Rectification of reservoir lining 1

[anjande]

We have an existing earthen raw water reservoir lined with LDPE liner sandwiched between two layers of sand( 100mm thick each) with precast concrete block lining fixed with mortar above it.This reservoir is undergoing huge seepage loss( either due to damaged precast blocks or LDPE lining). Please suggest a proposed rectification method with code references. Will a RCC lining of 100/150 mm thick over the existing precast block covers be sufficient to prevent seepage? If so, please suggest how a proper bonding can be ensured between the new concrete lining and existing precast blocks.

 

[Focht3]

The first thing you need to do is find the leak and understand what caused it. I think it's foolish to try to fix something whose failure isn't understood - that's a formula for a repeat failure...

And it's quite likely that the fix will be a lot cheaper than putting 4 to 6 inches (100 to 150 mm) of reinforced concrete over the entire pond. Unless, of course, you are certain that the site was unsuitable for the original design, and the sand/LDPE/sand/PCC block design was doomed anyway.

But I always worry about putting a rigid liner (PVC or reinforced concrete) where I expect the subgrade to move over time. Where is the site? What are the soil conditions? How large is the reservoir? How deep? Any special considerations?



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[cvg]

I agree with focht3. It seems that you should be able to find the leak in the liner and repair the liner. A leak in the block would not cause loss of water.

If I understand correctly, the block is a protective layer over the liner. Adding thicker concrete over the top of the block would not stop the leakage.

You might also be able to stop leakage using bentonite.

How much loss are you having?

 

[dicksewerrat]

Can you find the area of the leak? Can you empty the reservoir to work on it? I would find the leak first. then find out why the leak started. My guess would be movement of the PE and a big tear in it. A betonite based membrane could be overlayed on the offending area and replace the concrete protection.

 

[KRSServices]

After reading your thread, a few red flags appeared. How are you determining that a huge leak is present? Is the soils under the reservior very porous, such as sands or gravels or very low permeability such as clays? Are there failures in the berm or some other location that could help locate the location of the leak?





KRS Services

http://www.krs-services.com

 

[flexman]

I agree with the earlier posts.

How large is the reservoir? What is its configuration? What are the side slope ratios? How/where, etc. does the water enter and leave the reservoir? Under what conditions? What are the prevaling ambient conditions? Are there any strange chemicals or compounds in the water? Etc? Etc? (many, Many questions!)

I would not even consider re-lining the reservoir with a rigid, concrete lining system; it will be sure to leak over a period of time. I have been in the flexible membrane lining field for 40+ years and I can say that one of the largest markets that exist for flexible membrane liners are "cracked, leaking concrete reservoirs". Generally LDPE is not a good choice for a liner; HDPE or LLDPE would have been better choices. I would have predicted an early failure of LDPE due to its inherently poor environmental stress cracking resistance..

I would definitely re-line the reservoir with an appropriate flexible membrane liner/system.

There are many flexible membrane lining systems/products in the market which are good candidates for use in this fix.

Depending on the answers to some of my questions and the questions of earlier posters, the choice of which flexible membrane lining system/product would become indicated. Cost vs. long term performance is always a consideration.

Let me know if I can be of further assistance on this project; I will keep track of this post for a while.

Best Regards,
Flexman

 

[anjande]

Dear friends,

My sincere thanks to you all for responding to my thread and spending your valuable time.

There are two identical reservoirs side by side having common sloped bund in between, each having a plan size of 95m x 85 m approx. The side slope of bunds is 1.75 H : 1 V. The reservoir is holding water of approx. 5m height. The reservoirs are partly underground (about 2 m ) and partly over ground ( 3 m). The embankment is made of compacted clayey silt ( 20% clay and 50% silt) from Lignite mine overburden. Water input and output is through pumps. The site is located at a hot and extreme weather conditions.( Max temp 50 deg C in summer and Min 10 deg C in winter). No strange chemicals and compounds are there in the water retained. The area/site has suffered a major earthquake shock in 2001 (You must be aware of this earthquake in Gujarat, India), though it is not correctly known whether the seepage loss problem started exactly after the earthquake or not. This may be a reason for the tearing of the underlying LDPE liner – I guess.

