SEMI QUANTITATIVE AND QUALITATIVE ANALYSIS OF ROCKSLIDES AT PONOROGO - PACITAN
ROAD KM 226
Retno Puspa Rini
*1, Arief Rachmansyah
2, Eko Andi Suryo
31
Student , Master’s program, Departement of Civil Engineering, Faculty of Engineering, Brawijaya University
2,3
Lecturer, Departement of Civil Engineering, Faculty of Engineering, Brawijaya University
*Correspondence: [email protected]
ABSTRACT
Rock avalanches is one of the problems that is often faced along the Ponorogo - Pacitan road. The research location was at KM 226 which is located at coordinates 111°22'14.39" East Longitude and 08°01'14.73"
South Latitude. Semi quantitative analysis was conducted by using Geology Strength Index method, that is based on field identification. The classification results of rock mass GSI obtained value of 25 and was dacite rock type. The slope had angle of 75o and height of 12 meters, safety factor (FK) of 1.065 was obtained which made the slope unstable, so prevention was done by changing the slope angle geometry based on variations of 75 o - 35 o, so that (FK) 35o = 1.52
Keywords: safety factor, rock mass, rock slope stability.
1. INTRODUCTION
Pacitan Regency is located at the northwestern tip of East Java Province with low economic growth. This regency relies on agricultural, marine and mining products to support their economy. To market these products to other regions, only ground access is available. Based on the Geological Map of the Pacitan Sheet at scale of 1: 100,000 (Samodra, H et al, 1992), due to frequent landslides that affect slope stability, it is necessary to analyze rock mass to determine the safety factor value (FK) of the slope. Classification of rock mass quality can use the Geological Strength Index (GSI) analysis which is one of the geological classifications to determine the rock mass type where the GSI value ranges from 0 to 100, where the GSI value of 100 is equivalent to the mass of intact rock.
This research is very important to do to see the level of slope landslide vulnerability around the Ponorogo-Pacitan road, so that later it can be used as information source for related parties.
2. LITERATURE REVIEW 2.1. Rock Slide
Rock mass is the rock volume consist of rock material in the form of minerals, texture, and composition and consists of discontinuous planes, forming a material and interconnecting with all elements as a unit [1]. The strength of the rock mass is strongly influenced by the frequency and density of the discontinuity planes that are formed, therefore the rock mass will have less capacity when compared to intact rock. Rock masses are in-situ rocks that are discontinued by the rock structure system such as fracture, faults, folds and layering areas.
According to Priest (1993), the definition of discontinuous plane is any weak plane that occurs in the part that has the weakest tensile strength in the rock [2] Figure 1. According to Gabrielsen (1990), the occurrence of discontinuity plane is inseparable from the problems of changes in pressure, temperature, strain, mineralization, and recrystallization that
occur in rock masses over a long period of time [3].
Figure 1 Orientation of Discontinuity Plane
According to Hoek and Bray, 1981, in general, the orientation combination of rock discontinuities will form three main types of landslides / failure in rocks [4] Figure 2, namely:
- Plane sliding failure - Wedge sliding failure - Toppling failure
Figure 2 Main types of failures in rock
2.2. Rock Mass Classification Geological Strenght Index (GSI)
Geological Strenght Index (GSI) introduced by Hoek (1994), Hoek et al. (1995), Hoek & Brown (1997), Hoek et al. (1998) and Marinos & Hoek (2001), developed as tool for classification inability of Rock mass rating system of Bieniawski (1989) to determine the rock quality with bad quality. By estimating the constant values of mb, s, and a. Constants mb, s, and a are used in determining the strength of rock mass based on Hoek and Brown's (1980) failure criterion [5].
2.3. Rock Slope Stability
Usually in engineering practice, stability is defined as safety factor which is analytically the ratio between the strength of the material holding the landslide with the working force that causes the landslide due to its gravity.
FK =
The factors that influence the moment of holding force are: The type of rock that is getting fresher (fresh) and has not experienced weathering, the rock will be more stable than the rock that has experienced weathering.
