Erosion protection with Geomembranes

[Sparticus17]

Project Background

I am working on a project located in northern Alberta Canada. The current application is an existing wastewater lagoon constructed in 1982. The existing lagoon was constructed utilizing compacted clay liner medium to high plasticity (clay berms and clay bottom). A recent geotechnical investigation was completed on the existing berms to confirm the construction integrity and permeability of the existing lagoon berms. The study concluded the embankment core was constructed to an equivalent standard as that required for an earthen liner which exhibited a very low permeability (well below what is required under current standards). The berms within the storage cell were originally constructed to 4:1 sideslopes.

Site Investigation

All berms within the storage cell have been severely eroded leaving on average approximately 1:1 sideslopes throughout the storage cell. The wave action is predominantly acts on the east berms. A few years after the original construction in 1982 riprap armour was added to the east bank where the majority of the wave action is present. The remaining berms currently have not been armored and require rehabilitation.

The sloughing affecting the interior sideslopes of the storage cell containment berms is believed to be a result of surfacial erosion caused, in part, by wave action … the erosion is exacerbated by periodic/annual draw-down and/or emptying of the cells which:

1. eliminates a hydraulic pressure acting as a toe load supporting the side slopes;
2. leaves the exposed slopes saturated, heavy, weak; and,
3 leaves a relatively high residual pore pressure within the clay which is slow to dissipate.”

The traditional method for rehabilitation would be to restore banks to 3.5:1 or so and install (riprap,gabion baskets, concrete matting, etc) with a geotextile. The problem with this option is there is very little locally available 12" (300mm) riprap. The local gravel and riprap is extremely expensive to use as bank protection ~$50/m3 supply (not including install) and is still poor quality stone.

We have compared the HDPE liner option to the armoring the banks with riprap, gabion mats, armorflex, etc. and found HDPE liner to be a cost effective option.

I should mention the wastewater volumes flowing into the lagoon will be 50% light industrial and 50% residential.

I have a few questions below pertaining to the feasibility of using HDPE liners as bank protection. The questions below and advice I seek deal with installing the HDPE liner only on the sideslopes of the lagoon and NOT on the bottom as well. The installation will be from toe of berm to top of berm with anchor trenches installed at toe and top of berm.

1. Any problem with the liner dislodging or being damaged by wind (the city seems to be very windy at times)? Can vents be used to solve this problem? Are there any long term effects of wind on HDPE liner (fatigue)?
2. Will the liner float under certain circumstances? Is that a negative affect? Is there a way of preventing the liner from floating (i.e. a construction method or a design element). The groundwater table is about 2m below lagoon bottom and is a non issue.
3. The wastewater lagoon will be drained every 6 months about 2.7m water depth within three weeks. Does this rapid drawdown cause any issues with HDPE liners?
4. Example of wastewater lagoon projects that have used either HDPE liners or Environ liners preferably in cold climate applications along the banks only of lagoons.
5. Has anyone else encountered this problem before and if so what was the solution?

Thanks for you comments.

 

[Stego1]

Sparticus17,

I have worked on a number of lined lagoons and landfills, some in very windy parts of the UK!

It may be worth your while having a look at the technical information/technical documents bit of

http://www.geosynthetica.net

which may have some useful info for you.

My thoughts on your questions are.....

1) Wind problems will only really be a problem during installation and if you use a good installation contractor and ensure there are no unsecured edges that the wind can get under you should be OK. Best to specify no installation when wind speed is above a certain value though and weighting of unwelded seams with sand bags. Also ensure the anchor trenches are substantial and set back from the crest of teh slope, say 500mm to 1000mm deep and wide with full wrap around of the HDPE. Long term effects of wind are negligible in my experience, especially if the liner has very little movement. Sunlight can be a problem, check with manufacturer on this point.

2) HDPE has a density of about 930kg/m3 which is slightly less than water. However once anchored it shouldn't try to float. Make sure there is as little air between the liner and slope as possible, possibly install vents between the crest and the anchor to allow air to escape as water rises. (these would have to be properly formed to avoid subsequent tearing)

3) Rapid drawdown won't cause a problem with the HDPE itself, however there may be some reaction in the underlying med-high plasticity soils due to unloading especially if it is left drained for a long time.

4) We don't get the extreme cold that you probably get in Canada and so I have no experience of the long term effect on HDPE although I would imagine it may make it brittle. However we used to specify no welding of the liners below 0 degrees C (or above 35 degrees) as the seams would fail. Also no welding during periods of high humidity or rain/snow.

5) I haven't experienced degradation of slopes as you describe before, or been involved in this kind of remediation so my comments are only based on construction of new lagoons. It may be worth putting some instrumentation into the berms to get an idea of how conditions vary within them (pressure cell, piezo) these can be connected to a datalogger which can "phone" the data to you once a day or so giving you almost constant readout......much better than manual readings once a week!!

I hope this helps