Soil Compaction % Related to Bearing and Settlement 1

[JohnnnyBoy]

I have gone through a few previous posts and thought I would bring up the conversation again and try and get some better references and ideas behind compaction and bearing/settlement. First of all I want to note that each soil condition will result in separate results so lets try and stay as general as possible.

My firm has guidelines/good practices on when to specify 95,98 AND 100% SPMDD although I am not sure where this came from other than general practice and previous acceptance/performance.

Really my question is how much does the bearing capacity, settlement (differential or overall) and overall performance changes with compaction.

In my last conversation with a geotechnical engineer, I was trying to increase bearing capacity of the soil for my pre-cast foundation although I was told that this cannot be achieved even with going to a thicker sub-base and increasing compaction density. From my understanding bearing capacity is normally governed by settlement and many times governed by the underlying soil although this confuses me as the underlying soil sometimes has a lower compaction requirement. If it is governed by settlement increasing compaction should increase allowable bearing capacity although if its the underlying soil would increasing thickness not marginally help bearing or at the least if increasing compaction of the native soil should this not increase bearing.

Any literature on this matter would be much appreciated.

 

[oldestguy]

I'll throw out some comments. It's been some time since I have been closely with testing and bearing capacity.

To your question of bearing related to percent compaction. I doubt that there is much of any useful info out there. In my judgement the percent specified probably is termed "Good enough". I make a question up when a "know it all" on the job complains about not meeting the spec on compaction. "Go take a test on the undisturbed natural ground where the bearing is acceptable". When that test comes up with a percent under 90 by a lot, open mouths don't have an explanation.

 

[oldestguy]

OG once more with a story about compaction. At a site where classroom buildings were to be built the architect decided to dig some lakes for storage of collected roof water runoff and use that excavated material for compacted fill upon which buildings would be built. 95 percent compaction was speced. Roof drains were piped to the ponds. Also underfloor utility trenches back-filled with sand were connected to that system. Contractor was forced by the inspector to get that spec. on a lake clay deposit soil. You guessed it. Dumping that roof water in pipes connected to the underfloor system really did the job of adding water to that clay. Significant expansion took place and floors really raised up. Now pray tell where in the heck did the reasoning for that 95 percent come from?

 

[JohnnnyBoy]

Thanks oldest guy, that seems to be what I'm hearing a lot in the field and from other engineers as well. It's too bad there isn't more data on the actual relationship and how much affect it could have on building design or pavement design.

 

[geomane]

Check out the FHWA Soils and Foundations Manuals. They have tables (I think) comparing % compaction, friction angle, relative density, and SPT N values. You can then approximate settlement and bearing capacity.

 

[fattdad]

For granular soils, 95% compaction is roughly on par with 75% relative density. At 75% relative density, it's pretty likely you are below the critical void ratio for dilation on shear.

It's not too difficult to replicate 95% RC in the laboratory and actually measure your site soils and their measured strength and behavior (i.e. for fine-grained soils). Now you have a basis to specify something and know what to expect.

Bearing capacity of one layer is not telling. Same for the compressibility. You MUST take your foundation load, determine how it attenuates with depth and actually calculate expected performance! The bearing capacity of one unique layer is not enough!

Not to disagree with any of the above posts. . .

f-d

ípapß gordo ainÆt no madre flaca!

 

[BigH]

I have posted this in other threads years ago but thought it still be might useful.

 

[oldestguy]

A question for Professor fatdad: Exactly what adjective units do you measure in the lab for what you say here?

"It's not too difficult to replicate 95% RC in the laboratory and actually measure your site soils and their measured strength and behavio"

Perhaps in Pounds per square foot for strength. How about behavior? Site soils? What test machines to use? Units?

Of course the tests should then relate to compaction percentage, the subject of this post.

 

[fattdad]

If I understood the question, I'd answer.

All I was saying is if we base all our embankment fill design on compacted soil strength/behavior and we evaluate strength and behavior in the laboratory using remolds at 95 percent relative compaction, we should be benchmarking the properties that will be constructed.

Sorry if I confused. . . or continue to confuse?

f-d

p.s., not a professor and not a doctor and thinking of retirement!

ípapß gordo ainÆt no madre flaca!

 

[aeoliantexan]

One additional caution: If the fill may become saturated or nearly so in the future, you should base your design on the saturated strength. I have inspected compaction of a lot of silty clay to 95% Standard Proctor. It is usually stiff to very stiff, depending on the water content. Coming back after a rainy period, it was firm. Most compacted cohesive soils will become weaker and more compressible when saturated.

 

[fattdad]

In response to aeoliantexan: I only measure soil strength in the laboratory under saturated conditions. Field conditions (i.e., unsaturated) will therefore be conservative as unsaturated strength is greater than saturated strength.

Now, if they cited the wrong proctor or didn't get their moisture content right, there could be false hope passing the field-density test.

f-d

ípapß gordo ainÆt no madre flaca!