To calculate slope stability, software was used to facilitate planning. Rocscience, is one of the software programs especially for geotechnics which includes several programs in it consists of the Roclab program, software program for determining rock mass strength parameters, based on the latest version of the general Hoek-Brown failure criterion and also using the Rocplane program. which is interactive software that functions to analyze and design the stability of planar rock slopes. In 1995 Hoek, et al included the concept of the Geological Strength Index (GSI) which provides estimate of the reduction in rock mass strength due to differences in geological conditions. This criterion became known as the Generalized Hoek-Brown criterion with the equation :
(1)
Where dan is the maximum effective stress and minimum effective stress when the rock failures, while is the compressive strength of Uniaxial Compressive Strength (UCS). While mb and s is the rock constant value.
(2)
To get the value of this material parameter depends on the rock type (igneous, metamorphous, or sedimentary).
While s and a is the rock constant value using the following equation.
(3) (4)
Plane failure Wedge failure Toppling failure
(5) (6) The Hoek-Brown failure criterion also makes it possible to calculate the deformation modulus of the rock mass by the equation [6].
(7)
3. LOCATION AND METHOD
The research location was carried out along the Ponorogo-Pacitan road Figure 3. The Pacitan-Ponorogo route is one of the longest routes in the Pacitan area. The research area is at 7°58'20.0 "South Latitude and 111° 25'50.6"
East Longitude to 8°06'14.4" South Latitude and 111°09'50.2" East Longitude, more precisely at the coordinates 111° 22'14.39" East Longitude and 08°01'14.73" South Latitude.
Figure 3 The research location of the rock slope of KM 226 Ponorogo - Pacitan road
Figure 4. Plane sliding landslide at Slope of KM 226 Ponorogo-Pacitan Routes
4. RESULT AND DISCUSSION
The slope at KM 226 has slope of ± 75 ̊.
Based on direct analysis in the field the slopes experienced landslides which were included in the category of plane landslides which could be seen Figure 4, which had dacite igneous rock types that had undergone quite intense altration (high weathering level), causing them to change color to whitish brown so that the rocks become brittle and can be one of the causes of landslides.
To identify the landslide type, measurements of the slope span along 4 meters were carried out and to obtain this data, namely using geological compass, by measuring to find out the strike and dip data of slope N246oE / 46o by entering the data, kinematic analysis can be carried out in the form of stereographic method using software Dips to determine the
landslide type that occurred on the slopes of KM 226.
Figure 5. Stereographic Analysis Results of Fracture Projection By Using Software Dips.
From the analytical results in Figure 5 it can be seen and proven that the landslides that occurred on the slopes of KM 226 were planar landslides. This is because there was only one join set in the same direction or parallel to the slope plane. So it could be concluded and
known that the landslides that occurred on the slopes of KM 226 were planar landslides.
4.1. Semi Quantitative Analysis
Rock mass classification for the slopes of KM 226 was carried out using the Geological Strength Index (GSI) analysis using field analysis. GSI values were obtained by observations with geological observations in the field with the charts help so that what could be seen on the surface or the weathering level could be determined by the table / Figure 6 that had been determined in analyzing the rock mass quality. According to field observations, it was found the rock mass structure (dacite) is in the form of blocks arranged parallel, the surface was smooth and weathering occurrence was high. So that the results could produce GSI values that include, can be seen in Figure 7.
Figure 6 Assigning GSI Values to Rocks (Explanation : GSI assessment on dacite rocks that have undergone altration according to field analysis) (Hoek and Marinos, 2002)
Figure 7. GSI Value (Personal Document)
4.2. Quantitative Analysis
From the calculation of Hoek and Brown equation, obtained several values that would be inputted. In the analysis of the Roclab Program, obtained the figure 8 in the form of
major and minor graphs suitable with data input through the Hoek and Brown parameter criterion.
Figure 8. Major Minor Graph of Roclab Software
Based on the kinematic analysis that has been made with the stereographic method according to the data obtained, the safety factor analysis of slope stability could be assisted by using the RocPlane software application. Some
of the data required were in the form of slope tilt and height obtained based on observations at the KM 226 location, as well as several parameters that have been calculated based on the general criterion of Hoek and Brown.
