Footing Subgrade Compaction

[theclipper]

Good afternoon fellas,

Often times I run across specs that call for the top 6" of footing subgrade to be compacted to 95%.

As a structural engineer who doesn't have the full soil knowledge of a geotech this makes no sense to me. This requires a proctor to be pounded which I also feel is useless data since the insitu subgrade is not the same from location to location in the trench.

I want to rewrite our office spec so that it basically put the honus on the GC to achieve required design bearing capacity.

What are your guys' thoughts on this subject?

 

[Ron]

An age-old argument! Specifying the GC to achieve a certain bearing capacity is fine to protect you from a performance issue of bearing capacity failure, but the reason to specify near-surface compaction has more emphasis than just the bearing capacity. First of all, one of the most important reasons for compacting the near surface soils is to achieve uniformity of the bearing statum. Yes, soils can vary at different locations in a footing, but then it becomes necessary to decide only if the Proctor applies to that soil, and if not, another test is run in the lab. Most often, only one or two different soils are encountered in the confines of a building at the surface or at the bearing level.

In-place density tests are relatively inexpensive and quick to run. The only "in-place" bearing capacity test available is the Plate Load Test. This is not cheap, slow, and doesn't offer you nearly as much comfort as to uniformity as a simple density test.

The reason for requiring uniformity is to allow the soil strata to respond to the load in a more consistent, predictable manner. The near surface compaction effort may affect soils to a depth of 3 or 4 feet, yet we can usually tell if appropriate effort has been expended by the near-surface density tests. At the least, it tells us if the contractor did the minimum compaction.

Lastly, settlement from undercompaction is a routine complaint for structures where no density specification existed or the contractor ignored it. Those same structures didn't suffer a bearing capacity failure, just settlement due to further consolidation of the near-surface soils.

Also, I would specify compaction in the upper 12 inches, not 6 inches.

 

[Focht3]

An age-old argument, indeed!

This specification fits in with the plate load test specification (see link below for that discussion): in most circumstances it serves no real purpose. Other than to make someone feel more secure - falsely.

http://www.eng-tips.com/gviewthread.cfm/lev2/12/lev3/49/pid/261/qid/54742

Unless the footing in question is one foot - or less - in width, the allowable bearing pressure is controlled by the soils below this zone. You might reduce the settlement due to compression of soil disturbed by excavating with a toothed bucket, but that's about it. A better solution is to require the use of a smooth-mouthed bucket.

Putting the burden of requiring the general contractor to achieve a certain bearing capacity is consistent with requiring concrete strength, minimum F_y for steel, etc. But it won't hold up in court - at least if the GC finds a geotechnical expert that knows his/her stuff. The GC installs concrete, steel, asphalt, etc. - but they have to deal with the site "as is." It is up to the geotechnical engineer to determine the safe allowable bearing pressure, and no one else. If he/she makes the mistake of trying to hide behind a spec like this, they'd be eaten alive in court. And if the structural engineer makes the call ... well, he's practicing outside his area of expertise. At least in my areas of practice (Texas and California.)

For guidance on this topic, check with a local geotechnical engineer who belongs to ASFE. ASFE has some very well developed documents on this subject. Don't play ostrich on this one!

 

[MRM]

Compacting the upper 12" of the footing subgrade goes a long way toward helping to reduce settlement in more ways than simply "repairing" the subgrade due to construction disturbance. Continuing on from what Ron mentioned that compaction helps to create a more uniform and predictable subgrade, compaction also promotes a greater footing contact stress attenuation within that 12" (or perhaps a thicker compacted zone depending on the compaction equipment and technique). The new stress due to the footing at a depth of 12" with a compacted subgrade will be less than the new stress due to the footing at a depth of 12" with an uncompacted subgrade. The compacted zone tends to "take" more than its proportional share of the stress than the underlying uncompacted soil. In this way, a 3' or 4' footing would also benefit by soil compaction-not just a 1' footing. I agree with Ron that you should call for the upper 12" to be improved rather than only the upper 6" as this will result in better properties and end product and is usually within the capabilities of even a modest sized plate compactor on most sandy soils.

