several reasons such as having non-limestone ground, difficult access and security issues. Some GPR-surveyed schools have two to three GPR survey lines particularly those with numerous cracks in their building walls and floors. Areas with well-defined spacious cavern system based on the GPR radargram are classified as under High to Very High Subsidence Susceptibility. Those areas with multiple cavern systems resembling a network of cavities are also included under this category. Those areas relatively smaller multiple cavities based on the GPR radargram are classified under Moderate to High Subsidence Susceptibility. Twenty-four (24) schools have been identified under the High to Very High Subsidence Susceptibility; five (5) schools are under Moderate to High Subsidence Susceptibility; and one school is non-susceptible to subsidence.
Among the thirty-one (31) schools GRP-surveyed, twenty-four (24) schools were found exposed to High-Very High Subsidence Susceptibility namely; Liloy National High School (Fatima), Liloy National High School (Patawag), Liloy Central School, Comunal Elem School, Causwagan Elem School, San Francisco Elem School, Compra Elem School, New Bethlehem Elem School, Lamao Elem School, Punta Elem School, Santa Cruz Elem School, Gayam Elem School, San Miguel Elem School, Patawag Elem School, Sudiacal Elem School, Ganase Elem School, Kayok Elem School, Sto Nino Elem School, San Isidro Elem School, Cabangcalan Elem School, Mauswagon Elem School, Panabang Elem School, Goaw Elem School and Goin Elem School. The schools with exposure to Moderate to High Ground Subsidence Hazard are Baybay Central School, Tapican Elem School, Dela Paz Elem School, Timan Elem School and Canaan Elem School.
Foregoing considered, the following are the conclusion of the team:
1) About 75% of the total land area of the Municipality of Liloy is highly susceptible to subsidence hazard because of three (3) important factors, namely: I.) 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. Thirty-two (32) barangays have been identified with high exposure to subsidence hazard namely, Santa Cruz, Punta, Lamao, Villa M. Tejero, Villa Calixto Sudiacal, Fatima, Baybay, San Miguel, Banigan. Patawag, Ganase, Tapican, Comonal, Sto. Nino, Dela Paz, Timan, Kayok, Canaan, La Libertad, Candelaria, Cabangkalan, Causwagon, Mauswagon, San Francisco, San Isidro, Panabang, Mabuhay, Maigang, Silucap, New Bethlehem, Goaw and Goin. Most of these barangays have rolling terrain, thus, becoming vital areas for development. Infrastructural developments in Liloy are concentrated in barangays Fatima and Baybay. The barangays that are generally not susceptible to subsidence hazard are San Roque, Overview, Compra, Malila and El Paraiso.
2) The limestone terrains of Liloy exhibit a well-developed karst landscape. In the central and northwestern Liloy, subterranean river system is quite extensive. Surface run-off water usually get diverted underground that is why creeks rarely develop in these areas. In the east as well as in some portions of southwest Liloy, the primarily calcarenite composition of the terrain developed a well-defined surface drainage system although these are often dry.
3) Ground verification confirms about 70% of pre-delineated sinkholes by IFSAR-DEM of 2013 and identified 23 cave openings in the Municipality of Liloy. 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.
Based on the conclusions generalized from the findings and observations of the team, the following actions are recommended:
1) Regular monitoring of signs of sinkhole/cave collapse, and subsidence in the areas identified on the subsidence susceptibility map by concerned barangay officials. Monitor presence of tension cracks that are widening and/or progressing, and circular ground depressions that are subsiding, especially every after heavy rainfall and flood events. Report the same immediately to municipal authorities/MDRRMO.
2) Extra vigilance by residents during heavy, continuous rains brought by typhoon or monsoon rains, especially if areas with identified sinkholes are flooded. Divert run-off away from sinkholes.
3) In case of sinkhole and/or cave collapse, cordon off the area, report situation to municipal authorities/ MDRRMO. Do not cover sinkholes unless recommended by authority, especially if subterranean river or creek is noted beneath. This will result to ponding of subterranean river/creek and induce subsidence in the immediate vicinity.
4) Monitor displaced surfaces along national, provincial and barangay roads. Report situation to municipal authorities/MDRRMO. Limit the load capacity of vehicles passing on this type of road. Signages should be put up indicating the threat of subsidence. If possible, find alternative route to lessen the load brought by vehicles and heavy traffic.
5) Any proposed multi-storey buildings and heavy infrastructures should conduct subsurface studies in order to adopt appropriate building designs that mitigate subsidence hazard. Areas underlain by wider cavities should no longer be considered for heavy building structures. Residents should be made aware of the hazards and signs of ground subsidence. Collapse incidents could happen in snap of a second or in a very slow rate. Collapse could be caused by lowering of water table, heavy rainfall or earthquake.
6) Further infrastructure development in any of the subsidence susceptible areas should be well-planned and appropriately designed in order to reduce the load of the unstable ground, to ensure safety of school children, and to save on cost for mitigating measures.
7) Avoid disposing garbage and wastewaters to sinkholes and caves to prevent contamination of groundwater. Septic tanks should be well protected, and proper solid and wastewater management should be strictly implemented.
8) For rehabilitation and development of damaged roads and bridges, the concerned agency and LGUs should consult experts on rock and soil mechanics, foundation engineer, structural geologists, and engineering geologists. The subsurface condition and geological limitations should be considered in the design and cost of engineering interventions. For the meantime, it is recommended that warning signages should be installed to remind motorists of the presence of geologic hazards, decrease traffic load, and these roads/bridges should be closed during inclement weather condition.
9) Activate Barangay Disaster Risk Reduction and Management Council and establish an effective evacuation protocol. Evacuation and relocation sites as well as evacuation routes should be properly identified. Evacuation and relocation sites should be geologically assessed and suitable for the purpose.
10) Awareness campaign on subsidence hazards due to sinkhole collapse should be conducted to barangay officials and residents in Liloy.
(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 Liloy, Province of Zamboanga del Norte”.)