The causes of the slope failure must be investigated and this can prevent the slope failure from occurring again. Preliminary observation, site investigation, soil investigation, soil laboratory testing and cut slope stability design are the methodology of this project. This project started with a preliminary observation, which conducts the observation of the current condition of the cut slope, drainage system, vegetation type and the surroundings of the cut slope.
The site survey consists of topographic surveying to identify the actual cut slope geometry. A water pipe and an inclinometer were installed to monitor the groundwater level and measure the slope movement of the cut respectively. Laboratory soil testing is used to determine shear slope soil parameters and the list of tests are moisture content, atterberg limit, sieve and hydrometer analysis and triaxial test.
Morgenstern Analyze Method 35 Figure 4-17: FOS of the existing cut slope in failure mode, which is in Morgenstern Analyze Method 36 Figure 4-18: Modeling of slopes with stone filling 37 Figure 4-19: FOS of the modified slope of the cut in the collapse mode which v.
The localized slope failure incident in Block 14, UTP was identified in the second berm of the cut slope consisting of a three-level berm and occurred in February 2017. Soil condition, drainage system, high groundwater level and the type of vegetation in the cut slope area may be the cause of slope failure. Because of this, further investigation of the slope failure is required and the next step should be taken to ensure that the slope failure condition does not cause destruction to the public.
SCOPE OF STUDY
TYPES OF SLOPE FAILURES
1996) reported that landslides are types of shallow slope failures that are parallel to the slope face and can occur accidentally without any warning of potential slope failure. Spreading slope failure is defined as failure that involves movement of the extensional cohesive soil.
Earth flow is a downhill viscous flow when the volume of water contained in the coarse and fine grained is high. The repair of slope failure is very important in every slope failure cases involving many methods after obtaining the soil parameters from the soil investigation work. 1996), surface slope failures are common in Southern California and the repair method including the rebuild failure area, geogrid repair, soil-cement repair and also pipe piles and timber decay. Rebuild failed area methods are not effective because the shear strength of soil remains the same as the soil in the affected area is reused to bury the failed area.
The geogrid repair acts as a reinforcement and depending on the shear strength of the import fill, the slope of the slope and the thickness of the possible failure mass. The import fill is compacted to fill about 90% of the maximum modified proctor dry density, while for the soil-cement repair method there is a slight variation where the import fill is mixed with 6% cement. The last method of slope failure repair in Southern California is pipe piling and wood jamming.
This method used hollow galvanized steel pipe and wood cladding, which is placed behind the steel pipe as a holder with compacted fill. But the disadvantage of this method is that the steel pipe piles have a low capacity of bending strength, and the system is not designed properly, which causes failure.
- FLOW CHART
- PRELIMINARY OBSERVATION
- SITE SURVEY
- SITE INVESTIGATION
- DESIGN OF SLOPE STABILITY
A site survey is included in this project to identify the geometric measurements of the slope area and produce a topographic map to know the geomorphological features (Sew G.S. and Chin T.Y, 2000). A total station and a prism, as shown in Figures 3-2 and 3-3, are basic instruments that are very important for measuring accurate location coordinates. The purpose of the soil survey is to provide technical information and analysis from field and laboratory testing to enable the designer to decide and design the slope mechanism system of the proposed project.
The open pit and rotary sink sampling points are shown in Figures 3-4 and 3-5, respectively. Inclinometers were installed to record the movement of the slope, while water level tubes are used to measure the water level. The water level at the site is an extremely important input in design, as it can identify the soil conditions, and the water level is the influence on the stability of rock, waste rock and soil or fill slopes.
The function of laboratory soil testing is to determine soil parameters that will be used in design, such as unit weight, cohesion, friction angle and soil classification. In this experiment, four soil sample containers were used as shown in Figures 3-7 and 3-8 to obtain the moisture content where the calculation using Equation 3-1. 𝑀2 is the mass of the container and wet soil (in g) 𝑀3 is the mass of the container and dry soil (in g).
The plastic limit is defined as the percentage of moisture content when the soil is spun into 3.2 mm diameter yarn. Once the point of the cone is positioned on the ground surface as shown in Figure 3-10, it will penetrate the ground for 5 seconds. The triaxial test is used to measure soil mechanical properties such as shear angle and apparent cohesion.
Geoslope Studio software using the slope/w application was used to design the stability of the cut slope failure zone. Soil parameters such as unit weight, cohesion and friction angle are important as input to software for finding factor of safety. Morgenstern Price is best suited for regression solutions that have an interslice force direction.
The last type of analysis is the Spencer Method which has a constant relationship between the magnitude of the interlocking shear and the normal forces. The stability shear slope factor of safety should be in the range of 2.0 to 3.0 to avoid the occurrence of failure.
RESULTS & DISCUSSIONS
- DETAILED OBSERVATIONS
- EXISTING SLOPE SURVEY
- SUB-SURFACE SOIL AND GROUNDWATER LEVEL
- LABORATORY TESTING RESULT
- Open Pit Sampling
- Rotary Wash Boring Sampling
- Existing Slope Modeling with Lab Test Soil Parameter
- Existing Slope Modeling in Failure Mode as at the Site
- New Modified of Cut Slope with Rock Filling
The distance between the two wells is 20 m and the location is at the top of the slope as shown in Figure 4-7. The drilling result at points 1 and 2 are shown in Table 4-1 and the drilling log result can be referred to APPENDIX. After penetrating these samples, both ends of the samples were coated with a non-shrink wax to ensure a hermetic seal and the void at the top and bottom of the tube was then filled.
For borehole 1, the borehole depth at 7m has the same depth of the fault with the SPT value of 2, which means that the bearing capacity of the soil is weak. The measurement of the water table in the water level tube was carried out using a tape measure. The range of the result is from 5.9 to 6.5 m, and it can be said that the water table is not the cause of the slope error.
The characteristics of the soil is to examine the conditions of the subsoil and materials, which can assess the stability of the cut slope. As the depth of the sample increases, the value of sample depth (m) moisture content (%) Atterberg Limit Sieve & Hydrometer. As the depth of the sample increases, the value of moisture content and Atterberg limit fluctuates.
When designing slope stability, the residual soil parameter is used to determine the strength parameter of the failure zone. As indicated in Table 4-6, Mohr's circle in the critical state is shown in Figure 4-12, and the value of the friction angle is calculated by Equation 4.1, while the cohesion value is obtained from the intersection of the y-axis and the breakdown line under the effective stress. As indicated in Table 4-7, Mohr's circle in the critical state is shown in Figure 4-15, and the value of the friction angle is calculated using Equation 4.1, while the value of cohesion is obtained from the intersection of the y-axis and the effective stress breakdown line.
Slope stability design consists of checking the Factor of Safety (FOS) of the shear slope. Using the soil parameter in Table 4-8, the FOS values of the three methods show that the slope is in good condition as the FOS value is greater than 1. The landslide is located in the failure zone with a depth of about 4 m. Unfortunately, in reality the slope failure had occurred in the second cut slope berm and this has led to a re-analysis of the soil parameter.
Using the soil parameter in Table 4-9, the FOS values of the three methods show the low result of FOS which means the second level berm of the cut slope. The value of soil parameter is the use of residual soil which at the critical point uses 24% of axial strain. 2001), they mentioned that the critical condition of the residual soil is in the range of 10% to 30% of axial strain, as the shearing condition persists at constant volume and constant effective stress.
CONCLUSION & RECOMMENDATION
For the future work, the slope maintenance must be done frequently to prevent any blockage in the drainage channels. If it is not done, the slope failure will occur again and may damage the environment and surroundings.