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Analysis of slope stability of a segmental concrete block wall and geogrid in Meulaboh–Geumpang road

Landslide disasters are often a threat to the smooth transportation and economy of the community. Meulaboh-Geumpang crossroad, located in the western coastal region of Aceh Province, is one of the landslides-prone areas. Thus, the appropriate solution is necessary for preventing and handling the landslides in this area. This study conducted slope reinforcement using segmental concrete blocks combined with geogrids. The planned vertical distance of the geogrid (Sv) was 0.2 m, 0.4 m, and 0.6 m, while the length of the geogrid plan was 1 meter outside the failure envelope of the slip surface at the existing condition. The slope of the planned slope angle for the installation of the concrete blocks was 45°, 60°, and 90°. The dimensions of segmental concrete blocks were 20 cm high, 30 cm wide, and 40 cm long. The safety factor, at the condition after overall reinforcement, reached an acceptable amount for handling, only at STA 84 + 280 with a slope of 90°. A reinforced concrete beam was not added to the foot of segmental concrete blocks because it did not meet the acceptable safety factor, which was 1.32. The result of this analysis is to provide useful information about the form of a landslide that occurred. It will also inform the best combination of concrete block wall reinforced with geogrid for the reinforcement on the slopes. Thus, relevant parties can use it for landslide handling in these locations.


Introduction
Landslide is one of the geological disasters that occurred due to the movement of various types of rocks or soil masses, such as falling rocks or large chunks of rocks. A landslide or known as the mass movements of earth, rocks, or both earth and rocks, often occurs on natural or artificial slopes. It is a natural phenomenon, that is, nature seeks a new balance due to interferences or influencing factors.
There are internal and external factors causing landslides. Internal factors include the rising pore water pressure, the geotechnical condition, and the decreasing shear strength of the soil/rock. On the other hand, external factors are high rainfall [1], river erosion, and earthquakes.
Landslides frequently occur in Aceh, especially on the provincial roads [1]. In general, it begins with a high rainfall intensity a few days earlier. For example, a landslide occurred on Wednesday night (10/5/2017) around 11 pm, in which part of the road was eroded, as illustrated in Figure 1, leading to the disruption of the land transportation between Meulaboh and Geumpang on Thursday (11/5/2017) [2]. Meulaboh-Geumpang road is a road connecting between West Aceh and Pidie districts. The residents use this road to bring staples or vegetables from Pidie district to West Aceh district, or vice versa. For smooth traffic between Meulaboh and Geumpang, an appropriate reinforcement solution is necessary to overcome the landslide issue to avoid similar incidents that will distract the economic growth in the area.
Slope stability analysis plays a critical role in the planning of civil constructions, especially on the roads located in slope-prone areas. It is essential for understanding their performance and, in particular, their stability, reliability, and deformations. Thus geotechnical engineers often seek to calculate values of quantitative indicators of performance, such as the factor of safety (safety factor), lateral deformation, and the probability of failure and reliability index [3]. Slope stability analysis is undertaken to produce a safe plan for both natural and artificial slopes. The parameters resulted from the slope stability analysis are the shape of the slip surface and the safety factor. The safety factor is to identify the slope stability, defined as the ratio between the shear strength of the soil and the shear stress on the earth mass [4].
Slopes-prone landslides can be strengthened using several reinforcement materials. One of the reinforcement materials is geogrid. Geogrid is a geosynthetic material consisting of connected parallel sets of intersecting ribs with apertures of sufficient size to allow strike-through of the surrounding soil, stone, or other geotechnical materials [5]. There are three categories of geogrids, as illustrates in Figure  2, that are currently available: (1) unitized polyolefin, (2) coated yarn, and (3) polyester rod (or strap).
(a) unitized polyolefin (b) coated yarn (c) polyester rod (or strap) Figure 2. Various categories of geogrids [4] This study used segmental concrete blocks [6] combined with geogrids. The planned vertical distance of the geogrid (Sv) was 0.2 m, 0.4 m, and 0.6 m, and the length of the geogrid plan was 1 meter outside the failure envelope of the slip surface at the existing condition. The planned slope angle of the concrete block installation was 45 o , 60 o , and 90 o . The dimensions of the segmental concrete block were 20 cm high, 30 cm wide, 40 cm long.
This study aimed to analyze the stability of the existing and the reinforcement slope conditions using segmental concrete blocks combined with geogrids. The value of the safety factor after the overall reinforcement reached the permissible safety number for handling only at STA 84 + 280 with a slope of 90 o . No reinforced concrete beam was added at the foot of the segmental concrete block as it did not satisfy the permitted safety factor of 1.32.

