p–ISSN: 2723 - 6609 e-ISSN: 2745-5254
Vol. 5, No. 7 July 2024 http://jist.publikasiindonesia.id/
Jurnal Indonesia Sosial Teknologi, Vol. 5, No. 7, July 2024 3615
Analysis of the impact of dam slope modifications on the
sensitivity to rapid drawdown, earthquakes, and changes in
water level at the Jragung Dam in Semarang Regency
Rizal Undityo Rahardani
1*
, Trihanyndio Rendy Satrya
2
Institut Teknologi Sepuluh Nopember, Indonesia
Email:
*Correspondence
ABSTRACT
Keywords: Murugan
dam, slope stability, rapid
drawdown, slope
influence, OBE
earthquake.
Urugan-type dams are the most common type of dam to
work on in Indonesia, with a material composition that is
easy to obtain and relatively more economical. Based on the
results of slope stability, a sensitivity analysis will be carried
out based on changes in slope slope to post-construction
conditions, during reservoir operation water level and rapid
drawdown conditions. In this study, we will analyze the
changes in the value of safety factors that occur on the slope
of the body of the Jragung Dam due to rapid drawdown
events and seismicities (OBE and MDE) using the help of
seepage analysis calculation software and slope stability.
There are three models used, namely Model 1 using
geometry according to DED (slope of upstream slope 1: 3.0
and slope of downstream slope 1: 2.5), Model 2 of using
modification of field construction geometry (slope of
upstream slope 1: 2.5 and slope of downstream slope 1: 2.5),
Model 3 of using modification of field construction
geometry (slope of upstream slope 1: 2.5 and slope of
downstream slope 1: 2,25). From the three models, the
initial stage will be processed to find the phreatic line on the
dam body when drawdown and the next stage to find the
value of the dam slope safety factor. After the overall
analysis was carried out both in terms of seepage analysis
and stability analysis of the three simulation models, for the
construction in the field on the Jragung dam, the 3rd Model
can be used, namely the proposed geometric modification
with an upstream slope of 1: 2.5 and a downstream slope of
1: 2.25, and material parameters according to the availability
specifications at the work site. Regarding the stability of the
slope, it is noted that at the time of the MDE earthquake load
of 10,000 years, when the rapid-drawdown conditions y/h =
0.5h and y/h = 0.75h there is a possibility that the Jragung
dam will be damaged, but not collapsed.
Rizal Undityo Rahardani, Trihanyndio Rendy Satrya
Jurnal Indonesia Sosial Teknologi, Vol. 5, No. 7, July 2024 3616
Introduction
Urugan-type dams are the most common type of dam to work on in Indonesia, with
a material composition that is easy to obtain and relatively more economical than
concrete-type dams (Wicaksono, 2023). However, behind the advantages of this type of
dam, some things need to be considered in the design and construction process, for
example, related to the stability of the slope of the dam body.
The Jragung Dam is located in Candirejo Village, Pringapus District, Semarang
Regency. Geographically, it is located at coordinates 6°52'19.41"S and 111°15'30.20" E
located in the Jragung Watershed (Jragung Watershed Area 94.00 km²). The geographical
location of the Jragung watershed is in the northern part of Central Java which crosses 4
districts, starting from the widest Demak Regency, Semarang Regency, Grobogan
Regency, and Semarang City (PT. Indra Karya, 2019).
Urugan-type dams are the most common type of dam to work on in Indonesia, with
a material composition that is easy to obtain and relatively more economical than
concrete-type dams. However, behind the advantages of this type of dam, some things
need to be considered in the design and construction process, for example, related to the
stability of the slope of the dam body.
The Jragung Dam is located in Candirejo Village, Pringapus District, Semarang
Regency. Geographically, it is located at coordinates 6°52'19.41"S and 111°15'30.20" E
located in the Jragung Watershed (Jragung Watershed Area 94.00 km²). The geographical
location of the Jragung watershed is in the northern part of Central Java which crosses 4
districts, starting from the widest Demak Regency, Semarang Regency, Grobogan
Regency, and Semarang City (PT. Indra Karya, 2019).
The history of earthquakes around the location of the Jragung dam in the period
1600 – 2018, recorded at more than 4.00 magnitude; obtained from the USGS website.
