Unit weight of typical soils

[JrStructuralEng]

I'm looking for average unit weights (guidelines) for typical soils.

i.e. saturated and unsaturated unit weight of sand, silt, clay

 

[Teguci]

If the Geotech doesn't tell me, I use 120 pcf - fairly conservative for non-gravel soils.

However, the unit weights are all over the spectrum so an actual insitu test is best.

 

[JrStructuralEng]

I have dense compacted saturated sand. I need an conservative estimate of weight. I believe 22kN/m^3 should be conservative. Any conflicting thoughts?

 

[hokie66]

22 is conservative. I think 20 kN/m^3 is conservative enough. I normally use 19.

 

[msquared48]

Speak English!

Mike McCann
MMC Engineering

 

[palmahouse]

One thing you need to consider is using bounyant unit weight if you are dealing with a saturated sand, and depending on what type of analysis you are doing. One example of when you need to use a bouyant weight is if you are calculating vertical stress on a footings in sand under water (like for a bridge abutment)in order to calculate base friction.

A reasonable bouyant unit weight for sand is 70 pcf.

 

[rowingengineer]

Mineral Sand Ore and Ilmenite can weigh up to 2.7t/cu.m Just some food for thought.

When in doubt, just take the next small step.

 

[BigH]

Msquared48 - they are! I think you want "American"!! 10 kPa about 60 pcf (within about 3%). I hate it when they start using kg/m3 instead of kN/m3 for unit weights . . .

 

[swearingen]

I always rememebered it as 28 dynes/tablespoon...






If you "heard" it on the internet, it's guilty until proven innocent. - DCS

http://www.eng-tips.com/supportus.cfm

 

[dik]

Mike:

C a n I t a l k s l o w l y, t o o?

Dik

 

[JrStructuralEng]

thanks rowing eng. That is helpful

 

[Bagman2524]

I think most soils range in the 100 pcf to 120 pcf range. This isn't too much less than concrete which is typically 150 pcf.

 

[StructuralEd]

Saturated = adding water pressure or weight too, no?
Wouldn't drainage be a good idea?

 

[dgillette]

Expand Bagman's range a bit: Total unit weight can be 90 (aeolian silt with low water content) to 135 pcf (saturated compacted broadly graded fill).

How can you talk about "conservative" unit weights without stating the application? If it's active pressure on a retaining wall, higher is more conservative. If it's for footing bearing capacity, lower is more conservative.

 

[spats]

All you metric guys are confusing the heck out of me. Unit weight is mass/unit volume, or kg/m^3. A newton is a measure of force, not mass. What's up?

 

[dgillette]

Strictly speaking, unit weight has units of force/(length^3). Therefore, lb/ft^3 or, kN/m^3. Unit mass would be kg/m^3 or slugs/ft^3. Calculation of static forces and stresses is often easier with unit weight, since you don't need to multiply by 32.2 or 9.81.

Mention slugs to anyone, even U.S. engineers, and you are bound to get a bunch of blank looks. Slugs/ft^3 is most likely to be interpreted as how many slimy little creatures are in the garden.

 

[dik]

For SI stuff... density is usually defined as mass / cu (whatever), and the correct reference for soil in this application should be kg/m^3. It is often shown in reports (possibly incorrectly) as kN/m^3 with the idea that the weight of soil is a force and not a mass.

The Imperial confusion steps in when people use pounds (force) synonomously with pounds (mass). Dgillette is correct when he refers to Imperial mass as 'slugs' This is pounds (force) divided by g (32.2 approx) to convert it to a mass.

Dik

 

[civilperson]

Range in natural soils in my experience, 78 pcf to 147 pcf, with the majority under 110 pcf.

 

[dgillette]

147? Wow! What was it?

 

[civilperson]

Taconite tailings, (Minnesota) and basaltic rock, (Niagra Falls, Ontario).