Calculating the Unit Weight of In-Situ Soils 1

[Dasovich]

Can anyone out there give me insite on current testing methods for calculating the In-situ unit weight of soils? We are constructing a gravity sewer and would like to calculate the specific gravity of the material we are excavating, so we can backfill with a similar weight material. Any help would be greatly appreciated! Thank you!

 

[georam]

In-situ density tests are usually conducted using either a sand cone density apparatus (standard test ASTM D1556), or a rubber-balloon equipment (ASTM D 2167). Both are approved techniques by ASTM Standards. There is also a nuclear density test apparatus (standard test ASTM D2922), but equipment is more expensive and must be operated by specialist technicians.

the sand cone is basically a method to dig out the in-situ soil (and weight it), then fill the hole back with a known specific type of sand to get the volume of soil removed. So density = weight/volume. In balloon method, you fill the void created by digging using a rubber ballon filled with water. Volume of water in the balloon can be easily calculated (specific gravity of water = 1)

I don't think you are going to check the specific gravity of the soil.... just the unit weight or in-situ density. Specific gravity test is a different test... it is in a way similar to a measure of weight/volume of the actual mineral aggregates/soil, excluding the voids, etc.

Soil labs usually do the in-situ density tests for you. To check whether the backfill material has good density (good compaction)... or to compare it with the density of the original materials.

Regards

 

[Dasovich]

Yes, Sorry! I meant to type density instead of specific gravity in the 2nd sentance.

 

[KAM_imported]

If the soil is saturated,and you have measured the water content (a $10.00 test) you can assume a specific gravity for the soil (2.65 for sand and 2.7 for clays are typical values) and then use the following equation to estimate the total unit weight:

((1+w)(gamaw))/(w + 1/G) where G= specific gravity
gamaw =unit weight of water
w= water content of soil
If the soils to be excavated are cohesive, you can push Shelby tubes into the soil and determine the unit weight of the soil in the laboratory. There are correlations between the standard penetration resistance and soil unit weight in the literature--you may want to look in any basic soil mechanics textbook for these correlations.

 

What about insitu nuclear density measurement?

 

Hello, can anyone tell me the difference between unit weight and density?

 

[dirtsqueezer]

In practical terms, at least on planet Earth, they are the same.
The difference is actually that density is mass over volume, and unit weight is weight over volume. Since we are always going to be considering gravity, density will have to take it into account, also.
All ASTMs that deal with density are actually dealing in unit weight. Trust me! This is true. If you look at the proctor test, nuke test or the sand-cone test, all use WEIGHT over volume, and not mass over volume. So if you are looking at it from the angle of an ASTM, they are going to be the same there, too.