One of them occurred on Saturday, October 23, 2021, in the early morning at 00:32 WIB,
the Salatiga, Banyubiru, Bawen, and Ambarawa City areas in Central Java were shaken
by a tectonic earthquake. The results of BMKG's analysis show that this earthquake has
a magnitude of M = 3.00 to M = 3.30. The epicentre is located at coordinates 7,296 LS
and 110,385 BT, precisely on land at a distance of 13.00 km northwest of Salatiga city
with a hypocenter depth of 6.00 km. Refer to Figure 1.7 which explains the seismicity
map of Central Java province and its surroundings related to the relationship between
earthquake depth and magnitude. By paying attention to the location of the epicentre and
the depth of the hypocenter, the earthquake that occurred on October 23, 2021, was a type
of shallow earthquake due to activity in the active fault segment of Merapi-Merbabu (PT.
Rayakonsult KSO, 2021).
Analysis of the impact of dam slope modifications on the sensitivity to rapid drawdown,
earthquakes, and changes in water level at the Jragung Dam in Semarang Regency
Jurnal Indonesia Sosial Teknologi, Vol. 5, No. 7, July 2024 3617
Figure 1 Seismizity Map of Prov. Central Java and its surroundings Per. 1924-2020
(PT. Rayakonsult KSO, 2021)
There are 5 active faults adjacent to the location of the Jragung Dam including, the
Semarang fault, the Rawapening fault, the Merapi-Merbabu fault, the Pati fault, and the
Opak fault as shown in Figure 1.8. Based on the image shown in Figure 1.9, we can see
that the distance of the Jragung dam to the Merapi-Merbabu active fault is as far as 29.60
km (PT. Rayakonsult KSO, 2021).
Based on several lists of dam slope stability cases around the world, most
construction failures are caused by rapid drawdown, which is a condition when the water
level of a dam drops suddenly (Putra, Najib, & Hidayatillah, 2017). As is known, the
rapid drawdown of the water level can reduce the safety condition of the slope in saturated
conditions because the water pressure outside the dam body is reduced while the inner
pore water pressure does not support the stability of the dam body itself (Nainggolan,
2023).
Rizal Undityo Rahardani, Trihanyndio Rendy Satrya
Jurnal Indonesia Sosial Teknologi, Vol. 5, No. 7, July 2024 3618
For the case study of the Campolattaro dam investigated by (Sica, Pagano, & Rotili,
2019), the occurrence of an earthquake can reduce the safety of the upstream slope during
the rapid descent phase at y/h=0.50 when the speed > 1.00 m/day. The slower the descent
time, the more stable the dam will be (the SF value is higher). In contrast, sudden changes
in the water level on the slopes, without allowing time for drainage downstream, result in
unsafe conditions (Pramulandani, 2020). It can be seen in Table 1.13 that the safe
emptying of the reservoir y/h=1.00 due to the earthquake was identified within 1.00
m/day.
From the formulation of the problem above, several goals will be made to solve the
problem. The objectives of the preparation of this research include the following:
1. To determine the effect of seepage of the Jragung dam body (initial design and
modification proposal) during the water level condition of reservoir operation and
rapid receding operation (simulation of rapid drawdown conditions).
2. Determine the safety factors of the slope of the Jragung dam body (initial design and
modification proposal) at the time of simulation of the conditions of completion of
construction
3. Determine the safety factors of the slope of the Jragung dam body (initial design and
modification proposal) during the simulation of the water level conditions of reservoir
operation.
4. Determine the safety factors of the slope of the Jragung dam body (initial design and
modification proposal) during the simulation of rapid drawdown conditions.
5. To determine the relationship between the sensitivity of the slope of the dam body to
the variation of the operating water level, the speed of drawdown, when an earthquake
occurs, and the difference in slope slope.
Research Methods
A preliminary survey is carried out to find out the current condition of the project
location to be researched and identify problems in the field so that they can take steps to
find solutions to the problems that occur. A preliminary survey was also carried out to
collect technical data on the Jragung dam so that accurate data could be used in analyzing
slope stability.
The intended literature study is to collect materials that will be used as a reference
in conducting research. The study materials that will be used in this study include the
following:
1. References regarding slope stability calculations, seepage analysis and rapid
drawdown analysis on the dam body.
2. References on the operation of slope stability aids & seepage analysis.
3. References to the effect of earthquake loads on slope stability.
4. References to sensitivity analysis that affects dams.
5. Regulations in force in Indonesia are related to dam structures.
Analysis of the impact of dam slope modifications on the sensitivity to rapid drawdown,
earthquakes, and changes in water level at the Jragung Dam in Semarang Regency
Jurnal Indonesia Sosial Teknologi, Vol. 5, No. 7, July 2024 3619
Data Collection
Data collection is carried out by collecting related technical data that will be used
in the process of analyzing the slope stability of the dam body. The data needed in the
analysis is a type of secondary data. Secondary data is data obtained indirectly in the form
of records, survey results or research results from other agencies or parties.
