LANDSLIDES MECHANISM ANALYSIS AT KM 258 PONOROGO – PACITAN ROAD

LANDSLIDES MECHANISM ANALYSIS AT KM 258 PONOROGO – PACITAN ROAD

Miftahul Avidatur Rohmah

¹

, Arief Rachmansyah

²

, Harimurti

²

1Student, Civil Engineering, Engineering Faculty, Brawijaya University 2Lecturer, Civil Engineering, Engineering Faculty, Brawijaya University

Correspondence: miftahulavidaturrohmah@gmail.com

ABSTRACT

One of the locations where landslides occurred on the Pacitan - Ponorogo route was at KM 258 in December 2017. At that location there are 3 adjacent slope segments, the first segment and the second segment had landslides, to prevent landslide at the third segment investigation was carried out on the slope.

Kinematics analysis method was used using the DIPS program to determine the slope failure type and to obtain the circular failure type. The weighting of RMR rock mass on the slopes studied obtained value of 52 (class III) where the rock type is moderate, and the correlation with the SMR on class III RMR rocks is the recommended slope angle of 55°. After slope stability simulation at angles of 80°, 75°, 70°, 65°, 60° to 55°, the effective angle is 70° with an FS value of 1.6.

Keyword: Kinematics Analysis, RMR, SMR, Slope Stability

1. INTRODUCTION

The Pacitan – Ponorogo route, which is parallel to the grindulu river passes through hilly and mountainous areas is a route that often occurs landslides, such as that occurred at KM 258 in December 2017 [1]. The landslide made road access between Pacitan and Ponorogo obstructed due to soil debris and chunks of big stone covering the road.

At KM 258 there are 3 adjacent slope segments, the first segment has length of 52 meters, the second segment has length of 20 meters and the third segment has length of 33 meters. On the slopes of first segment there was landslide and countermeasures and prevention of another landslide has been carried out by making a retaining wall. In the second segment there was also landslide but did not get construction countermeasures as at the first segment, the slope angle at the second segment was 65° with slope height of 18 meters. In the third segment there has never been a landslide, the slope angle in the third segment is 80° with slope height of 18 meters. At the third segment there are residential areas around the top of the It needs to review the third segment as a form of landslide prevention considering that

the first and second segments have experienced landslides before. Research on the third segment of KM 258 is useful for traffic smoothness on the Pacitan - Ponorogo route and the safety of residents who live around the top of the slopes of the third segment.

2. LITERATURE REVIEWS 2.1. Rock slope failure

Instability caused by rock movement along the failure plane can occur in three ways:

flat plane failure, wedge failure, and circular failure. Inconsistency of slope surface and geometry that causes the slope instability can be determined by geometric method (stereographic projection).

2.1.1. Kinematics Analysis

Kinematics analysis is performed by using the stereographic projection method which can be used to collect and present data.

The data needed to make stereographic projection are the dip and dip direction of each discontinuous plane. The dip is defined as the maximum dip angle of the discontinuous plane structure measured toward the horizontal plane.

The dip direction is the direction of horizontal surface of the dip line measured clockwise

from the north.

Figure 1. Plot geological data using DIPS program.

After the geological data are collected, they are processed by computers, it is a considerable aid in planning information and significant interpretation. The figure below illustrates the concentration plots of the contoured poles and corresponding large circles generated by the DIPS program developed at the University of Toronto and now available at Rocscience. [2]

2.1.2. Plane Failure

Plane failure is very rare on rock slopes.

This plane failure should only be used assuming that the angle between the two wedges is close to 180°. The plane failure type can occur if it meets the following four criteria:

• The dip angle direction of the discontinuous plane is 20° from the slope surface (αp = f + 20°), or the strike of the discontinuous plane is less than the strike of the slope surface.

• The dip direction of the discontinuous plane must be less than the dip direction of the slope surface and there is clear gap ("daylight") on the slope surface.

• The dip direction of the discontinuous plane must be greater than the surface friction angle

• The lateral direction width of the mass failure potential must be known

2.1.3. Wedge Failure

A typical wedge failure occurs when rock mass moves along the intersection of two discontinuous planes to form wedge block. This failure can occur if the following conditions are met:

• The dip of the meeting line must be the same with the dip direction of the slope surface.

