ENVIRONMENTAL GEOTECHNICS. Landfills - 1. Prof. Ing. Marco Favaretti

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1 ENVIRONMENTAL GEOTECHNICS Landfills - 1 Prof. Ing. Marco Favaretti University of Padova Department of Civil, Environmental and Architectural Engineering, Via Ognissanti, 39 Padova (Italy) phone: marco.favaretti@unipd.it website: 1

2 INTRODUCTION Landfill design and construction technology has advanced rapidly in response to more stringent regulatory requirements and demands. A solid waste landfill must be able to: prevent groundwater pollution collect leachate permit gas venting provide for groundwater and gas monitoring. This part of the course focuses on the geotechnical aspects relating to the design and construction of the landfill containment systems (top cover and bottom liner). 2

3 INTRODUCTION A modern landfill is engineered work that consists primarily of a: composite liner system, leachate collection and removal system gas collection and control system final cover system. The adoption of suitable design and construction methods is essential not only to reduce design and construction costs, but also to minimize long-term operation, maintenance, and monitoring expenses. 3

4 Principal Landfill Requirements 4

5 Principal Landfill Requirements 5

6 Principal Landfill Requirements An engineered landfill is a controlled method of waste disposal. The site of the landfill must be geologically, hydrologically, and environmentally suitable. A landfill is not an open dump (with problems as smoke, odour, unsightliness, insect and rodent problems, seagull and other birds problems). Professional planning and engineering supervision are required. A landfill has a carefully designed and constructed envelope that encapsulates the waste and that prevents escape of leachate into the environment. The envelope consists basically of a top capping (or cover) and a bottom liner. Each of these two main components comprises a system of barrier and drainage layers as shown in the next Figures. 6

7 Landfill Components and Configuration Most MSW landfills are composed of the following elements or systems: (a) Bottom and lateral side liners system (b) Leachate collection and removal system (c) Gas collection and control system (d) Final cover system (e) Stormwater management system (f) Groundwater monitoring system (g) Gas monitoring system. 7

8 Landfill Components and Configuration The construction/design of a landfill requires that the following actions are taken: Landfill footprint layout Sub-base grading Cell layout and filling Temporary cover selection Final cover grading Final cover selection A schematic diagram of the development and completion of a solid waste landfill is shown in the next Figure. 8

9 A schematic diagram of the development and completion of a solid waste landfill is shown in the next Figures. Cell subgrade and leachate collection system 9

10 Placement of solid waste in landfill Closed landfill with final cover 10

11 Landfill Components and Configuration The interdependence between various landfill elements and the sequence in which they are considered plays an important role in landfill design. These elements can be combined in a variety of geometrical configurations or spatial arrays. The most common landfill types or geometrical configurations include: Area fill Trench fill Above and below ground level Valley Fill 11

12 Landfill Components and Configuration Area Fill. The landfill progresses with little or no excavation. This type of landfill is used in areas with shallow groundwater table or where soils are unsuitable for excavation. 12

13 Landfill Components and Configuration Trench Fill. Solid waste is filled in a series of deep and narrow trenches. It is generally used only for small waste quantities. This method is still used somewhere for hazardous waste landfills. 13

14 Landfill Components and Configuration Above and Below Ground Level. This type of landfill is like a combination of the two previously mentioned types. The excavation area is much larger than in a Trench Fill landfill. The depth of excavation normally depends on the depths of the natural clay layer and the groundwater level. 14

15 Grumolo delle Abbadesse (Vicenza, Italy) 15

16 Grumolo delle Abbadesse (Vicenza, Italy) 16

17 Grumolo delle Abbadesse (Vicenza, Italy) 17

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19 Landfill Components and Configuration Valley Fill. Solid waste is filled between hills (old quarry) or rolling soil. 19

It should prevent escape of leachate, limit rainfall

20 Landfill Envelope The envelope encapsulates wastes and isolates them from the surrounding environment. It should prevent escape of leachate, limit rainfall infiltration, and handle gas generation. TOP BOTTOM 20

21 Liner System The liner system is placed on the bottom and lateral sides of a landfill. It acts as a barrier against the advective (hydraulic) and diffusive transport of leachate solutes. Its main purpose is to isolate the solid waste and prevent contamination of the surrounding soil and groundwater. A liner consists of multiple barrier and drainage layers. 21

22 Leachate Collection and Removal System Leachate is generated from liquid squeezed out of the waste itself (primary leachate) and by water that infiltrates into the landfill and that percolates through the waste (secondary leachate). It consists of a carrier liquid (solvent) and dissolved substances (solutes). A leachate collection and removal system is used to collect the leachate produced in a landfill, to prevent the build-up of leachate head on the liner, and to dram leachate to a wastewater treatment plant by a sanitary sewer lime or a leachate storage tank for treatment and disposal. 22

23 Gas Collection and Control System MSW can generate large quantities of gas during decomposition. The two primary constituents in a landfill are methane (CH 4 ) and carbon dioxide (CO 2 ). The gas collection and control system is used to collect the landfill gas during decomposition of the organic components of the solid waste. Landfill gas can either be used to produce energy or flared under controlled conditions. 23

