Allowable Bearing Pressure For Backfill 3

[oxbridge]

Due to a metre of soft ground below the proposed basement, we've been advised in the geotech report to dig it out and replace with compacted material. Although the report gives allowable bearing pressures for the clay (that will be below the backfill) I guess we really need bearing pressures for the granular soil, as this is what the basement sits on. Is it possible to work on a typical bearing pressure for say medium/dense granular material and then state this is what we require on the drawings? Settlement must also be an important consideration in backfill and I'm presuming this can be calcuated and a geotech engineer is best suited to do this. Is that correct?

 

[csd72]

This is a question to ask your geotech and it should be confirmed by testing after the fill is compacted.

 

[jheidt2543]

I don't think the actual bearing capacity of the compacted backfill will be a problem, that material will easily give you 4 ksf or more. The compaction requirements are the key to the backfill and secondly, what does the compaction effort for 3 feet of fill do to the clay under it?

Compacting the first few lifts could pump the clay right into the fill. You might consider putting down a geotextile to keep the clay from pumping through, before you start backfilling.

 

[lovethecold]

Although the report gives allowable bearing pressures for the clay (that will be below the backfill) I guess we really need bearing pressures for the granular soil, as this is what the basement sits on.

Keep in mind that bearing capacity is not only a function of the soil immediately below the footings, but also of the soil through 2B of column footings and 4B of strip footings. My guess is that as the bearing capacity is of the clay below the footings, that is what you should go with. If you use an allowable bearing capacity based on the sand you may get excessive settlement, or bearing capacity failure (although not likely if this is a house).

If you have questions I recommend you go back to the geotech and ask why. For all we know there could still be relatively soft soils that are governing bearing capacity.

Hope this helps.

 

[fattdad]

Bearing pressure must consider ALL of the soil in the increased stress zone. If you have soft soil below the structure (i.e., with a low bearing capacity) and you place some strong granular soil immediately atop this soft layer, the resulting bearing pressure will still be influenced by the underlying soft soil.

It is often tempting to relate this directly to the strength of the soil layer. However, bearing pressure "failure" is rarely the governing mechanism for foundation design. Much more often, foundation design is based on performance (i.e., to limit settlement). As long as there is increased stress in the soft soil layer, you need to remain concerned about compression of this layer.

I just point out the obvious: You have a geotechnical engineer working on your project and are receiving feedback from civil/environmental and structural engineers. Two professions that I respect, but this is a geotechnical engineering issue.

f-d

¡papá gordo ain’t no madre flaca!

 

[BigH]

lovethecold - a bit of semantics, but bearing capacity doesn't go 2B and 4B deep - the zone of influence does - which is settlement oriented, viz., use of the words bearing pressure - whereas "capacity" is shear oriented (based on strength). My view anyway . . .

 

[msucog]

fattdad hit it on the head...again....

 

[lovethecold]

BigH - You're correct, I didn't explain it very well. Thanks for the correction. On the other hand, fattdad did a superb job of explaining.

 

[bkt1977]

what is the relationship between hand penetrometer readings and allowable bearing pressure for clay soil

 

[fattdad]

Pocket penetrometer gives unconfined strength (i.e., sigma 1 minus sigma 3), which is twice the value of the undrained shear strength (i.e., sigma 1 minus sigma 3 over 2). The ultimate bearing pressure for cohesive soil with no friction angle is about 5-1/2 the undrained shear strength. If you use a safety factor of three, that would make the allowable bearing pressure about two times the undrained shear strength or (full circle), the unconfined shear strength.

Now you need to figure out if the sample for the undrained shear strength represents the entire soil in the "increased stress zone." You also have to determine whether the foundation performance will be governed by consolidation (i.e., saturated conditions - 1-D consolidation) or compression (i.e., soil modulus). Typically the settlement derived from the empirical bearing formula is too great for servicability (i.e., the footing will settle too much).

Hope this helps.

f-d

¡papá gordo ain’t no madre flaca!