REKAYASA SIPIL / Volume 16, No.1 – 2022 ISSN 1978 - 5658 48
AN ANALYSIS OF CRITICAL SLOPES STABILITY IN BENDOSARI VILLAGE, PUJON, MALANG DUE TO THE
EFFECT OF RAINWATER INFILTRATION
Nindia Rizky Ismawan
*1, Eko Andi Suryo
2dan Arief Rachmansyah
31
Student, Master’s Program, Department of Civil Engineering, Faculty of Engineering, Brawijaya University
2,3
Lecturer, Department of Civil Engineering, Faculty of Engineering, Brawijaya University
Correspondence: [email protected]
ABSTRACT
A disaster is an incident which threatens and disrupts life. This research was purposed to determine the critical slope stability in Bendosari Village, Pujon due to rainwater infiltration. This research was conducted through ground laboratory test to investigate the type and characteristic of the soil, which then followed by testing of the geo-electric and topography to investigate the existing condition and the ground water level on the critical slopes and analyzed safety factor (SF) using SLOPE/ W for slope with rainwater infiltration effect.
The analysis results proved that rainwater infiltration in both conditions could reduce the SF (Safety Factor) value. This is because the water content in the soil increased the pore water pressure. In addition, high water content in the soil reduced the soil shear strength.
Keywords: Safety Factor, SLOPE/W,
1. INTRODUCTION
A disaster is an incident which threatens and disrupts life and livelihood of the society caused by either natural or non-natural factors as well as by human factors, resulting in human casualties, environmental damage, property loss, and psychological impact. It doesn’t merely occur without any causes, however, many factors of human error and neglect in anticipating the nature and possible disasters are given the occasion of it. Indeed, society living on steep mountain slopes faces the risk of landslides [1].
Landslide is the movement of slope-forming material in the form of rock, debris, soil or report material moving down or out of the slope. [2] The Ministry of Research and Technology (KRT) said that if the soil was cracked due to drought and it was suddenly hit by heavy rain, the land would have been landslide. Two factors related to rain give high occasion to landslides; they are high intensity
of rain in a short period of time and hitting areas with unstable soil conditions. This dry land becomes unstable and easily gets landslides when it rains. Another condition is the accumulation of rainfall in the rainy season on steep cliffs which causes it to collapse. This landslide is quite dangerous and can cause a lot of casualties [3].
2. BASIC THEORY
2.1. Research LocationPujon District, Malang is one of the districts which has several landslide prone areas and are quite dangerous. One of which has a high risk of landslides is Bendosari Village. In early 2018, a landslide occurred in February which resulted in heavy damage to one resident's house and partially slightly damaged with an estimated loss of around 100 million rupiah. This landslide incident occurred in Bendosari Village, Pujon, Malang.
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This research was conducted in BendosariVillage, Pujon District, Malang Regency. This village has a critical slope in the middle of densely populated settlements.
3. RESEARCH METHODOLOGY
Initially, this research established identification of the problem, the research data, then, were collected. The data collected were those related to problem solving of the incident.Topographic data, geotechnical data, and geo-electric data were directly obtained from the field data by conducting surveys and investigations, while rainfall data were obtained from the competent authority of research location. Furthermore, data analysis was carried out through two-dimensional software, Geostudio 2018.
3.1. SEEP/W Modeling
Slope stability were analyzed using Geostudio 2018 Software, SEEP / W and SLOPE / W. SEEP / W was used to determine the effect of rainwater infiltration on critical slopes, while SLOPE / W was used for the stability of embankment slopes or Safety of Factor for slopes both without rain and with rain.
3.2. Slope Function Modeling
Slope dimension modeling began with sketching an image of the model, which was a representation of the problem to be analyzed.
The modeling was created from the main toolbar in two different ways, using KeyIn toolbar.
The analysis phase began with creating a slope model using SLOPE/W. This model was obtained from geoelectric data thus the soil layer in each layer on the slope was visible.
3.3. Parameter Input
Several parameters which were to inputted to the slope analysis model were in Toolbar KeyIn, those are material, pore-water pressure, point/surcharge load, reinforcement load. The inputted parameters were according to the investigated soil data and other parameters obtained from SEEP/W analysis. Pore-water pressure was used to describe the state of the ground water level. Point/surcharge load was the load given to the slope, either centered load or even load. In this research, however, even load was treated to the road loads.
