Chapter 3 Research Methodology
3.4 Study Scheme for Sensitivity Analysis
The sensitivity analysis is categorized into 8 cases as shown in Table 3-5. For CASE 1, CASE 2, CASE 3, CASE 4, and CASE 5, the evaporative depth, the slope of LCRS, the hydraulic conductivity of LCRS, the hydraulic conductivity of waste, and the height of waste is varied, respectively. Moreover, as the waste reaches the design fill limit, CASE 6 is varied for the slope of LCRS; CASE 7 and CASE 9 is varied for the hydraulic conductivity of LCRS. In this table, the background in the cells of varia-tion is filled with grey.
CASE 1-1 is defined as the initial case. In this case, hydraulic conductivities of waste and LCRS are representative of the most suitable condition of landfills in Taiwan.
Furthermore, CASE 1-1 also equals to CASE 3-1, CASE 4-1, and CASE 5-1 because the parameters in CASE 1-1 are the initial value of evaporative depth, hydraulic con-ductivity of LCRS and waste, and height of the waste. Hence the series of CASE 3, CASE 4, and CASE 5 starts with CASE 1-1.
In the series of CASE 3, the average slope of LCRS will be applied in HELP.
Anding Landfill is not included in this case due to its homogeneous slope (0.5%).
From CASE 3-2 to CASE 3-8, the variation is the hydraulic conductivity of LCRS.
The hydraulic conductivity of LCRS will be applied from Kmax to 1 10-7 cm/s. From CASE 4-2 to CASE 4-5, the variation is the hydraulic conductivity of waste. From Case 5-1 to CASE 5-3, the variation is the height of the waste. This series of case is only applied for Toufen Landfill and Anding Landfill.
The series of CASE 6 and CASE 7 are all calculated as the waste layer reach the designed fill limit. In addition, the series of CASE 9 is calculated as the waste layer plus two more layer of waste. From CASE 6-2 to CASE 6-8, the variation is the slope of LCRS. The variation in series of CASE 7 and CASE 9 is the hydraulic conductivity
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of LCRS. Bali Landfill is close to the design fill limit, thus the variation of height of waste will not apply in the simulation of Bali Landfill.
Table 3-5: Values of Cases for Sensitivity Analysis Case No. Evaporative
Depth (cm) conduc-tivity of Waste (cm/s)
Height of Waste (m)
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Table 3-5: Values of Cases for Sensitivity Analysis (Continued) Case No. Evaporative
Depth (cm) conduc-tivity of Waste (cm/s)
Height of Waste (m)
*4: Various: slope of LCRS is different from zone to zone
*5: Kmax = max hydraulic conductivity of LCRS
*6: X = Not included in this case
*7: P = Present Height;
*8: C = Closed (Reach designed fill limit)
*9: C+2 = Closed with 2 more levels
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The result of daily and cumulative leachate production will be compared to the field data. The difference between simulation and field data will be quantified by root mean square method. The difference will be calculated from the first date of the field data.
Since there is no research on hydraulic conductivity of waste in landfills in Taiwan, the hydraulic conductivity has to be adopted from foreign researches of landfills (Table 2-4). The hydraulic conductivity of waste layer is assumed to be 1 10-3 cm/s for MSW and 1 10-2 cm/s for incinerator fly ash. In order to obtain conservative results, the evaporative depth is set to be 3 cm in the initial condition for ensuring the maximum leachate collection.
Derivation of Kmax
The total quantity of leachate is Q and the hydraulic conductivity of drainage layer is Kmax. As shown in Figure 3-18, the drainage layer consists of the drainage pipe and the loam material such as MSW hence the total quantity of leachate collection from drainage layer is also consisted from drainage pipe, Qpipe, and loam material, Qloam. Since:
Qmax = Qpipe + Qloam, ... (3.1) Kmax·i·Atotal=Kpipe·i·Apipe+Kloam·i·Aloam, ... (3.2) where Kpipe=hydraulic conductivity of drainage pipe, which is assumed as 1 m/sec;
Apipe=area of drainage pipe; Kloam=hydraulic conductivity of loam material of the drainage layer, such as MSW (K = 0.001 cm/s); Aloam=area of loam material of the drainage layer. Atotal=Total area of the drainage layer, which is: Atotal= Apipe+ Aloam.
The hydraulic gradient, i, is constant in drainage layer and it is also the same value in the drainage pipe and loam material. Therefore, both i are all removed from the
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equation and the equation shows:
K K A A K A A ... (3.3) The calculation result of Kmax for Bali Landfill, Toufen Landfill, and Anding Landfill are listed in Table 3-6.
Table 3-6: Result of Kmax
Name Bali Landfill Toufen Landfill Anding Landfill
Width of Area 144 m 169 m 137 m
Diameter of Drainage Pipe 400 mm 600 mm 300 mm
Kmax (cm/s) 0.175 0.3345 0.679
To get conservative result, the runoff is assumed as zero and the vegetation class is set as bare soil. Above two parameters of HELP is in order to produce the maximum amount of leachate.
Slope Stability Analysis
The profile of landfill is divided into three layers, waste layer, barrier layer, and the base layer. The base material of the landfill is assumed as soft rock. The unit weight of the soft rock is 24 kN/m3. According to the strength of classification from ISRM (ISRM, 1981), the uniaxial compressive strength of the extremely weak rock is 250 kPa to 1000 kPa. In this study, the undrained cohesion is 250 kPa for obtaining
conserva-Fill Material, such as MSW Drainage Pipe
Figure 3-18: Profile of Drainage Layer
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tional result. The stability analysis only calculates the factor of safety for the transla-tional failure, thus the material strength of base soil does not affect the result of anal-ysis.
The assumption of unit weight and shear strength are based on the result of field study in Taiwan (Fan and Shan, 2007). The material properties of waste are 10 kN/m3 for unit weight, 35° for friction angle, and 34 kPa for cohesion which are conducted from in-situ direct shear tests in Jhunan Landfill and Hukou Landfill.
Based on the research on interface strength of geosythetics, the interfacial shear strength between HDPE and soil is assumed as 15° for friction angle (Liu, 2004) and 0 kPa for cohesion. In additional, the internal friction angle is assumed as 8° for the condition of wetting under the geomembrane. Though geomembrane is only 2 mm, the ground surface is not completely flat in the landfill. Therefore, the thickness of liner system is set as 0.1 m.
The parameters for slope stability analysis are summarized in Table 3-7
Table 3-7: Summary of Parameters for Slope Stability Analysis Waste Layer Barrier Layer Base Layer
Unit Weight (kN/m3) 10 10 24
Cohesion (kPa) 34 0 250 (Su)
Friction Angle (°) 35 15
Su = Undrained Shear Strength
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