Mitigation of Slope Failure in Residential Area Located Southwest of Jakarta

A slope with soil from soft to medium consistency is at risk of slope failure. This situation occurred in a residential area located Southwest of Jakarta. Due to heavy rainfall, a slope next to an existing road which runs alongside a river experienced slope failure. Three alternatives were considered to prevent occurrence of slope failures. The methods considered were mechanically stabilized earth retaining wall, and sheet pile with two possible positions, i.e., crest of slope or mid-slope. In this paper, the three alternatives are presented. Due to a second slope failure which occurred, sheet pile installed at the crest of the slope was chosen as it provides the quickest installation time to stabilize the slope.


Introduction
Slope failures are often attributed due to rainfall [1].Rainfall intensity and duration are two factors that have to be taken into account when analysing the possibility of slope failure [2].In Indonesia, slope failures are one of the most commonly occurring disasters.Figure 1 shows the yearly occurrence of slope failures from 2008 to 2022 [3].It can be seen that there is an increasing trend in the yearly occurrence of slope failures.This increase can be caused by many reasons, such as better monitoring of slopes compared to the past, ongoing deforestation [4], climate change [5] and urbanization [6].To investigate the possibility of climate change, rainfall data from several sources: Statistics Indonesia (BPS) [7], Statista [8] and Nasa Satellite Giovanni [9] are also presented in Figure 1.From the 11 to 12 years data from Statista and Giovanni, Indonesia's annual rainfall ranges from 2500 mm to 3500 mm, without increasing trend in rainfall amount.However, this should not discount the possibility of climate change in contributing to the increasing number of slope failures.As climate change does not only mean an increase in rainfall, but also an increase in dry periods [10].Hence the effect of climate change may not reflect on annual rainfall, but on monthly rainfall.The effect of climate change on monthly rainfall is not within the scope of this paper.
In this paper, the focus is mitigation of a slope failure that occurred in a residential area located Southwest of Jakarta, Indonesia capital city.Figure 2 shows the location of slope failure that occurred.The slope has a height of around 10 m, with an overall slope angle of around 25º.When the slope failed, it damaged the powerhouse located on the crest of the slope.The owner of the residential area took a quick decision to demolish the powerhouse and relocate it elsewhere.Thereafter, the owner invited the author to design ways to mitigate future slope failure in the location.This paper shall cover the three design alternatives considered for stabilizing the slope.The chosen solution, as well as the reasons for the choice.3 shows the contour of the slope.As aforementioned, there was a powerhouse (gardu existing) at the crest of the slope (bottom of the figure).From site inspection, the critical plane was along the blue line.Hence, soil tests consisting of 6 cone penetration tests (CPT) and 1 borehole were carried out in the vicinity of the critical plane.The results of the soil tests were used to create soil profile (Figure 4).From the contour, despite the overall slope angle of 25º, the slope near the crest was quite steep, around 45º.The slope failure occurred in this upper portion of the slope.As for the soil profile, the soil layer consists of two layers, the soft layer (cone resistance < 1 MPa) and the stiff layer (cone resistance > 3.5 MPa).The thickness of the soft layer is around 4-5 m, whilst the stiff layer is underlain by the soft layer.Although there is presence of very stiff layers (cone resistance > 6 MPa) within the stiff layer, for simplicity of the design, the very stiff layers are considered as stiff.
The borehole result shows consistent results with the cone penetration tests, with the first 4 m showing soft consistency (SPT (N) Value < 5), thereafter being stiff (SPT (N) Value ≥ 8).The borehole result does detect presence of very stiff to hard layer (SPT (N) Value > 30) at depth 10 m and beyond.The very hard layer is not detected from the adjacent cone penetration test (S2).Undisturbed samples were also taken from the borehole for laboratory testing.At depth 1-5.5 m, triaxial CU (consolidated undrained) were conducted on 3 samples, while at depth 9-9.5 m, triaxial UU (unconsolidated undrained) was conducted.The results of the laboratory tests are summarized in Table 1.

Back Analysis
To obtain representative soil parameters, in the case of slope failure, back analysis can be carried out.Soil parameters can then be adjusted until the obtained factor of safety is approximately equal to 1.The slip surface of the slope obtained from the analysis can also be compared to the real slip surface.The back analysis was conducted using a limit equilibrium (LE) software, GEO5 [11].Figure 5 shows the LE model of the slope.The slope geometry follows that shown in Figure 4. To further simplify the model, the depth of stiff layer from CPT conducted at the crest and base of the slope are taken as linear, ignoring the CPT from the mid-slope.It is assumed that the slope failure occurred when the slope was in near-saturated conditions.After adjusting the parameters, the soil parameters that produced slip surface close to that observed in field are shown in table 2. Comparing the values in table 2 and table 1, there are differences in both friction angle and cohesion.The values of friction angle is higher, whilst the cohesion is lower.This discrepancies raise from degree of saturation of the samples tested in laboratorium.Often, it is not possible to reach 100% degree of saturation when conducting triaxial tests.Hence, lower friction angle and higher 'apparent' cohesion obtained from the results.

Design Alternatives
Three alternatives were considered in this case study, two of them are sheet pile installed at different position, whilst the third option is to build a retaining system using mechanically stabilized earth (MSE) system.Figure 6 shows the first alternative, with 8 m sheet pile installed in the mid-slope.In addition to the sheet pile, the existing upper slope is to be cut to form a gentler slope of 1V:3H.With this option, the factor of safety obtained in permanent conditions is 1.85 (Figure 6a).While in flood conditions, the factor of safety reduces to 1.70 (Figure 6b).The factor of safety obtained is sufficient to meet the required Indonesian National Standards [12] which recommends a factor of safety > 1.5 in permanent conditions.The second alternative is to install the sheet pile at the crest of the slope.The analysis was conducted using finite element software, Plaxis [13].In the second alternative, more extreme conditions with high water table, and allowing the soft front portion of the slope to fail was modelled.Despite the extreme conditions modelled, the factor of safety obtained with this alternative is nearly 1.5.However, deeper sheet pile is also required, from 8 m to 14 m.
The final alternative is to reinforce the base of the slope using MSE system (Figure 8).The bottom 2 m of the MSE is made entirely of gabions.The next 3 m of the MSE consist of gabion reinforced with 5 m geogrid.Similar to the first alternative, the upper portion of the slope is also trimmed to reduce the steepness of the slope.With this alternative, the factor of safety in permanent conditions is 2.2, while in flood conditions, the factor of safety drop to 1.4.This solution still meets the Indonesian National Standards [12], as factor of safety required for flood conditions is lower than 1.5, usually taken at 1.3.

Solution Chosen and Construction Photographs
Due to slope failure which occurred in the midst of design process, ultimately the second alternative was chosen to mitigate the slope failure.This is because the second alternative requires the least preparation and construction time.Figure 9a shows a photograph of the slope during construction, while Figure 9b shows a photograph of the slope after the construction is completed.Slope failures occur very frequently in Indonesia, averaging to 3 cases per day.This paper showcase one of such failure which occur in a residential area.Mitigating slope failures have a variety of options with their respective pros and cons.In this paper, 3 options with sheet pile and mechanically stabilized earth system are presented.Each solution is capable of mitigating the slope failure, meeting the required factor of safety stated in the Indonesian National Standards.Ultimately sheet pile installed at the crest of the slope was chosen due to ease of construction, allowing for faster completion.

Figure 2 .
Figure 2. Location of the slope failure, Southwest of Jakarta

Figure 3 .
Figure 3. Contour of the slope and location of soil tests

Figure 9 .
Figure 9. Photograph of the slope: (a) during construction; (b) after completion