1. Geological Setting
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 is encountered by a lowland known as Solo Lane (van Bemmelen, 1949). The area is divided into 3 (three) geomorphological units, i.e., the karst hills, the structural hills, and the lowlands. 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 volcanic clastic rocks formed by the deposition of gravity sediment that approximately 4000 meters thick. The study area compiled by the 4 (four) rock formations in the region, named respectively based on the age are Nglanggran Formation, Wonosari Formation, Young Merapi Volcanic Mount Deposition, and Alluvium Deposition.
Geological structures founded on the Geology Map of 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 patterns (van Bemmelen, 1949). The geological structures developed are shear faults and normal faults (Figure 3). The Opak Fault cut Yogyakarta and Wonosari old andesite as a constituent of fault cutting structures. Meanwhile, in the east of Opak River, there are Semilir and Nglanggran Formation that also involved in the fault system (Rahardjo et al., 1997).
The number of slopes found on the main roadside was 17 slopes with varying height and length. Each slope becomes an observation point so that in total 17 stations were analyzed in this study. The naming of these units was based on the lithology of the rocks. There were only 4 (four) geological units found from the 17 stations in the study area (Figure 4). These 4 (four) geological units, i.e., andesite, crystalline limestone, fragmental limestone, and reefal limestone (Rahardjo et al., 1977), used as the analysis unit in the analysis stage.
The number of stations has changed dramatically (closer to each other) start from the middle to the eastern end of the study area. However, the western part of the study area was still analyzed for some reason. The presence of volcanic rock outcrops with a significant height is a rarely found object in this area. Station 1, composed of an andesite unit, was expected to provide a distinct comparison of rock mass properties and quality with the three types of limestones units. Moving forward, composed of loose gravel to boulder sediments, most of the rock mass of the outcrops/slopes lied between Station 1 and 2 cannot be measured because the rocks were not compact, too soft, and easily destroyed. This road segment will still be mapped because several locations were considered to have the landslides potentials.
2. Rock Mass Rating basic (RMRb) Analysis
Based on Hardness Rebound (HR) values generated by Schmidt Hammer in the field measurement, the andesite unit has the highest UCS value of 131.20 MPa (very strong). The lowest average UCS value of 6.52 MPa (very weak) owned by the reefal limestone unit. Crystalline limestone unit included in the weak class with an average UCS value of 14.92 MPa. The fragmental limestone unit has a higher UCS value compared to other limestone types (crystalline and reefal) and categorized as the moderate class with the average UCS value of 35.33 MPa. The UCS values generated by Schmidt Hammer were equivalent to the UCS value resulted from laboratory tests (Table 12).
Table 12. Comparison of UCS values resulted from Schmidt Hammer and laboratory test
Unit
|
Average UCS value generated by Schmidt Hammer (MPa)
|
UCS value resulted from laboratory test (MPa)
|
Andesite
|
131.20 (1 stations)
|
122.30
|
Crystalline limestone
|
14.92 (5 stations)
|
14.61
|
Fragmental limestone
|
35.33 (6 stations)
|
36.52
|
Reefal limestone
|
6.52 (5 stations)
|
6.96
|
Indirect RQD measurements were carried out in all 17 stations because there was no borehole available. Based on the measurements, the RQD values in all stations categorized as good–very good. The 14 stations have very good RQD values, while 3 (three) other stations that are Station 1 (composed of andesite units) and Station 7 & 10 (composed of fragmental limestone units) have good RQD values. Table 13 shows the details of indirect RQD measurements in the study area.
