Rock Slope Stability Analysis in Melasti Beach Area: Ungasan, Bali

I W. Arya, I W. Wiraga, I G. A. G. S. Dwipa, I Nyoman Ramia

and I Made Wahyu Pramana

Civil Engineering Dept. Politeknik Negeri Bali, Bali, Indonesia

pramanawahyu@pnb.ac.id

Keywords: Rock Mass Classification, Excavation, Slope Stability.

Abstract: Melasti Beach is one of the main tourist attractions developed by Bali Government. This beach is located in

the southern area of Bali, precisely in Ungasan village, Badung Regency, Bali. At the moment, the region

administrator is constructing a tourism facility by excavating a rock mass near the beach. Therefore, to avoid

damage to nearby shops and beach facilities, a study is needed to be conducted about the safety of the rock

mass that being excavated. Direct observation on the rock mass and rock mass classification is conducted to

obtain the physic and mechanic characteristic of the rock. To do the stability analysis of the slope, Geo-Studio

2012 software is used by inputted the data obtained from the field and laboratory data. The result of this study

are mass rock in the Melasti beach is classified as limestone that has Fair rock classification and factor of the

safety of the rock slope / cliff is 1.631 which means the cliff is safe and no need to build an extra reinforcement

on the slope/ cliff.

1 INTRODUCTION

Melasti Beach is one of the main tourist attractions

which is currently developed by Bali Government.

This beach is located in the southern of Bali, precisely

in Ungasan Village, Badung Regency, Bali. The

number of tourists is increasing every year because of

its beautiful scenery and supported by a complete

tourism facility.

Figure 1: Melasti beach area.

Be advised that papers in a technically unsuitable

form will be returned for retyping. After returned the

manuscript must be appropriately modified.

At this moment, to develop the area, the region

administrator is constructing a tourism facility by

excavating a rock mass near the beach. The rock

Slope which is resulted from excavation activities can

be unstable because of activating moment of the rock

mass is greater than resisting moment. Therefore, to

avoid damage to nearby area due to excavation

activity, a study is needed to be conducted about the

rock mass that being excavated.

Figure 2: Excavation process in Melasti Beach.

In order to do the slope stability analysis, firstly, the

rock mass classification analysis is conducted to

obtain physical and mechanical characteristic of the

rock mass. Those characteristics determine the rock

mass’s shear strength. Shear strength is internal

resistance per unit area that the rock mass can offer to

1392

Arya, I., Wiraga, I., Dwipa, I., Ramia, I. and Pramana, I.

Rock Slope Stability Analysis in Melasti Beach Area: Ungasan, Bali.

DOI: 10.5220/0010966000003260

In Proceedings of the 4th International Conference on Applied Science and Technology on Engineering Science (iCAST-ES 2021), pages 1392-1396

ISBN: 978-989-758-615-6; ISSN: 2975-8246

 2023 by SCITEPRESS – Science and Technology Publications, Lda. Under CC license (CC BY-NC-ND 4.0)

resist failure and sliding along any plane inside it.

To determine the rock mass characteristic in

Melasti Beach Area, in this study uses RMR (Rock

Mass Rating) method by Bieniawski (1989). In RMR

method, field and laboratory test is performed. Field

test consist of: direct observation of the rock mass

condition, observation of cracks (discontinuities) that

occur in the rock mass, rock coring, observation of

the water table. Laboratory tests consist of: Uniaxial

Compression Test and Unit Weight Test.

Study about rock mass characteristic have been

conducted previously by some other researcher.

Ardana and Aribudiman (2017) have studied rock

mass in Pecatu Area, which is nearby the Melasti

Area and have a similar geological condition.

Therefore, the result of the previous study can be used

for controlling and comparation of the result in this

study.

2 METHODOLOGY

2.1 Location and Time of the Study

This study is located in Melasti Beach Area in

southern of Bali Province. Field observation and

sampling is done on April 17, 2020. Laboratory test

is conducted on April 20 – 30, 2020. The location of

this study is shown in figure below:

Figure 3: Location of Study.

