METHOD AND EXAMPLES OF CALCULATING THE STABILITY OF CAPS USING GEOSYNTHETICS

Transcription

1 METHOD AND EXAMPLES OF CALCULATING THE STABILITY OF CAPS USING GEOSYNTHETICS STUDY CONTENTS: 1. PROPOSED METHOD 2. PROGRAM INPUT DATA 3. ANGLES OF FRICTION BETWEEN DIFFERENT GEOSYNTHETICS 4. RESULTS 5. EXAMPLES 6. BIBLIOGRAPHY APPENDIX 1: CALCULATIONS FOR THE EXAMPLES 1. PROPOSED METHOD As the UNE Standard :2001 explains, it will always be necessary to study the stability of the slope, with this understood as covering three aspects: a) risk of loss of fines on the crown of the slope on hitting the geomembrane against the ground. In this case, an INTERDRAIN geotextile or geocomposite will be used. b) Landslide risk of the material forming the slope. In this case, a landslide calculation will be made using Mohr's circles, block landslide method, etc. c) Risk of landslides, basically in caps, because of the loss of friction between the different layers of geosynthetics and geosynthetic-covering soil contact. In this Technical Study, we will focus on point c). INTERMAS Geosynthetics uses the Veneer Reinforcement Stability Calculator program proposed by to calculate the stability of landfill cap slopes using geosynthetics. This calculation program: starts from the hypothesis that there is no hydrostatic pressure, so the proper dimensioning of the INTERDRAIN geocomposite as a drainage layer for infiltrated rain water must be ensured (see Technical study Dimensioning the INTERDRAIN geocomposite for draining off rain water infiltrating into a landfill cap, INTERMAS Nets SA, 2005). it starts from the hypothesis that there is no pressure exerted by gas, so the correct dimensioning of the INTERDRAIN geocomposite as a layer for capturing and draining gas must be ensured (see Technical study Dimensioning INTERDRAIN for capturing the drainage of biogas generated in landfill, INTERMAS NETS S.A., 2005). it takes into account the passive wedge generated at the bottom of the bank which provides a stabilizing force, so it must be ensured that this passive wedge actually exists. it calculates the safety factor for the stability of the slope without taking into account any additional reinforcement (Unreinforced Safety Factor). it calculates the safety factor of the bank considering additional reinforcement used (Reinforced Safety Factor), usually a reinforcing geomesh, so it is very useful for dimensioning long-term traction resistance (T allow ) of the reinforcing geomesh to ensure the long-term stability of the cap. Page 1 of 11

2 Figure 1. Diagram of forces in a cap. Source: 2. PROGRAM ENTRY DATA The input data sheet for the program is attached here: Figure 2. Input data for the Veneer Reinforcement Stability Calculator program. Source: Page 2 of 11

3 Notes: The cohesion of the covering soil (C) and the cohesion between the different geosynthetic layers (Ca) are not normal parameters. If it is assumed that the cohesion values are 0, the calculation gives conservative results. The recommended reinforcing geosynthetic is a reinforcing geomesh of polyester covered with PVC. The reduction factors for damage during installation (RF_in), for flux or creep (RF_cr), for chemical and biological degradation (RF_cbd) and for stitching joints (RF_sm) must be provided by the manufacturer. T ult is the maximum traction resistance of the reinforcing geomesh, according to the technical details. T allow is the long-term resistance to traction of the reinforcing geomesh, once T ult has been reduced by the reduction factors RF_in, RF_cr, RF_cbd and RF_sm. It is considered that an overall safety factor of 1.3 is enough to ensure stability. 3. ANGLES OF FRICTION BETWEEN DIFFERENT GEOSYNTHETICS In the calculation program the chosen interface friction angle (δ) must be the smallest friction angle between the different layers (geosynthetic-geosynthetic contact and geosynthetic-soil contact). As initial values, the values recommended by the UNE Standard :2001, shown in the following table, can be chosen: Table 1. Approximate friction coefficients between different types of soil and geosynthetics. Source: UNE Standard :2001. Synthetic materials. Set-up. Waste sealing systems with H.D.P.E. sheets. Page 3 of 11

4 These values are approximate, they depend on the manufacturer and the characteristics of the material, as well as the dampness conditions. Because of this, it is recommended to carry out a cutting table test with the materials that are going to be used in the most unfavourable conditions. Page 4 of 11