Table 13. Indirect RQD measurements
Sta.
|
Space of discontinuous planes (m)
|
Jv
|
RQD (%)
|
Description
|
Set 1
|
Set 2
|
Set 3
|
Set 4
|
1
|
0.415
|
0.200
|
0.240
|
|
11.560
|
76.850
|
Good
|
2
|
1.043
|
0.636
|
0.300
|
|
3.025
|
100.000
|
Very good
|
3
|
0.477
|
0.667
|
1.167
|
|
4.460
|
100.000
|
Very good
|
4
|
0.676
|
0.750
|
0.515
|
|
4.740
|
99.360
|
Very good
|
5
|
0.453
|
|
|
|
2.200
|
100.000
|
Very good
|
6
|
1.200
|
|
|
|
0.833
|
100.000
|
Very good
|
7
|
0.170
|
0.237
|
|
|
10.100
|
81.670
|
Good
|
8
|
0.580
|
|
|
|
1.720
|
100.000
|
Very good
|
9
|
0.560
|
0.820
|
|
|
3.000
|
100.000
|
Very good
|
10
|
0.380
|
0.730
|
0.710
|
0.165
|
11.400
|
77.400
|
Good
|
11
|
0.340
|
0.500
|
|
|
4.940
|
98.700
|
Very good
|
12
|
0.350
|
1.660
|
|
|
3.450
|
100.000
|
Very good
|
13
|
1.360
|
1.067
|
|
|
1.670
|
100.000
|
Very good
|
14
|
0.240
|
|
|
|
4.160
|
100.000
|
Very good
|
15
|
0.970
|
|
|
|
1.030
|
100.000
|
Very good
|
16
|
2.110
|
1.440
|
|
|
1.160
|
100.000
|
Very good
|
17
|
1.625
|
0.625
|
|
|
2.215
|
100.000
|
Very good
|
The spaces of discontinuous planes vary between 0.17–2.11 meters, with lengths between 0.47–7.49 meters, and various gaps from <0.01 to 76 millimeters. The surface roughness of the discontinuous planes is not the same between the sets. Some are very smooth, smooth, moderate to rough, with slightly to very weathered conditions. The filling of discontinuous planes varies from soft filling with <5 millimeters size to hard filling with >5 millimeters size, except discontinuous planes found at Stations 1, 3, and 8 that have no filling.
The groundwater conditions were observed based on general conditions found on rock surfaces of outcrops/slopes and categorized as dry, moist, wet, dripping, and flowing (Bieniawski, 1989). The groundwater conditions in the study area were dominated by humid and dry on rock surfaces, but there was 1 (one) exception station with a significantly different state. Station 1, which is composed of andesite unit, has a flowing groundwater condition (Figure 5). Meanwhile, the other slopes composed of crystalline, fragmental, and reefal limestone have between dry and moist groundwater conditions. Table 14 shows the groundwater conditions in the study area.
Table 14. Groundwater condition found on the slopes
General condition
|
Number of locations
|
Station
|
Dry
|
5
|
6, 7, 8, 9, 12
|
Humid
|
11
|
2, 3, 4, 5, 10, 11, 13, 14, 15, 16, 17
|
Wet
|
0
|
-
|
Dripping
|
0
|
-
|
Flowing
|
1
|
1
|
3. Stereographic projection analysis
A discontinuous plane and the kinematics mechanism can be analyzed using the stereographic projection (Goodman, 1989). According to Ragan (2009), the stereographic projection is a two-dimensional picture or a projection of a sphere surface that used to describe the geometry position or the orientation of the planes and lines. The method used in stereographic projection in this study was the Equal Area Projection using Schmidt Net as the projection plane and created using Dips v.5.1 software.
In this study, the stereographic projection was also carried out to determine the rockfall types that could or had occurred in the discontinuous planes on a slope, i.e., planar failure, wedge failure, and toppling failure, as mentioned in the Slope Mass Rating (SMR) analysis by Romana (1993). Furthermore, this stereographic projection analysis was also conducted to find out whether a rock volume from a slope could fall naturally due to the condition of the discontinuous planes or not. If the parallelism of the discontinuous plane orientation and the slope face orientation (resulted from the cutting hills) was known to be in a stable condition, then without any disturbance from natural phenomena or human disturbances, the rockfall could not happen by itself and vice versa.
