Volume 9, Number 2 (January 2022):3281-3291, doi:10.15243/jdmlm.2022.092.3281 ISSN: 2339-076X (p); 2502-2458 (e), www.jdmlm.ub.ac.id
Open Access 3281 Research Article
Identification of subsidence hazard zone by integrating engineering
geological mapping and electrical resistivity tomography in Gunung Kidul karst area, Indonesia
Wahyu Wilopo1,3*, Doni Prakasa Eka Putra1,3, Teuku Faisal Fathani2,3, Slamet Widodo4, Galeh NIP Pratama4, Maris S Nugroho4, Wisnu R Prihadi4
1 Universitas Gadjah Mada, Department of Geological Engineering, Yogyakarta, Indonesia
2 Universitas Gadjah Mada, Department of Civil and Environmental Engineering, Yogyakarta, Indonesia
3 Universitas Gadjah Mada, Center for Disaster Mitigation and Technological Innovation (GAMA-INATEK), Yogyakarta, Indonesia
4 Universitas Negeri Yogyakarta, Department of Civil Engineering, Yogyakarta, Indonesia
*corresponding author: [email protected]
Abstract Article history:
Received 3 September 2021 Accepted 25 November 2021 Published 1 January 2022
The presence of natural cavities in karst morphology may cause severe civil engineering and environmental management problems. Karst formations will limit the expansion of urbanization, especially infrastructure development in limestone areas. Geophysical methods, especially electrical resistivity tomography (ERT) techniques, are effective and efficient solutions to detect voids below the surface. This study aimed to develop a subsidence hazard map as basic information for infrastructure development. The identification was made by measuring electrical resistivity tomography on eight profiles in the infrastructure development plan. In addition, it was also supported by geological mapping, particularly the structural geology and types of rocks around the site. The research area consists of massive limestone, bedded limestone, and cavity limestone with generally north-south joints. The analysis of geological mapping data and electrical resistivity tomography measurements showed that the cavity limestone was identified with a north-south elongated pattern in line with the fracture pattern found on the surface at the research area. The surface lithology type, the geological structures density, and the subsurface lithology were used to develop a subsidence hazard map. This information is beneficial in determining the safe location of infrastructure development based on disaster risk mitigation.
Keywords:
electrical resistivity tomography engineering geological map karst morphology
subsidence
To cite this article: Wilopo, W., Putra D.P.E., Fathani, T.F., Widodo, S., Pratama, G.N.I.P., Nugroho, M.S. and Prihadi, W.R.
2022. Identification of subsidence hazard zone by integration of engineering geological mapping and electrical resistivity tomography in Gunung Kidul Karst Area, Indonesia. Journal of Degraded and Mining Lands Management 9(2):3281-3291, doi:10.15243/jdmlm.2022.092.3281.
Introduction
The natural pits and cavities beneath the surface cause serious problems for civil engineering and the environment. The term subsurface cavity denotes all subsurface structures, including caves, cavities, and sinkholes (Cui et al., 2015). The most naturally
occurring cavities in nature are developed by dissolution carbonate rocks and evaporation (Zhang et al., 2018). Karst's topography is created by the dissolution process of meteoric water percolation into the soil (Sun et al., 2018). A sinkhole is one of the karst morphologies due to ongoing loading and dissolving
Open Access 3282 processes (Font-Capo et al., 2015). The more acidic
the rainwater, the higher the dissolution process in carbonate rocks (Al-Zarah, 2007). The groundwater penetrating the weak zone of the limestone will accelerate the process of weathering and erosion produce of channels and cavities (Shah et al. 2018).
This process will continue until it reaches a critical condition, where the cave's roof cannot support the weight of the load on it. It will result in sinkhole development, which has adverse and catastrophic effects on the foundations (Pujades et al., 2012). The presence of cavities and caves causes limited land use and geotechnical hazards such as land subsidence, the collapse of surface structures, and cracks on building surfaces. Investigation of cavities is a challenge in many scientific and engineering fields (Abu-Shariah, 2009). The classic method for determining the presence of a cavity below the surface is drilling.
However, a more economical and effective solution is to implement geophysical surveys to reduce the number of drilling and increase the accuracy of test holes' placement for verification (Bacic et al., 2020).
In addition, engineering geological mapping is also beneficial for identifying the presence and pattern of geological structures observed directly (Martinez- Grana et al., 2013; Phillips et al., 2019). The identification of subsurface hazards is essential for foundation design and selecting areas to be developed.
