Leaking embankment
Leaking embankment
(OP)
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
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





RE: Leaking embankment
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!
RE: Leaking embankment
RE: Leaking embankment
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.
RE: Leaking embankment
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
RE: Leaking embankment
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.
RE: Leaking embankment
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
RE: Leaking embankment
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.
RE: Leaking embankment
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
RE: Leaking embankment
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 -
RE: Leaking embankment
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
RE: Leaking embankment
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 -