Increase in Safety Factor Value in Existing Conditions and Reinforcement of Gabion Type Retaining Wall Pt. Medco E&P Malaka Block A

. Indonesia is an archipelagic country with 34 provinces, which has a diverse soil profile and geological structure. This condition is an area where a slope is formed. The slope is a sloping land surface and forms a certain angle and has no force in resisting the occurrence of a slide. Landslide is a disaster that can cause casualties. Collapse (slide) occurs as a result of a driving force greater than the opposite force originating from the shear strength of the soil. The slide occurred at PT. Medco E&P Malaka Block A, Ranto Perlak District, East Aceh Regency, Aceh Province in the Siwah flow ROW STA 2+400. Landslides occur as a result of high rain intensity resulting in landslides. The safety factor analysis on the slopes was carried out using the Strength Reduction Method (SRM). In the existing conditions, the slope experienced a collapse (slide) with an SF value of 1.15, which did not meet the requirements of the safety factor value (SF≥ 1.25). Therefore, reinforcement was carried out on this slope by constructing a Gabion type retaining wall construction with a height of 3 m and a width of 1.5 m. From the analysis with Gabion type retaining wall reinforcement, the SF value was 1.51 which fulfilled the requirements of the Safety factor when strengthening (SF≥ 1.5). From this analysis, it was found that the SF value was different in the existing conditions, the SF value was 1.15 and in the strengthening condition, the SF value was 1.51. Where the SF value increases by 0.36 or 32%.


Introduction
Indonesia has 34 provinces which have diverse soil profiles and geological structures.This diverse condition is an area where slopes are formed in the form of artificial slopes and natural slopes (Haryadi, 2018).Das (1985) states that a slope is a sloping ground surface and forms a certain angle to the horizontal plane and has no force to resist collapse (slide).Landslides occur due to a driving force greater than the opposite force originating from the shear strength of the soil along the sliding plane.The slope where the slide occurs has a safety factor value that is not fulfilled, namely SF (safety factor) ≤1.25 (Sadat, 2018).Febrianti (2016) states that a stability analysis with an SF value ≥ 1 means that the excavation/fill slope is safe and for an SF value ≤ 1 indicates that the excavation/fill slope is not safe.Munirwansyah (2014) states that more land movements or landslides occur due to the capability factor of human resources that ignores natural factors such as topography, soil temperature, and weather which are closely related to balance, which is called the storie index factor.Landslide events are a threat to humans, loss of property, damage to infrastructure, and the occurrence of casualties around the occurrence of landslides.
The slide occurred at PT. Medco E&P Malaka Block A, Ranto Perlak District, East Aceh District, Aceh Province.Resulting in part of the road being eroded by the avalanche which resulted in the road at the location of this avalanche being impassable by the company's vehicles.Slides will continue to occur if there is no reinforcement action so that the slide will exacerbate the situation at the location of the landslide.This research will take the location of the landslide at ROW Alur Siwah KP 2+400.The failure occurred as a result of the high intensity of rain where the study site did not apply reinforcement on the slopes resulting in landslides.Of the problems that occurred at PT. Medco E&P Malaka Block A, it is necessary to strengthen the slopes to avoid landslides which further worsen the condition of the slopes.One of the ways to overcome landslides is to build a construction that is able to increase the stability of slopes or reinforcement on slopes.In general, reinforcement for handling slope stability is in the form of slope terracing, retaining walls (DPT), geotextiles, geogrids, and plaster walls.PT.Medco E&P Malaka Block A, especially at STA 2+400, will carry out a comparative analysis of the value of Safety Factor (SF) in the existing condition and with Gabion type retaining wall reinforcement using the Midas GTS NX software.