The geophysical methods have been extensively implemented to delineate cavities and weathered zones (Zhou et al., 2002; Abu-Shariah, 2009;
Vachiratienchai et al., 2010; Martinez-Lopez et al., 2013). The geophysical technique that is the most commonly used to identify subsurface structure is electrical resistivity tomography (ERT) (Metwaly and Alfouzan, 2013; Gabas et al., 2014). This method provides a fast and cost-effective alternative to identifying subsurface conditions from shallow subsurface with acceptable resolution (McCormack et al., 2017; Hussain et al., 2020; Wilopo et al., 2020).
The study area is located in the development area of Universitas Negeri Yogyakarta (Yogyakarta State University/UNY) campus in Pacar Rejo Village, Semanu District, Gunungkidul Regency, Special Province of Yogyakarta, Indonesia, as shown in Figure 1. This area has an elevation of about 200 meters above sea level (m asl) with an average temperature of 26.7
oC, while the amount of rainfall in 2020 was 2,327 mm and 150 rainy days (BPS-Statistics of Gunung Kidul Regency, 2021). The research area is included in the mountain zone of southern Java, located along the south coast of Java Island, from East Java Province to Yogyakarta Special Province. The most striking feature of this zone is the discovery of limestone that forms a karst landscape. The rock composition consists of the Wonosari Formation with a thickness of more than 800 m and ages from the middle Miocene to the end of the Pliocene (Surono et al., 1992). This formation is dominated by carbonate rocks consisting of bedded limestones and reef limestones. According
to the regional land subsidence hazard map in the Semanu district, the campus development is located in high susceptible zones of land subsidence with a collapse probability of about 34% per km2 (Widyaningtyas and Putra, 2014). Moreover, recently, near the location, several sinkhole collapses were reported by local communities within a one-kilometer radius. Therefore, this study aimed to develop a subsidence hazard map using the ERT method and geological mapping to ensure safe and sustainable future campus building usage.
Materials and Methods
The research was conducted using the ERT and geological engineering mapping. The ERT method analyses subsurface materials based on their resistivity values described in 1D, 2D, and 3D forms (Colella et al., 2004). This method requires multiple electrodes and a cable system to be installed over the strip to be profiled. The distance between the electrodes depends on the resolution and the depth of the particular objective being identified. In general, the shorter the distance between the electrode points, the greater the resolution, and the depth of interpretation accuracy decreases. Technically, the ERT surveys can be carried out using different electrode arrays such as dipoles, Wenner, and Schlumberger that span the surface of the area to be surveyed (Dahlin and Zhou, 2004; Rizzo et al., 2019). Electric current was injected into the ground, and the apparent electrical resistance and voltage were measured. The field measurements used the dipole-dipole method with an electrode distance of 5 meters and a line length of 250 meters. The ERT survey was carried out on eight lines (GL1 to GL8), as shown in Figure 1. The instrument used was an Mc- Ohm Resistivity meter model 2115 from OYO Corporation, Japan. The resistivity tomography profile was interpreted using the Oasis Montaj software version 8.4. The inversion method minimizes the difference between the observed and calculated resistivity values to construct a rock stratigraphy model based on the resistivity values (Loke et al., 2003).
Engineering geological mapping was carried out to support interpretation based on the tomographic resistivity data. This mapping emphasizes the lithology type and geological structures formed on the surface, such as cracks, joints, or the shape of cavities.
The observations of geological structures include location, direction, density, and distribution. One core drilling was conducted to identify the stratigraphy and cavities with a depth of 20 meters. It was located in the ERT line GL-02 at point 135, as shown in Figure 1.
The google earth images were used as a background for all thematic maps (Google Earth, 2021). There are 48 observation points (STA) for geological engineering mapping, as shown in Figure 1. The pattern and direction of the cracks were used to
Open Access 3283 determine the direction of the ERT line. The ERT lines
are perpendicular or parallel to the fracture pattern's direction to produce the best interpretation. The
geological engineering map was integrated with tomographic resistivity analysis to estimate subsurface lithology.
Figure 1. The location of the research area is based on google earth images (2021).