DISINTEGRATED Bad and heavily damaged
rock mass
POOR
The surface was smooth and very weathered with parallel layers GSI VALUE
The obtained is suitable with field analysis
= 25
From the data that has been obtained, slope modeling could be done using the Rocplane software can be seen in Figure 9 and Figure 10. The safety factor value of slope stability which was used as reference standard was FK ≥ 1.5 and in accordance with SNI -
03.1962 - 1990 complement of the book
"Landslide Prevention Procedures", namely
"Construction and Building Guidelines Pd T- 09-2005-B [7].
Figure 9. 2D Results of Slope Stability Analysis at KM 226 through the RocPlane 75Program
Figure 10. 3D Results of Slope Stability Analysis at KM 226 through the RocPlane 75Program In the analysis the original slope in the
field with angle of 75o using Rocplane software obtained the safety factor (FK) analysis result of 1.065 so that the slope could be said as very unstable so that it did not meet the predetermined standards.
4.4. Slope Angle Geometry Change
The slope at KM 226 has angle of 75o which is very risky for landslides so that failure prevention is needed, one of which is changing the angle of the slope 75o Figure 11 to reach the specified safety factor, cliffs that are prone to landslides and have slope angle greater than
the sliding angle in the soil can also be made ramp with fairly safe slope angle.
Determination of this method needs to consider the mechanism of slope failure that occurs.
Figure 11 Countermeasures by Changing the Slope Geometry
At the slope of KM 226, in order to make the slope stable and prevent landslides
occurred, it was done by changing and reducing the slope angle. In this case, to be reference for the slope angle, several determined tilt variations can be made namely angle of 75o to angle of 35 o. The analysis results using the RocPlane software with angle of 35 o can be seen in Figure 12 and Figure 13.
From the analysis of RocPlane program, slope improvement by trimming the slope can produce slope angle. In the variation of applied slope angle, it can be seen that the slope angle becomes stable at angle of 35o by referring to the safety factor value that meets the standard, namely FK = 1.5. To parse all safety factor data based on variations can be seen in Table 1.
Figure 12. 2D Results of Slope Stability Analysis at Point KM 226 Angle 35o
Figure 14 3D Results of Slope Stability Analysis at Point KM 226 Angle 35o the cut section
Table 1 The Result of Slope Angle Variation Slope Angle
() Safety
Factor
Explanation
75 1.06 Unstable
65 1.11 Unstable
55 1.17 Unstable
45 1.28 Unstable
35 1.52 Stable
4. CONCLUSION
The results of rock mass classification used the parameter of Geological Strength Index (GSI) = 25. The failure that occurred on the rock slope at KM 226 was plane landslide according to the strike and dip slope data N246oE / 46o. It is known the safety factor (FK)
= 1.065 so that the slope can be said to be very unstable. After changing the geometry of slope angle, the result is (FK) = 1.52 at angle of 35o. 5. REFERENCES
[1] Iswandaru, R. Rully, N., Rai, Made, A., &
Wattimena, K, Probabilistik Kelongsoran Lereng Tambang Terbuka Grasberg PT
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[2] Priest, S. D, The collection and analysis of discontinuity orientation data for engineering design, with examples. In Rock Testing and Site Characterization (pp. 167-192). Pergamon, 1993.
[3] Green, R. M., Kelly, K. M., Gabrielsen, T., Levine, S. R., & Vanderzant, C, Multiple intracerebral hemorrhages after smoking"
crack" cocaine. Stroke, 21(6), 957-962, 1990.
[4] Hoek, E., & Bray, J. D., Rock slope engineering. CRC Press, 1981.
[5] Hoek, E., & Brown, E. T., Empirical strength criteria for rock masses. J. Geotech. Eng. Div., Am. Soc. Civ. Eng.;(United States), 106, 1980.
[6] Hoek, E., Carranza-Torres, C., & Corkum, B. , Hoek-Brown failure criterion-2002 edition. Proceedings of NARMS-Tac, 1(1), 267-273, 2002.
[7] Umum, D. P., Pedoman Konstruksi dan Bangunan–Rekayasa Penanganan Keruntuhan Lereng pada Tanah Residual dan Batuan. Pd T–09–2005–B, 2005.
