Title: Geomembrane Slack

Geomembrane Slack

It is essential to include sufficient extra material in a geomembrane containment to accommodate thermal contraction.

Geomembrane materials expand and contract as the temperature changes. Thermal contraction can cause extensive damage to geomembranes if it is not taken into account.

Every geomembrane material requires sufficient slack during installation to be able to contract to the lowest temperatures expected in service without damage. In order to ensure that there is enough slack at low temperatures (in the winter), we must adjust the amount of slack installed at the warmer temperatures of installation (typically in the summer). Each geomembrane material has a characteristic thermal expansion/contraction coefficient that lets us calculate the amount of slack that will be required.

If a geomembrane material is installed without sufficient slack to attain the design low temperature, then the material will tighten until something “gives.” In many installations this means that the liner will be stressed to failure. Environmental Stress cracking failures in HDPE are commonly initiated by the stress of thermal contraction. Occasionally the anchor trench will pull out to relieve stress, or the corners of the liner will “bridge” or “trampoline” at cold temperatures. These stresses, if not corrected, can cause premature failure of the liner. The basic calculation for placing slack in a liner is:

Coeff Thermal Expansion x (Tinstall - Tdesign) x Distance x F.S.

We multiply the coefficient of thermal expansion/contraction by the difference between the installation temperature and the design low temperature, and then multiply by the distance between the points where we are measuring slack, multiplied by a factor of safety.

Layfield has found that too often a factor of safety is not included in slack calculations. In fact, many installers install lining materials with a complete disregard for thermal slack. Not placing slack in a liner may be suitable in other temperate climates, but in northern climates it is essential. We strongly recommend using a factor of safety of about 1.5. The table shown lists the required amount of slack for an exposed geomembrane in a range of factors of safety from 1.0 to 1.5.

In order to accurately measure thermal slack we use the actual temperature of the geomembrane, not ambient air temperature. Solar heating has been shown to raise the temperature of black lining materials to as much as 80ºC (176ºF) on a hot, clear day.


Polyethylene Slack Allowances
Intallation Sheet Temperature Design Low Temperature Slack Required over 23 ft Slack Required over 100 ft
80°C (176°F) -40°C (-40°F) 2.6" - 4.0" 12" - 17"
70°C (158°F) -40°C (-40°F) 2.4" - 3.6" 11" - 16"
60°C (140°F) -40°C (-40°F) 2.2" - 3.3" 9.6" - 14"
50°C (122°F) -40°C (-40°F) 2.0" - 3.0" 8.6" - 13"
40°C (104°F) -40°C (-40°F) 1.8" - 2.6" 7.7" - 12"
30°C (86°F) -40°C (-40°F) 1.5" - 2.3" 6.7" - 10"
20°C (68°F) -40°C (-40°F) 1.3" - 2.0" 5.8" - 8.6"
10°C (50°F) -40°C (-40°F) 1.1" - 1.7" 4.8" - 7.2"
0°C (32°F) -40°C (-40°F) 0.9" - 1.3" 3.8" - 5.8"
-10°C (14°F) -40°C (-40°F) 0.7" - 1.0" 2.9" - 4.3"
Please note the low temp installation restrictions on Polyethylene liners. See the Layfield Tech Note on Cold Temperature Installation.
-20°C (-4°F) -40°C (-40°F) 0.4" - 0.7" 1.9" - 2.9"
-30°C (-22°F) -40°C (-40°F) 0.2" - 0.3" 1.0" - 1.4"
-40°C (-40°F) -40°C (-40°F) 0 0

It is also important to understand the operating conditions of the lining material. If a liner is backfilled, or has a permanent liquid level the amount of thermal slack required can be reduced. The more of the liner that is exposed, the more thermal slack must be incorporated. A pond that has a 3 foot (1m) minimum liquid level will almost never have the submerged lining material go below 0C (32ºF) on the base. The requirement for slack in a backfilled liner is also reduced. Backfill “cushions” thermal shocks and prevents liners from reaching extreme low temperatures due to wind chill or other means. Alternately, in landfills, the backfilled liner can be subject to heating due to the decomposition of the landfill contents. In most cases a backfilled liner requires less slack. The flexibility of a lining material also affects the amount of slack used. Flexible, unsupported materials such as PVC and PP require less slack than stiff materials such as HDPE or supported materials. This is because the flexible, unsupported materials can stretch locally (up to 100%) to accommodate thermal contraction without damage, while very low extensions (as low as 2% at low temperatures) will damage both HDPE or supported materials.

Slack perpendicular to the seaming direction is installed by adjusting the amount of overlap while seaming. Slack can be incorporated in the longitudinal direction at toe tie-in seams, or in a center tie-in seam. Slack can also be released from the anchor trench prior to backfilling. It is important to prepare tie-in seams, or to backfill anchor trenches, at the coolest part of the day. The worst possible condition for a lining material is to be exposed, with no fluid in the pond. Typical slack placement methods do not include enough slack for a temperature of -40ºC (-40ºF) in an empty pond. In all cases a minimum water level (or backfill) would be required for a pond that must over-winter prior to operation.


PVC Slack Allowances
Intallation Sheet Temperature Design Low Temperature Slack Required over 100 ft
80°C (176°F) -40°C (-40°F) 10" - 15"
70°C (158°F) -40°C (-40°F) 9.2" - 14"
60°C (140°F) -40°C (-40°F) 8.4" - 13"
50°C (122°F) -40°C (-40°F) 7.6" - 11"
40°C (104°F) -40°C (-40°F) 6.7" - 10"
30°C (86°F) -40°C (-40°F) 5.9" - 8.8"
20°C (68°F) -40°C (-40°F) 5.0" - 7.6"
10°C (50°F) -40°C (-40°F) 4.2" - 6.3"
0°C (32°F) -40°C (-40°F) 3.4" - 5.0"
-10°C (14°F) -40°C (-40°F) 2.5" - 3.8"
Please note the low temp installation restrictions on Polyethylene liners. See the Layfield Tech Note on Cold Temperature Installation.
-20°C (-4°F) -40°C (-40°F) 1.7" - 2.5"
-30°C (-22°F) -40°C (-40°F) 0.8" - 1.3"
-40°C (-40°F) -40°C (-40°F) 0