 

[Focht3]

MRM:

The literature - and engineering analyses of settlements - don't agree with you. Compaction of the bearing surface of footings constructed in clays and cohesive soils certainly won't help reduce settlements to any significant degree. Even heavy compactors have a difficult time compacting more than about 8 inches in these soils. But you seem to be more interested in sands, so I'll focus on granular materials.

I will present your comments (in italics) followed by my comments (in plain text.)

Continuing on from what Ron mentioned that compaction helps to create a more uniform and predictable subgrade, compaction also promotes a greater footing contact stress attenuation within that 12" (or perhaps a thicker compacted zone depending on the compaction equipment and technique).

I've heard this claim before, and I don't believe it. Do you have any published references that include actual measurements to document this claimed "stress attenuation" effect? I haven't seen any - and where do the stresses go, anyway?

The influence of a relatively thin compacted zone of soil beneath a footing in sand is also questionable. Schmertmann produced an influence diagram for settlement calculations in sand that is roughly zero beneath the footing and reaches its' maximum value of 0.6 at about B/2 beneath the footing. And the influenced zone extends to a depth of 2*B. The performance of the first 6 inches of soil just beneath the footing doesn't control much of the total settlement of the footing.

In this way, a 3' or 4' footing would also benefit by soil compaction-not just a 1' footing.

Compaction might have some limited benefit for very small footings with widths of 3 or 4 feet, but how do you propose to get a modestly sized compactor - with a width of 5 to 6 feet - into the excavation? It seems that we should consider the minimum footing width to be at least 6 feet for the purpose of this discussion. After all, the compactor has to move around to "do its' thing." Or are you are talking about hand operated vibratory equipment?!

The new stress due to the footing at a depth of 12" with a compacted subgrade will be less than the new stress due to the footing at a depth of 12" with an uncompacted subgrade.

How do you propose the stresses get "locked in" in a thin granular veneer at the surface of an excavation? This might be true at the edge of the compacted area (although I doubt it) but it certainly isn't true in the middle of the footing. Again, any documneted support for this position in the literature?

The compacted zone tends to "take" more than its proportional share of the stress than the underlying uncompacted soil.

And where do you propose the stresses "taken" by this thin layer are transferred - to thin air? No - they are transferred to the underlying soils. This is elementary geotechnical engineering. If you don't believe me, you can confirm this for yourself by performing a finite element analysis on a two dimensional model of the soil beneath a footing. You can increase the shear modulus of the first 6 inches of soil to 10 times the underlying material, and only get a modest change in overall performance.

I'm sure that the practice of compacting the bearing area of excavated spread footings is standard practice in your area. And it's what you learned on your first job. And it has (some) limited value. But don't overestimate its' real importance in the performance of the footing. You could end up costing some poor contractor a lot of money!

 

[MRM]

Hi Focht3! Thank you for your response.
I was worried that I'd have to rummage through all my old papers to get all the perfect references, when suddenly, you seemed to agree with me-"Compaction might have some limited benefit for very small footings with widths of 3 or 4 feet...," you said. Well I never promised a miracle, all I said was that you could expect some additional stress attenuation within the compacted layer which may be 12" or greater depending on the compactor. The benefit would increase if you have, say, a smaller 2' footing, or the depth of compaction runs slightly deeper. This goes without saying, I suppose.

I'm interested in the finite element analysis you ran. Did you try it with a 12" compacted layer with a 2', 3', or 4' footing? What were your parameters? The main premise of your argument was that compaction at subgrade level would do no real good for anything other than a 1' footing, correct? Quite a few residential footings are even 16" or 18".

As other general advice, I wouldn't attempt to quantify this stress attenuation in my settlement analyses; It would be difficult and probably not representative. I simply know it exists so I am able to sleep more soundly at night. As engineers, we sometimes feel insignificant...I know I do when all I'm trying to do is prevent an additional 0.05" to 0.1" of settlement from taking place beneath a masonry wall footing-this may be the difference between a crack in the wrong place appearing or not appearing. (A crack is either there or it isn't) When they appear, some clients don't like it and may give the contractor or me a call back, as minor or harmless as it may be.

