Panel Fabrication

The final design aspect to consider is the FML panel layout of the facility. Three factors should be considered when designing an FML panel layout5 30:

1. Seams should run up and down on the slope, not horizontally.

2. The field seam length should be minimized whenever possible.

3. There should be no penetration of an FML below the top of the waste.

Panel number A Seam number

FIGURE 26.20 Panel-seam identification scheme. (Adapted from U.S. EPA, Requirements for Hazardous Waste Landfill Design, Construction, and Closure, EPA/625/4-89/022, U.S. Environmental Protection Agency, Cincinnati, OH, August 1989.)

Panels must be properly identified to know where they fit in the facility. Figure 26.20 depicts the panel-seam identification scheme used for this purpose. This numbering scheme also assures a high-quality installation, since seam numbers are used to inventory all samples cut from the FML panel during installation. The samples cut from the panels are tested to ensure that the installation is of high quality. Quality assurance and the panel-seam identification scheme are discussed in more detail in Chapter 7.

26.4 ELEMENTS OF LIQUID MANAGEMENT AT WASTE CONTAINMENT SITES

The drainage system for removing leachate or other aggressive liquids from landfills, surface impoundments, and waste piles is critically important. Even if a liner has no leaks, the phenomenon of molecular diffusion will allow some of the organics from the liquids ponded on top of the liner system to leach through the FML and the clay. The timely collection and removal of that leachate is at the heart of this section.

This section presents an overview of collector design and materials, followed by a discussion of the three parts of a liquid management system: the LCRS above the primary liner, the secondary leak detection, collection, and removal (LDCR) system between the primary and secondary liners, and the surface water collection system above the closure of the completed facility. The section concludes with a discussion of gas-collector and removal systems.

26.4.1 Overview

Leachate refers to rainfall and snowmelt that combine with liquid in the waste and gravitationally moves to the bottom of a landfill facility. During the course of its migration, the liquid takes on the pollutant characteristics of the waste itself. As such, leachate is both site-specific and waste-specific with regard to both its quantity and quality. The first part of the collector system to intercept the leachate is the primary leachate collection and removal (PLCR) system located directly below the waste and above the primary liner. This system must be designed and constructed on a site-specific basis to remove the leachate for proper treatment and disposal.

The second part of a leachate collection system is between the primary and secondary liners. Varying with State or region, it is called by a number of names including the secondary leachate collection and removal (SLCR) system, the leak detection network, or the leak questioning system. It will be referred to here as the LDCR system. The main purpose of this system is to determine the degree of leakage, if any, of leachate through the primary liner. Ideally, this system would collect only negligible quantities of leachate; however, it must be designed on the basis of a worst-case scenario.

The third part, called the surface water collection and removal (SWCR) system, lies above the waste system in a cap or closure above the closed facility. Its purpose is to redirect surface water coming through the cover soil from the flexible membrane in the cap to the outside perimeter of the system. The location of all three parts of the liquid management system is illustrated in Figure 26.21.

26.4.1.1 Drainage Materials

The drainage materials for the liquid management system must allow for unimpeded flow of liquids for the intended lifetime of the facility. In a leachate collection system, the drains may consist of pipes, soil (gravel), geonets, or geocomposites.

Perforated drainage pipes have the advantage of common usage and design, and they transmit fluids rapidly. They do, however, require considerable vertical space and are susceptible to particulate

Operational cover

Operational cover

FIGURE 26.21 Three elements of a liquid management drainage system in a double-lined solid waste facility. (Adapted from U.S. EPA, Requirements for Hazardous Waste Landfill Design, Construction, and Closure, EPA/625/4-89/022, U.S. Environmental Protection Agency, Cincinnati, OH, August 1989.)

FIGURE 26.21 Three elements of a liquid management drainage system in a double-lined solid waste facility. (Adapted from U.S. EPA, Requirements for Hazardous Waste Landfill Design, Construction, and Closure, EPA/625/4-89/022, U.S. Environmental Protection Agency, Cincinnati, OH, August 1989.)

clogging, biological clogging, and creep (deflection). Creep is of concern for both PVC and HDPE pipe materials.

The synthetic materials that best meet in-plane flow rate regulations are called geonets. Geonets require less space than perforated pipe or granular material, promote rapid transmission of liquids, and, because of their relatively open apertures, are less likely to clog. They do, however, require geotextile filters above them and can experience problems with creep and intrusion.

Another synthetic material is called a drainage geocomposite, many types of which are available. Geocomposites have most of the same advantages and disadvantages of geonets. They are generally not used for primary or secondary leachate collection systems, however, because of their relatively low crush strength. The crush strength, or normal strength perpendicular to the plane, of currently available products is not sufficient to carry the weight of a large landfill. Geocomposites are useful, however, for surface water-collector systems, where the applied normal stresses are quite low.

26.4.1.2 Filtration Materials

The openings in drainage materials, whether holes in pipes, voids in gravel, or apertures in geonets, must be protected against invading fine particle-sized materials. An intermediate material, having smaller openings than those of the drainage material, must be used as a filter. Commonly in a pipe or gravel drain, a medium-coarse to fine sandy soil is used as a filter. Sand, however, has the disadvantages of taking up vertical space and moving under various loading conditions.

Geotextiles used as filters avoid these problems. The open spaces in the fabric allow liquid flow while simultaneously preventing upstream fine particles from fouling the drain. Geotextiles save vertical space, are easy to install, and have the added advantage of remaining stationary under load. As with sand filters, clogging can occur, and because geotextiles are a new technology, much not is known about them. Geotextiles are being used more and more not only for filters, but also as cushioning materials above and/or below FMLs.