Reinforcement load was used to provide improvement parameters for slopes in accordance with the predetermined plan.
Figure 1. Research Location
Figure 2. Slope Modeling
Figure 3. Parameter Input of Slope Modeling
REKAYASA SIPIL / Volume 16, No.1 – 2022 ISSN 1978 - 5658 50 4. ANALYSIS AND DISCUSSION
4.1. Soil Test
Soil analysis and testing are two activities of soil sample analyzing to determine the soil state and characteristic, such as nutrient, contamination, compositions, acidity, and so on. It also determined the level of suitability of the soil to agricultural activities and the types of crops.
4.2. Specific Gravity Test
This test was aimed to determine the soil density. Specific gravity is value of ratio of the weight of soil grains to the weight of water with the same volume at a certain temperature.
This test was based on ASTM D-854 with the result presented on Table 1.
Table 1. Result of Specific Grafity Test (Specific Gravity)
No. Titik Kedalaman Berat Jenis
Toe 0.0 – 1.0 2.575
1.0 – 2.0 2.542
Crest 0.0 – 1.0 2.577
1.0 – 2.0 2.599 Sumber: Analisis (2019)
The specific gravity testing revealed that the soil sample contained mineral. According to Braja M. Das Book, the soil sample contained Hallosite mineral and Potassium feldspar.
4.3. Soil Classification
The soil testing was conducted based on Standard Sieve Analysis ASTM D-136 and Atterberg Limit ASTM D-423 and D-424 with the classification result as follow:
Table 2. Result of Specific G ravity Test (USCS Classification)
Point Depth USCS Classification
Toe 0.0 – 1.0 SC
1.0 – 2.0 CH
Crest 0.0 – 1.0 OL
1.0 – 2.0 CL
Toe point showed the soil classifications were SC (clay sand, mixed sand – loam) and CH (inorganic clay with high plasticity, fat clays). Crest point showed the soil classifications were OL (silt – organic and organic silty clay with low plasticity) and CL (from inorganic clay with low plasticity to gravel clay, sandy loam, silty clay, and “thin”
clay).
4.4. Sliding Angel
Slide angel test implemented Triaxial Test with ASTM D-2850-87 to investigate the shear strength parameter. The measurement was carried out by applying vertical pressure.
Table 3. Triaxial Test Result
No. Titik Kedalaman Triaxial Test C (kg / cm2) ф (0)
Toe 0.0 – 1.0 0.056 2.70
1.0 – 2.0 0.119 2.06
Crest 0.0 – 1.0 0.022 3.34
1.0 – 2.0 0.031 3.46
4.5. Geo-electric Test
One of the geophysical methods that can identify subsidence and landslides is the Geoelectric Method [4]. The geophysical method which was used to identify the subsurface geological structure of mud volcanoes was resistivity geo-electric method.
The study was conducted by taking four lines with sequence length each line 1, 2, 3, and 4 were 100 m, 75 m, 75 m, and 70 m. The configuration applied was Wenner Alpha configuration with the current electrode position (C) and the potential electrode (P) in sequence C1 P1 P2 C2 with a space of 5 meters between the electrodes and the shift.
4.5.1. Geo-electric Line 1
Figure 4. Lithological arrangement of rocks Line 1
After being correlated with the geological map, in line 1 there were 3 types of soil: sandy tuff, tuff, and sand. In this section, the soil composition was dominated by sandy tuff and tuff.
4.5.2. Geo-electric Line 2
Figure 5. Lithological arrangement of rocks Line 2
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After being correlated with the geologicalmap, in line 2 there were 4 types of soil: sandy tuff, tuff, and sand. In this section, the soil composition was dominated by sandy tuff and tuff.
4.5.3. Geo-electric Line 3
Figure 6. Lithological arrangement of rocks Line 3
After being correlated with the geological map, in line 3 there were 3 types of soil: sandy tuff/ aquifer, tuff, and sand. In this section, the soil composition was dominated by sandy tuff/
aquifer.