Earthquake Load Coefficient Analysis
This section aims to classify the earthquake zones and calculate the earthquake
coefficient that will be used in the area where the Jaragung dam construction project is
located. The earthquake analysis method used for modelling the Jragung dam is classified
into two, namely static earthquake analysis and modified earthquake analysis Operating
Base Earthquake (OBE) – Maximum Design Earthquake (MDE). The earthquake
coefficient obtained will later be applied as seismic input in software to help analyze the
stability of the dam body's slope.
Seepage Analysis
Seepage analysis was used to determine the water level (phreatic line) in the body
of the Jaragung dam. To help in modelling this phreatic line, the SEEP/W program is
used. Through the SEEP/W software, the phreatic line on the dam body can be
immediately known according to the material permeability coefficient.
Modelling during rapid drawdown in the SEEP/W software is carried out in a steady
state condition and followed by a transient condition. In transient conditions, the phreatic
line is reviewed that changes over time, therefore it is necessary to determine the
calculation of the time. In transient conditions, the water level is reviewed in a period
starting from the initial time, 30 days, 60 days, to 80 days.
Slope Stability Analysis
Slope stability analysis is used to determine the safety factor in the body of the
Jaragung dam. For the calculation of the safety factor, the SLOPE/W auxiliary program
will be used. So that the output of the SEEP/W that has been carried out is used as the
pore water pressure value in the analysis settings of SLOPE/W.
In modelling the stability of the slope, in addition to normal conditions (steady
state), the influence of static earthquake loads and modified earthquakes Operating Base
Earthquake (OBE) at the 100-year reage – Maximum Design Earthquake (MDE) at the
10,000-year reage will also be modelled. In earthquakes, the modified earthquake
coefficient applied uses a variation in earthquake depth y/h = 0.25; y/h = 0.5; y/h = 0.50;
y/h = 1.00; where H is the height of the dam.
Results and Discussion
Analysis of the Sensitivity of the Influence of Water Level on the Value of Slope
Stability Safety Factor in the Jragung Dam
The analysis of water level sensitivity due to the influence of elevation changes on
the value of dam body safety factors (SF) is an effort to understand how changes in water
level elevation can affect dam stability (Candra & Pratiwi, 2010). The purpose of this
analysis is to determine how sensitive the value of the safety factor of the body of the
Rizal Undityo Rahardani, Trihanyndio Rendy Satrya
Jurnal Indonesia Sosial Teknologi, Vol. 5, No. 7, July 2024 3620
Jragung dam is to changes in water level elevation, which is an important factor in the
management and operation of the dam (Juwono, Subagiyo, & Winarta, 2022).
In this analysis, it will be divided into two, namely the effect of water level change
on the upstream slope side and water level change on the downstream slope side. The
conditions used for this sensitivity analysis are only under normal conditions without the
influence of earthquake loads and will be applied to 3 simulation models (Rohmawati,
2022).
Analysis of the Sensitivity of the Influence of Upstream Water Level on Safety
Factor Value
To calculate the sensitivity value of the influence of water level on the safety factor
based on the existing data, we can use the elevation change (water level) as the variable
that affects (X) and the change in the Safety Factor (SF) as the variable that affects (Y).
With equations 2-27 in the previous Chapter 2, we can calculate the sensitivity value. The
preliminary data for the calculation of this sensitivity value can be seen in Table 1 and
the comparison graph in Figure 2.
Table 1
Data on Comparison of Safety Factor Values According to Water Level Elevation
Conditions (Upstream)
Water
Level
Elevation
Safety Factor Upstream Normal
Condition
Model 1
Model 2
Model 3
Empty
67.76
4.758
4.620
4.708
MAR
93.00
3.784
3.743
3.748
MAN
115.00
4.877
4.911
4.946
MAB
117.28
5.033
5.059
5.113
RDD
93.00
3.779
3.740
3.745
Figure 2 Comparison chart of safety factor values of water level elevation conditions
(upstream)
The next step is to calculate the relative change in water level elevation (ΔX/X)
followed by the calculation of the relative change in the safety factor (ΔSF/SF). Then
with the equation 2-27 in Chapter 2, the value of Sn is calculated.