26.4.1.3 Geosynthetics

Geosynthetic materials play a key role in liquid management systems. The five major categories of geosynthetics are5

1. Geotextiles

2. Geogrids

3. Geonets

5. Geocomposites.

Geotextiles are either woven or nonwoven fabrics made from polymeric fibers. Woven geotex-tiles are fabrics made up of webbed fibers that run in perpendicular directions. For filtration, the spaces between the fibers are the most important element. These spaces or voids must be large enough to allow unimpeded liquid flow but small enough to keep out invading particulates. The geotextiles must also be sufficiently strong to cover and reinforce the apertures, or openings, of the drainage materials they are meant to protect.

In nonwoven geotextiles, the fibers are much thinner but far more numerous. The various types are needle-punched, resin-bond, and melt-bond. All contain a labyrinth of randomly oriented fibers that cross one another so that there is no direct line of flow. The fabric must have enough open space to allow liquid to pass through, while simultaneously retaining any upstream movement of particles. The needle-punched nonwoven type is very commonly used as a filter material.

Geogrids are very strong in transverse and longitudinal directions, making them useful as reinforcing materials for either soil or solid waste. Generally, they are used to steepen the side slopes of interior cells or exterior containment slopes of a facility. Recently, they have also been used in the construction of "piggyback" landfills, that is, landfills built on top of existing landfills, to reinforce the upper landfill against differential settlements within the lower landfill.

Geonets are formed with intersecting ribs made from a counterrotating extruder. A typical geo-net is about 1/4-in. thick from the top of the upper rib to the bottom of the lower rib. The rapid transmission rate (1 cm/s permeability) is due to clear flow paths in the geonets, as opposed to particle obstructions in a granular soil material. There are two main concerns with geonets. First, the crush strength at the rib's intersection must be capable of maintaining its structural stability without excessive deformation or creep. Second, adjacent materials must be prevented from intruding into the rib apertures, cutting off or reducing flow rates.

Foamed geonets are relatively new products made with a foaming agent that produces a thick geonet structure (up to 1/2 in.) with very high flow rates. These improved flow rates result from the thicker product, but eventually the nitrogen gas in the rib voids diffuses through the polymer structure, leaving behind a structure with reduced thickness. The result over the long term is a solid rib geonet thickness equivalent to other nonfoamed geonets.

The fourth type of geosynthetic is a GM or an FML. It is the primary defense against escaping leachate and is of crucial importance.

The final category of geosynthetics is drainage geocomposites. These are polymeric materials with built-up columns, nubs, or other deformations that allow planar flow within their structure. A drainage geocomposite having 1-in. high columns can carry the flow of a 4-5-in. diameter pipe. Many products, however, have low crush strengths that are inadequate for deep landfills or surface impoundments. They are useful, however, for surface water-collector systems above the closed facility where they only need to support ~4 ft of soil and construction placement equipment.

26.4.1.4 Design-by-Function Concepts

Whatever parameter of a specific material one is evaluating, a required value for the material must be found using a design model, and an allowable value for the material must be determined by a test method. The allowable value divided by the required value yields the design ratio (DR), or the resulting FS. This design-by-function concept is necessary to design and evaluate new materials that are both feasible and safe for a variety of situations.

In evaluating drainage and filtration materials, an allowable flow rate is divided by a required flow rate to obtain the DR or FS according to the equations below5:

1. For drainage

reqd where DR is the design ratio, q is the flow rate per unit width, and Y is the transmissivity.

2. For filtration q'

q reqd

reqd where DR is the design ratio, qr is the flow rate per unit area, and Y' is the permittivity.

FIGURE 26.22 Variables for calculating in-plane flow rates (transmissivity). (Adapted from U.S. EPA, Requirements for Hazardous Waste Landfill Design, Construction, and Closure, EPA/625/4-89/022, U.S. Environmental Protection Agency, Cincinnati, OH, August 1989.)

FIGURE 26.22 Variables for calculating in-plane flow rates (transmissivity). (Adapted from U.S. EPA, Requirements for Hazardous Waste Landfill Design, Construction, and Closure, EPA/625/4-89/022, U.S. Environmental Protection Agency, Cincinnati, OH, August 1989.)

Transmissivity is simply the coefficient of permeability, or the hydraulic conductivity (k), within the plane of the material multiplied by the thickness (T) of the material. Because the compressibility of some polymeric materials is very high, the thickness of the material needs to be taken into account. Darcy's law, expressed by the equation Q = kiA, is used to calculate the rate of flow, with transmissivity equal to kT and i equal to the hydraulic gradient (see Figure 26.22):

where Q/w is the flow rate per unit width and 0 is the transmissivity.

With a liquid flowing across the plane of the material, as in a geotextile filter, the permeability perpendicular to the plane can be divided by the thickness, T, to obtain a new value, permittivity (see Figure 26.23). In crossplane flow, T is in the denominator; for planar flow, it is in the numerator. Crossplane flow is expressed as

where y is the permittivity and Q/A is the flow rate per unit area (flux).

FIGURE 26.23 Variables for calculating crossplane flow rates (permittivity). (Adapted from U.S. EPA, Requirements for Hazardous Waste Landfill Design, Construction, and Closure, EPA/625/4-89/022, U.S. Environmental Protection Agency, Cincinnati, OH, August 1989.)

Thus, both transmissivity and permittivity values allow for the thickness to be avoided in subsequent analyses.

Table 26.5 shows some of the ASTM test methods and standards for drainage and filter materials used in primary leachate collection and leachate detection and collection systems.