4.6. Analysis of SEEP/W and SLOPE/W Slope stability is one of the most important factors in implementing safe, productive and environmentally friendly mining [5]. Slope stability analysis at Dadapan Bendosari Village , Pujon implemented Geostudio 2018 software, which applied two features of the 2018 Geostudio; SEEP/ W and SLOPE/ W.
those two features were integrated so that the analysis result of SEEP/ W could be used for stability analysis of SLOPE/ W. The calculation of the safety of factor for a slope used two conditions:
1. Rainless condition 2. Rain condition
4.6.1. Data Analysis
The analyzed data of stability with SEEP/
W and SLOPE/ W features were topography data, rainfall data, and soil parameter.
Table 4.Parameter Data of SEEP/W
Table 5. Parameter Data of SLOPE/W
4.6.2. Analysis Result of Safety of Factor Safety of Factor is known as a terminology in determining the stability of slope, which is the ratio between the holding force and the driving force.
1. Rainless condition
The SF analysis of a condition without rain using SLOPE/W as follow:
Figure 7. Total force and slope of the slope without rain
Table 6. The total of force and moment of slope in rainless condition
Method Morgenstren-Price
Safety of factor 1.781
Volume total 56.001 m3
Weight total 920.78 kN
Holding moment total 6925.6 kNm Driving moment total 3888.5 kNm Holding force total 495.63 kN Driving force total 278.18 kN
Radius 13.250
Circle center (44.004; 44.387)
Fig. 7 presents the slip plane lines and pore water pressures in an analysis using SEEP/W and SLOPE/W. It can be seen that the right of the pore water in all layers is negative. The total force and moment are shown in Table.6.
The SF result is 1.781.
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2. Rain conditionThe SF analysis of a condition with rain using SLOPE/W and SHEEP/W as follow:
Figure 8. Total slope force and moment with rain
Table7. The total of force and moment of slope in rain condition
Method Morgenstren-Price
Safety of factor 1.353 Volume total 59.007 m3
Weight total 968.64 kN
Holding moment total 5621.4 kNm Driving moment total 4153.6 kNm Holding force total 385.77 kN Driving force total 285.25 kN
Radius 13.859
Circle center (43.977; 44.524)
Fig. 8 presents the slip plane lines and pore water pressures in an analysis using SEEP/W and SLOPE/W. It can be seen that the pore water pressure in all layers is negative. The total force and moment are shown in Table.6.
The SF result is 1.353.
The analysis results proved that rainwater infiltration in both conditions could reduce the SF (Safety Factor) value. This is because the water content in the soil increased the pore water pressure. In addition, high water content in the soil reduced the soil shear strength.
5. CONCLUSION
Reviewing the result of research in laboratory, field, and established analysis, the researcher concluded:
1. After conducting soil observations, soil laboratory analysis, geoelectrical testing, and analysis using the Geoslope software, the dimensional critical slope model was in accordance with the analysis results.
2. The original critical slope in conditions without rain had a safety factor of 1.781, while the critical slope with rainwater infiltration is 1.353. Slopes without rain and with rainwater infiltration were in unstable or critical condition.
3. Critical slopes are advised to be repaired using soil reinforcement at the bottom of the slope and plant reinforcement on the slope body.
6. SUGGESTION
1. It needs to take more analysis of slope stability when it rains at various times, thus, more variations and possibilities of slope stability are obtained and recognized.
2. Further studies are needed to be conducted to obtain the most economical and efficient comparison of slope improvement.
7. BIBLIOGRAPHY
[1] Ramli, Soehatman. Pedoman Praktis Manajemen Bencana, Jakarta, Dian Rakyat.
2010.
[2] Nur, Firman, Arif, Analisis Kerawanan Tanah Longsor Untuk Menentukan Upaya Mitigasi Bencana Di Kecamatan Kemiri Kabupaten Purworejo, 2015.
[3] Majid, Kusnoto Alvin. Tanah Longsor dan Antisipasinya, Semarang, 2008.
[4] Santoso, Budy, Penerapan Metode Geolistrik-2D untuk identifikasi Amblasan Tanah dan Longsoran Tol Semarang, Vol. 15, 2015.
[5] Salam, Abdul, Kesatabilan Lereng Menggunakan Program SLOPE/W pada PIT GN-10 Pulau GAG Kabupaten Raja Ampat Papua Barat, Vol.6, 2018,
