Best tests for soil profile under Recharge Basin

[canardiniIEUA]

Hi,

I work at a municipal water district and have been assigned to a project at one of our recharge basins - however, I don't have much experience in the geotechnical side of things.

We want to see if there is anyway we can improve the percolation rates, and are going to be using shallow backhoe excavations for a surface survey of the soil and sonic drill boring for the deep excavations.

Anyway, my question is what tests would be best to determine the soil profile for this case? We are working with limited funds, so need to choose only the most relevant.

From my understanding, the most applicable in this case seem to be tests such as Consolidation, lab Density/Moisture tests, maximum density, sieve analysis (coarse fraction or fine?), etc., and the less applicable would be Expansion, Shear, etc?

 

[oldestguy]

I'd not run most of those tests. However, you might indicate how large an area you are working with. One of the first things I would do is either use an available map or actually map the area for pedologic soil series. Chances are it already is mapped. This is in the realm of soil scientists. Soil series descriptions usually get into the subject of infiltration rates. The local US Dept of Agriculture office or state agencies may have plenty of soil series data from which to plan any field investigations. One possible test is done in the field called a double ring infiltration test. If this site is not in the USA, it still is possible to do the mapping job any place on earth. Without pedologic mapping you can waste a lot of time and money.

I am surprised how so few soil mechanics courses even mention the word pedology. Check agronomists at any university devoted to agriculture.

 

[oldestguy]

Also spend some time looking into what happens with time for different types of soil. For instance, cycles of wetting and drying change things, especially for clays, sometimes for the better as to infiltration. Take that word pedology, since it springs from "peds" the little globs of soil that develop into what soil scientists call "structure". So, it is not a simple quick fix thing and it may take time.

 

[cvg]

OG has some good points on regional characterization of the soil. also, it depends on how you are recharging. if you plan on injection wells, then boring down to a good recharge layer and drilling logs are essential. if this is an existing basin that has degraded, it is likely that surface soils which originally had good percolation will be covered with silt or clay which has reduced the rate of infiltration. if that is the case, then scarification / removal of the top layer might be necessary and also perhaps filtration or other treatment process to remove the fine particulates from the water before putting it into the basin.

if you want to do subsurface investigation without drilling, you might think about seismic refraction or reflection

http://www.enviroscan.com/html/seismic_refraction_versus_refl.html

 

[canardiniIEUA]

Thank you both.

It is a basin we currently use (dry at the time being, drought in CA), with pretty decent perc rates, but we are doing everything we can with our basins to get as much out of them as possible. We are already planning on drilling down to a variety of depths using backhoe and sonic drilling, removing the top silt/clay layer, etc., My question is referring more specifically what tests would be best to perform on the samples we pull out at different locations from the surface to 100 ft, to determine an accurate soil profile/soil characteristics.

From discussion with a colleague in ground water recharge, it seems sieve analysis would be one of the few useful tests?

 

[oldestguy]

Assuming test borings, etc. the gradation tests are very useful along with hydraulic conductivity (permeability) tests on undisturbed cohesive samples. Be aware that if the area was at any time influenced by water flow and/or deposition (lacustrine), if you miss even thin layers of fine grained material in your test program, it can throw off your calculations by a significant factor, sometimes in orders of magnitude. It will be the fine grained materials that will control what happens.

 

[fattdad]

Hvorslav has a series of equations for borehole permeability testing. One example is for water flowing from the bottom of the casing. You can do these tests as you advance a cased boring. (Just don't use mud and advance casing.)

You can go to fatt-dad.com and click on my hydrology link for a graphic correlation between D10 or D20 and permeability.

I'm not a fan of percolation testing. It makes great sense for drainfield design, as it relates to the influx of (grey) water. You are really talking about feet of ponded water and to what extent it'll flow into the "formation" soil. There may be confining layers (horizontal) at depth below the pond bottom. So, as the first water infiltrates, it quickly has more to do with horizontal and vertical flow under saturated conditions.

Injection wells (e.g., holes drilled and filled with gravel) follow the usual equations for withdrawal wells. Also at my link you'll see, "dewatering and ground water control," which provides all sort of equations for withdrawal wells. Just use those equations and accept that little h is bigger than big h.

the county of Fairfax, Virginia has a test for infiltration testing that's not half bad. I personally would rather just do borehole permeability tests that return a "k" value for permeability.

f-d

¡papá gordo ain’t no madre flaca!

 

[canardiniIEUA]

Thanks to all for the info, and fattdat for the link, lots of good info. Will take all into consideration