26.4.2 PLCR Systems

The various design options for primary leachate collection systems are granular soil drains, perforated pipe collectors, geonet drains, sand filters, and geotextile filters. Figure 26.24 shows a cross section of a primary leachate collection system with a geonet drain on the side slope leading into a gravel drain at the bottom. This gravel drain then leads into a perforated pipe collector. A geotextile acts as a filter protecting the geonet, and sand acts as a filter for the drainage gravel. Quite often, the sideslope geotextile extends over the bottom sand filter as shown in Figure 26.24.

FIGURE 26.23 Variables for calculating crossplane flow rates (permittivity). (Adapted from U.S. EPA, Requirements for Hazardous Waste Landfill Design, Construction, and Closure, EPA/625/4-89/022, U.S. Environmental Protection Agency, Cincinnati, OH, August 1989.)

TABLE 26.5

Test Methods and Standards for Drainage and Filter Material

ASTM Test

Designation

(or Other)

Used to Determine

Material

D2434

Permeability

Soil

D2416

Strength

Underdrain pipe

F405, F667

General specification

HDPE pipe

D4716

Transmissivity

Geonet, geocomposite

D4491

Permittivity

Geotextile

D4751

Apparent opening size

Geotextile

CW-02215a

Gradient ratio

Geotextile

GRI-GT1»

Long-term flow

Geotextile

Source: U.S. EPA, Requirements for Hazardous Waste Landfill Design, Construction, and Closure, EPA/625/4-89/022,

U.S. Environmental Protection Agency, Cincinnati, OH, August 1989. a U.S. Army Corps of Engineers Test Method. b Geosynthetic Research Institute Test Method.

Value Used for

PLCR, LDCR PLCR, LDCR PLCR, LDCR PLCR, LDCR PLCR filter PLCR filter PLCR filter PLCR filter

Source: U.S. EPA, Requirements for Hazardous Waste Landfill Design, Construction, and Closure, EPA/625/4-89/022,

U.S. Environmental Protection Agency, Cincinnati, OH, August 1989. a U.S. Army Corps of Engineers Test Method. b Geosynthetic Research Institute Test Method.

FIGURE 26.24 Cross section of primary leachate collection systems. (Adapted from U.S. EPA, Requirements for Hazardous Waste Landfill Design, Construction, and Closure, EPA/625/4-89/022, U.S. Environmental Protection Agency, Cincinnati, OH, August 1989.)

26.4.2.1 Granular Soil (Gravel) Drainage Design

The MTG regulations require that granular soil drainage materials must5

• Have 1 cm/s (2ft/min) permeability (hydraulic conductivity)

• Include perforated pipe

• Include a layer of filter soil

• Cover the bottom and side walls of the landfill.

To calculate the required flow rate, Q, in granular soil drainage designs based on MTG values:

26.4.2.2 Perforated Collector Pipe Design

The original perforated collector pipes in landfills were made of concrete like those used in highway underdrain systems. As landfills became higher, the strength of such pipes became inadequate. Today, perforated PVC pipes are commonly used, as are HDPE pipes. New regulations require that all materials be tested for chemical resistance as part of the permit-approval process. The three steps in designing perforated collector pipes are as follows:

1. Obtain the required flow.14

2. Obtain the required pipe size using the required flow and the maximum slope.31

3. Check the pipe strength and obtain its ring deflection to determine tolerance against crushing.32

26.4.2.3 Geonet Drainage Design

Table 26.6 presents a compilation of various geonets. The structure and properties of each are also identified. Geonets used in drainage design must be chemically resistant to the leachate, support the

TABLE 26.6

Types and Physical Properties of Geonets (All are PE)

TABLE 26.6

Types and Physical Properties of Geonets (All are PE)

Width/

Thickness

Aperture

Manufacturer/

Product

Roll Size

Length

Approximately

Size

Agent

Name

Structure

(ft)

(m)

(mil)

(mm)

(in.)

(mm)

Carthage Mills

FX-2000

Extruded ribs

7.5/300

2.3/91

200

5.1

Geo-Net

FX-2500

Extruded ribs

7.5/300

2.3/91

250

6.3

Geo-Net

FX-3000

Extruded ribs

7.5/220

2.3/67

300

7.6

Geo-Net

Conwed Plastics

XB8110

Extruded ribs

6.9/300

2.1/91

250

6.3

0.3 x 0.3

8 x 8

XB8210

Extruded ribs

6.9/300

2.1/91

160

4.1

0.35 x 0.35

9 x 9

XB8310

Extruded ribs

6.9/300

2.1/91

200

5.1

0.3 x 0.4

8 x 10

XB8410

Extruded ribs

6.9/220

2.1/67

300

7.6

0.25 x 0.25

6 x 6

XB8315CN

Extruded ribs

6.9/300

2.1/91

200

5.1

0.3 x 0.3

8 x 8

Fluid Systems Inc.

TN-1001

Extruded ribs

7.5/300

2.3/91

250

6.3

Tex-Net (TN)

TN-3001

Extruded ribs

7.5/300

2.3/91

200

5.1

TN-4001

Extruded ribs

7.5/300

2.3/91

300

7.6

TN-3001 CN

Extruded ribs

7.5/300

2.3/91

200

5.1

Poly-Net (PN)