2.00
3.00
4.00
5.00
6.00
0 1 2 3 4 5 6
Safety Factor
Model 1 Model 2 Model 3
Analysis of the impact of dam slope modifications on the sensitivity to rapid drawdown,
earthquakes, and changes in water level at the Jragung Dam in Semarang Regency
Jurnal Indonesia Sosial Teknologi, Vol. 5, No. 7, July 2024 3621
Table 2
Calculation of Sn Value of Simulation Model 1 Effect of Water Level Elevation
(Upstream)
Water
Level
Elevation
Model 1
Δx
Δx/X
ΔSF
ΔSF/SF
Sn
(X)
(SF)
Empty
67.76
4.758
MAR
93.00
3.784
25.240
0.372
- 0.974
- 0.205
- 0.550
MAN
115.00
4.877
22.000
0.237
1.093
0.289
1.221
MAB
117.28
5.033
2.280
0.020
0.156
0.032
1.613
Average Sn in Simulation Model 1
0.762
Table 3
Calculation of Sn Value of Simulation Model 2 Effect of Water Level Elevation
(Upstream)
Water
Level
Elevation
Model 2
Δx
Δx/X
ΔSF
ΔSF/S
F
Sn
(X)
(SF)
Empty
67.76
4.620
MAR
93.00
3.743
25.240
0.372
-
0.877
- 0.190
-
0.51
0
MAN
115.00
4.911
22.000
0.237
1.168
0.312
1.31
9
MAB
117.28
5.059
2.280
0.020
0.148
0.030
1.52
0
Average Sn in Simulation Model 2
0.77
7
Table Error! No text of specified style in document.
Calculation of Sn Value of Simulation Model 3 Effect of Water Level Elevation
(Upstream)
Water
Level
Elevation
Model 3
Δx
Δx/X
ΔSF
ΔSF/SF
Sn
(X)
(SF)
Empty
67.76
4.708
MAR
93.00
3.748
25.240
0.372
- 0.960
- 0.204
- 0.547
MAN
115.00
4.946
22.000
0.237
1.198
0.320
1.351
MAB
117.28
5.113
2.280
0.020
0.167
0.034
1.703
Average Sn in Simulation Model 3
0.836
Table 5
Calculation of Average Sn Value Influence of Water Level Elevation (Upstream)
Model
Sn Value
Simulation Model 1
0.762
Simulation Model 2
0.777
Simulation Model 3
0.836
Average Sn Value
0.791
Rizal Undityo Rahardani, Trihanyndio Rendy Satrya
Jurnal Indonesia Sosial Teknologi, Vol. 5, No. 7, July 2024 3622
From the three simulation models, the average Sn value will be taken due to the
influence of the water level on the upstream slope, so that from Table 4.54 the average
Sn value = 0.791 is obtained (Nugraha, Prayudha, Ibrahim, & Riyadi, 2017). The
sensitivity value (Sn) between data points starting from the empty water level after
construction to the flood water level is 0.791. This means that a 1% change in elevation
leads to a change of about 0.791% in the Safety Factor value. The greater the absolute Sn
value, indicating that the safety factor is very sensitive to changes in water level elevation
(Kusnandar, 2019). The smaller the absolute Sn value, indicating that the safety factor is
not too sensitive to changes in water level elevation. The negative Sn value indicates that
the safety factor decreases when the water level elevation increases.
Sensitivity Analysis of the Influence of Downstream Water Level on Safety Factor
Value
The same application applies to the downstream slope side, we can use the elevation
change (water level) as the variable that affects (X) and the change in the Safety Factor
(SF) as the variable that affects (Y). The preliminary data for the calculation of this
sensitivity value can be seen in Table 6 and the comparison graph in Figure 3.
Table 6
Comparative Data on Safety Factor Values According to Water Level Elevation
Conditions (Downstream)
Water
Level
Elevation
Safety Factor Downstream Normal
Condition
Model 1
Model 2
Model 3
Empty
67.76
5.228
5.405
5.859
MAR
93.00
3.653
3.822
3.791
MAN
115.00
3.606
3.725
3.707
MAB
117.28
3.475
3.778
3.685
RDD
93.00
3.654
3.824
3.787
Figure 3 Comparison Chart of Safety Factor Values of Water Level Elevation
Conditions (Downstream)
The next step is to calculate the relative change in water level elevation (ΔX/X)
followed by the calculation of the relative change in the safety factor (ΔSF/SF). Then
with the equation 2-27 in Chapter 2, the value of Sn is calculated. Details of the
calculation will be presented in Table 7 to Table 10.