• The dip direction angle of the meeting line is less than the dip direction angle of the slope surface.

• The dip angle of the meeting line must be greater than the friction angle of the failure plane.

Figure 2. Basic Mechanism of rock slope failure (Hoek, 1991)

2.1.4. Circular Failure

Circular plane failure occurs along circular slip paths that occur due to weathering, cracking or weak rock mass. In general, structural discontinuities such as joints and rock layers do not form certain pattern that forms slip trajectory and develops into kinematic failure.

2.1.5. Overturning Failure

Overturning failure occurs if the following conditions are met:

• Strike of rock layers parallel to the slope surface.

This directional difference must be less than 20°.

• The dip direction of the layer must be in

the slope surface.

• The discontinuous condition must satisfy the following equation:

[ (90° - ϕp) ≤(ϕf – φp)]

with, ϕp = plane dip direction of plane, ϕf = dip direction of slope surface, and φp = friction angle of plane. [3]

2.2. Mohr Coulomb Criteria

To simplify calculations in rock mechanics, the Mohr sheath is considered as a straight line and the equation is expressed as the Mohr Coulomb criteria [4]. Following are Mohr Coulomb's criteria:

τ = C + μσ (1) τ = Shear stress

σ = Normal stress C = Cohesion

μ = Coefficient of internal friction of rock μ = tan ϕ

2.3. Slope Stability of Fellenius Method Slope stability analysis using the Fellinius method is assumed that the forces acting on the right - left side of any slice have zero resultant in the direction perpendicular to the landslide plane. [5]

The equation for the safety of factor Fellinius method is:

F = (2) F = Safety factor

c = soil cohesion (kN/m²) ϕ = friction angle in the soil (°)

ai = the length of the circle arc at the I^th slice (m)

Wi = weight of I^th soil slice (kN)

ui = pore water pressure at I^th slice (kN/m²) θ = defined angle (°)

2.4. RMR

Geomechanical rock classification system is a rock mass quality classification that takes into account rock mass strength, discontinuous plane orientation, hydrological conditions, and fracture geometry. This method was developed by the South African Council for Scientific and Industrial Research (CSIR).

Bieniawski (1976) published in detail about this rock mass calcification which became known as the Rock Mass Rating system or better known as the RMR system.

This method was later redeveloped with several improvements to the parameters used in

the classification system [6]. The parameters used for rock mass assessment are given as follows:

a. Whole rock mass strength based on uniaxial compressive strength.

Table 1. Whole rock strength weighting

Parameter Compressive strength of whole rock

Weight

Value Interval

PLI (MPa) UCS (MPa)

> 10 > 250 15

10 – 4 100 – 250 12

4 – 2 50 – 100 7

2 – 1 25 – 50 4

For low compressive

strength need UCS

25 – 5 2

5 – 1 1

< 1 0

b. The quality of the rock mass determined from the RQD value.

Table 2. RQD Weighting

Parameter RQD (%) Weight

Value Interval 90 - 100 20

75 - 90 17

50 - 75 13

25 - 50 8

< 25 3

RQD calculations are usually obtained from direct calculations from rock outcrops that experience cracks based on the Hudson formula (1979, in Djakamihardja & Soebowo, 1996):

RQD = 100 (0.1 λ + 1) e^-0.1λ

λ is the ratio of the number of joints to the length of the scan-line (joints/meter). [7]

c. Distance between discontinuous planes Table 3. Weighting distance between

discontinuities

Parameter Joint distance Weight Value

Interval

> 2 meter 20

0.6 – 2 meter 15

0.2 – 0.6 meter 10 0.06 – 0.2 meter 8

< 0.06 meter 3

The distance between the discontinuous planes is the distance between one crack plane and another in the rock object under consideration.

d. Discontinuous plane condition

The condition of discontinuous plane which includes the length of the discontinuous plane, gaps, crack filler, roughness, degree, weathering.