Land Slide
Soil is the top layer of the earth's layer, soil has different properties between the soil in one location and another.Soil is in the form of relatively weak bonds between grains caused by carbonates, organic matter, and oxides that precipitate between the particles (Hardiyatmo, 1992).The earth's surface has different contours and elevations, it is these differences that can cause landslides.According to Arsyad (1989) landslides occur as a result of sliding of the volume of the topsoil which is impermeable to water, a layer consisting of soil containing clay content causing the sliding of the soil (landslide).Landslides have several types of slides, namely translation, rotation, block movement, rockfall, soil creep, and debris flow.At this research location, this type of slide is a rotational slide, as shown in figure 1 below.Suryolelono (2005) suggests that slope failure occurs due to a reduction in shear strength and an increase in shear stress in the layers of soil that form the slope.The increase in shear stress occurs due to the phenomenon of variations in intergranular forces caused by the water content in the soil which causes the pore water pressure in the hydrostatic pressure in the soil to increase.The increase in the weight of the load on the slopes caused by rainwater infiltrating into the soil on the exposed slopes will result in an increase in the water content in the soil, so that the weight of the soil volume increases and the weight on the slopes also increases.Mohr (1910) states that a theory of failure of the material occurs in a plane caused by a combination of normal stresses or shear stress factors alone.The relationship between normal stress and shear stress at the failure plane is presented in Equation (1) [3].

Mhor-Coulomb Failure
Soil shear losses occur due to the crushing factor of the grains but are caused by the relative movement between the soil grains.The shear strength of the slope material is explained by the Mohr-Coulomb theory, expressed in equation ( 2) [3].

Slope stability consept
In general, slope safety has a value of SF ≥ 1.25, which is a normal design to provide an estimate of the safety factor value in slope stability analysis.Ramadhan et al (2015) stated that the design of the slope factor safety analysis is used to ensure that the slope design is safe to prevent unplanned factors and prevent the occurrence of factors that are out of design.Duncan (2005) states that the resulting parameters in the analysis of slope stability are the form of failure and the factor of the slope.The safety factor is used to identify slope stability which is used as a comparison between the soil shear strength and the shear stress acting on the soil mass, as shown in Equation (3) [3].
The value of the safety factor in the computer software used is highly dependent on the quality of the data from the results of soil investigations and the experience of the planner.The lower the quality of the soil investigation and the experience of the planner, the greater the value of the safety factor taken (Duncan, 1996).Bowles (1993) provides a recommendation that the safety factor value of a slope based on research on slope failure is shown in table below.The value of the safety factor in the software that will be released is very dependent on the quality of data from the results of soil investigations and the experience of planners in designing reinforcement.Bowles (1993) provides a recommendation that the safety factor of a slope based on research (strengthening) regarding slope failure is shown in the following table.Less than 1,0 Not safe

Type of retaining wall
A retaining wall is a building that functions as a building to stabilize certain soil conditions, which is generally built in areas with unstable slope conditions (Nur et al, 2010).field conditions to which a retaining wall will be applied (O'Rourke and Jones, 1990).In general, there are several types of retaining walls, namely gravity walls, semi-gravity walls, cantilever walls, counterfort walls, gabion walls and buttresses.walls).Figure 2 shows the types of retaining walls.

Gabion retaining wall
Gabion type retaining walls or also commonly called gabions are a type of retaining wall in the form of boxes resembling a ladder consisting of steel wire which is filled with rock inside as strength to resist slope failure.The visible properties of gabion wire include having high strength, having a hexagonal woven shape with double windings, and having tight coils (SNI 03-0090-1999).Figure 3 shows in below the shape of a gabion type retaining wall.Gabions have properties that are almost the same as gravity type retaining walls, which have a way of working the structural load will withstand the load of the material behind it so that the material does not slide.

Midas GTS NX
Midas GTS NX is program based on finite element method for the use of analysis in farious geotechnical (santoso, 2016).This software will include the geometry of field conditions using AutoCAD software, As shown in Figure 4 in below.

