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Raising Crest Elevations

Raising Crest Elevations

Raising Crest Elevations


I am looking for suitable material to add on top of the crest and enhance the freeboard requirements as the water level is going up or the crest elevation is abraded by continuous traffic. This is for few small dams (less than 6 m high) with relatively shallow depth of water, which is collected from mining processes.
If the materials have too much fine, there would be problems during raining when driving on the crest. If the materials are too coarse, they would inhibit access and would allow water to pass through if water level goes up. The site may be covered with snow for 1/2 year.
My questions are:
What would be the optimum gradation for the materials?
How much fine is desirable, if granular materials are placed on the crest?
What others do in this circumstances?
Any publication available?

RE: Raising Crest Elevations

typically use a good, dense graded aggregate such as what is used for base material under pavement

RE: Raising Crest Elevations

I would suggest removing any gravel surfacing, placing a high fines material to above the desired elevation of the top of the dam, then place a waring/driving surface of gravel.

Mike Lambert

RE: Raising Crest Elevations

Few questions from the responses:

  • cvg...a good, dense graded aggregate,.. Does it mean uniform graded or gap graded aggregate? In a project I was involved in the past, the base materials were crushed aggregate of relatively similar grain size. Does it mean uniform graded or gap graded.
  • GeoPaveTrafic... placing a high fines material to above the desired elevation of the top of the dam, ... Raising the dyke/dam embankment is to maintain a freeboard, which should mean that the "desired elevation of the top of the dam" would be the elevation of the top of the freeboard height. Hope I understood correctly. However, when it rains, the water penetrates through the surface gravel and is collected on top of the high fine materials. Wouldn't that promote a sort of a pumping action when subject to driving?
  • Could a well graded till material be used and compacted?
  • Is there any stabliser to secure the surface materials from rolling down of the embankment by trafic?

RE: Raising Crest Elevations

If you are concerned about the fine material pumping under traffic, then you need a thicker road base section. Also, if traffic needs to pass over the dam while the water level is near the top of the dam, you need a taller dam. I would suggest that the crest of the dam (minus the roadway section) should be at least 1 meter above the highest expected water level. That should give you 1 meter of clayey material that will not saturate due to water behind the dam.

As for using till, it is possible; all depends on the material in the till. As for a "stabliser", not sure what you are getting at.

One thing that has not been discussed is the purpose of the dams. If they are collecting mine runnoff, then I would expect that discharges could be quite damaging. In which case it would seem that a taller structure would be necessary.

Mike Lambert

RE: Raising Crest Elevations

do not use uniform or gap graded material, use "well graded, aggregate base course material" which will form a very dense compacted layer that is that is stable. if you want a rock surfacing, uniform crushed rock on top of the aggregate base course could be used.

look in any highway department specification for aggregate base course and use that. It will have a maximum particle size of about 50 and should be well graded all the way down. do not use natural material such as till, it will not likely be suitable. recommend no more than about 5% silt or clay size particles.

best way to avoid the material rolling down is to make the top of the embankment wider so that traffic is not near the edges. stabilizer then would not be needed. but if this has heavy traffic than you will need to surface dress it from time to time.

other suggestions such as 1000 freeboard are also recommended. maximum anticipated water level should remain below the structural road section.

RE: Raising Crest Elevations

cvg recommend no more than about 5% silt or clay size particles.

I understand this is for the materials located above the maximum water level. In fact this will be the materials that will make the freeboard body. So why the fine% should be limited to 5%. As there is a chance of water raising along the freeboard and more fine materials would guarantee a more impermeable freeboard and can better prevent water perclating down the embankment (in the freeboard height. however, the surface rock better to have as little fines as possible. Hope I am not missing any point here as there is also a suggestion to use high fine materials to above the desired elevation of top of the dam. In other words, should the soil which makes the freeboard body be fine or coarse and if there is any recommended gradation. Thank you again to allow me to understand the issue.

RE: Raising Crest Elevations

if you want a strong stable driving / surface wearing course, than you need to limit the fines. If you want to build an impermeable core for your dam, than use clay. you might need two layers buth you cant do both with the same material

RE: Raising Crest Elevations

Use Ministry of Ontario Granular A (Ontario Provincial Standard Specification OPSS) . . when compacted well, it will not pass water (this is highway crushed aggregate base course). you can google the specifications - I think the number is something like 1010.

I just googled it - I was right about 1010 - not bad after nearly 20 years.


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