Filter thickness of a earthen dam 8

[pelelo]

Hello,

There are several references (USACE, USBR, Cerdegren) related to the filter design for earth dams.

I noticed that these references state very vagley how to calculate the thickness of a filter drain.

Does anyone any reference that provides details (or maybe examples) of how to compute the thickness for a filter?.

Thanks.

 

[BigH]

See Earth Filters for Dams by Talbot and Pabst (you can find it on the net) - The Journal of Dam Safety - Fall 2006

On a Dam on which I am aware - > 275 m in height - the Bid Design calls for 10 m thick. Contractors of course will want to minimize this.

 

[GeoPaveTraffic]

No reference, but my experience is that the filter width is more a function of constructability. Theoretically, any thickness of filter is thick enough. For a 200 ft. dam in NW Arkansas, we used a two stage filter with both stages 4 feet thick. That allowed both stages to be placed simutanously using an 8 ft wide box with a divider in the middle and then compaction by standard width equipment.

Filters are typically the most expensive material to produce for a dam, so keeping the width small is important from a cost view point.

One last thought. When working in the Pacific NW (USA), where seismic loading/displacments were large, we used wider filters to account for lateral displacement due to seismic activity.

Mike Lambert

 

[BigH]

Mike - the dam to which I was referring is in a very highly seismic area.

 

[pelelo]

Thanks a lot for your responses.

Cerdegren in "Seepage, Drainage and Flow Nets", 3rd Edition, provides an approach of how to compute the thickness of a chimney drain (upper and lower portion) see attached.

Does anyone know how he computes the cross-sectional area normal to the direction of the seepage within the chimney (Ac) is computed?. He states is 11 SF. I am not sure what is the support for this.

Thanks.

 

[GeoPaveTraffic]

pelelo,

I think the 11 SF is the 12 ft. width of the chimney drain corrected for it not being vertical.

Mike Lambert

 

[GeoPaveTraffic]

BigH,

Makes sense that the filter zone could be that thick in an area that you describe.

Mike Lambert

 

[geotechguy1]

This probably wont be to helpful but I was involved in construction of a small water retention earth dam in the oil sands region of Alberta, and the vertical chimney drain / filter was built with what was called a 'christmas tree' approach; by excavating back into each 1m thick lift of the main dam material in a relatively approximate manner which allowed the center of the drain to wander a bit. It seemed that in the mind of the designers the specified width (maybe 2-3m or so) wasn't really necessary as long the thing was continuous. Could never get them to give me a minimum width though.

 

[BigH]

Problems with filter and filter construction - which is why I like thicker filters - see attached paper.

 

[Milliontown]

I work for one of the reference agencies, and this has been a discussion in our office as well. When I asked at a seepage training, the answer I got was mostly based on constructability. You want it thick enough that there isn't a possibility of the filter being thinner than designed because of construction tolerances, etc. My personal preference is no thinner than 6 inches (mostly for places with tight clearances and usually hand placed/compacted), but really I like starting at 18". As long as your filter thickness meets your permeability and flow requirements, it comes down to construction and constructability.

I have gone thinner than 6", however that was a coarser 2nd stage filter to a perforated pipe in a trench. The placement was 3" vertical and not ideal, but it works.

 

[jgailla]

18 inches for a horizontal drain and 36 inches for a vertical drain. The vertical drain can end up with discontinuities if it is too narrow.

 

[GeoEnvGuy]

Filter thickness is

based on your application of appropriate engineering judgment.

Below is a theoretical minimum is based on the depth of penetration into the base soil, this is briefly discussed in geotechnical engineering of dams.

 

[BigH]

See:

https://scholarsmine.mst.edu/cgi/viewcontent.cgi?article=1485&context=icchge

Figure 5 for what can happen when placing filters

 

[theCorkster]

Should you not design the minimum width of a filter based on 1) the estimated seepage volume coming from the finer material the filter is protecting?

 

[BigH]

Filters are present not only with respect to seepage and migration of fines. They also serve as crack sealers - upstream filters, when the core has experienced hydraulic fractures at the upstream face, need to be able to flow into the cracks of the core to seal the cracks.

 

[cvg]

in addition to hydraulic fractures, crack sealers are common in flood retarding structures and protect against cracks caused by drying shrinkage, settlement and subsidence. minimum width is often set at two feet and placed wider due to constructability. Five or six feet is about the minimum achievable with heavy equipment

 

[geotechguy1]

https://www.fema.gov/sites/default/files/2020-08/filters_embankment_dams_update.pdf

I'm not deeply familiar with it but a colleague suggested the FEMA manual on filters for embankment dams, see link above.

 

[BigH]

FEMA document written by the same chap who wrote Chapter 5 of USBR DS-13