More information obtained from the stereographic projection of the 17 stations was that the rockfall type of wedge failure has more potential and occurred more frequently than the planar failure and toppling failure types. It was resulted due to a large number of discontinuous plane sets, which have many variations of orientation and causing intersections between discontinuous planes. Some examples of stereographic projections from 3 (three) stations were shown in Figure 6. The detail of slopes and discontinuous planes orientation (strike and dip), and also the failure type of each station were shown in Table 15.
Table 15. Slopes and discontinuous planes orientations (strike/dip) and the failure types
Sta.
|
Slopes orientation (strike/dip)
|
Discontinuous planes orientation (strike/dip)
|
Set 1
|
Set 2
|
Set 3
|
Set 4
|
1
|
N150⁰E/85⁰
|
N220⁰E/40⁰
(wedge)
|
N50⁰E/30⁰
(wedge)
|
N67⁰E/15⁰
(wedge)
|
|
2
|
N87⁰E/88⁰
|
N343⁰E/90⁰
(toppling)
|
N37⁰E/67⁰
(wedge)
|
N80⁰E/35⁰
(wedge)
|
|
3
|
N120⁰E/85⁰
|
N62⁰E/20⁰
(planar)
|
N240⁰E/84⁰
(wedge)
|
N43E⁰/87⁰
(wedge)
|
|
4
|
N/200⁰E/70⁰
|
N193⁰E/62⁰
(planar)
|
N290⁰E/28⁰
(wedge)
|
N110⁰E/50⁰
(wedge)
|
|
5
|
N105⁰E/80⁰
|
N120⁰E/42⁰
(planar)
|
|
|
|
6
|
N93⁰E/76⁰
|
N97⁰E/24⁰
(planar)
|
|
|
|
7
|
N132⁰E/64⁰
|
N48⁰E/33⁰
(wedge)
|
N222⁰E/47⁰
(wedge)
|
|
|
8
|
N144⁰E/76⁰
|
N253⁰E/70⁰
(planar)
|
|
|
|
9
|
N183⁰E/78⁰
|
N324⁰E/18⁰
(planar)
|
N115⁰E/66⁰
(planar)
|
|
|
10
|
N53⁰E/66⁰
|
N143⁰E/6⁰
(planar)
|
N317⁰E/48⁰
(planar)
|
N138⁰E/42⁰
(wedge)
|
N143⁰E/63⁰
(wedge)
|
11
|
N83⁰E/82⁰
|
N87⁰E/22⁰
(planar)
|
N355⁰E/90⁰
(toppling)
|
|
|
12
|
N24⁰E/83⁰
|
N28⁰E/3⁰
(planar)
|
N341⁰E/47⁰
(planar)
|
|
|
13
|
N182⁰E/74⁰
|
N87⁰E/85⁰
(toppling)
|
N189⁰E/65⁰
(planar)
|
|
|
14
|
N124⁰E/76⁰
|
N132⁰E/32⁰
(planar)
|
N48⁰E/61⁰
(wedge)
|
|
|
15
|
N87⁰E/75⁰
|
N274⁰E/82⁰
(toppling)
|
|
|
|
16
|
N127⁰E/84⁰
|
N37⁰E/33⁰
(wedge)
|
N213⁰E/52⁰
(wedge)
|
N220⁰E/11⁰
(planar)
|
|
17
|
N110⁰E/70⁰
|
N116⁰E/53⁰
(planar)
|
N24⁰E/33⁰
(planar)
|
|
|
Rose Diagram, which is a graphical form that concludes the entire stereographic projection analysis of 17 stations, is shown in Figure 7. These Rose Diagrams were created using Dips v.5.1 software and carried out to determine the dominant strike orientation of the discontinuous plane in the slopes. The diagrams show that the dominant strike orientation of the discontinuous planes was trending northeast-southwest (N 60⁰E) in the andesite unit (Figure 7a), northeast-southwest (N 45⁰E) in the crystalline limestone units (Figure 7b), northwest-southeast (N 325⁰E) in the fragmental limestone units (Figure 7c), and northeast-southwest (N 40⁰E) in the reefal limestone units (Figure 7d). In general, the most dominant strike orientation of all discontinuous planes in the study area was trending northeast-southwest (N 45⁰E).
