DOI: https://doi.org/10.21203/rs.3.rs-17254/v1
Rockfall is one type of mass movement that occurs in the form of rock lumps collapse from a slope, this phenomenon occurs quickly both vertically and sub-vertically (Goodman, 1989; Budetta, 2004). The danger of rockfall has the potential to cause negative impacts on human life for a certain period of time (Westen et al., 2010). The threat of rockfall hazard is suspected from the natural physical characteristics of the region in the form of hilly areas (Hizbaron et al., 2010), especially in areas experiencing sloping cuts for transportation facilities in the form of road networks. Although rockfall is one of the most potential hazards occurring along a section of road that formed by hill cuts, not all of these roads have the same hazard degree. The potential risks and hazards of rockfall vary in each affected area (Budetta, 2004; Ansari et al., 2016).
Parangtritis Road is the main access to the famous Parangtritis beach tourism along the coast of the Southern Mountains of Yogyakarta Province. This road access has a fairly dense activity and frequently visited by local and international visitors. The Girijati Fault which has 250 meters of steep slope is estimated to be the main trigger of the rock mass movement in the area (Husein et al., 2010). This fault movement activates the Parangtritis Fault as the extension in the west. Both of these faults produced a rock-shaped semi-circular landslide crown (Husein et al., 2010) that was traversed by the main road access connecting both villages. Meanwhile, to entering the area, karst mountain cutting is needed to support the growth of the tourism sector. These cuttings produce several very steep and long slopes along the sides of the road.
To find out the rockfall hazard potential in the study area (Fig. 1), we need to conduct the study of the geological setting of the area, Slope Mass Rating (SMR) of the slopes, and other parameters that influence it. The rockfall hazard zonation will be created using a weighting system of hazard parameters obtained from field data collection.
The required field equipment consists of general tools used to conduct field measurements, but there is special equipment used such as the Global Positioning System (GPS), geological hammer and compass, and Schmidt Hammer. The software used in this study are ArcMap 10.6.1, Global Mapper 19, and Dips v.5.1.
Data obtained through direct measurements in the field including dip and strike from the slope and discontinuous plane, Uniaxial Compressive Strength (UCS) of intact rock, Rock Quality Designation (RQD), the space of discontinuous planes, the conditions of discontinuous planes, groundwater conditions, slope height, and the size of rock blocks (Fig. 2). Except for slope height and block size data, they are all used to produce SMR values. SMR values along with slope height and block size data are used as hazard parameters of rockfall hazard zonation. Secondary data used in the data processing stage are including rock lithology, geological structures, and rainfall amounts obtained from relevant government agencies.
The data analysis stage was preceded by laboratory tests to obtain UCS values which were then converted to MPa units according to the weighting of UCS by Bieniawski (1989). Followed by the geomechanics analysis of Rock Mass Rating basic (RMRb) with the weighting method by Bieniawski (1989), then proceed with SMR analysis with the weighting method by Romana (1993). The next step is the making of stereographic projections from discontinuous planes in two dimensions to model discontinuous planes on slopes or outcrops found in the field, as well as to determining rockfall types that can occur in a planar, wedge, or toppling type.
The weighting of rockfall hazard parameters is carried out using rockfall hazard classification which is a modified weighting method of Pierson's Rockfall Hazard Rating System (RHRS) (1991) and Budetta (2004). Some of the data processing was done with Geographic Information System (GIS) applications, as well as the maps produced in this study. The verification phase is carried out by confirming the accuracy and the conformity of the data and the resulted map with actual conditions at study area. If there is a discrepancy, repeated field investigation and re-analysis need to be carried out.
The study area is included in a series of Southern Mountains hills along the west to the east of the south coast of Java and the northern part of the area was encountered by a lowland known as Solo Lane (Van Bemmelen, 1949). The area is divided into three geomorphological units, they are the karst hills unit, the structural hills unit, and the lowlands unit.
