Slope Failure Analysis to Restore and Increase the Slope Stability in Flores,  East Nusa Tenggara

ABSTRACT: When modifying natural slopes into steeper ones due to construction demand, slope failure may be more likely to happen if no additional actions are performed. Excavation and removal of slope’s soil from the modification process will create new internal instability, which in turn will increase the potential of slope failure. This potential could be reduced by performing additional reinforcements for the slope, which functioned as an internal stability system during and after the excavation activities. In this paper, the case study would be performed on slope failure that happened in Flores, Nusa Tenggara Timur, Indonesia. Control slope failed after it was excavated for approximately 20 meters with an inclination of 60o-70o. This paper simulated a combination of reinforcements – Ground Anchor, Soil Nailing, and Crib Wall depending on the location which required reinforcement(s) as the independent variables. Comprehensive analysis of the slope reinforcement was done by using Finite Element Method in Plaxis 2D and 3D program. Design parameter was obtained from the result of soil investigation and field observation from the study area. Process analysis is done by gradually excavating the slope, while back-filling the studied slope as counterweight, followed by installing the previous combination of reinforcements to the slope with considerations toward study area’s slope failure pattern. Results from the slope analysis model is as follow: for slope failure condition with crack around the collapsing area, Safety Factor is less than 1 before the reinforcements. After said combination of reinforcements were installed, Final Safety Factor obtained increased to more than 1.5, which is in a safe condition.

Keywords: Slope Stability, Finite Element Method, Ground Anchor, Soil Nailing, Crib Wall.

INTRODUCTION

The study was located in Flores – East Nusa Tenggara with existing steep slope about 30o to 45o. To achieve the elevation level of construction demand during that time, existing slopes were excavated for about 20 meters – with an inclination of 60o-70o. The slopes are in an unstable condition. There is some indication that show the slope in an unstable condition before the construction started. Therefore, the construction stage should consider the geotechnical design of slope stability to avoid slope failure potential.

In the Construction Phase, the work to be done was to strip existing slopes to the planned elevation level while ensuring availability of temporary road below main road. During that process, reinforcements were not carried out, which made the slope prone to failures. In the end, the potential slope failure did occured. This failure was caused by overstraining the axial and lateral extension of the slope from said work. From this accident, it could be learned that slope excavation may be very dangerous to be worked upon without additional mitigatory actions.

Prevention of slope failure have to be done by reinforcing the slope before the construction continued. During the process of restoring the slope, it is necessary to observe the fractures formed by slope failures periodically. The Slope reinforcement could create by backfilling and gabion installation at the bottom of the slope to the collapsing area which serving as counterweight, then install an anchor at the top of the slope, and lastly install soil nailing to the planned elevation.

  1. PREMILINARY STUDIES

2.1      Regional Geology

Based on the geographical position, the study location is in the Physiographic Zone of the Lesser Sunda Islands, where the Small Islands physiographic zone includes Bali, West Nusa Tenggara and East Nusa Tenggara. These zones consist of small islands that are formed by tectonic activity of Indo-Australian plate that moves northward and urging the Eurasian plate. Such urging resulted in the originally below average land surface to rise up and formed a group of Lesser Sunda Islands. In addition, the increased volcanic activity

also contributed to the formation in some parts of the archipelago due to its position on Sunda Volcanic Arc (Van Bemmelen, 1949)[1].

The study location composed by Tertiary Volcanic Rock Formation. The stratigraphic aspect of the study area structured by one formation – The Dacitic Tuff Unit, based on the Geological Sheet Map of Komodo, Nusa Tenggara. The Dacitic Tuff Unit (Tmdt) generally consists of volcanic sediment material, composed by dacitic tuff that is mostly bedded and partly massive. It contains intercalations of green tuff, calareous tuff, limestone, tuffaceous sandstone, breccia, and lava. The composition of the lava is partly dacite and partly andesite. This unit is predicted to be aged in the middle of Miocene with marine depository environment. It is not deposited harmoniously with Volcanit Rock Units (Tmv) composed of lava, dacitic breccia and Laminated Limestone (Tml) units – composed by layered limestone with intercalations of tuffaceous limestone, quartz sandstone, tuffs, and conglomerates (Ratman et al., 1978)[2].

2.2      Site Observations

Based on field observations, the slopes are in an unstable condition before constructions phase started. This shown by signs of deformation of the slope, including initial crack above the study area, repaired road and tilted electric pole due to sliding, and road rectified due to slope deformation. Excavating such slope should intuitively require combination of reinforcements in the form of crib wall, soil nailing, and drainage at the edge of the highway to stabilize and reduce erosion on the slope

[1] Van Bemmelen . R. W, (1970). ”Geology of Indonesia. Vol 1. General Geology Adjaeent Archipelago,” Government office. The Haque.

[2] Ratman, Nana & Yasin, Aswan, (1978). “Geologic Map of Komodo Quadrangle, Nusa Tenggara,” Geological Survey of Indonesia, Indonesia.

Figure 1:  Study area condition (Source: Google Earth).

Figure 2:  Initial crack above the study area

Post-slope failure observation

When the excavated slope collapse, many of the cracks appear in some location around the crown. It is one of an indication that the slope still in unstable condition since it is very probable for further slope failure potential. Considering that is necessary to conduct intensive observation of the slope deformation during the process of slope reinforcement until the construction phase is complete.

Figure 3:  Post-Slope failure Condition.

The slope failure pattern can be classified into Multiple Rotational Debris Slides. It is because the slope geometry consists of one crown and multiple fractures that forms a crown behind the collapsing area (shown in figure 3).