Result and Discussion
Soil parameters are the data needed to obtain the results of the slope stability analysis calculations.The data needed for slope stability analysis using the Midas GTS NX software are soil gamma ( ), cohesion (c), and soil shear angle () obtained from the results of testing soil samples on the right of way (ROW) Alur Siwah 2+400 slope as shown in Fig. Table 3 poison ratio (υ) and also soil elastic modulus (E), and other soil data obtained based on samples taken at the research location and testing.From figure 5 can be seen that 5 layers obtained from drill data, with a drill data depth of 17 m.The first layer is silty clay, second layers is silty sand, third and fourth layer is stiff sand, and the fifth layer is dense sand.Hasyim (2007) states that the value where the slope is 1.2 is a medium safety value, so there is concern that landslides will occur.Based on the results of the analysis of slope stability using the Midas GTS NX software, the existing condition shows that the slope conditions are unstable and deformation occurs with an SF value of 1.15, which is shown in Figure 6.The results of the analysis of stability in the existing conditions using the Midas GTS NX software by giving a vehicle load of 147.10 kN/m2 and given a rain load for 3 days the output obtained is an SF value of 1.15.Analysis using the Strength Reduction Method (SRM) method has a requirement that the allowable slope safety value in the existing condition is SF ≥ 1.25.Based on Figure 6 and 7 which is the output of the Midas GTS NX, it shows that the slope in the existing condition is deformed and will collapse.The results of calculations using the Midas GTS NX software at STA 2+400 show an SF value of 1.15 where this value does not meet the requirements for slope safety in the existing conditions, namely SF ≥ 1.25, therefore at this location reinforcement will be carried out using retaining walls of the type Gabions.The gabion wall is built as high as 3 m with a total of 6 traps from the gabion.The height of 1 gabion trap is 50 cm which is built to resemble a ladder.The parameters of the gabion type retaining wall to be inputted into the Midas GTS NX are shown in Table 4. Analysis with reinforced gabion type retaining walls shows that there is an increase in the FS value of 1.51 as shown in Figure 7.This SF value indicates that the condition of the slope that has been reinforced with a gabion retaining wall meets the requirements, where the condition of the slope that has been reinforced must have an SF value ≥ 1.5.Reinforcement with a gabion type retaining wall at this location is considered very suitable where the gabion wall functions as a barrier from collapse, but need to increas more stabilize by using 5 m micro pile beneath gabion.The gabion wall itself has properties similar to the gravity type retaining wall which utilizes the load from the wall itself as a function of resisting failure.

Conslusion
From the calculation of slope stability using Midas GTS NX software at PT. Medco E&P Malaka Block A the conclusions obtained are as follows: 1.The main factor for landslides at this location is due to the rain factor which results in an additional load on the soil resulting in a collapse on the slope 2. The safety factor in the existing condition (natural condition) with the Midas GTS NX computer software at STA 2+400 is 1.15 where this value indicates that the slope condition is not safe (SF ≤ 1.25).3. The increase in the SF value occurred due to reinforcement with a gabion type retaining wall, the SF value became 1.51 where this value met the slope requirements in a reinforced condition (SF ≥ 1.5).

4.
The SF value has increased by 0.57 from the existing condition where this increase occurred due to reinforcement with Gabion type retaining walls, if gabion wall is built only high as 3 m with a total of 6 traps from the top of road.

Fig. 4 .
Fig. 4. Slope geometry model inport from AutoCAD Parameters used Mohr-Coloumb metodh, Midas GTS NX can model with coherent parameter values (c), soil gamma ( , friction angle (), elastic modulus, (E), and poissong ratio (V).Based on the results of soil investigation in location obtained 5 layes soils, with different parameters.Figure 5 shows the 5 layers soil and the input parameters that have been inputted in the Midas GTS NX software.

Fig. 5 .
Fig. 5. Soil layers at the research location

Fig. 6 .Fig. 7 .
Fig. 6. Results of total displacement calculations and results of calculations with the safety factor of the existing ROW of Siwah flow STA 2+400

Table 1 .
Safety factor in existing conditions

Table 2 .
Slope safety factor based on research

Table 4 .
Gabion type retaining wall parameter input on Midas GTS NX