The earlier lining was with LDPE with sandwiched sand cushion, over which Concrete pre-cast blocks were laid with mortar joints.

Now, to prevent seepage and to have no disturbance to the existing lining ( for faster rectification), I have the following proposal.

1. Keep the present lining and protection as it is, with minor local repair of the damage blocks, if necessary.

2. Provide a fine sand layer over it and lay a new HDPE lining (0.75 mm to 1.0 mm) above the new sand layer with proper anchorage and joints.- What should be the sand layer thickness ???

3. Lay one more fine sand layer above the HDPE liner - What should be the sand layer thickness ???

4. Provide the brick lining with mortars to fix above the new HDPE lining as new protective cover.

Your comments/observations on my proposal and a prompt reply to this thread will be highly appreciated.

Best Regards

Anjan De

 

[Focht3]

Generally, the proposed fix looks reasonable. But I have a few concerns.

First, you don't know how much damage has been done to the soils beneath the existing liner system. Since you described the problem as a "huge seepage loss", I'm worried about soil erosion and piping beneath the existing reservoir. This should be thoroughly investigated before proceeding with any repairs. Have you evaluated the dispersive properties of the soils at this site?

Second, I'd leave out the grout between the bricks. It makes the liner too rigid and brittle, and can mask bad problems until they become catastrophic in size. And the grout won't really reduce the seepage enough to offset the other risks -

Keep us posted on the progress of the investigation and repairs, please. Your problem sounds very interesting.



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[flexman]

I agree with the earlier posts.

I would expect the sand to slide off of the slope walls which have a 1.75:1 ratio. (pretty steep!) Some HDPE liners are available with a rough, "textured" surface which would minimize this, but the sand should be assumed to settle to its natural angle of repose, in a saturated state. (?)

I would suggest a minimum of 6" of sand over the existing system. I would suggest that great care be given the grading of the sand under the new liner to insure that it is slightly sloped from a low point or low line in the center of the reservoir towards the sides of the reservoir to prevent gas from becomeing trapped under the new liner.

It is my opinion that a 0.75 mm thick HDPE liner is too thin for reliable long term robust performance. This is somewhat subjective.

As an alternate to the over-cover of a layer of sand and "brick lining with mortars", I would suggest you consider a thicker, more robust liner such as 60 mil. (1.5 mm) (or preferrably thicker) thick HDPE which does not have any over-cover. Of course you would want to be sure that the supplier of the HDPE liner will approve of their liner being left exposed to the elements (water, weathering, etc.);proper compounding should provide satisfactory resistance to the elements. It is done all the time in the USA.

If you definitely want to put a protective cover over the new liner, then I would suggest that you consider placing a thicker layer of compactable sand or base rock mixture over same, and leave out the "brick lining with mortars". Remember that the overburden must be planned to assume its natural angle of repose, in a satruated condition (which will vary, based on the specific soils properties of the overburden locally available). This could result in the thickness of the overburden to be a little thicker at the base of the slopes than at the tops of the slopes. If available, I would select the "textured surface" type of HDPE for the slopes, to help hold the overburden from sliding down the slope. Certainly the "brick lining with mortars" add a major component of liner protection and if you can handle the cost of same, then that is a value judgement you must make.

I agree with the earlier post that you should expect the mortar lines of the "brick lining with mortars" to fracture over time.

Well, keep us posted - - -
Best Regards,
Flexman

 

[envscout]

Have spoken with someone at Quality Lining Company in the past and they were very helpful. Don't have their website address or contact info readily available but should be able to find them through a search engine.

 

[layfieldman]

There are also other membranes available besides all the polyethelyne derived materials (HDPE, LLDPE, LDPE) such as polypropylene (which can come reinforced with a scrim as well as approved for potable water use) and pvc and it's alloys as well. These other choices are much more flexible then HDPE and have much better elastic properties, which will help prevent cracking if there is more settlement.