Leaking embankment 1

[Eduardo5000]

Hi all! There is an old embankment 3H 1V, build circa 40's. Its size an heigth have been changed with the time, due to it moves down, it protects some lowlands and suffers the subsidence effect. Because of that, its internal structure is not what it was in the past. There are sections that have no impervious core or internal drain, and for the lasts 4 months it has a very small "spring" on its toe that can be easily seen, I saw it and I estimated that around 1 litre/5 min goes out.
I know that every dam has its own hydraulic equilibrium, but additionally I know that the internal slope of the dam has to be dry. If the pore pressure increases, shear strenght will decrease and the slope could seen affected (specially if an earthquake happens).
So, this week we will perform some CPT tests in there, and all the readings that have to be taken will be taken. I would like to know if there is something very important that can not be forgotten (something I should not miss), we already have piezometric data that is measured all the time on the site, I haven't seen it but I will. This will be my first CPT experience, people who work with me have performed lots of CPT's, but anyway if there is anybody who can give me an advice I will really appreciate it.
Probably I did not give enough details, so let me know what is missing. Thanks

 

[Focht3]

Small springs have a nasty habit of growing up. Your existing "small" flow is enough to be a concern, particularly if it is fairly concentrated.

We do need more data. Where is the project located? (Don't tell me a marsh - I know that already!)

How high is the embankment? Any slope failures in the area of the leak? When (month) does the embankment face the highest water level?

Be careful punching holes in a leaking embankment! Grout the holes - completely - if you must use CPT. Otherwise, you leave a weak point for future seepage and embankment failure.

Or you can turn the CPT holes into standpipe piezometers. The more piezometric data, the better!

 

[Daryoush]

The maximum height of the embankmant and the maximum depth of water behind the dam would usually be two of the the controlling factors in the downstream seepage. Embankment dams have failed in the past as a result of internal erosion (piping). Piping failure starts with washing out of soils particle through narrow flow paths (pipes) which get enlarged as more soils particles are washed out. This process leads to caving and collapse of a zone and uncontrolled release of large flows under upstream pressure heads and failure of the dam. Perform a thorough reconnaissance of the downstream and upstream slopes as well as thalwegs, and record any signs of irregularities, animal borrows, etc. Large settlements (distortions) may cause development of fissures which would then lead to other problems such as leakage. Has the seepage flow rate changed over time, and do you have records of water level history (even within the last 12 months) behid the dam? I suggest installation of a flow measurement facility which would also indicate outwash of any soil (like a holding tank). Although it is good to check piezometric head, piezometers may not necessarily provide the indicative data where piping is a problem. Please provide more information about the project if you would like additional asistance.

 

[VAD]

Is this the first time the dam is to be investigated for its composition and the characteristics of its underlying soils?.

As pointed out by Focht3 and Daryoush the spring observed should be of concern as it may be a sign of pending problems especially if it is related to seepage along the base of the dam or through some pervious foundation component.

You may need to address this asap by using a bandaid until you complete your investigation and analysis. Filter fabric and rocks over the spring location should assist in keeping any fines and providing some weight to help in avoiding piping.

In addition to CPT tests I would want to obtain samples of the dam and foundation materials to correlate with the CPT results and also for physical examination and possible lab testing.

The test holes as indicated by Focht3 are to be backfilled by grouting immediately following completion of drilling. I would do CPT first and follow later with conventional sampling, if possible.

The CPT should give you a wealth of data for evaluation and assessment of the subsoil condtions.

 

[Eduardo5000]

Hi again, thank you very much for your answers!!!!

This is an old embankment that doesn't have a homogeneous design, originally it was built during the 40's, following some techniques that were developed by then (wood barriers and some walls). With the time its size has had to be increased (and an embankment shape appeared) in order to protect the area, so, the original embankment started to be covered by different materials which have been tested lots of times. The thing is that is very big, so is very hard to keep an eye everywhere.

Focht3, the embankment is located in a lowland site, and actually, it is part of the lowland, it is constantly moving down (due to the subsidence), and the source of water is the sea (infinite). If you see the embankment from downstream (where we are 6 meters below sea level) it is 8 meters high, and upstream it is 2 meters over the sea level.

At the moment I can't tell you where the embankment is located, I could be fired!!! but I hope that saying that it protects a big area of getting flooded by sea water, you can have a clearer idea.

The slope is quite big, the cross section of the embankment is very large ( it could has around 110 m upstreamtoe-downstreamtoe). It has a few kilometers in length. It only presents a few stability problems due to some stormwater drains which are a bit deteriorated, but in general it looks fine.

The water level that the embankment has to face, changes a lot. Because it depends on the sea tides, what I can tell you is that the sea is very quite there, no waves at all.
I know that they have pizometric data, and the intention is that more pizometers have to be installed. Hopefully I'll go to the site this week and I'll check all of it.

I really appreciate the information you gave me, specially that refers to punching holes and internal erosion (piping). I hope I can give you more details, ask me if you want! Thanks indeed VAD, Focht3 and Daryoush!


