Existing Wastewater Lagoon Compacted Clay Liner Erosion Issues

[Allan17]

The existing lagoon was constructed in 1982 and is currently operational receiving wastewater flows from residential and industrial developments. The lagoon existing containment berms were constructed as compacted clay liners with compaction of 97% or higher standard proctor density. The noted the composition of the berm core is fairly uniform comprised of medium to high plasticity clays.

Since the Lagoon construction the compacted clay liner banks have been eroding and sloughing resulting in vertical faces of up to 1.5m in the most severe locations. The majority of the lagoon banks exhibit vertical faces in the range of approximately 0.5m to 0.75m.

A geotechnical investigation was completed for this lagoon located in northern Alberta. The causes of erosion in his report were stated as follows:

The sloughing affecting the interior side slopes of the storage cell and facultative cell containment berms is believed to be a result of surfacial erosion, in part, by wave action. The exposed faces of the berms in un-armored sideslope areas are comprised of silty clay and clay soils with a fair to poor resistance to erosion. The erosion is exacerbated by periodic/annual drawdown and/or emptying of the cells which: a) eliminates a hydraulic pressure acting as a toe load supporting the side slopes, b) leaves the exposed slopes saturated, heavy and weak, and c) leaves a relatively high residual pore pressure within the clay which is slow to dissipate. The combined affects are resulting in the slumping of the interior slopes. The slumping is resulting in the formation of near vertical faces. This in-turn is developing further instability of the interior sideslopes (in unarmored areas) in the form of tension cracking which, eventually, will lead to further slumping. The slump debris will tend to be washed away by wave action as the cells are once again refilled.

The project requires to rehabilitate the clay lined banks. The first step proposed is to bench 1.4m steps into the existing clay lined berm and rebuild it using clay and recompacting at 150mm lifts to a 3.5:1 sideslope.

The armouring options for the rehabitated sloped surface is where I require additional input. Many option exist for armoring but I am comapring two currently. The first option is riprap which is a tried tested method of erosion control but is very costs in this area since no locally available riprap exists. Contractors are also charging a heavy sum to haul the material in and place it.

The inovative option is to fine grade the slope berm surface with 50mm sand after the surface has been rebuilt to 3.5:1 with compacted clay. The next step is to install 80mil HDPE liner on top of the sand from top of bank to bottom of bank and 5m into the lagoon contstructing an anchor trench at the top of berm and bottom of lagoon floor. The majority of the lagoon floor would be left as compacted clay liner but the banks would be protected with HDPE liner. Can anyone comment on using HDPE as a erosion armoring tool in this application. The HDPE including bank recampaction can be installed for $950,000 where as riprap with compaction is $2,031,750. I am aiming for a solution that works to mitagate errosion and is also cost effective. The storage cell dimmension is 375mx375m and 2.7m depth from top of berm to lagoon bottom. The high water level is 2.2m deep. The lagoon becomes full every 6months and is drained within 3 weeks.

Thanks you in advance for your input. Your help is appreciated.

Allan

 

[civilperson]

Rip rap is more common and probably as effective for erosion control from wave action. The costs of rip rap should be less than liner at any distance from the source.

 

[Allan17]

civilperson... I have received quotes from contractors in that area for both options and the above price is what I have received. If rip rap was the least amount then the HDPE option would not have be evaulated as an option. Has anyone seen HDPE been used as bank armoring for swmf ponds or lagoon applications?

Thanks

Allan

 

[cvg]

with a lagoon of this size, wave action (for only 3 weeks out of the year) would seem to be very minimal. It sounds like the primary cause of your problems is slope stability due to rapid drawdown of the water level. Is the 3.5:1 flatter than it was before? If so, has a stability analysis been done to calculate the FS of the new slope? Perhaps you could get by cheaper by flattening the slope and foregoing the bank lining alltogether. I would go back to your geotech and see if perhaps the interpretation of the actual failure mechanism has more to do with stability than it is with erosion.

 

[Allan17]

The lagoon storage cell is filling up approximately 370mm/month until it reaches a high water level of 2.2m. Once 2.2m is attained (approximaetly every 6 months)the lagoon is fully drained. Wave action is occuring throughout the filling process. The geotech is currently preforming slope stability calculations. The 3.5:1 is the same slope as previous. Good suggestion ... flatter slopes would help but it would require hauling in additional clay from offsite and would reduce the storage cell volume. I hope this clarifies the issue.

 

[TDAA]

If you are just trying to protect the slope, the 80 mil is way overkill, in my opinion. Also, in my experience, the HDPE is not as UV stable as the LDPE or VLDPE materials. I would go with something along the lines of a 40 mil VLDPE material unless you are getting a rejected batch (for really cheap) that did not meet QC specs. I have used 40 mil VLDPE as the primary liner in a leachate lagoon, and do not feel there would be any reason you could not use it in this case.

Also, I would forgo the sand. Just be sure the surface is smooth, and you should be OK from a puncture point of view. If you were providing drainage for potentially saturated materials, you would need a piping system to remove accumulated water.

 

[GabionGuy]

What thickness and size of rip rap were you looking at? A gabion mat revetment may be 3rd option for you. Depending upon the thickness of your rip rap layer, you may be able to get a similar level of protection, without the UV issues, using less material with the gabion mats.