2.00
3.00
4.00
5.00
6.00
7.00
0 1 2 3 4 5 6
Safety Factor
Model 1 Model 2 Model 3
Analysis of the impact of dam slope modifications on the sensitivity to rapid drawdown,
earthquakes, and changes in water level at the Jragung Dam in Semarang Regency
Jurnal Indonesia Sosial Teknologi, Vol. 5, No. 7, July 2024 3623
Table 7
Calculation of Sn Value of Simulation Model 1 Effect of Water Level Elevation
(Downstream)
Water
Level
Elevation
Model 1
Δx
Δx/X
ΔSF
ΔSF/
SF
Sn
(X)
(SF)
Empty
67.76
5.228
MAR
93.00
3.653
25.240
0.372
-
.1.575
-
0.301
-
0.80
9
MAN
115.00
3.606
22.000
0.237
-
0.047
-
0.013
-
0.05
4
MAB
117.28
3.475
2.280
0.020
-
0.131
-
0.036
-
1.83
2
Average Sn in Simulation Model 1
-
0.89
9
Table 8
Calculation of Sn Value of Simulation Model 2 Effect of Water Level Elevation
(Downstream)
Water
Level
Elevation
Model 2
Δx
Δx/X
ΔSF
ΔSF/SF
Sn
(X)
(SF)
Empty
67.76
5.405
MAR
93.00
3.822
25.240
0.372
- 1.583
- 0.293
-
0.786
MAN
115.00
3.752
22.000
0.237
- 0.097
- 0.025
-
0.107
MAB
117.28
3.778
2.280
0.020
0.053
0.014
0.718
Average Sn in Simulation Model 2
-
0.059
Table 9
Calculation of Sn Value of Simulation Model 3 Effect of Water Level Elevation
(Downstream)
Water
Level
Elevation
Model 3
Δx
Δx/X
ΔSF
ΔSF/SF
Sn
(X)
(SF)
Empty
67.76
5.859
MAR
93.00
3.791
25.240
0.372
-.2.068
- 0.353
-
0.948
MAN
115.00
3.707
22.000
0.237
- 0.084
- 0.022
-
0.094
MAB
117.28
3.685
2.280
0.020
- 0.022
- 0.006
-
0.299
Average Sn in Simulation Model 3
-
0.447
Table 10
Calculation of the Average Sn Value of the Effect of Water Level Elevation (Upstream)
Model
Sn Value
Rizal Undityo Rahardani, Trihanyndio Rendy Satrya
Jurnal Indonesia Sosial Teknologi, Vol. 5, No. 7, July 2024 3624
Original Model
- 0.899
Alternative Model 1
- 0.059
Alternative Model 2
- 0.447
Average Sn Value
- 0.468
Sensitivity Analysis of the Effect of Rapid Drawdown Time Speed on Normal
Conditions on Safety Factor Value
To calculate the sensitivity value of the effect of the speed of the rapid drawdown
time on the safety factor based on the existing data, we can use the change in the rapid
drawdown time as the variable that affects (X) and the change in the Safety Factor (SF)
as the variable that affects (Y). With equations 2-27 in the previous Chapter 2, we can
calculate the sensitivity value.
Sensitivity Analysis of the Effect of Rapid Drawdown Time Speed on the
Downstream Side Conditions of Static Earthquake Effect on Safety Factor Value
The same application applies to the downstream slope side during earthquake
conditions, we can use the change in the time of rapid ebb and flow as the variable that
affects (X) and the change in the Safety Factor (SF) as the variable that affects (Y). With
equations 2-27 in the previous Chapter 2, we can calculate the sensitivity value.
Sensitivity Analysis of the Effect of Earthquake Load on the Value of Slope Stability
Safety Factor in the Jragung Dam
The next sensitivity analysis is the effect of earthquake load on the value of the
slope stability safety factor at the Jragung dam, which is an evaluation process to
understand how changes in earthquake load intensity affect slope stability. The
earthquake load that occurs can significantly affect the stability of the slope, especially in
conditions that are already critical. It can also be useful for identifying potential risks of
slope failure caused by earthquake loads.
This analysis is only carried out on the upstream slope side and will be divided into
two, namely: when the water level is normal and the water level is rapidly receding (rapid
drawdown) with the influence of the 100-year OBE earthquake load and the 10,000-year
MDE earthquake load and will be applied to 3 simulation models.
Sensitivity Analysis of Normal Water Level Conditions The Effect of OBE 100 Years
and MDE 10,000 Years Earthquake on Safety Factor Values
To calculate the sensitivity value of normal water level conditions of the influence
of OBE earthquake load of 100 years and MDE 10,000 years on the safety factor based
on existing data, we can use the change in the earthquake coefficient as the influencing
variable (X) and the change in the Safety Factor (SF) as the influencing variable (Y). In
this analysis of the sensitivity of normal water level conditions, the earthquake coefficient
used was 100%. With equations 2-27 in the previous Chapter 2, we can calculate the
sensitivity value. The initial data for the calculation of this sensitivity value can be seen
in Table 11 and the comparison graph in Figure 4.
