DESIGN METHODS & CONCEPTS

The primary purpose of the geocomposite in a leachate collection system (LCS) is to provide in-plane transmission of leachate that reaches the geocomposite after permeating through the overlaying waste and protective soil cover. The function of the LCS is to convey leachate while controlling the head on the underlying geomembrane. The maximum head must be smaller than the thickness of the drainage system and must be smaller than the required maximum value, typically 0.3 m (1 ft).  Since the geocomposite is commonly designed to convey flow in an unconfined condition (i.e., the liquids are completely maintained within the composite), the buildup of the head is very small.  

 

In a typical landfill cell configuration, the LCS consists of two sections with two different slopes: a steeper upstream side slope; and a flat downstream cell slope.  Figure 5.1 shows a typical cross-section of a landfill leachate collection system with two slopes:

LandfillLeachate_Figure5.1.png

FIGURE 5.1 – TYPICAL LANDFILL LEACHATE COLLECTION AND REMOVAL SYSTEM CONFIGURATION

The required transmissivity for geocomposite in the upstream and downstream section can be calculated as follows:

​θ       = 

up_req

q  L 

h

up

sinβ

up

EQUATION 5.1

sinβ

q  (L   +L     )

​θ         = 

down

h

up

down

down_req

EQUATION 5.2

WHERE

θ            = Design required transmissivity at upstream section

θ                 = Design required transmissivity at downstream section

L      = Length of the liquid collection layer in upstream section

L           = Length of the liquid collection layer in downstream section

β      = Slope for the upstream section

β          = Slope for the downstream section

up

down

up

down_req

up_req

down

The ultimate transmissivity for the two section can be calculated using equation (5.3) and (5.4)

θ       = θ       × RF  × RF  × RF  × RF  × FS

up_ult                 up_req                      in                    cr                    cc                   bc

EQUATION 5.3

θ         = θ         × RF  × RF  × RF  × RF  × FS

down_ult               down_req                     in                    cr                    cc                    bc

EQUATION 5.4

WHERE

θ            =

Specified 100-hour transmissivity of a candidate geocomposite layer at upstream section

θ                 = 

Specified 100-hour transmissivity of a candidate geocomposite layer at downstream section

RF      = Reduction factor of intrusion

RF      = Reduction factor of creep

RF      = Reduction factor of chemical clogging

RF      = Reduction factor of biological clogging

FS = Overall factor of safety

up_ult

down_ult

in

cr

cc

bc

GRI-GC8 does not make recommendations for the impact of temperature on the transmissivity.  It is the responsibility of the user to perform additional testing to calculate a suitable reduction factor for temperature where long term exposure to high temperatures is expected.

LANDFILL LEACHATE

COLLECTION & REMOVAL DESIGN

 

LANDFILL LEACHATE

EQUATION SHEET

Input Parameter Values

q   =

h

(m/s)

Impingement rate

Upstream

Downstream

β     =

up

o

Slope for the upstream section

L     =

up

m

Length of the liquid collection layer in upstream section

RF    =

in

Intrusion Reduction Factor

RF    =

cr

Creep Reduction Factor

RF    =

cc

Chemical Clogging Reduction Factor

RF     =

bc

Biological Clogging Reduction Factor

FS=

Overall Factor of Safety

0.38 RAD

β         =

down

o

Slope for the downstream section

L         =

down

m

Length of the liquid collection layer in downstream section

Permeability of the adjacent soil

RF    =

in

Intrusion Reduction Factor

RF    =

cr

Creep Reduction Factor

RF    =

cc

Chemical Clogging Reduction Factor

RF     =

bc

Biological Clogging Reduction Factor

FS=

Overall Factor of Safety

0.021 RAD

SOLUTION (Upstream)

θ           =

up_req

Design required transmissivity at upstream section

m  /s

2

θ           =

up_ult

m  /s

2

Specified 100hr transmissivity of a candidate geocomposite layer at upstream section

SOLUTION (Downstream)

θ             =

down_req

Design required transmissivity at downstream section

m  /s

2

m  /s

2

Specified 100-hour transmissivity of a candidate geocomposite layer at downstream section

θ            =

down_ult

Chemical Clogging Reduction Factor