PN-1000

Foamed and

6.75/300

2.0/91

250

6.3

0.3 x 0.3

8 x 8

extruded ribs

PN-2000

Extruded ribs

6.75/300

2.0/91

160

4.1

0.3 x 0.4

9 x 9

PN-3000

Extruded ribs

6.75/300

2.0/91

200

5.1

0.35 x 0.35

8 x 10

PN-4000

Foamed and

6.75/300

2.0/91

300

7.6

0.25 x 0.25

6 x 6

extruded ribs

Geosynthetics

GSI Net 100

Foamed and

250

6.3

extruded ribs

GSI Net 200

Extruded ribs

160

4.1

GSI Net 300

Extruded ribs

200

5.1

Gundle

Gundnet XL-1

Extruded ribs

6.2/100

1.9/30

250

6.3

0.3 x 0.3

8 x 8

Gundnet XL-3

Extruded ribs

6.2/100

1.9/30

200

5.1

0.3 x 0.3

8 x 8

Low Brothers

Lotrak 8

Extruded mesh

6.6/164

2.0/50

120

3.0

0.3 x 0.3

8 x 9

Lotrak 30

Extruded mesh

6.6/164

2.0/50

200

5.2

1.2 x 1.2

30 x 27

Lotrak 70

Extruded mesh

6.6/164

2.0/50

290

7.3

2.8 x 2.8

70 x 70

Tenax

CE 1

Extruded ribs

4.8/66

1.5/20

250

6.3

0.3 x 0.25

8 x 6

CE 2

Extruded ribs

7.4/82

3.8/25

200

5.1

0.3 x 0.35

9 x 9

CE 3

Extruded ribs

7.4/82

2.2/25

160

4.1

0.3 x 0.25

8 x 6

CE 600

Extruded ribs

5.5/100

1.67/30.5

160

4.1

0.3 x 0.25

8 x 6

Tensar

DN1-NS1100

Extruded ribs

5.2/98

1.6/30

220

5.6

0.3 x 0.3

8 x 8

DN3-NS1300

Extruded ribs

6.2/98

1.9/30

150

3.8

0.3 x 0 3

8 x 8

-NS1400

Extruded ribs

6.2/98

1.9/30

200

5.1

0.3 x 0.3

8 x 8

Source: U.S. EPA, Requirements for Hazardous Waste Landfill Design, Construction, and Closure, EPA/625/4-89/022, U.S. Environmental Protection Agency, Cincinnati, OH, August 1989.

Source: U.S. EPA, Requirements for Hazardous Waste Landfill Design, Construction, and Closure, EPA/625/4-89/022, U.S. Environmental Protection Agency, Cincinnati, OH, August 1989.

entire weight of the landfill, and be evaluated by the ASTM test D471633 with regard to allowable flow rate or transmissivity. This allowable value must then be compared with the required value in the design-by-function equation presented earlier.

In the D4716 flow test,33 the proposed collector cross section should be modeled as closely as possible. The candidate geonet will usually be sandwiched between an FML beneath and a geotex-tile above. Soil, perhaps simulating the waste, is placed above the geotextile, and the load platen from the test device are placed above the soil. Applied normal stress is transmitted through the entire system. Then planar flow, at a constant hydraulic head, is initiated and the flow rate through the geonet is measured.

The required flow rate can be calculated (1) directly from MTG or (2) on the basis of surface water inflow rate. To be conservative, all three calculations should be performed and the worst-case situation (e.g., that with the highest flow rate) used for the required flow rate. The various equations for determining the required flow rate or transmissivity appear below:

1. Geonet must be equivalent to MTG regulations for natural materials:

where Q is the surface water inflow, C is the runoff coefficient, I is the average runoff intensity, and A is the surface area.

Generally, geonets result in high factors of safety or DRs, unless creep becomes a problem or if adjacent materials intrude into the apertures.

26.4.2.4 Granular Soil (Sand) Filter Design

There are three parts for an analysis of a sand filter to be placed above drainage gravel. The first determines whether or not the filter allows adequate flow of liquids through it. The second evaluates whether the void spaces are small enough to prevent solids being lost from the upstream materials. The third part estimates the long-term clogging behavior of the filter.

In the design of granular soil (sand) filter materials, the particle-size distribution of the drainage system and that of the invading (or upstream) soils is required. The filter material should have its

2. Based on surface water inflow14

Gravel Sand

Silt

Clay

Particle size (log)

FIGURE 26.25 Design based on particle-size curves. (Adapted from U.S. EPA, Requirements for Hazardous Waste Landfill Design, Construction, and Closure, EPA/625/4-89/022, U.S. Environmental Protection Agency, Cincinnati, OH, August 1989.)

Gravel Sand

Silt

Clay

Particle size (log)

FIGURE 26.25 Design based on particle-size curves. (Adapted from U.S. EPA, Requirements for Hazardous Waste Landfill Design, Construction, and Closure, EPA/625/4-89/022, U.S. Environmental Protection Agency, Cincinnati, OH, August 1989.)

large and small size particles intermediate between the two extremes (see Figure 26.25). Adequate flow and adequate retention are the two focused design factors, but perhaps the most important is clogging. There is no quantitative method to assess soil filter clogging, although empirical guidelines are found in geotechnical engineering references.

26.4.2.5 Geotextile Filter Design

Geotextile filter design parallels sand filter design with some modifications. The three elements of adequate flow, soil retention, and clogging prevention remain the same.

Adequate flow is assessed by comparing the allowable permittivity with the required permittivity. Allowable permittivity uses the ASTM D4491 test method,34 which is well established. The required permittivity utilizes an adapted form of Darcy's law. The resulting comparison yields a DR, or FS, that is the focus of the design.5

reqd where Yallow is the permittivity from ASTM Test D4491, Yreqd = (Q/A)(1/hmax), Q/A is the inflow rate per unit area, and hmax = 12 in.

The second part of the geotextile filter design is determining the opening size necessary for retaining the upstream soil or particulates in the leachate. It is well established that the 95% opening size is related to the particles to be retained in the following type of relationship.

where O95 = 95% opening size of geotextile (U.S. Army Corps of Engineers CW 02215 test method),35 d50 = 50% size of upstream particles, CU is the uniformity of the upstream particle sizes, and DR is the relative density of the upstream particles.