As for a reference, Poulos and Davis did a lot of work in this area. You'll likely see one of their famous figures or graphs in most entry-level soil mechanics and foundation engineering texts.

There are a lot of considerations to sizing a footing and recommending site prep. Remember to be mindful of punching shear, among other things, if the underlying soils are very loose on your site.

 

[Focht3]

I re-read theclipper's original post; he seemed - to me - to be talking about "commercial" construction (office buildings, warehouses, schools, etc.) rather than "residential" construction (houses, some apartment buildings.) He said, I want to rewrite our office spec so that it basically put the honus on the GC to achieve required design bearing capacity. This sounds like commercial work to me. While parts of my posts apply to shallow footings of all sizes, I was clearly focused on the "bigger stuff."

I'm glad you mentioned Poulos and Davis; as I recall, they had some charts for elastic stresses and settlements beneath a footing in a two-layer soil. The upper layer thickness is expressed in terms of the footing width B. The analysis won't be exact, of course, because it will assume the layer is continuous - and it isn't. Use the footing depth D equal to B and the upper layers' thickness equal to 1.1*B. Do the same analysis but with the upper layers' thickness equal to B. You won't find much difference.

But the simplest, most compelling thing to do is to draw the footing and layers to scale on a sheet of graph paper. Look at it. Do you really think that a 6 inch compacted zone will really have a significant impact on a 6 ft footing? 10 ft? 30 ft?

You don't need an engineering degree to get that answer.

 

[MRM]

Hey Focht3,
No, I don't think a 6" compacted zone will have any significant impact on a 6', 10', or 30' footing. I never claimed it would. That's crazy talk.

It sounds like theclipper was looking for input on general office building specs, meaning all sizes. Be careful when making generalizations. Offices come in different sizes, some with small structural element spacings and loads, some with greater. Our single story office building has column loads of perhaps 15 kips or less and walls with 1 to 2kips/ft maximum. Why? The entire thing is a maze of hallways with very small foundation element spacings, yet it's still an office building.

 

[BigH]

I would like to make a few points, if I could. I've read the various comments by others and have found an interesting discourse/debate about compaction of the upper zone etc. I'll leave that as is.

My added point is: The footing has been placed (designed)at the chosen level by the designer (enshala) based on the recommendations of the geotechnical engineer (or is/should-be usually the case). If the geotech wasn't happy with the founding level soil, would he not have (1) taken the footing deeper to more competent layer; (2) dug out 12 inches or so of the material and replaced with crushed stone (or sand and gravel); (3) chosen a different support type. So - let's assume he is happy with what he has for the soils. Now - what would the purpose of compacting 6 inches be? I would say that in nearly all excavations, the mechanics of the digging and cleaning out always leaves a bit of disturbed material - usually in the range of 6 inches or less. How many times have we been to site and asked the contractor to skim off a softened/disturbed zone in clay? Or a rutted/loosened surface in sands? Several or more times I would think. So, it appears to me that the original intent of the spec (to compact 6 inches) is really not to "improve" the founding stratum's overall support capability - but to rectify disturbed material due to excavation. That's my view. Is it necessary? Yes and No. I would say that contractor should be given choice to rectify by removing the disturbed material (that he disturbed) at his own cost and make it up using a blinding coat (mud mat) or crushed stone. Or, he could compact to a "uniform density". That is my term - I like it better than achieving a specified 95% or 98% (modified or standard?). In other words to uniformity that would make any small deviations less problematic in differential (of course, for purists of the "contract law" - how does one measure uniform density, etc. - a bone of contention on my present contract).

In summary - I don't find that the compaction is for the purposes of increasing the bearing - it is simply to rectify disturbance due to excavation techniques. Taking this into account, theclipper could rewrite his specs along the lines suggested above.

