graded gravel fill under placed soil 1

[aliceholt]

Hello,

I currently work in land restoration to woodland. We usually specify placement of up to 2 m depth of soil over landfill or capped sites to allow for planting trees (according to drought and rainfall data for the particular site). I would like comments, please, or some idea about the suitability of a layer of gravel fill placed below the soil to improve drainage. What sort of grade fill would be required and would geotextiles be necessary to prevent infill by soil? I'm envisaging a gravel layer of approx 0.3 m thick would be appropriate. The soils used can range from fine sands to silts to clays and are usually placed by loose tipping to minimise compaction.

Thanks in anticipation.

 

[KAM_imported]

If the gravel is to be used for drainage, then yes, you should include an appropriate geotextile between the gravel and soil to act as a separator. The fabric should be designed to accomodate the retained soil (AOS/EOS). There are numerous references available that tell you how to verify that the pores in the fabric are large enough to permit water flow thru the fabric, but small enough to retain soil particles. You will need grain size distribution data on your retained soil. Consult Bob Koerner's geotextile book for more information.

 

[aliceholt]

Does it then matter what grade of stone you place? Would it be possible to avoid using geotextile by graduating the stone fill, using fines above the coarser grade material?

 

[redgreen]

I don't do much landfill work, but don't you typically cover landfills to prevent water from seeping into the landfill?

Each site is different, however, if the landfill or cap is meant to act as an impermeable layer, then you wouldn't want any water draining into or out of landfill.

I have found that without a proper impermeable layer, the trees have more opportunity to draw up some of the unwanted moisture (contaminated) and sometimes cant survive.

 

[aliceholt]

This gravel and soil would be placed on top of an impermeable clay cap or butyl liner, basically to encourage drainage of the rooting zone but with no ingress of water into the landfill. The soil is usually ridged at a 30m to 60 m wavelength to encourage lateral drainage into the lower-lying furrows. However, some sites remain flat and the drainage is poor, hence the need to place underlying gravel.

 

[BillHolt]

Alice,

The sands are easy to deal with, so far as the drainage in concerned, as are the clays for different reasons. The sands will drain into the gravel layer, the clays eventually won't - or at least not much.

The silts, if truely silts, are potentially a mechanism for failing your drainage system. Geotextiles that are tight enough to retain silt particles will eventually be blinded by them, virtually eliminating the ability of the textile to convey flow. Geotextiles that are open enough to allow silt particles to migrate through can work by allowing the soil matrix to develop an envelope, of sorts, where the particles nearest the interface are lost through the system but those further back are retained by structural interlock. Geotextiles that are too open allow the silt to flow as it erodes.

Check Terzagi and Peck or some of the Corps of Engineers pubs where they talk about internal drainage systems for dams to get an understanding of this. Of course, the head driving flows through your system may be so small as to reduce all of this to irrelevence, but that's a function of your weather, geometry and vegetative cover system.

I'd be astonished to see trees be a good idea on top of a system of this sort if long term durability is important. Landfills settle differentially in suprising ways. Trees die, fall over and make holes. That's real bad for cover systems.

Bill Holt