5 3. EXAMPLES 3.1. CAPS WITH HDPE GEOMEMBRANES AS A SEALING SYSTEM Let us suppose that a cap is made up of the following layers (from the bottom upwards): Waste Earth regularization layer (30 cm) INTERDRAIN GMG 512 geocomposite for draining gas HDPE layer, rough on both sides INTERDRAIN GMG 512 drainage geocomposite for draining off rain water Covering soil largely made up of sand, 80 cm thick γ=17 kn/m 3 and internal angle of friction F = 20º Hypotheses: It is considered that C=0 and Ca=0. Type of INTERDRAIN geocomposites: If an INTERDRAIN geocomposite with standard (heat-welded) geotextiles is used, the friction angle between INTERDRAIN and the rough membrane is 24º, and the friction angle between INTERDRAIN and the covering soil is 22º. If an INTERDRAIN geocomposite with non-heat-welded geotextiles is used, the friction angle between INTERDRAIN and the rough membrane is 26º, and the friction angle between INTERDRAIN and the covering soil is 26º. EXAMPLE 1: Characteristics of the slope: L = 20 m Angle of inclination = 18º Using INTERDRAIN with standard geotextiles on both sides, the most unfavourable friction angle is 22º (between the covering soil and INTERDRAIN) Using the Veneer Reinforcement Stability Calculator program, the safety factor is: Unreinforced SF = 1.350, so the cap is stable and does not require further reinforcement. EXAMPLE 2: Characteristics of the slope: L = 20m Angle of inclination = 25º Using INTERDRAIN with non-heat-welded (non-standard) geotextiles on both sides, the most unfavourable friction angle is 26º (between the covering soil and INTERDRAIN and between INTERDRAIN and the rough geomembrane) Using the Veneer Reinforcement Stability Calculator program, the safety factor is: Unreinforced SF = < 1.3 (unacceptable). Additional reinforcement must be fitted. If a polyester reinforcing geomesh covered with PVC of 80 kn/m 2 and with typical reduction coefficients of RF_id = 1.3, RF_cr = 2, RF_cbd = 1.3 and RF_sm = 1.3 is used, the safety factor then is: Reinforced SF =1.347 (the cap is stable). Page 5 of 11

6 3.2. CAPS WITH BENTONITE LAYERS AS WATERPROOFING SYSTEMS Let us suppose that a cap is made up of the following layers (from the bottom upwards): Waste Earth regularization layer (30 cm) INTERDRAIN GMG 512 geocomposite for draining gas Bentonite layer (GCL) INTERDRAIN GMG 512 drainage geocomposite for draining off rain water Covering soil largely made up of sand, 80 cm thick γ=17 kn/m 3 and internal friction angle F = 20º Hypotheses: It is considered that C=0 and Ca=0. Type of INTERDRAIN geocomposites: If an INTERDRAIN geocomposite with standard (heat-welded) geotextiles is used, the friction angle between INTERDRAIN and the rough membrane is 20º, and the friction angle between INTERDRAIN and the covering soil is 22º. If an INTERDRAIN geocomposite with non-heat-welded geotextiles is used, the friction angle between INTERDRAIN and the rough membrane is 22º, and the friction angle between INTERDRAIN and the covering soil is 26º. EXAMPLE 1: Characteristics of the slope: L = 30 m Angle of inclination = 18º Using INTERDRAIN with non-heat-welded (non-standard) geotextiles on both sides, the most unfavourable friction angle is 22º (between INTERDRAIN and the Bentonite layer). Using the Veneer Reinforcement Stability Calculator program, the safety factor is: Unreinforced SF = 1.311, so the cap is stable and does not require further reinforcement. EXAMPLE 2: Characteristics of the slope: L = 10 m Angle of inclination = 28º Using INTERDRAIN with non-heat-welded (non-standard) geotextiles on both sides, the most unfavourable friction angle is 22º (between INTERDRAIN and the Bentonite layer). Using the Veneer Reinforcement Stability Calculator program, the safety factor is: Unreinforced SF = (unacceptable). Additional reinforcement must be fitted. If a polyester reinforcing geomesh covered with PVC of 80 kn/m 2 and with typical reduction coefficients of RF_id = 1.3, RF_cr = 2, RF_cbd = 1.3 and RF_sm = 1.3 is used, the safety factor then is: Reinforced SF =1.363 (the cover is stable) This example shows the importance of the passive wedge at the bottom of the bank as a stabilizing force in short, steep slopes. Page 6 of 11

7 7. BIBLIOGRAPHY EPA (1998) Eseverri, A (2004) Eseverri, A (2005) Koerner, R. M. (1998). R. M. Koerner, and T-Y. Soong, 1998 Thiel, R.S. (1998), UNE AASHTO (1996) m Linning of Waste Containements and other Impoundment Facilities. Cincinati. Ohio, Soluciones drenantes con georredes. Revista Subsuelo y Obra Urbana nº 9. Junio 2004 Sellado del Vertedero de Serrallarga. Revista Infoenviro.nº7. Agosto 2005 Designing with Geosynthetics. 4th Edition, Prentice Hall, New Jersey, USA "Analysis and Design of Veneer Cover Soils". Proceedings of 6 th International Conference on Geosynthetics, Vol. 1, pp. 1-23, Atlanta, Georgia, USA. "Design Methodology for a Gas Pressure Relief Layer Below a Geomembrane Landfill Cover to Improve Slope Stability", Geosynthetic International, Vol. 5, No. 6 pp Materiales sintéticos. Puesta en Obra. Sistemas de impermeabilización de Vertederos de residuos con láminas de polietileno de alta densidad. Standard Specification for Geotextiles, Designation: M American Association of State Transportation and Highway Officials, Washington, D.C. Website dedicated to dimensioning with geosynthetics in landfills. Page 7 of 11

8 APPENDIX 1. CALCULATING THE STABILITY OF THE EXAMPLES 1. CAPS WITH HDPE GEOMEMBRANES AS A WATERPROOFING SYSTEM 1.1. EXAMPLE 1 Page 8 of 11

9 1. CAPS WITH HDPE GEOMEMBRANES AS A WATERPROOFING SYSTEM 1.2. EXAMPLE 2 Page 9 of 11

10 2. CAPS WITH BENTONITE SHEETS AS WATERPROOFING SYSTEMS 2.1. EXAMPLE 1 Page 10 of 11

11 2. CAPS WITH BENTONITE LAYERS AS WATERPROOFING SYSTEMS 2.2. EXAMPLE 2 Page 11 of 11