The O95 size of a geotextile in the equation is the opening size at which 5% of a given size glass bead passes through the fabric. This value must be less than the particle-size characteristics of the invading materials. In the test for the O95 size of the geotextile, a sieve with a very coarse mesh in the bottom is used as a support. The geotextile is placed on top of the mesh and is bonded to the inside so that the glass beads used in the test cannot escape around the edges of the geotextile. This particular test determines the O95 value. To verify the FS for particle retention in the geotextile filter, the particle-size distribution of retained soil is compared with the allowable value using any of a number of existing formulae.

The third consideration in geotextile design is long-term clogging. The test method adopted by ASTM is called the Gradient Ratio Test. It was originally formulated by the U.S. Army Corps of Engineers and is listed in CW 02215. In the test, the hydraulic gradient of 1 in. of soil plus the underlying geotextile is compared with the hydraulic gradient of 2 in. of soil. If the gradient ratio is <3, the geotextile will probably not clog. If the gradient ratio is >3, the geotextile will probably clog. An alternate to this procedure is a long-term column flow test that is also performed in a laboratory. The test models a given soil-to-fabric system at the anticipated hydraulic gradient. The flow rate through the system is monitored. A long-term flow rate at a constant value indicates an equilibrium between the soil and the geotextile system. If clogging occurs, the flow rate will gradually decrease until it stops altogether.

26.4.2.6 Leachate Removal Systems

Figure 26.26 shows a low-volume sump in which the distance from the upper portion of the concrete footing to the lower portion is ~1 ft. One foot is an important design number because U.S. EPA regulations specify a maximum leachate head of 1 ft. Low-volume submersible sumps present

FIGURE 26.26 Leachate removal system with a low-volume sump. (Adapted from U.S. EPA, Requirements for Hazardous Waste Landfill Design, Construction, and Closure, EPA/625/4-89/022, U.S. Environmental Protection Agency, Cincinnati, OH, August 1989.)

operational problems, however. Since they run dry most of the time, there is a likelihood of their burning out. For this reason, landfill operators prefer to have sumps with depths between 3 and 5 ft instead of 1 ft (Figure 26.27), even though the leachate level in a high-volume sump will be greater than the 1-ft maximum.

The leachate removal standpipe must be extended through the entire landfill from liner to cover and then through the cover itself. It must also be maintained for the entire postclosure care period of 30 years or longer.

FML

JffW

Compacted clay

FIGURE 26.27 Leachate removal system with a high-volume sump. (Adapted from U.S. EPA, Requirements for Hazardous Waste Landfill Design, Construction, and Closure, EPA/625/4-89/022, U.S. Environmental Protection Agency, Cincinnati, OH, August 1989.)

26.4.3 LDCR Systems

The LDCR system is located between the primary and secondary liners in landfills, surface impoundments, and waste piles. It can consist of either granular soils (i.e., gravels) or geonets.

26.4.3.1 Granular Soil (Gravel) Drainage Design

As with the primary leachate collection system above the liner, LDSs between liners are designed by comparing allowable flow rates with required flow rates. The allowable flow is evaluated as discussed in the section on granular soil (gravel) drainage design for PLCR systems. The required flow is more difficult to estimate. This value might be as low as 1 gallon/acre/day or many times that amount. It is site specific and usually is a rough estimate. Past designs have used 100 gal/acre/ day for the required flow rate. Data from the field monitoring of response action plans (RAPs) will eventually furnish more realistic values. A pipe network for leachate removal is required when using granular soils.

26.4.3.2 Geonet Drainage Design

For a geonet LDCR system, the flow rate for the geonet is determined in the laboratory by using the ASTM D4716 test method, and the value is modified to meet site-specific situations. The geonet flow rate DR is then determined in the same way as for the granular system. No pipe network is needed.

A concern when using geonets with a composite primary liner design is the effect of geotextile intrusion and creep on the allowable flow rate. In composite primary liner systems, the geonet is placed immediately below a clay liner with a geotextile as an intermediate barrier. The design of this geotextile is important because clay particles can go through large voids in an open woven geotextile, necessitating the use of a needle-punched nonwoven geotextile of at least 8-10 oz/yd2. Even with this precaution, the laboratory test to evaluate the allowable flow rate should simulate the anticipated cross section in every detail.

26.4.3.3 Response Time

U.S. EPA specifies that the minimum detection time for the leachate entering the LDS of an LDCR system is <24 h. Response time calculations are based on velocity in the geonet and/or granular soil drainage layer. Darcy's law is used to calculate flow velocity in the geonet, and a "true" velocity must be used for granular soil.

26.4.3.4 Leak Detection Removal Systems

LDRS require monitoring, sampling, and leachate removal. Any leachate that penetrates the primary liner system and enters the secondary system must be removed. During construction, the LDCR system may accept runoff water, but once the landfill is in operation it only removes any leakage coming through the primary liner. The most common removal system consists of a relatively large diameter pipe running down the side wall between the primary and secondary liners to the low point (sump) in the LDCR. The pipe must penetrate the primary liner at the top. A submersible pump is lowered through the pipe periodically for "questioning" of the quantity of fluid coming into the system. The choice of monitoring and retrieval pump depends on the quantity of leachate being removed.

An alternate system, one based on gravity, requires penetration of both the FML and clay components of the secondary composite liner system. It also requires a monitoring and collection manhole on the opposite side of the landfill cell. The manhole and connecting pipe, however, become an underground storage tank that needs its own secondary containment and LDSs.