24 Final Cover System The final cap or cover system consists of barrier and drainage layers. A cover soil layer is also included in order to protect the underlying layers against intrusion, damage, and the effects of frost. The main purpose of a landfill final cover is to minimize water infiltration into the landfill to reduce the amount of leachate generated after closure. 24

25 Composite Liners Proper functioning of a liner system is critical to the containment effectiveness of a landfill. Early liners consisted primarily of a single liner composed of a clay layer or a synthetic polymeric membrane. During the past few decades the trend has been to use composite liner systems comprising both clay and synthetic geomembranes together with interspersed drainage layers. The following is an approximate chronology showing the introduction date for each of these approaches: 25

26 < 1982 Single compacted clay liner 1982 Single GM liner 1983 Double GM liner 26

27 1984 Single composite liner system 1987 Double composite liner system 27

28 Composite Liners A composite liner works in a mutually supportive way to minimize and offset the limitations (excessive seepage, holes, tears) associated with single liner systems. A composite liner works in different way respect to an individual GM or a soil (compacted clay) liners as well. If there is a hole in a GM, liquid will move easily through the hole, assuming the subgrade soil does not impede seepage. With a soil liner alone, seepage takes place over the whole area of the liner. With a composite liner only a limited amount of liquid will pass through any hole in the GM, but it will then encounter low permeability clay. 28

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30 Composite Liners The low-permeability clay will not allow further migration of the limited amount of liquid passing through the holes. Good composite action requires good hydrauilc contact between the geomembrane and underlying clay soil. The geomembrane should not be separated from the clay with permeable material, such as a bed of sand or a thick geotextile, because this would jeopardize intimate hydraulic contact. If stones of a size and shape that could puncture the geomembrane exist in the clay soil layer, the stones must be removed. To achieve intimate contact, the surface of the compacted clay soil on which the geomembrane is placed should be smooth-rolled with a steel-drum roller. Also, the geomembrane should be placed and backfilled in a way that minimizes wrinkles (Daniel, 1993). 30

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32 Seepage rates through GM liners, soil liners, and composite liners may be calculated using equations published by Giroud and Bonaparte (1989a) and Giroud et al. (1989). The following example calculations are presented to compare calculated flow rates through different lining systems. Assumptions made for the calculations are that the head of liquid, h, is 300 mm; the hydraulic conductivity k of the soil liner is m/s (best case), 10-9 m/s (average case), or 10-8 m/s (worst case); the GM contains holes with an area of 0.1 cm 2 and the number of holes per hectare is 2 (best case), 20 (average case), or 60 (worst case). 32

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34 Flow rate (Liter/hectare/day) Best case Average case Worst case GM alone Holes/ha Compacted Clay alone k (m/s) Composite 0, Holes/ha k (m/s) Calculated flow rates through the composite liner are typically at least 100 times less than through the GM or soil liner alone. Further, even if the soil liner is relatively permeable (10-8 m/s) and there are 20 holes/ha in the GM, the calculated flow rate through the composite liner is far less than the flow rate through relatively good-quality soil liners or GM liners on their own. 34

35 A compelling attribute of composite liners is that neither the clay soil nor the geomembrane component needs to be constructed perfectly to realize excellent performance of the composite liner. This fact is critical to designers who cannot always be assured that the liner will be built to the desired high standards. The performance of composite liners to date has generally been very good. 35

36 ITALY - Decreto Legislativo 13 gennaio 2003, n.36 Top Containment System Landfill type Inert wastes Not dangerous wastes Dangerous wastes Top protection layer 1 m 1 m 1 m Drainage layer 0,5 m 0,5 m 0,5 m Clayey Barrier layer 0,5 m (low k) ( 0,5 m) (k 10-8 m/s) ( 0,5 m) (k 10-8 m/s) Biogas collection layer - 0,5 m 0,5 m Base Containment System Landfill type Inert wastes Not dangerous wastes Dangerous wastes Drainage layer 0,5 m 0,5 m Synthetic layer HDPE (if necessary) HDPE (if necessary) Clayey Barrier layer (s 1 m) (k 10-7 m/s) (s 1 m) (k 10-9 m/s) (s 5 m) (k 10-9 m/s) 36

37 Top Containment System according to D.LGS. 36/2003 (Not dangerous wastes landfill) 1. Top Protection Layer t 1 m 2. Rainwater Collection Layer t 0,5 m 3. Compacted Clayey Layer t 0,5 m k 1x10-8 m/s 4. Biogas Collection layer t 0,5 m 5. Regularization Layer MSW 37

38 Base Containment System according to D.LGS. 36/2003 (Not dangerous wastes landfill) 1. Leachate Collection Layer t 0,5 m 2. Geomembrane 3. Compacted Clayey Layer t 1 m k 1x10-9 m/s Geological Barrier 38

Geotechnical aspects of landfill design

Geotechnical aspects of landfill design Dr.-Ing. B.V.S. Viswanadham Associate Professor, Department of Civil Engineering IIT Bombay Email: viswam@civil.iitb.ac.in Introduction Up to 75 % the solid waste