4. Slope Mass Rating (SMR) analysis
Road segments without slopes were not classified because the assessment of the slope mass quality cannot be conducted without the rock slopes as the observed objects. The summary of the classified slopes according to the Slope Mass Rating (SMR) by Romana (1993) is shown in Table 16. Overall, there were none of good quality mass slopes in the study area. The measured SMR score ranged from normal to very bad.
Table 16. The summary of measured SMR
Sta.
|
Lithology
|
RMRbscore
|
SMR Adjustment Factors
|
SMR score
|
Class
|
F1
|
F2
|
F3
|
F4
|
1
|
Andesite
|
60.33
|
0.72
|
0.56
|
-60.00
|
15.00
|
51.27
|
Normal
|
2
|
Crystalline limestone
|
61.33
|
0.72
|
0.90
|
-48.33
|
0.00
|
30.19
|
Bad
|
3
|
Crystalline limestone
|
67.33
|
0.67
|
0.80
|
-53.33
|
0.00
|
38.87
|
Bad
|
4
|
Crystalline limestone
|
61.33
|
0.72
|
0.80
|
-56.67
|
0.00
|
28.87
|
Bad
|
5
|
Crystalline limestone
|
58.00
|
0.70
|
0.85
|
-60.00
|
0.00
|
22.30
|
Bad
|
6
|
Crystalline limestone
|
68.00
|
1.00
|
0.40
|
-60.00
|
0.00
|
36.50
|
Bad
|
7
|
Fragmental limestone
|
60.50
|
0.57
|
0.85
|
-60.00
|
0.00
|
31.17
|
Bad
|
8
|
Fragmental limestone
|
73.00
|
0.85
|
1.00
|
-25.00
|
0.00
|
51.75
|
Normal
|
9
|
Fragmental limestone
|
71.50
|
0.57
|
0.57
|
-60.00
|
0.00
|
51.66
|
Normal
|
10
|
Fragmental limestone
|
59.50
|
0.15
|
0.75
|
-57.50
|
0.00
|
53.03
|
Normal
|
11
|
Fragmental limestone
|
55.00
|
0.57
|
0.70
|
-55.00
|
0.00
|
35.16
|
Bad
|
12
|
Fragmental limestone
|
68.00
|
0.57
|
0.57
|
-60.00
|
0.00
|
48.16
|
Normal
|
13
|
Reefal limestone
|
53.00
|
0.92
|
1.00
|
-37.50
|
0.00
|
18.31
|
Very bad
|
14
|
Reefal limestone
|
48.00
|
0.57
|
0.85
|
-60.00
|
0.00
|
18.67
|
Very bad
|
15
|
Reefal limestone
|
56.00
|
0.85
|
1.00
|
-25.00
|
0.00
|
34.75
|
Bad
|
16
|
Reefal limestone
|
54.15
|
0.43
|
0.62
|
-60.00
|
0.00
|
38.15
|
Bad
|
17
|
Reefal limestone
|
53.00
|
0.92
|
0.85
|
-60.00
|
0.00
|
5.82
|
Very bad
|
The andesite unit represented by 1 (one) station of observation point has SMR value of 51.27 and categorized as normal class. The RMRb value of andesite units was not the largest of the entire sample, but the type of slope excavation method (natural) in this station gives an extra point to the SMR score. Natural exposed slope was only found at this station.
The crystalline limestone unit represented by 5 (five) stations of observation point had SMR score of 22.30–38.87 and categorized as bad class. Furthermore, the fragmental limestone unit represented by 6 (six) stations of observation point has SMR score of 31.17–53.03 and classified as normal to bad class. The highest average score of RMRb and SMR were found in fragmental limestone unit, proven by hard and compact rock conditions and non-complex discontinuous plane conditions.
The reefal limestone unit represented by 5 (five) stations of observation point has SMR score of 5.82–38.15, which is classified as bad to very bad. This unit has the lowest average score of RMRb and SMR, proven by weaker rock hardness, accompanied by holes of water dissolution, and complex condition of the developed discontinuous planes. The result of the SMR zonation is shown in Figure 8.
