value of requiring compaction measurement for footings for light frame construction in good soil? 8

[Engineerataltitude]

In the jurisdiction where I do most of my practice (Sierra Nevada in CA), the soil is really very good for footings for light frame construction. All the preliminary geotechnical investigations I have seen in the past 30+ years have either 2000 or 3000 psf soil bearing allowable for locations where there are no troublesome soil indicators. The local building departments in this area are very reasonable about not requiring geotech reports unless there is some kind of indicator for atypical soil.

With that in mind, is there much or any value to requiring compaction testing for the bottom of footings placed in competent native soil placed at the prescribed embedment depth (either 18" or 24")? Seems like overkill testing to me. My understanding is that if you have excavated into untouched native soil, it is usually compacted adequately for building light frame construction. Correct?

Any benefit to compaction testing I might be missing? I try to be sensitive about not requiring costly inspections if there is limited value to either safety or quality.

btw, I am a civil engineer in practice in this area for the past 30+ yrs. For whatever that's worth.

 

[r13]

Based on your time in service and familiarity with the local soil condition, the reasons I can find that support soil testing are "formality" and "just in case".

 

[GeoEnvGuy]

I always find the foundation inspection letters are just a check box the building department file away. I believe the only value is the building department knows that a professional engineer actually visited the site and confirms the foundation was placed on the expected material. If they removed this requirement it would allow for contractors to cut corners.

 

[Ron]

Engineerataltitude.....there are several reasons that requiring compaction testing, even for light frame construction, is a good idea.

I have seen, particularly over the past 30 years, a decline in the quality of knowledge of contractors. They have gone from constructors to construction managers. They often do not vet their subs for quality and most of the time they get by with it. That premise alone has led to numerous construction defects and failures. I have seen contractors with reasonably good reputations make really bad calls with respect to soil conditions, occasionally with catastrophic results. I see these results as a failing of the following:

1. Construction education has moved to a trend of management rather than a strong technical background. There is little emphasis on learning to build rather than learning to manage a process.
2. Construction laws have followed this trend or vice versa. In most jurisdictions, a license is required for the primary contractor, the electrical contractor, the plumbing contractor, the roofing contractor and the HVAC contractor. Most often other subs of the prime contractor are required to have no training or competency licensing. This leaves the foundation, the framing, the exterior envelope and numerous other subcontracted tasks to potentially untrained or apprentice trained by ignorance (we've always done it that way....how many times have you heard that?).
3. Many in the construction field tout that they have "20 years of experience" when in fact they have one year of experience 20 times. There's a difference.
4. Soil conditions vary, even within the same site. I have seen small, light frame construction buildings settle on one corner because of an isolated soil condition. This is not uncommon.
5. One primary reason for compaction of a site is to provide uniformity. To verify this uniformity requires compaction testing.

My primary practice is construction and structural forensics. I spent 25 years in a testing atmosphere with large geotech/materials firms and had my own testing lab for 6 years. I have run thousands of tests myself. Additionally I teach part time at a regional university in the Construction Management degree program and am the primary author of a textbook on commercial construction that is used by over 50 colleges and universities in the US and Canada, so I have seen all of what I noted firsthand. Not just pulling this from the posterior

 

[oldestguy]

Ron has hit it again. His posts usually are a few words, all great. Now all great and that voice of experience is valuable stuff here. I can only add "it keeps people honest".

 

[r13]

"it keeps people honest".

How true!

 

[r13]

In the 90s, I was in China for a co-gen project, which was a design-built project by the Chinese for an American company. Soil lab was scarce, wait time was long and often missing scheduled date. In order to avoid delay, an old Chinese engineer insert a steel rod a few inches into the excavated ground, and dropped a weight to observe and measure the settlement. Per his claim, he had performed this simple test and used it as the base for acceptance/rejection criterion for over 30 years. His test results were latter upheld by the lab compaction tests.

 

[Okiryu]

Hi Ron...

I like your statement " ...Many in the construction field tout that they have "20 years of experience" when in fact they have one year of experience 20 times..."

I do not have that much of "overall" experience yet but I can see a lot of "seniors" with that kind of experience you mentioned... !

Compaction tests are not expensive and give proper indication for subgrade preparation... so I am not sure why people is planning to delete them rather than increase them !

 

[Engineerataltitude]

Thank you for the responses.

Ron, I really appreciate your perspective. You best reason for the compaction testing seems the most realistic. I have found the same reduction is subcontractor expertise. Not much "journeyman" education being done in any of the trades these days. It's really too bad since so much experience is being lost as good contractors age out of the industry. Don't get me started...

One of the things I have going for me in the area where I do most of my work is that there are only 4 Building Inspectors in both the County and City where I practice and they are all quite good and very experienced. I believe their expertise counteracts the effect of foundation subcontractors not knowing what they are doing. They routinely write up "footing base not on competent native soil per plan" Making the contactor either compact the bottom of the footings or dig deeper.

Okiryu mentioned that "Compaction tests are not expensive and give proper indication for subgrade preparation". Actually, in the rural area where I am located, compaction testes are quite expensive and are hard to schedule. Only one firm does compaction testing for two counties. The owner of the firm (not a geotechnical engineer) knows it and charges a lot for the testing and reporting.