The subsidence hazard level was determined by a combination of surface lithology, joint density, and sub-surface lithology data. Surface lithology joints density and subsurface lithology were classified into three classes. High class with a value of 3 means very significant to the occurrence of subsidence. A value of 2 is moderate, and a value of 1 is low, which is not significant. Then the three parameters were overlaid to produce a subsidence hazard map. The total value of 7 to 9 is a high hazard, moderate with a total value of 5 to 6, and low if the total value is 3 to 4.
Results
Engineering geological mapping
The engineering geological mapping showed that the study area is composed of limestone. Limestone in the research area can be divided into three based on texture
and structure in the field, namely massive limestone, bedded limestone, and cavity limestone, as shown in Figure 2. The bedded limestone in the study area generally has a brownish white color, as shown in Figure 3(a). Bedded limestones have a layering orientation of east and northwest with a layer thickness ranging from 20 cm to 50 cm. Bedded limestones were found on the northwest and southeast sides of the study area.
The fresh cavity limestone has white and yellowish-white color, as shown in Figure 3(b). The cavity structure in the cavity limestone is caused by the infiltration of rainwater and groundwater in the weak zone. The size of the cavity found in the field ranged from 1 cm to 35 cm. This cavity will be larger due to the weathering and dissolution process by both rainwater and groundwater flow. Massive limestone is white and yellowish-white, like cavity limestone, as shown in Figure 3(c). However, what distinguishes
Open Access 3284 this rock from cavity limestone is its massive structure
without any layers or cavities. However, in some places found many fractures and joints in this rock unit. Over time, these cracks will form larger holes due to high weathering and dissolving processes comprising cavity limestones. Apart from rock types,
the presence of geological structures was also observed. The distribution of joints in the study area is shown in Figure 2. Many joints were found in the center of the study area, as shown in Figure 3(d). This joint structure is commonly found in bedded and massive limestones.
Figure 2. The geological engineering map is plotted in the google earth images (2021).
In the cavity limestone, fractures and cracks have generally undergone weathering and erosion that form holes. The joint direction is used to determine the resistivity survey line, where the direction of the line is made perpendicular to the existing joint pattern. The existing joint pattern influences the cavities/caves pattern formed below the surface. In addition, there are also resistivity tomography lines made parallel to the joint pattern. The joint direction has a different orientation in each location, but the most dominant is N150oE to N 180oE and N330oE to N0oE. The main pattern of joint direction found is N 345oE. The joint directions plotted on the rose diagram shows that the influencing geological structure has a North-West to South-east direction, following the study area's regional geological structure trend (Surono et al., 1992). Based on the pattern and direction of the existing joints, it is estimated that if there were
cavities, caves, or underground rivers, the main direction would follow the existing geological structural pattern.
Electrical resistivity tomography survey
The ERT measurements were carried out on eight lines with the line directions considering the pattern of geological structures, as shown in Figure 1. The 2D resistivity tomography data processing has a root means square error (RMSE) value of less than 10%. It means that the resistivity model is acceptable (Amaya et al., 2016). The resistivity values can be divided into less than 40 ohmmeters, 40 ohmmeters to 100 ohmmeters, and groups of more than 100 ohmmeters.
Groups less than 40 ohmmeters generally indicate a relatively soft rock and contain water. The resistivity group of 40 to100 ohmmeters indicates a medium to hard rock and holds water.
Open Access 3285 Figure 3. (a) Outcrop of massive limestone (STA 21); (b) Outcrop of bedded limestone (STA 38); (c) Outcrop of
cavity limestone (STA 40); and (d) Joint/crack in the massive limestone (STA 22).
A resistivity group of more than 100 ohmmeters generally indicates a hard rock or has many cavities/caves. The line of the GL-01 is located north of the research area. The resistivity value is varied, with the highest reach 720 ohmmeters situated in the middle of a section. The highest resistivity value is probably due to cavity limestone, as shown in Figure 4(a). In the GL-02 measurement line, there are two points of primary concern. First, in the middle, at a depth of 25-40 meters, an isolated group of 40-100 ohmmeter resistivity can be seen, indicating rocks with trapped water content as shown in Figure 4(b). The second point is to the east, where the resistivity value is very high at a depth of 15-20 m from the surface.
The resistivity value suggests that the rock has a huge cavity or a small cave. The results of the GL-03 line show that it has an incline the resistivity 60 to 100 ohmmeters in the west area indicates a geological structure filled by groundwater from the surface or weathered rock, as shown in Figure 4(c). In addition, a low resistivity reaching 40 ohmmeters with a circle shape indicates a cave with water in the east. Line GL- 4 is relatively similar to line GL-03. However, the east of the circle shape has a high resistivity value, probably due to the dry cave, as shown in Figure 4(d).
