# 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:

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

# 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)