Based on the Geological Map of Yogyakarta sheet (Rahardjo et al., 1977), the western part of the Southern Mountains is composed of volcanic rocks, volcanic clastic rocks, and carbonate rocks. Most of the volcanic clastic rocks formed by the deposition of gravity sediment produce approximately 4000 meters thick. The study area compiled by the four rock formations in the region, namely respectively based on the age are Nglanggran Formation, Wonosari Formation, Young Merapi Volcanic Mount Deposition, and the youngest is Alluvium Deposition.
To determine di geotechnical units in the study area (Fig. 3), the distribution of rock lithology and the results of weathering and falling rocks were observed. The naming of these units is using the rock or grain size of the material from each lithological unit. The geotechnical units (with the at the study area are andesite, breccia andesite, layered limestone, crystalline limestone, fragmental limestone, reefal limestone, silt - boulder, clay - sand, and fine - medium sand unit. Meanwhile,
Based on Hardness Reborn (HR) values obtained through field measurement using Schmidt Hammer, the andesite unit has the highest UCS value of 131.2 MPa (strong), while the lowest UCS value is owned by the reefal limestone unit of 5.8 MPa (weak). Crystalline limestone unit also includes in the weak class with an average UCS value of 14.92 MPa. The fragmental textured limestone unit has a higher UCS value compared to other limestone units, categorized as the intermediate class with the UCS value of 35.33 MPa.
Geological structures founded on the Geology Map Yogyakarta Sheet (Rahardjo et al., 1977) are joints, faults, and folds. The folds consist of anticline and syncline which are having a general direction northeast-southwest and east-west and some other trending is northwest-southeast. Faults are generally a normal fault with antithetic fault block pattern (Van Bemmelen, 1949). The geological structures developed are shear faults and normal faults (Fig. 4). The Opak Fault cut Yogyakarta and Wonosari old andesite as a constituent of fault cutting structures, while in the east of Opak River there are Semilir and Nglanggran Formation which is also involved in the fault system (Rahardjo et al., 1997).
Stereographic projection analysis in the form of Rose Diagrams (Fig. 5) is also used to determine the orientation of the dominant discontinuous plane in the slope composing rocks. These Rose Diagrams created using Dips v.5.1 software. The diagrams show that the relative orientation of the discontinuous plane in the andesite unit is trending northeast-southwest (N 60⁰E). Furthermore, also trending northeast-southwest (N 45⁰E) for crystalline limestone unit, northwest-southeast (N 325⁰E) for fragmental limestone unit, and northeast-southwest (N 40⁰E) for reefal limestone unit. In general, the most dominant relative direction of the discontinuous field at the study area is a northeast-southwest direction (N 45⁰E).
To obtain the SMR value, the value of RMRb (consist of UCS, RQD, the space of discontinuous planes, the condition of discontinuous planes, and the groundwater conditions) are calculated with SMR Adjustment Factors using the formula SMR = RMRb - (F1 x F2 x F3) + F4. SMR Adjustment Factors are additional weights given based on the orientation of the discontinuous plane on the outcrop or the slope (Romana, 1993). The SMR Adjustment Factors consist of F1 which reflects the parallelism between the joints direction and the slope direction, F2 which shows the steep of the slope, F3 which shows the relationship of the sloping joints and the sloping slope, and F4 which is an adjustment for the excavation method on the slope. The resulted SMR zonation map is shown in Fig. 6.
Based on the SMR values obtained, andesite unit has a normal slope mass quality (51.27), crystalline limestone unit has bad slope mass quality (22.30 to 38.87), fragmental limestone unit has bad to normal slope mass quality (31.17 to 53.03), and reefal limestones unit has very bad to bad slope mass quality (5.82 to 38.15). Overall there were no good quality mass slopes at the study area. However, the present study also uses two other parameters that considered as intrinsic factors which also affect the degree of rockfall hazards.