As required Topsoil layer

> Frost p^traton Hg CoVer soil lay

As required Topsoil layer

As required pp^nc > GM

0.6 m /■> CCL, k £ 10-7 cm/^/ ^ Composite barrier i J

As required Gas drainage layer

As required pp^nc > GM

0.6 m /■> CCL, k £ 10-7 cm/^/ ^ Composite barrier i J

As required Gas drainage layer

FIGURE 26.28 Final cover system. (Adapted from U.S. EPA, Requirements for Hazardous Waste Landfill Design, Construction, and Closure, EPA/625/4-89/022, U.S. Environmental Protection Agency, Cincinnati, OH, August 1989.)

26.4.4 SWCR Systems

The third part of liquids management is the SWCR system. It is placed on top of the completed facility and above the cover FML. The rainwater and snowmelt that percolate through the top soil and vegetative cover must be removed to a proper upper drainage system. Figure 26.28 illustrates the major components of a surface water-collector system. The design quantity for the amount of fluid draining into the surface water-collector system can be determined by either a water balance method or the computer program HELP (Hydrologic Evaluation Landfill Performance Model)36 37 (see Figure 26.29).

Surface water drainage systems can be composed of granular soils, geonets, or geocomposites, but the majority of drainage systems use granular soil. This is particularly true in frost regions where it is necessary to have 3-6 ft of soil above the FML to satisfy the requirements for frost

FIGURE 26.29 Design methodology to estimate cover soil infiltration into the SWCR system. (Adapted from U.S. EPA, Requirements for Hazardous Waste Landfill Design, Construction, and Closure, EPA/625/489/022, U.S. Environmental Protection Agency, Cincinnati, OH, August 1989.)

penetration. In such cases, 1 ft of granular soil thickness can serve as the surface water collector. If good drainage materials are not available, if the site is too extensive, or if natural materials would add undesired thickness, a geonet or geocomposite can be used. The advantage of drainage geocom-posites is their higher flow rate capabilities over geonets or granular soils. All geocomposites systems have polymer cores protected by a geotextile filter. Although many of the polymers cannot withstand aggressive leachates, this is not an issue in a surface drainage collector where the only contact is with water. The crush strengths of the geocomposites are generally lower than that of geonets, but that too is not a problem in a surface water collector. The heaviest load the geocompos-ite would be required to support probably would be construction equipment used to place the cover soil and vegetation on the closed facility.

The design for the surface water-collector system is determined by an allowable flow rate divided by a required flow rate. Allowable rates for geocomposites are determined experimentally by exactly the same method as for geonets. The specific cross section used in the test procedure should replicate the intended design as closely as possible. For the required flow rate, Darcy's law or HELP36,37 can be used. Then the design-by-function concept is used to determine the DR, or FS.

required

26.4.5 Gas Collector and Removal Systems

Degradation of solid waste materials in a landfill proceeds from aerobic to anaerobic decomposition very quickly, thereby generating gases that collect beneath the closure FML. Almost 98% of the gas produced is either carbon dioxide (CO2) or methane (CH4). Because CO2 is heavier than air, it will move downward and be removed with the leachate. However, CH4, representing about 50% of the generated gas, is lighter than air and, therefore, will move upward and collect at the bottom of the facility's "impermeable" FML. If the gas is not removed, it will produce a buildup of pressure on the FML from beneath.

In gas-collector systems, either a granular soil layer or a needle-punched nonwoven geotextile is placed directly beneath the FML or clay of a composite cap system. Gas compatibility and air transmissivity are the design factors that must be considered. Methane, the most predominant gas, should be compatible with most types of geotextiles including PET, PP, and PE.

The thickness design should be based on gas transmissivity tests. Since water has a viscosity that is 1000-10,000 times that of gas, qallow for gas flow should compare very favorably with the results of a water transmissivity test. Alternatively, one could look directly at permeability coefficients where geotextile air flow is several orders of magnitude greater than the MTG-required values. In the test method, the geotextile specimen fits underneath a load bonnet. Then the load, equivalent to the cover soil, is added and gas is brought to the inside of the geotextile. The gas flows through the geotextile and into a shroud that goes on the outside of the flanges and registers on an air meter. The resulting applied stresses, gas pressures, and gas permeabilities are then recorded and, if necessary, converted into gas transmissivity. The allowable gas transmissivity is then divided by the required gas transmissivity to yield the DR, or FS.

Gas generation occurs over a period of 70-90 years, so gas-collector and removal systems must work for at least that long to avoid gas pressure on the underside of the cover.

Gas generation might also cause problems in "piggyback" landfills, landfills that have been built on top of one another. It is still unknown what happens to the gas generated in an old landfill after a new liner is placed on top of it. To minimize problems, the old landfill should have a uniform slope and possibly an accordian-pleated bottom cross section. Then the gas could escape from the underside and be collected from the high gradient side of the site.

As seen in Figure 26.30, the details of a gas collection system are quite intricate and yet very important to the proper functioning of the system.

FIGURE 26.30 Miscellaneous details of a gas-collector system. (Adapted from U.S. EPA, Requirements for Hazardous Waste Landfill Design, Construction, and Closure, EPA/625/4-89/022, U.S. Environmental Protection Agency, Cincinnati, OH, August 1989.)

26.5 SECURING A COMPLETED LANDFILL

This section describes the elements in a closure or cap system of a completed landfill, including flexible membrane caps (FMCs), SWCR systems, gas control layers, biotic barriers, and vegetative top covers. It also discusses infiltration, erosion control, and long-term aesthetic concerns associated with securing a completed landfill.

FIGURE 26.31 Typical geosynthetic cell profile. (Adapted from U.S. EPA, Requirements for Hazardous Waste Landfill Design, Construction, and Closure, EPA/625/4-89/022, U.S. Environmental Protection Agency, Cincinnati, OH, August 1989.)