Table 11
Comparative Data on Safety Factor Values of Normal Water Level Conditions Affected by
the 100-Year OBE Earthquake and the 10,000-Year MDE Earthquake
Conditions
Earthqua
ke
Coefficie
nt
Safety Factor Upstream
Model 1
Model 2
Model 3
Normal Conditions
-
4.877
4.911
4.946
Analysis of the impact of dam slope modifications on the sensitivity to rapid drawdown,
earthquakes, and changes in water level at the Jragung Dam in Semarang Regency
Jurnal Indonesia Sosial Teknologi, Vol. 5, No. 7, July 2024 3625
Static Earthquake Conditions
0.090
3.074
3.285
3.297
OBE Earthquake 100 Th 0.25 H
0.125
5.685
5.314
5.315
OBE Earthquake 100 Th 0.50 H
0.104
3.328
3.372
3.400
OBE Earthquake 100 Th 0.75 H
0.095
2.598
2.601
2.609
OBE Earthquake 100 Th 1.00 H
0.086
3.126
3.611
3.348
MDE Earthquake 10000 Th
0.25H
0.873
1.581
1.546
1.545
MDE Earthquake 10000 Th
0.50H
0.729
0.990
0.973
0.977
MDE Earthquake 10000 Th
0.75H
0.664
0.700
0.758
0.779
MDE Earthquake 10000 Th
1.00H
0.600
0.923
1.067
1.066
Figure Error! No text of specified style in document. Chart of Safety Factor Values of
Normal Water Level Conditions Affected by the 100-Year OBE Earthquake and the
10,000-Year MDE Earthquake
Analysis of the Sensitivity of Rapid Drawdown Water Level Conditions The Effect
of OBE 100 Years and MDE 10,000 Years Earthquake on Safety Factor Values
Furthermore, the same application also applies to calculations during rapid
drawdown conditions, we can use changes in earthquake coefficients as variables that
affect (X) and changes in Safety Factor (SF) as variables that affect (Y). In this analysis
of the sensitivity of the rapidly receding surface condition, the earthquake coefficient used
was 50%. With equations 2-27 in the previous Chapter 2, we can calculate the sensitivity
value.
Sensitivity Analysis of the Effect of Slope Slope on the Value of Slope Stability Safety
Factor in the Jragung Dam
The sensitivity analysis of the influence of slope slope on the value of slope stability
safety factor at the Jragung dam is an evaluation process to understand how changes in
slope slope affect slope stability. The slope of the slope is one of the important factors
that affect stability, as small changes in slope can significantly affect the likelihood of
landslides or other instability. The slope coefficient is based on the value of the planned
slope angle on the Jragung dam.
0.00
1.00
2.00
3.00
4.00
5.00
6.00
0 2 4 6 8 10 12
Safety Factor
Kondisi Beban Gempa
Model-1 Model-2 Model-3
Rizal Undityo Rahardani, Trihanyndio Rendy Satrya
Jurnal Indonesia Sosial Teknologi, Vol. 5, No. 7, July 2024 3626
In this analysis, the sensitivity calculation is carried out on the upstream slope side
and the downstream slope side with the influence of operating water level conditions and
fast low tide water level and will be applied to 3 simulation models. As a limitation for
sensitivity analysis due to the influence of slope, this does not take into account the
influence of earthquakes.
Sensitivity Analysis of the Effect of Slope of Upstream Side Slope on Safety Factor
Value
To calculate the sensitivity value of the influence of slope on the safety factor based
on existing data, we can use the change in the slope angle as the variable that affects (X)
and the change in the Safety Factor (SF) as the variable that affects (Y). With equations
2-27 in the previous Chapter 2, we can calculate the sensitivity value. The initial data for
the calculation of this sensitivity value can be seen in Table 12 and the comparison graph
in Figure 5.
Table 12
Comparative Data on Safety Factor Values Affected by Upstream Slope Slope
Slope
Angles
Safety Factor Upstream Normal Condition
Description
Empty
MAR
MAN
MAB
RDD
1:
3.00
18.000
4.758
3.784
4.877
5.033
3.779
Model 1
1:
2.50
22.000
4.620
3.743
4.911
5.059
3.740
Model 2
1:
2.50
22.000
4.708
3.748
4.946
5.113
3.745
Model 3
Figure 6
Comparison Chart of Safety Factor Values Affected by Slope Upstream
Sensitivity Analysis of the Influence of Downstream Side Slope Slope on Safety
Factor Value
The same application applies to the downstream slope side, we can use the change
in the slope angle as the influencing variable (X) and the change in the Safety Factor (SF)
as the influencing variable (Y). With equations 2-27 in the previous Chapter 2, we can
calculate the sensitivity value. The preliminary data for the calculation of this sensitivity
value can be seen in Table 13 and the comparison graph in Figure 7.