It shows in the middle like a basin, probably a cave filled with water or weathered rock. The GL-05 line measurement location is located in the south of the research area. The results of the GL-05 line measurement show that the section is dominated by a resistivity value of more than 100 ohmmeters, where a value of 40-100 ohmmeters is found at the bottom line, shown in Figure 4(e). It indicates a cave fully
saturated. It shows that the cavity from the north continues to the south; however, some cave is filled by water, decreasing resistivity.
The GL-06 line consists almost entirely of resistivity groups >100 ohmmeters, as shown in Figure 5(a). A very high resistivity value of more than >1,440 ohmmeters indicates rocks with many cavities or caves. In addition, a low resistivity with a circle shape revealed a cave with water in the south of the section.
The results of the GL-07 line are similar to GL-06, as shown in Figure 5(b). Probably the continuity of rock and cavity continues from GL-06 to GL-07. The GL- 08 (8) results are relatively similar to the GL-07 line, showing the domination of resistivity of more than 100 ohmmeters. In addition, a resistivity value with a range of 40-100 ohmmeter was found in the north, as shown in Figure 5(c). It indicates a cavity with water content.
A resistivity fence diagram was developed from all geoelectrical lines, as shown in Figure 6(a). It was used to determine the distribution and relationship of resistivity values at the study site. The fence diagram shows that the study area is dominated by more than 100 ohmmeters resistivity value, indicating cavity rock or very hard rock. The limestone with more than 100 ohmmeters resistivity in the northern part is more dominant than in the southern area. Some places have resistivity values of more than 720 ohmmeters, indicating many voids or small caves in the north part of the study area. The southern part of the research area is dominated by rocks with a 40-100 ohmmeters resistivity value, indicating that the rock is saturated cavity limestone. This data follows the fact that groundwater is easier to find in the south. Figure 6(b)
(a) (b)
(c) (d)
Open Access 3286 shows a subsurface geological model where massive
limestone predominates in the study area. Cavity limestone dominates in the north to the middle of a research area. The result of the geoelectrical analysis was verified with drilling data (BH 1) at line GL-02.
The drilling log show soil was found at 0 to 1 meter, 1 to 6.5 meters massive limestone, 6.5 – 11 cavity
limestone, 11-12 massive limestone, 12 – 14.5 meters laminated limestone, and 14.5 to 20 meters cavity limestone. In addition, it shows that cavities were found at a depth of 6.5 meters, 16.5 meters, and 19 meters, with a thickness of approximately 0.5 meters for each. The presence of a cavity will increase the resistivity value.
Figure 4. Resistivity values of west-east direction from north to south. (a) Line GL-01; (b) Line GL-02; (c) Line GL-03; (d) Line GL-04; and © Line GL-05.
(a)
(b)
(c)
(d
(e)
Open Access 3287 Figure 5. Resistivity values of south-north direction from west to east. (a) Line GL-06; (b) Line GL-07; and (c)
Line GL-08.
Subsidence hazard map
The subsidence hazard map was determined based on the characteristics of surface lithology, joint density, and subsurface lithology revealed from the resistivity tomography survey. Each parameter was divided into three classes, with the value of each class starting from 1 to 3. The first parameter was surface lithology, where massive limestone has a low level of influence on the occurrence of subsidence (value 1), medium bedded limestone (value 2), and high cavity limestone (score 3). Figure 7(a) shows the distribution of the threat of subsidence based on the type of surface lithology. The second parameter was fracturing, processed using kernel density functions and geostatistics to produce fracture data per square kilometer. Figure 7(b) shows the distribution of the influence of geological structures on the potential for subsidence based on the fracture density factor on the surface. The figure shows that most areas have low values because massive limestone dominates in the study area. The third parameter classified was the distribution of rock types below the surface. Subsurface rock types were based on resistivity surveys. Subsurface conditions with the dominance of massive limestone have a score value of
1, medium bedded limestone value 2, and high cavity limestone value 3. Figure 7(c) shows the subsurface conditions in the southern part of the study area, which are dominated by bedded limestone. Cavity limestones dominate the western and northern parts of the study area. There are several locations where the top is composed of massive limestone, but the bottom is composed of cavity limestone. This combination makes the potential for subsidence in low massive limestones increase to moderate. All parameters were combined and assessed based on the total value of the three subsidence parameters. Figure 8 shows a subsidence hazard map of the three parameters used.