E5000

 

[BigH]

Eduardo's comments on the age of the dam and confirmation is what I would expect - a heterogeneous structure, most likely, built by the locals, maybe, with whatever material they could get their hands on. Some nasty failures in Upstate NY, if I remember over the years - Teton started as a small spring (too, if I remember). The springs bother me. Focht3 is right about the grouting - but, as I had experience with an artesian head - I couldn't grout up the damn hole no matter how 'ard I tried!

Eduardo - I take it you might be in Mexico or SA - you really should be getting an experienced earth dam geotechnical engineer to review the data you have, to help you set up a program of investigation, and to assist (if not take over and do) a remedial design. This could turn into the "soup" if you aren't careful. Take care and protect (1) the public and (2) yourself - then the company.

 

[VAD]

It looks like your concern is as a result of rising sea levels which would have an impact on the stability of the structure. Is it subsidence or rising sea level?

I am sure you have a monitoring program in place so that the upstream end can be reviewed when the tide has receeded from the slope. If there are no perceptible problems then it is difficult to convince any one to do rehabilitation works.

Based on the situation described it would seem to me that monitoring, on going analysis of stability, repairs to isolated failing areas and a strategy for a second line of sea defence is what will be required for some time since renewal of infrastructure costs money and can be a highly political issue.

What are low tide and high tide levels with respect to the level of the lowland on the downstream side. Is your embankment on a straight alignment or are there many bends. What is the wave height due to wind

 

[Daryoush]

I suggest, as a first priority, sample the water from the downstream source of leakage and run tests (through a qualified lab)to identify source and sediment content. At the same time, install an appropriate flow meter and regularly monitor the seepage flows. You can also teach a local person to perform regular monitoring. A simultaneous sea level measurement would be necessary.
Although it is difficult and time consuming, thorough reconnaissance of dams provide very useful data in safety evaluation of the facility and understanding of the problems to be addressed. By the description of the project, it seems to me that there is a strong possibility that large displacements have led to fissures and leakage through fissures. The dam in my opinion should be surveyed accurately (relatively low cost). The present shape should be compared with the old shape if any staged as-built plans are available. Also recommend periodical precision surveys to monitor any ongoing vertical and horizontal movements. The monitoring data would be very useful in determination of the nature and hopefully location of the problem.
If the problem is narrowed down to piping, tracing of the piping channel may be performed by dye injection. Once you lacate the flow path, it could be grouted or blanketted if necessary.

 

[Eduardo5000]

Hi all. Thanks for your answers, I'm checking all the stuff you have told me, and all your advices are helping me a lot, specially because there are a few new terms that I didn't know.

In fact, we have subsidence there. The land moves down at a rate of 2 to 5 cm per year (I know is a lot, but it really happens), because of that, the shape and size of the dam is modified at least every ten years. The sea there is very quite, I'm pretty sure that waves and current effects are not affecting us ( a thick layer compounded by big rocks protects the dam upstream). However, I would like to compare piezometric data with tides levels, I am sure they have all of that data there.

BigH is right, the dam started to be built by locals, when the problem wasn't totally understood. Later, they realized what was happening and lots of professionals designed an effective embankment. At the moment there are many people working on that, but the thing is that not many advicers have been hired, and I think that most of the times there are people who help you to see what you can't, even if the problem has been in front of you. I'll take care of people and myself as you said.

Daryoush, I will suggest what you said (water test), I think that they have asap to perform a precision survey.

So, thank you very much indeed! I hope to see you soon.

E5000

 

[Focht3]

Is the subsidence due to groundwater withdrawal, or just settlement due to the added load? I doubt that rising sea level is haveing much of an impact, although it certainly isn't helping things.

Keep us posted on what you find - and try to get your boss to let us know approximately where the site is located. The additional information may help us to help you -

 

[Eduardo5000]

Hi Focht3, hope you are ok. In fact the subsidence is due to the withdrawal of water, it has been reduced but it won't stop.
There is something interesting here, the dam is an homogeneous block compounded by a CL. There is no impervious core or something like that. The material is very dense and well compacted, it doesn't give the impression that it can leak (it may do it of course).
People I work with, said that some CPT's will be performed, all they want to know is the pore pressure. I think, that we can do SPT before, then we can measure some properties using CPT, what do you think? I am telling you this, because the material is very hard. Are there some parameters of soil hardness that indicate the usefulness of CPT? is there any http address where I can get this? Additionally, one of them said that we could just drill and then use the CPT, is that OK?. Do we need to recover some samples (using SPT)? will be there significants? Thanks for the tip about the tides. Hope to see you soon.

E5000

 

[Focht3]

You do need samples to test. I strongly prefer Shelby tube samples to SPT - high quality tube samples will give you more testing options and you can get a better look at the material. You don't need SPT blow counts - CPT data will give you much better strength data.

The CL classification concerns me. You need to run crumb tests on a large number of recovered samples from the dam/levee - and the soils directly beneath the embankment. If at least 30% of the crumb tests show the CL clays are dispersive, you may have a problem with dispersive clays.

Anyway, let us know what happens -