Most of the slopes at the study site need special attention from the community and local government because they have poor slope strengths and unstable conditions. These conditions may increase the risk and hazard level of rockfall and lead to physical and social losses. SMR values obtained in this study can be used as a reference to find the most suitable slope reinforcement method. The relationship between the SMR scores with the recommended slope reinforcement method is presented in Table 17.
Slope reinforcement should be installed on a slope with SMR value below 80 (good–very bad classes of SMR). That means even though a slope has a stable condition, slope reinforcement still needs to be installed to avoid the rockfall threats from external factors such as natural disasters and human disturbances. Fence, nets, ditch need to be installed on slopes that could potentially drop small rocks. These types of supports suitable for installation in Stations 8, 9, and 12. Shotcrete was needed to cover fragile and weak parts of the slopes and suitable for Stations 3, 7, and 10.
Anchors and bolts can be installed on a slope with very hard composing rock as in Station 1. Meanwhile, the systematic shotcrete or the concrete coating method should be installed on slopes formed from weathered and/or softer rock types and easily destroyed, such as in Stations 2, 4, 5, 6, 11, 15, and 16. This fragile character is often found in excavated karst hills as in the study area. If the slopes have badly weathered and/or has experienced a lot of rockfalls, then the most needed slope reinforcement is an anchored wall or even need re-excavation as in Stations 13, 14, and 17.
Table 17. Recommended support according to SMR score (Romana, 1993).
SMR Score
|
Support
|
91–100
|
None
|
81–90
|
None; scaling
|
71–80
|
None; toe ditch or fence; spot bolting
|
61–70
|
Toe ditch or fence; nets; spot or systematic bolting
|
51–60
|
Toe ditch and/or nets; spot or systematic
bolting; spot shotcrete
|
41–50
|
Toe ditch and/or nets; systematic bolting;
anchors; systematic shotcrete; toe wall
and/or dental concrete
|
31–40
|
Anchors; systematic shotcrete; toe wall
and/or concrete;
re-excavation; drainage
|
21–30
|
Systematic reinforced shotcrete; toe wall
and/or concrete;
re-excavation; deep drainage
|
11–20
|
Gravity or anchored wall; re-excavation
|
However, not only SMR scores used to assess rockfall hazards in this study. Two other parameters (i.e., slope height and rock block size) were also considered as intrinsic factors that could significantly affect the level of rockfall hazards. Slope height and rock block size will be explained further in the next section.
5. Slope Height and Rock Block Size
Rockfall that occurred from a higher slope possesses greater energy than those that occurred from a lower slope. So it is necessary to measure the slope height considering a higher slope was expected to have a higher level of hazard. Based on the field measurements, the crystalline limestone unit has moderate to high slope height hazard category (Figure 9a). Reefal limestone unit has varied slope height hazard categories from low, moderate, and high. A significant difference in slope height between the units will affect the rockfall hazard weight.
The block size is a very significant parameter in rock mass behavior (Barton, 1991) and considered practically affect the rockfall hazard assessment in the study area because of its variations. The rock block size on the slopes varies with the rock diameter size of 0.2 to 1.3 meters. A very high hazard category of rock block size was found in Station 13 (Figure 9b). From the evidence and remnants, rockfall was indicated to had occurred in the location. Rock block size in other stations varies with a very low to high hazard category. Combined with SMR and slope height values, the weighting of rock block size will result in more variations of the rockfall hazard assessment.
6. Rockfall Hazard Zonation
The very low class of rockfall hazard zonation has the greatest percentage of 83.83%, road segments without slopes were also classified in this class. The second-largest percentage is the moderate hazard class by 7.16%, followed by low hazard class by 4.82% and high hazard class with the smallest portion of 4.19%. On the different scenario, a different result was shown when the roads without slopes were not included. The largest portion owned by the moderate hazard class by 36.60%, followed by low hazard class by 24.64%. Third, there was high hazard class with a percentage of 21.39%, followed by very low hazard class with the smallest portion of 17.38%. The most significant parameter that influences the rockfall hazard zonation was the SMR, with a percentage of 53.42% of total rockfall hazard weight. Followed by slope height and rock block size with the percentages of 24.27% and 22.30%, respectively. Figure 10 shows the Rockfall Hazard Zonation Map.
