Low Unit Weight Soils (76 pcf)

[cpdonahue]

Looking for any data or experience working with low unit weight soils. I drilled a site for a new project which will have earthwork cuts and fills. We ran Standard Proctor tests on two bulk samples and got very low unit weights, 76 pcf and 83 pcf with optimum moistures around 30 percent. Further testing and samples of the site indicated a bulk of proposed cut materials falling in this 76 pcf to 89 pcf range. The soils were sandy silts, with around 60 percent passing the #200 sieve. These are residual soils. In-situ moistures were quite high, upwards of 40 to 60 percent. We are awaiting some triaxial shear test results on remolded samples.

Possible concerns on these soils are are they suitable to be used as structural fill under slabs, roads, structures? Will they be overly susceptible to deterioration during construction? Will it be realistic to dry these soils to be used as fill? Assuming I get decent results back from the Triaxial testing, should the contractor plan a very large contingency to deal with trying to build with these soils? For reference, typcial soils in our area are silty sands or sandy silts with max. dry density's per Standard Proctor of 95 pcf and greater.

 

[EireChch]

I had to convert to SI units, so the range is approx 11 - 13kN/m[sup]3[/sup]

I have never encountered cohesive material with such low unit weight. BS8004 indicates for SILT/CLAY the typical lower bound unit weight is 14kN/m[sup]3[/sup].

Not saying you are wrong but you should definitely triple check everything when it comes to this site as it does seem funky.

With water contents of 40-60% you are talking about a void ratio of 1-1.6 (assuming specific gravity of 2.67).

You have some very poor material on your hands.

What are your atterbergs like? Are you sure you are dealing with SILT and not some funky mixed CLAY with portions of kaolinite or montmorilinite. I would check the minerology.

I would not consider it suitable as structural fill. Can definitley be used for general filling such as landscaping but not for slabs and roads etc.

I think your contractor will have a very difficult time drying the material out to its OMC of 30%. OMC also seems etremely high for SILT and CLAY. See below.

 

[Ron]

Can you identify the parent material? Is it a micaceous silt? Calcareous silt? Diatomaceous earth?

 

[cpdonahue]

It's definitely not a CL, we rarely have any of that around here. No errors in the math or test, we have ran into this lightweight material a few times in our region, but this is the first job we've had where they need to use it as fill (or face a costly export/import scenario).

I need to brush up on my geology to identify parent material. It does have some mica, but not as much as some samples I have seen. There are several mines in the area where they mine Quartz, Feldspar, and Mica.

 

[GeoEnvGuy]

Not much to add, just have you tested the specific gravity of the material?

You need to communicate to your client that when you put imported granular A or what the convention is in your region under a building as structural fill you are going to get reliable performance. If your using substandard material you will likely not get the same performance.

 

[cpdonahue]

The building is likely going on stone columns anyways due to high loads. Going back and forth and whether only foundations or both slabs and foundations will need to be supported by the stone columns. Earthwork fills will include filling for a parking lot and some 2H:1V fill slopes. Some of the fill is supposed to be exported and used to raise grades on a school football field with turf covering. We can utilize geogrid in these areas to help stabilize. Mostly concerned over how well this material will be for the contractor to work with (will it be a pain to dry). We live in a place where it rains quite a bit.

 

[GeoPaveTraffic]

I echo the other comments. Some unusual material there. Do more lab testing to better understand the material.

 

[geotechguy1]

If you run the atterbergs with and without drying do you get completely different results?

Residual / volcanic clays (halloysite / allophane) will plot below the A line and may also have low unit weights. Also the behaviour is totally different if you oven-dry it before doing the proctor or atterberg (example:

https://www.icevirtuallibrary.com/doi/pdf/10.1680/jgele.18.00056

also look up Laurence Wesley's work).

 

[cpdonahue]

An update on this....lab testing indicated high strenght parameters on remolded samples. Performed CU triaxial on samples remolded to 95% of ASTM D 698 and got results of around 36 degrees which is just confounding to me. I still think the lightweight will make it challenging for the contractor to work with.

 

[EireChch]

Id love to see a PSD curve and atterberg data.

I think the below paper is probably of use to you also.

Drained Shear Strength Parameters for Analysis of Landslides Timothy D. Stark ; Hangseok Choi ; and Sean McCone

 

[geotechguy1]

cpdonahue - not surprising to me that the friction angle is quite high. This is the type of behaviour that's commonly reported in the literature for the type of material I described (I think has far back as Terzhagi geotech engineers have been confused by the behaviour and performance of these type of soils). Unit weight is low, moisture content is high, but the strength and stiffness characteristics are often quite good. Do you have any labs that can do testing to determine allophane or halloysite content?