If I may, I have the same type of spec for compacting the upper layer of the clayey foundation soil for a highway embankment. We are to achieve 95% of the modified Proctor (heavy tamping). The foundation clay is highly plastic, soft to firm and is 4 to 6m thick. So, what good will compacting the upper 500mm of this layer be? The embankment (to be compacted to 95% heavy tamping) won't care - and the settlements will be predominantly of the foundation clay so even the specs on the embankment (lower zone) is somewhat dubious. But, this points out how specs get applied from one job to another by people who have no real clue on why they are cutting and pasting - and many times the cutting and pasting makes no sense as the conditions are not the same.

 

[VAD]

Excellent response BigH, I was wondering when someone would discuss why compaction was required at the footing level. We should also realize that silty soils, high water tables may cause us grief if we were to apply this specification carte blanche. Yes indeed this taken for granted compaction has been and will perhaps will continue to be controversial. In relation to the highway embankment heavy compaction if the soils are swelling clays could result in creating a medium that will swell and result in future pavement distress. Very often one wants to have the moisture on the high side of the determined optimum to minimize the tendency for swelling.

In cut areas some specs require that the top 2 ft of the grade be excavated or ripped and be compacted. This again should not be used indiscriminately. The concept is to provide a road bed upper layer that has material that is of some uniformity by removing in some materials stratification etc that may be consistent with the ground immediately outside of the road prism and as such in cut areas minimize the ability for example in having a continuous seepage path. As another example, this reworking would allow weak pockets to be removed.

However, if I am in a cut that the material is determined to be of uniform consistency or sand,or gravel, I would not undertake this reworking because there would be no benefit. Note that I would inspect the material before making my decisions. Specs are often written for guidance only and judgement has to be used. Of course, it is necessary when in doubt to determine what you are about to undertake is not something which is againt the premise of the design. By the way does any one go out with a copy of the design in the field?

However, this is carrying logic too far as specs seem to but just thrown in as no one wants to create work for themselves.

Some resort to Special Provisions to ensure that there is some connection with what has been designed to what has to be done. However, I have rarely noted such related to compaction in highways.

Here is one for you. I once asked a question of why the Proctor density and optimum moisture was specified for compaction of granular backfill against multiplate culverts when no tests are done in the field to verify that satisfactory compaction was achieved. My idea was to think about a method that could be used without geting in the way of the often conjested work area. I had suggested the Clegg hammer which seemed to be a hneat device. I was told that the specifications was just to make sure that you could have something to wave to the contractor if he was not doing what you thought was acceptable.This answer and specs came from the Bridge Group of a Govt Organization where every group was an Empire and everyone had their own specs.

Yes, indeed we always seem to say that the Contractor must comply when we do not even have the ability to determine how it can be done. In my books one must be able to concieve the construction before even taking on the task of supervision or designing. However, we all have to learn but unfortunately with the present approach today one hears that the contractor is required to take all the responsibility and so you get today many CYA (Cover your rear) clauses. The govt beauracracy is full of that stuff propogated by singular individuals on many occasions. So sometime you need to know that when you look at some specs.

Returning to the footing compaction, I fully agree with BIG H's comments and want to leave these questions for some thought. Do we normally determine the density of the foundation for small or medium sized buildings during our geotechnical investigation. Do we normally undertake laboratory compaction tests to assess the likely density that can be achieved. If we do then do we adjust the moisture in the field to suit the concept of compacting to achieve this density. What is the practicality of doing this on all sites.

Is it not interesting to know that in many cases we rely on SPT counts to determine the state of consistency and compactness of soils and use those to determine what values of allowable bearing pressure we recommend. Where does the density and moisture fit in. Do we realize that very often the physical state of the stratum is a function of its geologic history. Do we investigate this aspect always. Perhaps we should be always using tube samples to detremine this and other charactersitics, but the price goes up for such investigations and assessments. We have all the tools available but these have become less important. Let cracks occur in a house and see how the owner reacts. Yes it may be to some a run of the mill investigation that should be cheap but it is also one that result in numerous litigations. Have we caused this based on our practice.

Ripping and compacting often destroys such natural characteristics. This does not mean that one should not compact but one should examine each situation in the context of the design etc.