Table 4. Weighting of discontinuous plane condition

Parameter Joint distance Weight

Interval Value

Very rough, not continuous, no separation, stone walls are not weathered

30

Slightly rough, separation < 1 mm, walls slightly weathered

25 Slightly rough, < 1 mm separation, very weathered walls

20

Slicken sided / thick gouge <

5 mm, or 1 – 5 mm separation, continuous

10

Thick soft gouge > 5 mm or separation > 5 mm, continuous

0

e. Ground water surface condition Table 5. Ground water weighting

Para- meter

Ground water weight

Interval value

stream /10m tunnel length (lt/min)

Joint water pressure mask σ1

General condition

None 0 Dry 15

< 10 < 0.1 Moist 10 25 - 10 0.1 – 0.2 Wet 7 25 – 125 0.2 – 0.5 Dripping 4

> 125 > 0.5 flow 0

3. RESEARCH METHODOLOGY 3.1. Research location

Figure 3. Research Location Map The Pacitan-Ponorogo route is one of the longest routes in the Pacitan area. The research area is located at 111°20'07.26" East Longitude and 08°10'49.64" South Latitude. The location of this research is at KM 258 Ponorogo – Pacitan which is located in Gegeran Village, Arjosari Sub District, Pacitan Regency.

Figure 4. Slope at the study location In Figure 4 is the slope under consideration, the slope has height of 18 meters and length of 33 meters.

This research is divided into several steps, among others :

1. Field Survey, which is to determine the study location to be researched.

2. Data collection, the data collected are primary data and secondary data. Primary data includes measurement of joint orientation, soil and rock sampling, soil and rock properties data. Secondary data includes geological maps and maps of research locations.

3. Laboratory tests, from soil and rock samples that have been obtained, soil and rock parameters needed for slope stability analysis can be searched, namely rock uniaxial, rock density, soil density, soil shear angle, and soil cohesion.

4. Kinematics analysis of rocks on the slopes, this stage is to determine what types of landslides have the potential to occur on the slopes under review.

5. Weighting RMR and SMR, this stage serves to determine rock quality and can be a reference for slope improvement if the slope is unstable.

6. Modeling of landslide mechanisms, the modeling is supported by using geostructural applications.

7. Based on SNI-8460 of 2017 recommended safety factor value of rock slope is 1.5 [8]

4. RESULTS AND DISCUSSION 4.1. Identification of Landslide Types With

Kinematic Analysis

In Figure 5, the dip and dip direction data are taken from rocks to see the

tendency

of the slope landslide mechanism. The length of the rock span under consideration was 4 meters.

The tools used at this stage are a meter to measure the distance from the starting point to the end point and see the distance between the discontinuity planes, and a geological compass to find the strike and dip direction of rock discontinuities. In Table 6 are the results of dip data taking and the dip direction of rock KM 258.

Figure 5. KM 258 Rock Slope

Table 6. Data Dip and Dip direction

Distance (m) Dip Dip Direction

0 257 73

0.2 294 70

0.2 111 42

0.3 95 42

0.45 8 63

0.6 302 64

0.7 283 23

0.7 265 19

1.1 347 66

1.6 231 12

2.2 110 65

2.6 317 83

2.6 129 50

3.3 335 66

3.6 324 72

4 325 77

Figure 6. Stereographic projection using the dips program

Figure 7. Orientation Plot Point Distribution Data Using the DIPS App

After getting the results of the stereographic projection figure and the distribution of the dip and dip direction data orientation plot points, then look for the tendency of the figure to lead to the failure type, which is guided by Figure 6 and Figure 7, the distribution of orientation plot points in the research location, tends to be similar to Figure 2 part c, where the distribution of the dip orientation plot points and dip direction is spread in all directions and the prediction of slope failure is circular failure.

4.2 RMR

The RQD measurement in this study used the scan line method, which was to calculate the discontinuities contained in the research object. Thus RQD was calculated using the following formula:

RQD = 100e-0.1λ (0,1λ+1)

With λ is the average of the discontinuous frequencies per meter (resulting from the probability density function). In field

measurements, 16 discontinuities are found in span of 4 meters, so :

λ = = 4

RQD = 100e-0.1(4) (0,1(4)+1) RQD = 93,84%

Referring to Table 2, the RQD results are included in the 90% – 100% classification, with weighted result of 20.

