Hi dear researchers,
First I want to know what the essential problem with filter contamination into clay core (or vice-versa) in earth/rock fill dams ?
please find the attached image, which in it, you see a filter material hill near to clay core. as you see the filter material are spread (to a distance) on a compressed core clay layer. Is this kind of placement will lead to a big issue (problem for stability, extent pore water pressure). and in this dam (image) I saw there's no geomembrane or something to cover the haul when trucks move from other layers to core.
could you explain the problems which may occur (with this working style) for the dam.
thanks in advance
Dear Hamed,
You know very well the clay core is to be impermeable so that there would not be any seepage at center of the core area. If there is a chance for seepage then definitely pore water pressure is going to be developed and questionable on the stability of the dam. I recommend you to evaluate the compaction characteristics of the clay material to be used in the dam and choose proper moisture content for laying the core layer. Also i recommend you to study the chemical characteristics of clay core, mainly to see weather the material is swelling type or not. If it is swelling type clay then proper precautions to be taken. Avoid using swelling type of clay for construction of the core.
Dear Karthigeyan,
Thanks for your respond. As you see in picture there's a distance near the clay that filter material are spread. Someone said it could even be a transition and there's no problem for this contamination.
I thinks the placement construction would not shape as designed and we will see some differences in clay and filter width. it may also shape the Christmas-tree efect.
I wanted to know these kind of contamination need a big consideration or not as problematic.
and also If you could say your idea about the "haul" which I've mentioned in the question I'd be appreciate it.
Thanks in advance
Dear Hamed,
Thanks for your compliment. Now, I am getting your question by look into the picture the material is in dry state. In dry condition we may not have any big issue but during raining season the main question is the chance for seepage into the clay core or not. If at all seepage problem is there then the contaminated particles may also seeping into the clay core. Depend on the type of contaminated soil the clay core may react on contamination along with water. In this situation, the performance of clay core may affect in over all stability of the dame. However, it purely depends on the kind reaction is to takes place b.w the clay and contaminated material. Since, the clay core is impermeable and assuming that there is no seepage along the core. Then, there may not be any big issue in it, as you also mentioned that there may not be any big issue in this case. But, before completing the work you have to ensure that the performance of the dam is good in during rainy season.
Gentlemen
This is all about quality control and adequacy of supervision during construction. The photo indicates that there are several areas of concern with the construction methodology currently being employed. The most obvious are:
- problem with filter layer thickness control - both variability and excessive thickness
- tipping of filter material using the size of truck seen in the photo will be problematic, i.e. difficult to control the quantity of material deposited by end tipping at each point
- trucks delivering the filter material should travel on the rockfill shoulder and not along the core
- arrangements for filter crossings should have been addressed in the specification: clearly, they must be well-controlled, limited to one crossing point for each lift and inspected before the next filter layer is placed on it.
You did not mention the geometry of the clay core, but it looks like it is a central, vertical clay core (?). In this case the "Christmas Tree effect" is inevitable. The important thing will be to ensure that the minimum thickness of the filter is achieved. The "Christmas Tree effect" can be limited by controlling both the lift heights and level differences between the clay core and filter zone.
Thanks for your thorough respond dear Geoff Wilson
Yes the geometry of clay core is vertical. According to what you have mentioned and the image attached, the problem is occurring Christmass Tree effect. after construction how this Tree would affect the stability of the dam?
Hamed
There should be no effect on the embankment stability. The deviation from the theoretical geometry of the dam cross section that is caused by the "Christmas Tree" effect will be on a micro scale when compared with the length of the theoretical slip surface.
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