2.2 Type of Data and Data Collection

Technique

Data obtained in this study consist of primary and

secondary data. Primary data in this study are field

observation result, laboratory test and discontinuity

measurement. Secondary data are previous study and

literatures.

2.3 Data Analysis and Calculation

Technique

This study is quantitative and applied studies because

it can solve a problem that can occurs in the field

(Yusuf, 2016). This study consists of field test of the

rock mass, rock core sampling, laboratory test, and

slope stability analysis. This study is conducted as

follows:

1. Direct observation of the rock mass, analyze and

measure the crack discontinuities which occur on

the rock mass.

2. Rock core sampling. This sampling is used to

obtained undisturbed sample of the rock core and

the RQD (Rock Quality Designation) value of the

rock mass. Sampling is performed is 4 different

point to obtain the representative result.

3. Unit weight and Uniaxial Compression Test

(UCT). The rock core obtained from the field is

tested with Unit weight and UCS test. Both of

those tests are done in laboratory.

4. Rock mass classification analysis. The data

obtained from previous step is used to determine

the characteristic of the rock mass. The RMR

method is used in this study. RMR method gives

rating from 1 to 100; according to some rock mass

properties such as: RQD value, UCS value, crack

spacing, condition of crack, water condition.

5. Rock slope stability analysis. The slope stability

analysis is done by using GeoStudio 2012. The

rock’s physical and mechanical properties is

obtained from previous analysis. The geometry of

the slope are right angle slope with height of 100

m. This height is the highest excavation plan on

the Melasti Area.

Rock Slope Stability Analysis in Melasti Beach Area: Ungasan, Bali

1393

Figure 4: Rock Slope Stability Analysis Flowchart.

3 RESULT AND DISSCUSION

3.1 Rock Mass Observation

Observation of rock mass type is conducted by using

direct visual observation and measure the crack on the

rock mass. The result of rock mass observation are:

rock mass is formed by million years-sedimentation

of sea organism; has a rough surface and spacing

between crack (discontinuities) is 6 cm – 20 cm.

Figure 5: Crack or discontinuities conditition on the rock

mass in Melasti Beach area.

3.2 Rock Core Sampling

Rock core sampling is done to obtain undisturbed

sample of the rock core and determine the RQD value

of the rock mass. Coring is done in 4 point; 2 in the

top of the slope and 2 on the bottom of the slope. RQD

value is determine by:

𝑅𝑄𝐷 

     

  

𝑥100% (1)

The result of the rock core sampling is shown in table

as follows:

Table 1: Rock Core Sampling Result.

Number Total Length (cm)

Rock Core

Len

th (cm)

1 35 20

2 26 7

3 30 28

4 27 26

𝑅𝑄𝐷 

20  7  28  26

35  26  30  27

𝑥100%

𝑅𝑄𝐷  68 %

Thus, the RQD of the rock mass in melasti beach area

is 68%.

3.3 Laboratory Test

The sample obtained from rock core sampling is

tested in laboratory. The result of the laboratory test

is shown in table below:

Start

Literature Review

Field Test:

1. Direct Obervation of Rock Mass

2. Rock Core Sampling

Laboratory Test:

1. Unit Weight Test

2. Uniaxial Compression Test

Rock Mass Classification Analysis

Rock Slope Stability Analysis

End

iCAST-ES 2021 - International Conference on Applied Science and Technology on Engineering Science

1394

Table 2: Laboratory Test Result.

No Sample

Specific

Gravit

Unit Weight

/m3)

UCS

(MPa)

1 2.67 2140 16

2 2.68 2396 31

3 2.69 2512 59

Average 2.68 2349 35

the equation as a graphic and insert it into the text

after your paper is styled.

