Unit weight of soil in the absence of water table 3

[ak.t]

Hi All,

Is the 'bulk density of soil' and the 'saturated density of soil' the same thing?

Which unit weight of soil (dry, saturated or submerged) to be used for Footing/retaining wall back fill calculation, in the absence of water table?

Thank u

 

[r13]

A soil is a complex three phase system - comprising of soil solids, water and air. Bulk density (aka total density) is the total mass of the soil (ie mass of soil solids, water and air) per unit volume, while a dry density is mass of soil solids only per unit volume of the soil.

The bulk density is determined by taking field samples and tested in the lab. You won't have a saturated soil without water source. The unit weight for design shall be furnished by your soil lab.

 

[ak.t]

The soil report states that the ground water level was not encountered at the desired level of footing base. But it doesn't furnish data on density. Hence, I considered 18kN/m3 and mentioned it as dry density of soil.

 

[r13]

Although the water table seems low, the backfill can get moist/wet during the retaining wall service lift. I don't think it is correct to use the dry weight for design. You shall ask/demand the soil lab to do their job.

 

[jdonville]

Saturated unit weight is the unit volume weight of soil plus water when all the void space is occupied with water. By definition "saturated".

Dry unit weight is the total unit volume weight of soil (no water present). By definition "dry".

The unit weight of a soil above the groundwater table will generally be somewhere between these two extremes.

 

[GeoEnvGuy]

Jdonville has it right the bulk density is somewhere in between based on the saturation ratio of the soil.

To address you real question of what unit weight to use for the wall backfill calculation. Well if you backfill the wall loosely with native silty sands 18 kn/m3 is probably a reasonable estimate. If you backfill granular A in 6 inch lifts to 100% standard proctor it will probably be near 22 kn/m3. So how much effort is the contractor backfilling the excavation going to do, in my experience on small residential and commercial developments near zero compaction effort is performed until the top layer.

 

[ak.t]

Thank you for all your inputs.
retired13, Will ask Geotechnical specialist for exact data and confirm.

jdonville & GeoEnvGuy, Thank you. This is for external compound wall footing calculation. The soil is medium dense, brown, silty, fine to medium grained SAND upto the desired footing base level (1.25m from Natural ground level.

 

[MTNClimber]

You shall ask/demand the soil lab to do their job.

In my experience it’s typically the geotechnical engineer’s job to prescribe a moist unit weight to use for retaining wall design. The lab has nothing to do with design recommendations.

 

[r13]

In my area, third party geotechnical engineers usually associated/worked for the lab.

 

[ak.t]

Confirmed and included in Soil report by geotechnical consultant. Density to be used for this particular project is 18 kN/m3.

Yeah, as retired13 said, here also, we get it from lab only through geotechnical engineer.
Thank u all.