Slope Stability Analysis

Slope Stability Analysis

1. General

         Intake PDAM is planned to take water from the Progo River, and is on the edge of the river bordering the slopes as high as 19.65 m with an average slope of 2 H: 3 V. Therefore to obtain the space of Intake PDAM requires slope cuts as high as 19.65 m with an average slope of 1 H: 4 V. 11.5 m deep intake construction is carried out with 90 ° upright excavation as shown in Figure 1. Slope and quarry safety analysis studies were conducted to determine potential lumbar hazards that may occur in the slope cutting process , excavation of intake and during the construction of the intake and also the talud on the slope. This report contains slope stability analysis and excavation using finite element method (FEM), on slope cutting and intake excavation.

Figure 1. Cross section of talud construction and intake.

2. Slope and Soil Layer Slope Existing Condition

       To evaluate the condition of the existing slopes surveyed work location. Figure 2 shows visually the slope is covered by trees, so the impression is quite gentle, however the actual situation is quite steep because the trees grow in a horizontal direction. The surface soil is a brown sandy earth and there is a tree root at the top. The result of measurement of topographic detail indicates that the slope has an average slope of 2 H: 3 V as shown in Figure 1. In-depth drilling tests are conducted on the slope and river bank of 1 point and 2 (two) sondir points and 1 (one ) shallow drill points are performed at the pump house location. The drill test results show that the soil layer on the riverbed is in the form of coarse sand dotted with gravel and boulder with high density indicated from Nspt value> 50. The result of shallow sondir and drill test shows hard soil layer with end value of qc resistance > 220 kg / cm2 is at a depth of 2-3 m above ground level. Generally from the results of land and location investigation it can be estimated that the soil layer on the slopes of the intake area consists of 2 layers: 1) surface soil in the form of sandy soil with thickness of 2 - 3 m and 2) sand soil mixed with gravel and large stones at a depth of 2-3 m above ground level.

Figure 2. Survey location.

3. FEM analysis
        FEM analysis is performed to model the construction stage of slope cutting and intake excavation. Each phase is a security number analysis, SF that describes the level of slope security as well as compared to the requirements of the security number to be met.

3.1. Geometry Model.
        Based on the results of the site survey, soil investigation and topographic measurement, existing soil layer conditions are modeled in FEM analysis as a geometry model as shown in Figure 3. In the geometry model the soil layer consists of two layers: the soil mixed with silt soil hereinafter referred to as surface soil, and mild "sated" boulder sand. Both soil layers are modeled with the mohr-coloumb model.

        The soil parameters used in the analysis were obtained from return analysis with the assumption of safe number, (safety factor, SF) of the existing slope conditions 1,3, also taking into account the results of field and laboratory testing. The stone pair is modeled as a ground with a modulus of elasticity, the cohesion and the friction angle in very high. The soil parmeter values ​​used in the analysis can be seen in Table 1.

        The limitation of the number of points (only one drill point on the slope foot) causes the elevation of the soil layer in the slope area to be unknown, therefore two simulations are performed simulation 1. The assumption that hard sandy soil mixed with mild "sated" boulders is present in the elevation shallowly follow slope and simulation form 2. It is assumed that the hard sandy soil mixed with the mild "lactated" boulder on the lreng portion is almost horizontal with an elevation similar to that of the mild "light" sintered layer of milky soil sand on the slope foot.

Table 1. Land Parameters.

Lapisan Tanah	Berat volume kering, gk (kN/m3)	Berat volume basah, gb (kN/m3)	Modulus elastisitas, E (kN/m2)	Kohesi, c (kN/m2)	Sudut gesek dalam, f (o)
Pasir Permukaan	17	20	20000	25	40
Pasir Dalam	17	20	120000	45	25
Pasangan Batu	24	24	200000	200	45

(a)

(b)

              

Figure 3. Model geometry a) simulation 1 and b) simulation 2

3.2. Slope Stability Analysis

Stage 1

Slope cuts up to 5 m above as shown in Figure 4. The results show that for the simulation of 1 maximum deformation that occurred is 54.65 mm and safe number SF = 1.36 while the simulation of 2 maximum deformation that occurred is 95,10 mm and safe number SF = 1.43. 

(a)   

                                                   (b)                                                    

Figure 4. Deformation stage 1 a) simulation 1 and b) simulation 2

Stage 2

Slope cuts up to 9 m upper as shown in Figure 5. The results show that for the simulation of 1 maximum deformation that occurs is 53.62 mm and safe number SF = 1.38 while the simulation of 2 maximum deformation that occurs is 95.36 mm and safe number SF = 1.43.

(a)

  (b)

                                                                                          

Figure 5. Deformation stage 2 a) simulation 1 and b) simulation 2

Stage 3

Slope cuts up to 13 m above as shown in Figure 6. The results show that for the simulation of the maximum deformation that occurred was 47.54 mm and the safe number SF = 1.25 while the simulation of 2 maximum deformation that occurred was 83.51 mm and the safe number SF = 1.25. .

(a) 

                                                 (b)

Figure 6. Stage deformation a) simulation 1 and b) simulation 2

Stage 4

Slope cuts up to the top 17m as shown in Figure 6. The results show that for the simulation of 1 the maximum deformation that occurs is 43.22 mm and the safe number SF = 1.07 whereas the simulation of 2 maximum deformation that occurs is 82.95mm and safe number SF = 1.07.

(a) 

                                                                                 (b)

Figure 7. Deformation stage 4 a) simulation 1 and b) simulation 2

Stage 5

Slope cutting up to the upper slope elevation or slope foot as shown in Figure 7. The results show that for the maximum 1 deformation simulation that occurs is 43.33 mm and the safe number SF = 1.07 while the simulation of 2 maximum deformation that occurs is 85.45mm and safe number SF = 1.07.

.

(a) 

                                               (b)

Figure 8. Deformation stage 5 a) simulation 1 and b) simulation 2

Stage 6

The intake error up to -6.0m from the upper talud elevation is shown in Figure 7. The results show that at this stage there is a collapse of soil on the slope both in simulation 1 and simulation 2.

 (a)

                                              (b)

Figure 9. Deformation stage 6 a) simulation 1 and b) simulation 2

Summary of slope stability analysis of existing slope to 6 can be seen in Table 2, Figure 8 and Figure 9. In slope cutting stage 4 and stage 5 both simulated the hard sand location of the sucked sand following slope (simulation 1) and almost parallel elevation of sand hard soil tersensasi on the edge of the slope (simulation 2) obtained the value of the safe value, SF the same slope that is 1.07. This indicates that from stage 4 the simulation of the hard sand of the saturated grain has no effect on the stability / safety of the slope. The results of the second slope stability analysis of the simulation also show that the collapse occurs at stage 6, ie when the excavation work intakes up to -6.0 m from the upper talud elevation.

Table 2. Figures safe, SF analysis results.

	Angka aman, SF
Tahap konstruksi	Lereng Eksisting	1	2	3	4	5	6
Simulasi 1	1,30	1,36	1,38	1,25	1,07	1,07	Runtuh
Simulasi 2	1,32	1,43	1,43	1,25	1,07	1,07	Runtuh

Figure 8. Figures safe, SF results stability analysis of existing slope conditions - stage 6 simulation 1

Figure 9. Safe figures, SF results of slope stability analysis of existing conditions - stage 6 simulation 2

4. Conclusions

1. The collapse occurs at the intake dump stage up to -6.0 m depth from the upper talud elevation.
2. The safe number, the smallest SF of 1.07 occurs in stage 4 when slope cuts reach a height of 8-9 m above the upper talud elevation.