Table 13
Comparative Data on Safety Factor Values Affected by Downstream Slope Slope
3.00
3.50
4.00
4.50
5.00
5.50
KOSONG MAR MAN MAB RDD
Safety Factor
Model-1 Model-2 Model-3
Analysis of the impact of dam slope modifications on the sensitivity to rapid drawdown,
earthquakes, and changes in water level at the Jragung Dam in Semarang Regency
Jurnal Indonesia Sosial Teknologi, Vol. 5, No. 7, July 2024 3627
Slope
Angles
Safety Factor Downstream Normal
Condition
Description
Empty
MAR
MAN
MAB
RDD
1: 2.50
22.000
5.228
3.653
3.606
3.475
3.654
Model 1
1: 2.50
22.000
5.405
3.822
3.725
3.778
3.824
Model 2
1: 2.25
24.000
5.859
3.791
3.707
3.685
3.787
Model 3
Figure 7
Comparison Chart of Safety Factor Values Affected by Downstream Slope Slope
The next step is to calculate the relative change in the slope angle coefficient
(ΔX/X) followed by the calculation of the relative change in the safety factor (ΔSF/SF).
Then with the equation 2-27 in Chapter 2, the value of Sn is calculated. Details of the
calculation will be presented in Tables 14 to 19.
Table 14
Calculation of Sn Value in Post-Construction Conditions – Downstream Slope Side
Slope
Angles
Empty
Δx
Δx/X
ΔSF
ΔSF/SF
Sn
(X)
(SF)
1: 2.50
22.000
5.228
1: 2.50
22.000
5.405
2.000
0.091
0.177
0.034
0.372
1: 2.25
24.000
5.859
2.000
0.091
0.631
0.117
1.284
Sn Average on The Downstream Side Of Normal Post-Construction
Conditions
0.828
Table 15
Calculation of Sn Value in MAR Condition – Downstream Slope Side
Slope
Angles
MAR
Δx
Δx/X
ΔSF
ΔSF/SF
Sn
(X)
(SF)
1: 2.50
22.000
3.653
1: 2.50
22.000
3.822
2.000
0.091
0.169
0.046
0.509
1: 2.25
24.000
3.791
2.000
0.091
0.138
0.036
0.397
Sn Average on The Downstream Side Normal Low Water Level Conditions
0.453
Table 16
Calculation of Sn Value in MAN Conditions – Downstream Slope Side
Slope
Angles
MAN
Δx
Δx/X
ΔSF
ΔSF/SF
Sn
(X)
(SF)
1: 2.50
22.000
3.606
1: 2.50
22.000
3.725
2.000
0.091
0.119
0.033
0.363
3.00
4.00
5.00
6.00
7.00
KOSONG MAR MAN MAB RDD
Safety Factor
Model-1 Model-2 Model-3
Rizal Undityo Rahardani, Trihanyndio Rendy Satrya
Jurnal Indonesia Sosial Teknologi, Vol. 5, No. 7, July 2024 3628
1: 2.25
24.000
3.707
2.000
0.091
0.101
0.027
0.298
Sn Average on The Downstream Side Of The Normal Condition Normal
Water Level
0.331
Table 17
Calculation of Sn Value in MAB Conditions – Downstream Slope Side
Slope
Angles
MAB
Δx
Δx/X
ΔSF
ΔSF/SF
Sn
(X)
(SF)
1: 2.50
22.000
3.475
1: 2.50
22.000
3.778
2.000
0.091
0.303
0.087
0.959
1: 2.25
24.000
3.685
2.000
0.091
0.210
0.056
0.611
Sn Average at The Downstream Side Of The Normal Flood Level
0.785
Table 18
Calculation of Sn Value in Rapid Drawdown Conditions – Downstream Slope Side
Slope
Angles
RDD
Δx
Δx/X
ΔSF
ΔSF/SF
Sn
(X)
(SF)
1: 2.50
22.000
3.654
1: 2.50
22.000
3.824
2.000
0.091
0.170
0.047
0.512
1: 2.25
24.000
3.787
2.000
0.091
0.133
0.035
0.383
Sn Average on The Downstream Side Of Normal Rapid Low Tide
Conditions
0.447
Table 19
Calculation of the Average Sn Value of the Effect of the Slope of the Downstream Slope
Slope
Sn Value
Description
Empty
MAR
MAN
MAB
RDD
1: 3.00
-
-
-
-
-
Model 1
1: 2.50
0.372
0.509
0.363
0.959
0.512
Model 2
1: 2.50
1.284
0.397
0.298
0.611
0.383
Model 3
Average Sn Value
0.569
From the three simulation models, the average Sn value will be taken due to the
influence of slope on the downstream slope side, so from Table 4.109, the average Sn
value = 0.569 on the influence of safety factor is obtained. This means that a 1% change
in the amount of the angle of inclination coefficient causes a change of about 0.569% in
the Safety Factor value. The greater the absolute Sn value, indicating that the safety factor
is very sensitive to changes in the slope angle. The smaller the absolute Sn value,
indicating that the safety factor is not too sensitive to changes in the slope angle. If the Sn
value is negative, it indicates that the safety factor decreases along with the increase in
the slope angle. Meanwhile, if the Sn value is positive, it will indicate that the safety
factor increases along with the increase in the slope angle.