In this study, rock mass classification is

determined by the RMR method. RMR method gives

rating from 1 to 100; according to some rock mass

properties such as: RQD value, UCS value, crack

spacing, condition of crack, water condition. The data

result from field and laboratory test is used to give the

rock mass its rating. RMR value of rock mass in

Melasti Beach area is shown in table below:

Table 3: Rock Mass Rating Result.

Parameter Descri

tion

Rating

Nilai UCS 35 MPa

RQD 68 %

Discontinuity

acin

0.06 – 0.2 m

Discontinuity

condition

Rough Surface,weathered,

aration 1

–

5 mm

Water condition

dry

Total Ratin

Rock mass in Melasti Beach area obtained total rating

of 52 and categorized as fair rock. Mechanical

parameter suitable for fair rock are: (φ) : 25° - 35° and

cohesion : 200 KPa – 300 KPa.

3.4 Slope Stability Analysis

Rock slope stability analysis in this study use

GeoStudio 2012 software and use Morgenstren –

Prince calculation method. The height of the slope is

100 m, which is the highest excavation plan on

Melasti beach area. Input geometry on Geostudio is

shown in figure below:

Figure 6: Geometry input on GeoStudio.

The input of the rock mechanical and physical

properties is obtained from previous analysis on this

study. Input parameter is shown in table and figure

below

Table 4: Rock Parameter.

Parameter Description

Unit Weight 2349 kg/m3

Phi 30

Cohesion 250 KPa

The selection of phi and cohesion value is based on

range value that correspond with the type of rock in

Melasti Beach area.

Result of calculation shows that slope has factor

safety (FS) of 1.63. FS >1 value means that the slope

resulted from excavation of the rock mass is stable

and don’t need any stabilization or external support.

The results of the calculation are shown in figure 8.

Figure 7: Rock Parameter Input On Geoslope.

Table 5: Tabulation of slope stablility calculation.

Item Data

Slip Surface 90

Factor of Safety 1.63

Volume 6,571 m³

Weight 137,997kN

Resisting Moment 12,254 kN-m

Activating Moment 7,515,343. kN-m

Resisting Force 91,282 kN

Activating Force

55,980 kN

Exit

(240.144, 50) m

Entry (112.5, 150) m

Radius 106.38839 m

Center

(218.80441,

154.22625)

Rock Slope Stability Analysis in Melasti Beach Area: Ungasan, Bali

1395

Figure 8: Slope stability analysis output on GeoStudio.

4 SUMMARY

From field test, laboratory test and analysis that have

been conducted in this study, can be summarized as

follows:

A. The type of rock in Melasti Beach area is

sedimentary rock formed by sea organism with

classification of fair rock.

B. The physical and mechanical properties of rock

mass are:

• Internal agle of friction (φ): 30

• Cohession (c): 250 Kpa

• Unit Weight (γ) : 2349 kg/m3

C. Factor of safety of the excavation plan is 1.63 (FS

= 1.631)

D. The rock slope doesn’t need any stabilization or

external support.

ACKNOWLEDGEMENTS

The author wish to thank the others member of KBK

geotechnics in Civil Engineering Departement – PNB

for their help throughout the course of this work.

REFERENCES

Sivakugan, N., Shukla, S.K. dan Das, B.M. (2013). Rock

Mechanics an Introduction. USA: CRC Press.

Das, Braja M. (1993). Principal of Geothechnical

Engineering. Canada:Nelson

Bieniawski, Z. T. (1989). Engineering Rock Mass

Classification. New York: Wiley

Ardana, M.D.W dan Aribudiman, I.N. (2017). Analisis

Karakteristik Batuan Kapur (Limestone dan Chalk) di

Kawasan Bukit Pecatu Kabupaten Badung Bali.

Yusuf, A. M. (2016). Metode penelitian kuantitatif,

kualitatif dan penetilian gabungan. Prenada Media.

Abbas, S. M dan Konietzky. H.H. (2015). Rock Mass

Classification Systems.

1.631

Distance

0 100 200 300

Elevation

100

200

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