Based on the subsidence hazard map, the areas with the highest hazard level (total score 7-9) are in the northwest and north of the study area. It is due to the high fracture density of the cavity limestone on the surface and below the surface. Areas with low subsidence hazards are on the study area's east, northwest, and southwest sides, with a total score of 3- 4. The site has a relatively low hazard level due to massive limestone that dominates on and below the surface and lacks fractures. Other areas that are not classified as high and low hazards are moderate hazards.
(a)
(b)
(c)
Open Access 3288 Figure 6. (a) Fence diagram of resistivity values from all geoelectrical profiles; (b) Fence diagram of lithology
based on the resistivity value interpretation. All of them are plotted in google earth images (2021).
(a)
(b) (b)
Open Access 3289 Figure 8. Land subsidence hazard map with the background of the google earth images (2021).
(a) (b)
(c)
Figure 7. Subsidence parameter maps (a) Surface lithology map; (b) Joint density map;
and (c) subsurface lithology map. All of them are plotted in google earth images (2021).
Open Access 3290 Discussion
The mapping of potential subsidence hazards carried out by previous researchers was only based on information from the surface (Putra et al., 2011;
Widyaningtyas and Putra, 2014) or only based on subsurface aspects (Martinez-Lopez et al., 2013, McCormack et al., 2017; Hussain et al., 2020). The combination of surface and subsurface data will give a better result for subsidence hazard assessment. The results of previous studies indicate that the location of the planned campus of UNY is included in the area with a high level of vulnerability to subsidence. The method used in the previous mapping is only based on surface information, including surface lithology type, distance to lineaments, and slopes, without considering the subsurface lithology. Therefore, it does not represent subsurface conditions and is also not suitable to be implemented in relatively flat areas such as the location of the planned UNY campus. In general, subsidence is triggered by the presence of spaces/cavities below the surface either due to human activities (subsurface mining) or natural processes of rock dissolution (Font-Capo et al., 2015). The degree of dissolution will be significantly influenced by the type and characterization of the rock (Zhang et al., 2018). The dissolution rate will be more intensive if the rock is fractured or jointed (Sun et al., 2018).
Information on the types and characteristics of subsurface rocks is important as one of the parameters to determine the level of vulnerability to subsidence, not only data from the surface. Therefore, in this study, the parameters of surface lithology, density of geological structures, and subsurface lithology were used. Identification of subsurface rock types and characters is based on geophysical surveys.
Meanwhile, the lithology types and surface geological structures were obtained from direct mapping in the field. The results showed that not all locations of the planned development of the UNY campus were at a high potential for subsidence hazard as the results of previous studies (Widyaningtyas and Putra, 2014). The subsidence potential map using surface and subsurface parameters has better accuracy and more detail than previous studies. Areas with a high potential for subsidence are located only in the northern and central parts (red zones), as shown in Figure 8. It causes by the presence of cavities limestone on the subsurface with intensive joints. The information has also been verified by drilling data on the geoelectric line GL-2. It shows that many cavities were found at a depth of 6.5 meters, 16.5 meters, and 19 meters.
Conclusion
The result shows that the research area consists of bedded limestone, cavity limestone, and massive limestone. The cavity will increase the resistivity value. A combination of surface and subsurface data gives more accurate and detailed information for
assessing subsidence hazard maps. The highest subsidence hazard level area is located in the northwest and north of the study area. It is caused by the presence of cavity limestone both on the surface and subsurface with a high joint density. The areas with a low subsidence hazard are located on the east, northwest, and southwest sides. The low hazard level is caused by massive limestone that dominates on the surface and sub-surface and lacks fractures. The information on surface geological conditions, geological structures, and subsurface geological conditions show a better engineering geological picture than only geophysical methods or surface data interpretations. It is recommended that a high level of subsidence hazard area should be avoided for building construction. In addition, to minimize the load on the building's foundation, the number of floors should be limited and use a light material.
Acknowledgements
This study was financially supported under the MoU between the Faculty of Engineering, Gadjah Mada University (UGM) and Faculty of Engineering, Yogyakarta State University (UNY) with contract number 7839/UN1/FTK/LKFT/HK.08/2021.
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