From a professional perspective, I am keenly aware of the crisis in affordable housing in the state of CA and try as a licensed civil specializing in structural work to only require on my projects that which is needed for either safety or quality.

My understanding is that soil compaction is independent of soil bearing capacity. Correct? Soil bearing capacity is based on the type of material in the soil. Compaction is how well that constituent material is compacted together, i.e. removing air between the soil particles. Correct?

I'm trying understand the risk associated with poor compaction. I understand that the risk associated with overloading the bearing capacity of the soil is settlement. What is the risk associated with poorly compacted soil? Settlement again? Something else?

 

[GeoEnvGuy]

To respond to the latest post by the OP

My understanding is that soil compaction is independent of soil bearing capacity. Correct? Soil bearing capacity is based on the type of material in the soil.

Bearing capacity and settlement are a function of the materials relative density more so than type. By compacting soil you increase the relative density of the material, in turn you increase the amount of effort required to shear the soil (AKA bearing capacity) and you over-consolidate the soil so the new load will be less than that applied during compaction thus reducing potential settlements.

Compaction is how well that constituent material is compacted together, i.e. removing air between the soil particles. Correct?

Soil is a three phase system comprised of solids, water, and air and will have a bulk density typically between 1.6 and 2.2 g/cm3. The solid portion will have a typical density between 2.6 and 2.7 g/cm3 water will be at 1 g/cm3 and air is around 0.00012 g/cm3. Compaction removes both air and water in partially saturated materials with diminishing effects further from the compaction.

I'm trying understand the risk associated with poor compaction.

Bearing capacity failure as a result of insufficient shear resistance

By reducing the compaction the shear surface will offer less resistance and the footing will rapidly displace down. EDIT If the material is a soft clay or a sand which has a really low relative density or founded at greater depths you can have a punching shear failure.

Depth of influence for settlement from a footings width

If your material is not compacted above the expected bearing pressure in the upper zones which have the highest levels of influence over the amount of settlement, increases in load until the construction is completed will cause significant settlement. So when the footing is poured and you attach your first level of foundation walls additional loads will likely cause differential settlement thus potentially causing issues in the foundation walls.

 

[Okiryu]

Engineerataltitude, understand about the lack of testing companies in your area...I assume they do sand cone tests so perhaps a nuclear gauge equipment can help in this case...

 

[Engineerataltitude]

Thank you for the responses and figures GeoEnvGuy. Very helpful. Would you mind sharing the source of that depth of influence for settlement figure? If a textbook or a good reference book, I'd like to get it.

So as to my original question, trying to understand the risk associated with poorly compacted soil, sounds like it is unplanned settlement. Correct?

So my risk assessment on a structure that didn't get compaction testing on the soil at the bottom of footings would be what would happen to it if some or all of the soil under the footings settles differentially. Is that right?

 

[oldestguy]

OG here for comment. Whether by this diagram of stress or not, here is the rule I use as recommended by B.K. Hough in his text. "Basic Soil Engineering". When the added pressure under the footing due to its load is below 10 percent of the existing soil pressure, that's sufficient depth to be computing the influence of the footing below that point. Maybe call it depth of influence. So, if the footling is deep, the depth effect of it is less than if the footing was shallow. So that pressure diagram has to be used along with the natural soil pressure from earth above the footing, etc.

 

[Ron]

@OG....good clarification. Agree.

 

[Okiryu]

OG, I recall you suggested that in previous posts....thanks for the reminder !

 

[GeoEnvGuy]

The image of influence for settlement is a bousinesq stress plot from google, it is a common figure in many textbooks. Also the plot changes whether you have a circular square or strip footing. You would also need to consider the depth of the footing as described by OG above.

You are also interested in a good reference textbook there are many free publications from department of transportations in the US. In Canada the primary reference is the Canadian foundation engineering manual. In most universities I believe they are typically using current textbooks authored by Dr. Braja Das.

 

[Engineerataltitude]

Excellent technical advice from everyone. As always I can count on the ETF experts for well-reasoned and experienced advice in engineering areas I'm not as strong in. So glad this forum exists. Thank you very much.

 

[BigH]

Love my mates Ron, Oldest Guy and others, but I’d like to come at this “issue” of compaction of footing foundations from another perspective.

First, one should know, from the geotechnical report, the nature of the founding material. Firm (medium stiff for the Yanks) silty clay or clayey silt, compact (or medium dense) granular for example. This should be established and a thorough confirmation is to be made on site at the time the proposed foundation level is reached. At the foundation level, is the material the same as what the geotechnical report reported? Perhaps the geotechnical report identifies that the founding material is a mottled brownish stiff clayey silt but, upon exposure, the actual material encountered is a dark grey pocket of fibrous peat (Don’t laugh, it has happened!). This would set off a light-bulb that something, at this footing location is not correct. Corrective/design change measures are needed. It could be, too, that the material appears to be the same, but instead of being stiff, it is firm. Again, this has a design implication.