Based on the result of monitoring the slope deformation carried out at 6 monitoring point, showing that the slope deformation in the cracks had decreased. The observation after 9 days post-slope failure showed a downward trend on the deformation until it stops. From this result, the slope deformation can be classified as an ‘Extremely Slow’ deformation – with an average deformation speed of 9,65 x  10-9mm/sec » 0.3mm/year

DATA AND METHOD

The approximate of soil parameter and cross section for slope stability analysis assumed from the geotechnical investigation, consists of 11 deep boring and laboratory tests, and also the monitoring survey post-slope failure. The slope stability analysis based on finite element method, which performed on the Plaxis 2D and Plaxis 3D program.

To calculate the required reinforcement for the slope stability design, the soil parameter required as input for the Plaxis’s analysis. Analyses are carried out by considering one condition: is it a long term or a drained condition on the program. The Program’s input parameters use main parameters such as (c ‘, j’, E ‘, n’) for the modulus value in the excavation are used unload-reload modulus whose value is assumed to be three to four times the effective modulus.

Due to slope movements in the field, the back analysis of soil parameters reduced on the deformation plane by adding an interface line on the layer based on the slope failure and crown patterns that occur around it. The existing safety factor obtained will be lower than the equilibrium condition (SF = 1).

The reinforcement alternatives used in slope back analysis such as fill material, Ground Anchor, Soil Nailing, Crib Wall, and Gabion.

The Slope stability improvement analysis shall meet the requirement for the safety factor condition with a factor value of 1.5.

Result and discussion

5.1      Back Analysis Result

The back analysis in Plaxis programs were done in various phases. These phases are performed within the Construction Stage in the field if the analysis results have been qualified. The slope geometry was based on post-slope failure cross section.

  1. In the early phase was the initial load or gravity load stage in the post-slope failure condition. This stage still includes the traffic load of main road and the temporary road to the location.

2. The second phase is the early treatment phase Treatment was done by backfilling the fill material that serves as counterweight up to +28.7m and installation of gabion on the toe of counterweight. The main road shifted and the road temporarily stopped at this stage.

3. The Slope reinforcement phase 1: Reinforcements were started by installing the combination of ground anchor and crib wall for slopes close to the main road.

4. Slope reinforcement phase 2: Installing the combination of soil nailing and crib wall down to +18.5m elevation. In the installation of soil nailing and crib wall, the counterweight excavated gradually then followed by installing the soil nailing and crib wall.

5. Final Slope reinforcement phase 3: The installation of soil nailing and crib wall down to +9.8m elevation and external load (traffic on the main road started to run normally once more).

6. The last phase: Application of earthquake load ah= 0.288g.

The safety factor results of back analysis with Plaxis 2D and Plaxis 3D, in various phases is as below

           Early Warning System

For Further construction and post-construction, it required to install early warning system (EWS) for hazard mitigation of slope failure. The EWS instruments consist of piezometer, crackmeter, extensometer, tiltmeter, potensiometer, and accelerometer, which giving the access for monitoring remotely whenever it is required. These instruments record any indication of slope movementand groundwater level increment, then ring an alarm if there are abnormalities within the monitored parameters.

  1. CONCLUSION

The slope failure that occurred in study area, caused by its loss axial and lateral extension (stress release) due to the excavation work without installing additional reinforcement to the slope. According the analysis of restoring collapsed slope need backfill serves as counterweight, excavating gradually, and followed by installing the combination of ground anchor, soil nailing, and crib wall.

It is concluded by the back analysis using Plaxis 2D and Plaxis 3D that the SF of final condition had reached the requirements of safety (SF = 1.5) in final condition. For dynamic condition (earthquake load), safety factor is slightly lower for Plaxis 2D (SF = 1.15) while SF of Plaxis 3D still meets the requirement. The difference between the safety factor result of Plaxis 2D and Plaxis 3D is SF of Plaxis 3D larger than SF of Plaxis 2D. Based on these results, it is concluded that additional reinforcements can help to increase the strength of the collapsing area and also increase the slope stability. The synchronization between geotechnical slope stability design and construction activity are required to minimize the slope failure potential.

In addition, it is recommended to install EWS to eliminate unnecessary risks of construction accident due to slope deformation or failure.

  1. REFERENCES

Brinkgreve, RBJ, ”PLAXIS 2D ver 8: User’s manual”, Delft University of technology & PLAXIS b.v. Belanda.

Carter, M. & S.P. Bentley, ”Correlations of Soil Properties”.

CGGS, (2012). ”Manual Kestabilan Lereng”, Universitas Katolik Parahyangan, Bandung.

GEC, (2001). ”Insitu Testing and Soil Properties Correlations”, Insitu, Bali.

Holtz, Robert D., William D.Kovacs & Thomas C.Cheahan, ”An Introduction to Geotechnical Engineering”, 2nd Edition.

Hunt, Roy E., (2005). ”Geotechnical Engineering Investigation Handbook, Second Edition”, Taylor and Francis Group.

Ratman, Nana & Yasin, Aswan, (1978). “Geologic Map of Komodo Quadrangle, Nusa Tenggara”, Geological Survey of Indonesia, Indonesia.

Sanglerat, G., (1972). ”The Penetrometer and Soil Exploration”, Amsterdam.

Sanglerat, G., Gilbert Olivari & Bernard Cambou, (1984). ”Practical Problems in Soil Mechanics and Foundation Engineering”, Amsterdam.

Van Bemmelen . R. W, (1970). “Geology of Indonesia. Vol 1. General Geology Adjaeent Archipelago,” Government office. The Haque.



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