Table 7. Weighting RMR of KM 258 Slope

Parameter Value Weight

Compressive strength 10.32 MPa 2

RQD 93.84% 20

Distance Between Discontinuities

0.2 – 0.6 meters 10 Discontinuity Plane

Condition

Slightly rough,

< 1 mm separation, very weathered walls

20

Groundwater flow 0

RMR 52

Based on the analysis of rock mass classification that has been carried out using the RMR classification, the rock RMR value was 45, so it can be concluded that the rock belongs to group III with moderate rock mass quality..

Slope Mass Rating is the application of the RMR value to estimate the dip angle of the stripping slope. Laubscher (1975) discussed the relationship between RMR and SMR [7], for an RMR value of 41-60 the recommended slope angle (weighted slope mass, SMR) is 55°.

4.3 SLOPE STABILITY

To calculate slope stability, slope geometry data and slope constituent materials are needed. The materials that make up the slopes on the slopes of KM 258 have 2 layers, namely rock and soil. At altitude of 0-14 meters the building material for the slopes is tuff rock, and at altitude of 14-18 meters the constituent material is soil. The following data properties of soil and rock making up the slopes.

Table 8. Soil Properties Data

ɣ soil (kN/m³) 16.5

c (kN/m²) 48.05

Φ (°) 13.93

ɣ sat soil (kN/m³) 18.13

Table 9. Rock Properties Data

ɣ soil (kN/m³) 22.21

c (kN/m²) 61

Φ (°) 31.11

ɣ sat soil (kN/m³) 22.79

The required slope geometry data is in the form of slope height of 18 meters and slope profile as shown in Figure 8 below.

Figure 8. Slope Geometry

After knowing the slope profile and the slope building material, the slope safety factor can be found by simulating the Geostructural program. Based on the results of the identification of the landslide type with kinematics analysis using the DIPS program, it was found that the landslide type was circular, then the slope stability simulation with the Geostructural program used the fellenius method.

In this case, 6 models were simulated, including when the slope in the existing condition, namely when the slope angle is 80°, and slope stabilization experiments when the slope angle is 75°, 70°, 65°, 60° to 55°.

Figure 9. Graph of Slope Angle Based on the simulation results, the results as Figure 9 are in the form of graphs, it can be seen that the slope angle that meets the minimum slope safety requirement of 1.5 is start from slope angle of 70°, with safety factor of 1.6.

Annual Journal of Hydraulic Engineering, JSCE, VOL.42, 1998, February

5. CONCLUSIONS

1. Identification of landslide types by using strike and dip measurement methods in the field and using the DIPS application, then the landslide type at the study site is circular landslide.

2. Classification of rock mass using the Rock Mass Rating method at the study location obtained weight value of 52, where this value is in class III, namely moderate rock types.

3. Based on the slope stability simulation at Geostructural program, the slope is safe at 70° sloping conditions, namely with SF value of 1.6.

6. REFERENCES

[1] Sutondo, Yuniardi, “Jalur Pacitan-Ponorogo Masih Banyak Sisakan Longsoran”, https://bangsaonline.com/berita/39900/jalur-pa citan-ponorogo-masih-banyak-sisakan-longsor, an , accessed on 12 July 2021 at 17:13 WIB,

2017.

[2] Hoek, Evert, Practical Rock Engineering.

Notes, Evert Hoek Consulting Engineer Inc., Canada. http://www.rockscience.com, 2007.

[3] Muntohar, Agus S, Tanah Longsor Analisis, Prediksi, Mitigasi. Yogyakarta: Institue for Educational Research and Community Development, Universitas Muhammadiyah Yogyakarta, 2010.

[4] Rai, M. A., Kramadibrata, S. dan Wattimena, R. K., Mekanika Batuan. Bandung: ITB Press, 2019.

[5] Hardiyatmo, Hary C., Mekanika Tanah II.

Yogyakarta: Gadjah Mada University Press.

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[6] Bieniawski, Z.T, Engineering rock mass classifications. New York: Wiley, 1989 [7] Hirnawan, R.F., & Zakaria, Z., Geoteknik dan

Geomekanik, Geology Engineering Lab , Geology Engineering Study Program, FTG-Unpad, 2002.

[8] National Standardization Agency. SNI 8460, Tentang Persyaratan Perancangan Geoteknik.

Jakarta: Badan Standarisasi Nasional, 2017.