Going on taking the material as “matchking” the geotechnical report, secondly though, is the material disturbed due to the excavation process? Obviously digging a trench for a strip footing with an excavator or backhoe will leave teeth marks in cohesive materials and “remain” whereas in sandy/granular soils, it will leave marks but they can be leveled rather easily. How deep did the disturbance go? 50 mm? 100 mm? More? Less? This is important, in my view.

Thirdly, “compaction” and/or compaction testing of the foundation level – how necessary is it? Posts above, say it is absolutely necessary. I can see that there are benefits IF the founding material is granular as some of the above are familiar in Florida, New Jersey coastal cities and the like. Most footing sizes will only permit “compaction” of the foundation level with small equipment, i.e., jumping jacks, vibrating plate compactors – that are able to be able to fit in the trench. Such equipment does have a limited depth of compaction. If one does a field density test, one must understand that he/she is only testing a small distance below the granular foundation level. Troxler’s test probe typically goes no deeper than 200 mm (at least when I was using them). Sand cone tests or balloon tests go about 150 mm typically. Limited depth! How does this compare to the depth of influence of a footing? 2x width for square footings or 4 x width for strip footings. Footing width of 450 mm means that the zone of influence is between 900 to 1800 mm. One has now confirmed the upper 150 to 200 mm; one has not confirmed the zone below this testing depth. So the question remains, what is the efficacy of the compaction test in the sandy soil? (and remember, sandy soil needs confinement; when I tested sandy soils for roadways, general thick fills, upfill for building floors, I have always thought that the previous layer should be the layer for which the compaction should be checked. If this wasn’t the specified testing for compliance, I would, like most, do the sand cone at the surface and if it didn’t pass, say 93% rather than 95%, I would carefully dig out 50 to 75 mm and do the sand cone from the “lowered” level – which, of course, is still within the layer being placed. Invariably the test results would be fine.) Again, experience and engineering judgment must be exercised.

Going on to cohesive founding materials and assuming that the material encountered is as expected, what is the effectiveness of compaction or “compaction” testing? In my view, not much. Care must be taken upon exposure to carefully trim the teeth marks to undisturbed condition. Compaction using jumping jacks or vibrating plate will have little effect on cohesive soils if encountered. One seldom uses smooth drum rollers in such materials – pad foots are used for kneading purposes. In cohesive soils, the use of hand pushed probes is more effective to ascertain the level of disturbance. One probably, as well, does not have a Proctor value for a small project for cohesive soils. One could, too, use torvanes or miniature vanes.

In summary, while one can see benefit of compaction of foundation level and/or carrying out field density tests for sandy/granular soils for the purposes of surface disturbance, the same efficacy cannot be said for cohesive soils. The most important point to keep in mind – Is the material exposed for the foundation the same and in the same “condition” as that given in the geotechnical report? Has the material been disturbed? Granular and cohesive soils behave differently and this should be taken into account.
Anyway – these are some of my thoughts.

 

[Ron]

Excellent post, Big H! Loaded with common sense, practicality and sound technical information. Completely agree that the encountered material will dictate. My only reservation is that while I would trust your judgment without question....not so of a contractor with no particular background in soil mechanics.

 

[Engineerataltitude]

BigH, "2x width for square footings or 4 x width for strip footings. Footing width of 450 mm means that the zone of influence is between 900 to 1800 mm. One has now confirmed the upper 150 to 200 mm; one has not confirmed the zone below this testing depth" I had the same thought looking at that zone of influence diagram.

I work in an area where the soil is almost always cohesive. Occasionally I encounter areas of fill or sandy areas near former creek beds or areas on steep slopes that need specialized foundation design attention. But for the most part the native soil (even on slopes with topography) is naturally very compact when we dig down 24 inches to get to the frost line foundation embedment.

I am the only licensed civil doing structural work in 2 counties, so I get most of the small-to-medium sized development work. I am the only one who does insurance damage inspections in the area. In my 30+ yrs I have never seen a building of any size develop structural problems due to settlement unless it was near a stream bed or was something built 50+ yrs ago before we had decent building inspection (i.e. old cabins) or if the proper embedment on a slope was not attained. (CBC: All footings shall be embedded deep enough that a 5 ft minimum horizontal distance to daylight from bottom of footing is attained). Structural failure due to settlement problems is exceptionally rare around here.

I never engineer a foundation on a site I haven't inspected. I frequently bring and use a shovel.

What I consider in lieu of requiring compaction testing of the bottom of footings is what would be the effect on the structure if the soil was poorly compacted. The answer is different for continuous footings than for isolated footings. An inverted T continuous footing is a really strong concrete beam. It can actually span a decent sized hole beneath it if necessary. It is different for isolated footings. So I do a practical evaluation of settlement risk based on what effect settlement might have on the structure. Sometimes I bump up the footing size of isolated footings if they are located in area of the building more sensitive to settlement (i.e. posts under decks built on a slope) The difference in cost is the increased size of those bigger footings vs. the cost (and schedule delay) of getting compaction testing. I think the increased cost of slightly bigger footings is the lower cost and the better way to go.