Rockfall that occurred from a higher slope possesses greater energy than those that occurred from a lower slope. So it is important to measure the rate as the higher slope was expected to have a higher degree of hazard. Based on the field measurements, the crystalline limestone unit has intermediate to high slope height hazard category (Fig. 7.a). Reefal limestone unit has varied slope height hazard categories from low, intermediate, and high category. The big difference of slope height between the units will affect the rockfall hazard weight on each rock unit.
The rock block size can be considered fairly affect the rockfall hazard assessment in the study area because of its variations. The size of the rock block on the slopes varies with the rock diameter size of 0.2 to 1.3 meters. Station 13 has a rock block size with very high hazard categories (Fig. 7.b). From the evidence and remnants, rockfall is indicated to had occurred in the location. Other stations have various rock block sizes with a very low to high hazard category. Combined with SMR values and slope height, the weighting of rock block size will result in variations of the rockfall hazard assessment.
Rockfall hazard class that has the greatest percentage is very low hazard class with a percentage of 83.83%, road which does not have slopes is also categorized in this class. The second-largest percentage is the intermediate hazard class by 7.16%, and then low hazard class with 4.82% and high hazard class with the smallest percentage equal to 4.19%. On the different scenario, a different result is shown when the roads that do not have slope is not included. The largest percentage owned by the intermediate hazard class with 36.60%, followed by 24.64% for low hazard class, then high hazard class with a percentage of 21.39%, and very low hazard class with the smallest percentage of 17.38%. Table 1 provides the summary of each rockfall hazard class conditions and Fig. 8 shows the Rockfall Hazard Zonation Map.
| Hazard Class | Lithology | SMR | Slope Height | Rock Block Size | Number of Locations |
|---|---|---|---|---|---|
| Very low | Fragmental limestone | 51.66–51.75 (normal) | 2.85–4.57 m | 0.2–0.3 m | 2 stations |
| Low | Andesite, crystalline limestone, and fragmental limestone | 31.17–53.03 (bad - normal) | 3.52–5.28 m | 0.2–0.7 m | 5 stations |
| Intermediate | Crystalline limestone, fragmental limestone, and reef limestone | 5.82–38.15 (very bad - bad) | 4.26–8. 96 m | 0. 3 − 1.0 m | 7 stations |
| High | Crystalline limestone and reef limestone | 18.31–36.50 (very bad - bad) | 3.62–7.82 m | 0.7–1.3 m | 3 stations |
There are four rockfall hazard classes of at the research location, i.e. very low, low, intermediate, and high. Very low rockfall hazard class also includes roads that do not have slope. This class has the highest percentage that is equal to 83.83%. In this hazard class, the slope strengthening method is not yet needed, but wire mesh can be installed to hold small debris rocks that potentially fall out. If the percentage of rockfall hazard classes only calculated based on the total length of the slopes and roads that do not have slope is not included, then the largest percentage owned by intermediate hazard class with 36.60%, followed by a low hazard class and high hazard class with percentage of 24.64% and 21.39%, and very low hazards class with the smallest percentage of 17.38%. The slope strengthening method is important to be installed at the intermediate, high, and very high hazard class locations. The shotcrete method can be used on slopes that formed from soft rock types or easily destroyed. The fragility nature like this one is often found in karst mountain cutting, as in the study area. If the slope has degraded and has experienced a lot of rockfalls, then the most needed slope strengthening method is wall coating with cement or concrete.
Availability of data and materials
The DEM data utilized in this study are freely available from NASA Shuttle Radar Topography Mission Version 3.0 Global 1 arc second (https://earthexplorer.usgs.gov). The other datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.
The authors declare that they have no competing interests.
Research Center of Oceanography, Indonesian Institute of Science
Research Center of Geotechnology, Indonesian Institute of Science
Contributions
KT conducted field investigation in 2014, took samples, created the maps, analysed and compiled the research work. KH carried out some geotechnical analyses in the research work. Both authors read and approved the final manuscript.
Acknowledgements
The author thanked to Dr.rer.nat Doni Prakasa Eka Putra and Dr. Wahyu Wilopo for giving the substantial inputs, and also to Dr. A'an Johan Wahyudi for the constructive comments.