Materials and Method
This research was conducted at two points, namely Km 84 + 280 and Km 84 + 910. The coordinates of the research location are presented in Table 1. The data required for this study was in the primary and secondary data. Primary data included the measurements of slope geometry, location coordinates, documentation, data of soil sample testing in the laboratory, and modeling of the slope condition. The secondary data was the map of West Aceh district, map of the research location, the parameter of young modulus value, permeability, and Poisson ratio.
The data was used to analyze the slope stability employing the finite element method on the existing slope condition to determine the value of the safety factor and the shape of the slip surface. The finite Element Method is an elasto-plastic method widely accepted in geotechnical area. It is much economical and time effective with iterative capability [7] [8]. Furthermore, reinforcement was provided at the existing slope location that did not meet the safety factor of slope stability. The stages of slope stability analysis are described in the following section.

Data input stage
The analysis started by data input, consisting of slope geometry input, the geometric depiction of the slope, load input, material data input, and mesh generation. The data of the existing slope at STA 84 + 280 were, as follows: the height of the slope was 9 m, and a slope angle of 61 o . Meanwhile, at STA 84 + 910, the slope height and angle were 8 m and 70 o , respectively. Figures 3a and 3b illustrate the existing conditions at the research location. The geogrids used in this study was a uniaxial geogrid, as shown in Figure 4. The planned vertical distance of the geogrid (Sv), as illustrated at Figure 4, was 0.2 m, 0.4 m, and 0.6 m while the length of the geogrid plan was 1 meter behind and below the failure envelope of the slip surface at the existing slope condition without reinforcement after being analyzed by the finite element method. Meanwhile, the concrete blocks used were fabricated segmented units with dimensions of 20 cm high, 30 cm wide, 40 cm long.

Data calculation stage
Data calculation conducted on the slope stability analysis consisted of several stages, including: (i) The initial condition that was the calculations before load working (ii) The construction stage, the parameter activated by ΣMweight and ΣMload (iii) The calculation of deformation and total displacement using the load advancement ultimate level. (iv) The security factor analysis was using phi-c reduction.
(v) The reinforcement stage, that was the stress and strain phases due to the self-load, the external load and loads of concrete block and geogrid walls (vi) The security value calculation process.

Data output stage
The output of the slope stability analysis consisted of, as follows: (i) The safety factor of the existing condition and the condition of the segmental concrete blocks walls reinforced by geogrids; (ii) Slope deformation is shown based on the total displacement; (iii) The graph of safety factors

Results and discussion
The soil parameters used in this analysis are displayed in Table 2.

The Results of Slope Stability Analysis
Initially, slope stability analysis was conducted on two different slope conditions without-and withreinforcements. This analysis was carried out at two separate locations, which are at STA 84 + 280 and 84 + 910. Overall, the results indicated that the slope safety reached an acceptable safety factor on slope conditions with reinforcements, which was SF ≥ 1.5. The results are shown in Tables 3 to 5. The slope stability analysis of the existing conditions at STA 84 + 280 and STA 84+910 suggested critical to unsafe conditions with a safety factor of 1.14 and 1.16, respectively. Thus, reinforcement is required at both STA 84 + 280 and 84 + 910. The analysis results of the safety factors of the existing conditions at STA 84 + 280 and 84 + 910 are shown in Table 3. Further slope stability analysis was carried for the slope with reinforcement. Thirty-six (36) models of reinforcements were analyzed, as shown in Tables 4 and 5.  The results of this further slope stability analysis showed that the closer the geogrid vertical distance on the slope produces higher the slope safety factor, similarly for the slope of the concrete blocks. The more declivous of the concrete block results in higher the value of the safety factor. The addition of a reinforced concrete beam at the foot of the segmental concrete blocks also increased the safety factor value, as shown in Table 5.

The Comparison of Safety Factor, Vertical Geogrid Distance and Slope of Concrete Blocks Based on the Analysis of Finite Element Method
Another slope stability analysis was carried out by adding geogrid reinforcement. The slope stability analysis with additional geogrid reinforcement on the Meulaboh-Geumpang road, at STA 84 + 280 and 84 + 910, was conducted for the geogrid (Sv) vertical distance of 0.2 m, 0.4 m, and 0.6 m, and the slope of segmental concrete blocks of 45 o , 60 o , and 90 o to achieve a safe and appropriate safety factor ( Figure  5 and Figure 6).