I would like to leave a little story which is less than a week old. I was asked by a company if I would look at providing geotechnical recommendations for a building site sice they undertook the investigation under the context of only doing soils survey? investigations and this was conveyed to the Client.

On reading the report I could not help seeing the terms geotechnical. In fact there was a heading - Geotechnical evaluation which stated that the purpose of the investigation was to undertake the evaluation of ground water and soil characteristics and went on to say that the Contractor needs to use the report to develop his own design. Needless to say the builder found this report not to be the norm as he was expecting recommendations on foundation types etc.

In discussing the issue with the owner, I determined that there was no real outline provided on what was to be done or intent of the infrastructure. I advised that in order for me to look at the problem, while I could use some of the information of the soils survey company? I would have to undertake my own investigation which would cost x dollars. Of course, this was about five times of what was charged by the previous company and as well other company X could do it for one-half my quote or better yet, one could purchase a previous geotechnical report from someone who had one done for a nearby site. This last one was from a Contractor.

Very interesting indeed as to the state of the practice in the real world but quite common place. Well, I said my price was my price and that while I respect the other companies who can do it cheaper, I will not lose any sleep over the matter.

As you will note there are different guises in relation to geotechnical investigation. Mind you some insurances regard soils investigations as being geotechnical investigations and I am not sure how soil survey fits in. You see all we have to do is to have an auger and being able to send someone out and put a report with glitz of covers and photos and we are able to compete. A few lunches, drinks at X-mas, picnics at the lake, fishing trips, a ticket or two to a vacation hideaway and nothing can stop us. Aren't the bigs boys doing this successfully? Check the daily news.

Well I must close by providing my first smileys. To BigH and to the rest

 

[BigH]

VAD - muchas gracias. Maybe we should start a thread on professional practice - the meaning of terms, expectations, etc.

One point you make is the "specification" as a guideline. It is how I thought, how I think. But, in working my current project, many "mouth" the guideline aspect - but come the auditors, whoa! We had some HDPE pipe to test. Normal practice on impact test is to drop a 5kg weight 1 metre. Well, the spec wrote 5kg weight dropping 1 mm. Could I change the spec?? NO - it might have cost implications came the Sr. Quantity Surveyor. So, the testing company dropped it the 1mm as "specified" and then the 1m as "practice". There are so many more of these types of stories. These adherences to specs become controlling event though they are (1) wrong - outright so (2) 6 of one/half dozen of another (3) not necessary. Auditors can make life difficult. We got hit because out of 1600 concrete tests, 1 test of 20MPa (cube - or 15MPa cylinder) had a 28-day value more than the specified (-15%) off the average of the three 28-day specimens. (It was (-16%) off). Yes, we got written up!!! Still the average 28 day strength was 20% higher than required.

to all !

 

[VAD]

Could the auditors not have used some rational engineering judgement. I presume they were Engineers not bean counters.
Yes It is realized that specifications must be adhered to for the reason that some people may make uninformed decisions and the idea is perhaps to hold everyone at the same level. This does not give credit to those with experience and perhaps we all behave that way depending on whose side we are on. It would be a good day when we can discuss the implications of one slightly low result in an intelligent fashion with out pulling the specs only. You know though that the practice with the end result specs gives a penalty or bonus for tests that fall short of or exceed the specifications. I am not sure how this is exact for man made materials. What we tend to produce to day are Engineers and Techs who are skilled in the ar of producing fiction. Of course this is much liked by the Contractor who is today working as they say much better to give better results. I often wonder what percentage is cooked as has been discovered as well.

There was a case of a Contractor's QC personnel invading the Clients QA lab in the night and applying his skills to allow the results to be acceptable. Of course where there is money to be made there are crooks as well and this is not only attributed to dope peddlars. Are we recognizing that there is an old saying smarter the government wiser the population.

 

[BigH]

So right - and I've caught them at it too! Even when the auditors are engineers - they, too, are so afraid, they just go by the book. Sad. Best to all. I've a presentation to the government tomorrow on fly ash in embankment construction - found out about it yesterday. Fun day off!