Figure 26.31 shows a typical landfill profile designed to meet U.S. EPA's MTG requirements.38 The upper subprofile comprises the cap, or cover, and includes the required 2-ft vegetative top cover, 1-ft lateral drainage layer, and low-permeability cap of barrier soil (clay), which must be more than 2 ft thick. This three-tier system also includes an optional FMC and an optional gas control layer. The guidance requires a 40-mil thick FMC.

26.5.1 Flexible Membrane Caps

FMCs are placed over the low-permeable clay cap and beneath the SWCR system. FMCs function primarily in keeping surface water off the landfill and increasing the efficiency of the drainage layer. U.S. EPA leaves operators with the option of choosing the synthetic material for the FMC that will be most effective for site-specific conditions. In selecting materials, operators should keep in mind several distinctions between FMLs and FMCs. Unlike an FML, an FMC is usually not exposed to leachate; so chemical compatibility is not an issue. Membrane caps also have low normal stresses acting on them in comparison with FMLs, which generally carry the weight of the landfill. An advantage FMCs have over liners is that they are much easier to repair, because their proximity to the surface of the facility makes them more accessible. FMCs will, however, be subject to greater strains than FMLs due to settlement of the waste.

26.5.2 SWCR Systems

The SWCR system is built on top of the FMC. The purpose of the SWCR system is to prevent infiltration of surface water into the landfill by containing and systematically removing any liquid that collects within it. Actual design levels of surface water infiltration into the drainage layer can be calculated using the water balance equation or the HELP model.36 37

Errors in grading the perimeter of the cap often integrates (or cross-connects) the SWCR system with the secondary LDRS, resulting in a significant amount of water infiltrating the secondary detection system. This situation should be remedied as soon as possible if it occurs. Infiltration of surface water is a particular concern in nuclear and hazardous waste facilities, where gas vent stacks are found. A containment system should be designed to prevent water from entering the system through these vents.

In designing an SWCR system above an FMC, three issues must be considered5:

1. Cover stability.

2. Puncture resistance.

3. The ability of the closure system to withstand considerable stresses due to the impact of settlement.

26.5.2.1 Cover Stability

The stability of the FMC supporting the SWCR system can be affected by the materials used to construct the drainage layer and by the slope of the site. In some new facilities, the drainage layer is a geonet placed on top of the FMC, with the coefficient of friction between those two elements being as low as 8-10°. Such low friction could allow the cover to slide. One facility at the Meadowlands in New Jersey is constructed on a high mound having side slopes steeper than 2:1. To ensure adhesion of the membrane to the side slopes of the facility, a nonwoven geotextile was bonded to both sides of the FMC.

26.5.2.2 Puncture Resistance

FMCs must resist penetration by construction equipment, rocks, roots, and other natural phenomena. Traffic by operational equipment can cause serious tearing. A geotextile placed on top of or beneath a membrane increases its puncture resistance by 3 or 4 times. Remember, however, that a geotextile placed beneath the FMC and the clay layer will destroy the composite action between the two. This will lead to increased infiltration through penetrations in the FMC.

26.5.2.3 Impact of Settlement

The impact of settlement is a major concern in the design of the SWCR system. A number of facilities have settled 6 ft in a single year, and 40 ft or more over a period of years.5 The Meadowlands site in New Jersey, for example, was built at a height of 95 ft, settled to 40 ft, and then was rebuilt to 135 ft. Uniform settlement can actually be beneficial by compressing the length of the FMC and reducing tensile strains. However, if waste does not settle uniformly, it can be caused by interior berms that separate waste cells.

In one current closure site in California, a waste transfer facility with an 18-ft wall is being built within a 30-ft trench on top of a 130-ft high landfill. The waste transfer facility will settle faster than the adjacent area, causing tension at the edge of the trench. Electronic extensometers are proposed at the tension points to check cracking strains in the clay cap and FMC.

Settlements can be estimated, although the margin for error is large. Secure commercial hazardous waste landfills have the smallest displacement, <1.5%. Displacements at new larger solid waste landfills can be estimated at 15%, while older, unregulated facilities with mixed wastes have settlements of up to 50%.

26.5.3 Gas Control Layer

Gas-collector systems are installed directly beneath the low-permeability clay cap in a hazardous waste landfill. Landfills dedicated to receiving only hazardous wastes are relatively new and gas has never been detected in these systems. It may take 40 years or more for gas to develop in a closed secure hazardous waste landfill facility. Because the long-term effects of gas generation are not known, and costs are minimal, U.S. EPA strongly recommends the use of gas-collector systems.

Figure 26.30 shows details of a gas vent pipe system. The two details at the top left of the illustration show close-ups of the boot seal and flange seals located directly at the interface of the SWCR system with the FMC. To keep the vent operating properly, the slope of the closure system should

FIGURE 26.32 Water traps in a gas-collector system. (Adapted from U.S. EPA, Requirements for Hazardous Waste Landfill Design, Construction, and Closure, EPA/625/4-89/022, U.S. Environmental Protection Agency, Cincinnati, OH, August 1989.)

never be less than 2%; 5-7% is preferable. A potential problem with gas-collector systems is that a gas venting pipe, if not properly maintained, can allow surface water to drain directly into the landfill waste.

Figure 26.32 illustrates two moisture control options in gas-collector systems. Gas-collector systems will tolerate a large amount of moisture before air transmissivity is affected. Condensates from the gas-collector layer that form beneath the clay and FMC can also be taken back into the waste, since most hazardous wastes are deposited very dry.