Conclusion
In Simulation Model 1, when the water level conditions are normal and the water
level recedes rapidly (rapid drawdown), the filtration seepage that occurs in the dam core
and the lower foundation is within the safe limit because it does not exceed 0.05% of the
reservoir capacity. The filtration flow rate also remains below the critical speed, making
it safe from piping hazards. The safety factor (FK) value for piping is 5,702, which is
Analysis of the impact of dam slope modifications on the sensitivity to rapid drawdown,
earthquakes, and changes in water level at the Jragung Dam in Semarang Regency
Jurnal Indonesia Sosial Teknologi, Vol. 5, No. 7, July 2024 3629
higher than the minimum requirement of 4,000, indicating that the dam is in safe
condition. In Simulation Model 2, filtration seepage conditions also remain safe during
the operation of the reservoir at normal water level and fast receding. The filtration flow
rate that remains below the critical speed indicates that the dam is safe from piping
hazards. The FK value for piping of 4,401 meets the minimum safety requirements.
Simulation Model 3 shows that the filtration seepage condition remains within the safe
limit at normal water level conditions and recedes rapidly. The filtration flow rate that
remains below the critical speed also indicates that the dam is safe from piping hazards.
The FK value for piping of 4.046 meets the minimum safety requirements. The slope
stability analysis in Simulation Model 1 shows that the dam is stable under normal
conditions, static earthquakes, and 100-year OBE earthquakes, with a safety factor (SF)
higher than the minimum value. However, the dam was not safe in a 10,000-year MDE
earthquake with an SF of 0.700 which was lower than the minimum value of 1,000.
Nonetheless, the deformation still shows that the damage will not cause the dam to
collapse The slope stability in Simulation Model 2 shows that the dam is stable under
normal conditions, static earthquakes, and 100-year OBE earthquakes, with a safety factor
higher than the minimum value. However, the dam was not safe at the 10,000-year MDE
earthquake with an SF of 0.758 which was lower than the minimum value of 1,000.
Nonetheless, the deformation shows that the damage will not cause the dam to collapse.
In Simulation Model 3, the slope stability shows that the dam is stable under normal
conditions, static earthquakes, and 100-year OBE earthquakes, with a safety factor higher
than the minimum value. However, the dam was not safe in a 10,000-year MDE
earthquake with an SF of 0.779 which was lower than the minimum value of 1,000.
Nonetheless, the deformation shows that the damage will not cause the dam to collapse.
Sensitivity analysis shows that the safety factor against changes in water level elevation
on the upstream side is quite large, but not too significant on the downstream side. Rapid
drawdown time negatively affects the safety factor on the upstream side, but the effect is
small. The earthquake load shows that the safety factor is very sensitive to changes in
earthquake intensity, with a greater influence on OBE earthquakes than MDE
earthquakes. The slope of the upstream slope does not affect the safety factor too much,
while the slope of the downstream slope is quite sensitive to changes in the angle of
inclination. Overall, the Jragung Dam was declared safe against piping hazards and stable
under various simulated conditions. However, the 10,000-year MDE earthquake requires
special attention because although the dam was damaged, it did not cause a complete
collapse.
Rizal Undityo Rahardani, Trihanyndio Rendy Satrya
Jurnal Indonesia Sosial Teknologi, Vol. 5, No. 7, July 2024 3630
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