Most rockfall hazard zonation class at overall stations experience reduced levels compared to their SMR class. Only Station 2 and Station 6 that classified in the same class level (high) in both the SMR and rockfall hazard zonation classes. It proves that the slope height and rock block size parameters affected the final result of rockfall hazard zonation. For example, Station 3 has a bad SMR class, but the size of the rock block at this station has low weight because the largest diameter found was only 0.6 meters (classified as low hazard of rock block size). Therefore, the total weight of the rockfall hazard zonation at Station 3 was classified as low class. Another example, Stations 8 and Station 9 have normal SMR class, but the slope height at both stations did not reach 5 (five) meters (classified as low hazard of slope height). Hence, the final result of rockfall hazard zonation at Station 8 and Station 9 was classified as very low class. Table 18 provides a summary of rockfall hazard classes with the condition of each parameter.
Table 18. The summary of rockfall hazard class and the parameters.
Hazard class
|
Lithology
|
SMR score
|
Slope height (m)
|
Size of rock block (m)
|
Stations
|
Very low
|
Fragmental limestone
|
51.66–51.75 (normal)
|
2.85–4.57
|
0.2–0.3
|
8, 9
|
Low
|
Andesite, crystalline, and fragmental limestones
|
31.17–53.03 (normal–bad)
|
3.52–5.28
|
0.2–0.7
|
1, 3, 7, 10, 12
|
Moderate
|
Crystalline, fragmental, and reefal limestones
|
5.82–38.15 (bad–very bad)
|
4.26–8. 96
|
0. 3–1.0
|
4, 5, 11, 14, 15, 16, 17
|
High
|
Crystalline and reefal limestones
|
18.31–36.50 (bad–very bad)
|
3.62–7.82
|
0.7–1.3
|
2, 6, 13
|
There was a 'landslide potential' category with purple color on the Rockfall Hazard Zonation Map. The slopes could be assumed to have landslide potential if there was evidence of occurred landslide and also none of the slope reinforcement installed yet on the slopes. The slope mass quality at the location automatically could not be measured because the rocks that composed the slopes had been destroyed and/or collapsed. Therefore, slopes with landslide potential were not included in the rockfall hazard classes, but they still need extra awareness and marked.
The extension of Girijati and Parangtritis Fault which forms a semi-circular crown structure (Prasetyadi et al., 2011) produced paleo-landslides deposits with estimated dimensions of 2,700 m long, 1,500 m wide, and 810 million m3 of landslide volumes (Husein et al., 2010) in a gravel–boulder grain size. These landslide deposits are very prone to move and/or collapse and believed to be one of the causes of most recent landslide events in the study area. On these landslide potential areas, the stakeholders should consider to re-excavating or at least covering the slopes with an anchored wall. Figure 11 below shows the occurred landslides at several locations in the study area.
Historical rockfall points were overlaid over the Rockfall Hazard Zonation Map to validate the predicted hazard zones, and the number of rockfalls that occurred in each hazard class was calculated. Information about occurred rockfalls was obtained from the local authorities of transportation, facilities, and infrastructure. In some stations without administrative data sources, the number of rockfalls estimated based on observations around the stations and information from residents. The estimation was carried out by observing the holes in the former slope or the rock blocks found on the trench/ditch. From this validation, rockfalls have occurred in the classes of very low, low, moderate, and high with the percentages of 1.75%, 7.02%, 26.32%, and 64.91%, respectively (Figure 12). Since 91.23% of the rockfall occurred in the moderate and high hazard classes, the Rockfall Hazard Zonation Map considered reliable to predict future rockfall.