26.5.4 Biotic Barriers

A biotic barrier is a gravel and rock layer designed to prevent the intrusion of burrowing animals into the landfill area. This protection is primarily necessary around the cap but, in some cases, may also be needed at the bottom of the liner. Animals cannot generally penetrate an FMC, but they can widen an existing hole or tear the material where it has wrinkled.

Figure 26.33 shows the gravel filter and cobblestone components of the biotic barrier and their placement in the landfill system. The proposed 1-m thickness for a biotic barrier should effectively prevent penetration by all but the smallest insects. Note that the biotic barrier also serves as the surface water collection/drainage layer. Biotic barriers used in nuclear caps may be up to 14 ft thick

Vegetation

Topsoil (60 cm)

Topsoil (60 cm)

FIGURE 26.33 Optional biotic barrier layer. (Adapted from U.S. EPA, Requirements for Hazardous Waste Landfill Design, Construction, and Closure, EPA/625/4-89/022, U.S. Environmental Protection Agency, Cincinnati, OH, August 1989.)

with rocks several feet in diameter. These barriers are designed to prevent disruption of the landfill by humans both now and in the future.

26.5.5 Vegetative Layer

The top layer in the landfill profile is the vegetative layer. In the short term, this layer prevents wind and water erosion, minimizes the percolation of surface water into the waste layer, and maximizes evapotranspiration, the loss of water from soil by evaporation and transpiration. The vegetative layer also functions in the long term to enhance aesthetics and to promote a self-sustaining ecosystem on top of the landfill. The latter is of primary importance because facilities may not be maintained for an indefinite period of time by either government or industry.

Erosion can seriously effect a landfill closure by disrupting the functioning of drainage layers and surface water and LCRSs. Heavy erosion could lead to the exposure of the waste itself. For this reason, it is important to predict the amount of erosion that will occur at a site and reinforce the facility accordingly. The Universal Soil Loss Equation shown below can be used to determine soil loss from water erosion5:

where X is the soil loss, R is the rainfall erosion index, K is the soil erodibility index, S is the slope gradient factor, L is the slope length factor, C is the crop management factor, and P is the erosion control practice.

Figure 26.34 can be used to find the soil loss ratio due to the slope of the site as used in the Universal Soil Loss Equation. Loss from wind erosion can be determined by the following equation:

100 200 300 400 500

Slope length (ft)

FIGURE 26.34 Soil erosion due to slope. (Adapted from U.S. EPA, Requirements for Hazardous Waste Landfill Design, Construction, and Closure, EPA/625/4-89/022, U.S. Environmental Protection Agency, Cincinnati, OH, August 1989.)

100 200 300 400 500

Slope length (ft)

FIGURE 26.34 Soil erosion due to slope. (Adapted from U.S. EPA, Requirements for Hazardous Waste Landfill Design, Construction, and Closure, EPA/625/4-89/022, U.S. Environmental Protection Agency, Cincinnati, OH, August 1989.)

where X' is the annual wind erosion, I is the field roughness factor, K is the soil erodibility index, C is the climate factor, L is the field length factor, and V is the vegetative cover factor. There are many problems in maintaining an agricultural layer on top of a landfill site, especially in arid or semiarid regions. An agricultural layer built on an SWCR system composed of well-drained stone and synthetic material may have trouble supporting crops of any kind because the soil moisture is removed. In arid regions, a continuous sprinkler system may be needed to maintain growth on top of the cap, even if the soil is sufficiently deep and fertile. A final problem involves landfills built on slopes greater than 3:1. Equipment necessary to plant and maintain crops cannot operate on steeper slopes.

Operators should contact their local agricultural extension agent or State Department of Transportation to find out what kinds of vegetation will grow under the conditions at the site. The impact of the SWCR system on the soil layer should also be studied before vegetation is chosen. Usually native grasses are the best choice because they are already adapted to the surrounding environment. Sometimes vegetation can overcome adverse conditions, however. At one site in the New Jersey Meadowlands, plants responded to excess surface water by anchoring to the underlying waste through holes in an FMC, creating a sturdy bond between surface plants and underlying material.

For sites on very arid land or on steep slopes, an armoring system, or hardened cap, may be more effective than a vegetative layer for securing a landfill. Operators should not depend on an agricultural layer for protection in areas where vegetation cannot survive. Many States allow asphalt caps as an alternative to vegetative covers. Some closures at industrial sites have involved constructing hardened cap "parking lots" on top of the cap membrane and clay layers. A chip seal layer over the asphalt prevents ultraviolet degradation of the pavement. These caps, however, need to be maintained and resealed every 5 years. At some sites, a fabric incorporated into the top of the asphalt minimizes cracking and water intrusion.

26.5.6 Other Considerations

Filter layers, frost penetration, and cap-liner connections are other factors to consider in designing the closure system for a hazardous waste landfill. Before using geotextiles for filter layers in closures, one should conduct pressure tests and clogging tests on the material. Freeze-thaw cycles probably have little effect on membranes, but their impact on clay is still not known. Because of this lack of knowledge, membrane and clay layers should be placed below the frost penetration layer. Finally, a cap membrane should not be welded to the primary FML. Differential settlement in the cap can put tension on the cap membrane. In such a situation, the seam could separate and increase the potential for integration of the surface water collection system into the LDS.

26.6 LINER COMPATIBILITY WITH WASTES

This section discusses chemical compatibility (resistance) of geosynthetic and natural liner materials with wastes and leachates. Even in a relatively inert environment, certain materials deteriorate over time when exposed to chemicals contained in both hazardous and nonhazardous leachate. It is important to anticipate the kind and quality of the leachate a site will generate and select liner materials accordingly. The chemical resistance of any FML materials, geonets, geotextiles, and pipe should be evaluated before installation.39

Chemical

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