Footing Inspection 2

[visitor]

My company is active in performing roadway inspection so we have many nuclear gauges and DCPs. We are now getting into building inspection. My question is regarding equipment for footing inspections. It seems that in our region, the pocket penetrometers are typically used for this test. From what I have read, it seems that the static cone penetrometer is used less often but is actually more accurate. Do these two pieces of equipment give roughly the same results? From what I have seen, there is no ASTM standard for footing inspection and therefore no specified equipment (unless specifically sited in specs). Are there any sources of info out there on standard procedures for this testing. We have people who have done this work at other companies, I am just looking for some documentation of the proper procedures and equipment.

 

[Ron]

Footing inspections should only be done under the direction or supervision of a qualified geotechnical engineer. The equipment is not necessarily the issue...the evaluation of the conditions is the key.

 

You are looking for a specific relative proctor density. This is what matters, but the most accurate method used to co-relate nuclear and/or DCP is the sand cone method. This can be used for checks and balances. A vane can be used too but has to be co-related to lab tested density vs shear strength.

 

[TAC]

I agree with Ron. The geotechnical engineers at our company routinely perform footing inspections. The primary purpose of the inspection is to verify that the near surface soils in the excavations are consistant with those encountered in the geotechnical exploration. Without a soils exploration, you are limited to what can be observed in the sides and bottoms of the excavation. If conditions vary significantly with depth, problems could arise.

I tend to prefer the hand penetrometer on clayey soils. Mostly because we utilize this instrument in the lab and I have a good feeling for the correlation between the instrument reading and soil strength. Because bearing capacity is not a property of the soil but is effected by many factors including soil strength, footing size, and depth of footing. Knowing how to interpret the readings from the instuments and understanding their limitations is the real key.

 

[dhooper]

Sticking a pocket penetrometer into a bottom of a footing excavation as a test for bearing capacity is not reasonable, yet I have seen it done. The zone of influence is not even going to extend beyond the limit of backhoe teeth.

Testing the density of foundation soils through the nuclear method or other method only makes sense if the geotech design engineer has specified a minimum density. Because bearing capacity is dependent on so many factors, a specific requirement for soil density as a determination of B.C. is rare.

The DPC can give a warm and fuzzy feeling and some numbers to show testing was performed. The real issue is having a qualified geotechnical engineer responsible for overseeing the evaluation. Preferably this engineer should have been responsible for the geotechnical recommendations and understand what are the important subsurface issues.

 

[72shortbox]

a few of you refer to a DCP I an assuming that means dynamic cone penitrometer. I have heard them refered to as thumpers or smashers. ( a 10 or so lb weight dropped driving a 1 inch or so dia rod into the ground)

1) Is there a standard piece of equipment
2) where can I buy one
3) are there any correlation tables

 

[Ron]

DCP's are standard equipment and there is an ASTM standard. There is also a correlation to SPT (George F. Sowers, Ga. Tech, sometime in the 1960's). It uses a 15 lb. weight falling 20 inches to drive a 1.5-inch diameter, 60 degree cone.

Many of the soil equipment suppliers probably have them. Try Durham-GEO in Stone Mountain, Georgia (

http://www.durhamgeo.com)

 

[isola]

very interesting -thank you

 

[DRC1]

Footings on soil for buildings are typically designed to bear on natural ground and compacted fill is usually avoided, unlike roadway construction where building on compacted fill is common. Therefore for the footings, troxler gauges are not required. Where we have used them is inside the footings,gravel is placed to a specifed density for the floor slabs. Although monitering is not usually done in residential, it is done a lot for industrial or other commercial applications where large floor loads are anticipated.

 

[nbryan]

Try this as a reference for performing dynamic cone penetrometer testing:

ASTM Special Technical Publication No. 399

 

[cslaude]

I know I'm probably posting to this thread long after it is relative, but what I've read scares me.

It is critical (and required by code) that a licensed geotechnical engineer that has experience with the local soil conditions inspects all foundations. Since the foundation is the base upon which the building will be constructed, doesn't it make sense to hire a competent inspector? If not, why bother to perform an inspection at all?

None of the posts so far show even the slightest grasp of the basic concepts of foundation design or inspection. I would be alarmed if I knew that any of the other persons posting in this thread were providing geotechnical engineering services in my geographic region. For anyone who has the slightest interest in getting a foundation inspected, I suggest you avoid anyone other than a licensed, experienced geotechnical engineer in the geographic region you are building in.

To those professional engineers posting in this thread, I urge you to read the code of professional conduct and state regulations under which you are licensed. Your comments are irresponsible. They can be construed as practicing outside your area of experience and expertise, and you can be reported to your board. If the board finds that this is the case, you can and probably should lose your license to practice engineering.

 

[Focht3]

[blue]cslaude[/blue]:

First, Welcome to [green]Eng-Tips[/green]!

While I agree entirely with your sentiments, don't paint with too wide a brush. Please re-read the comments by [blue]Ron[/blue], [blue]TAC[/blue] and [blue]dhooper[/blue] - they said essentially the same thing that you did, although with less passion. (I like to see engineers that are passionate about quality and ethics!) The rest, unfortunately, seem to fit quite nicely in the lump of "I'm just a tester" mentality. What a shame.

Keep in mind that a good number of posters to this site are not licensed engineers - hell, a good many aren't engineers at all! So I'd suggest that you "pick your fights carefully", or you'll burn out quickly. I would also caution you about getting too personal. (Not that you have - just a caution.) Inappropriate posts are 'red-flagged' and the site's management can (and has) removed members for inappropriate behavior. It's an easy line to cross. (I've gotten pretty close a few times, myself.)

And use the "mark this post as a helpful/expert post!" link when you think that a poster has done a particularly good job of addressing an issue.

Again, welcome to the site. I look forward to reading your posts.



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[MRM]

cslaude,
Hi. I'm kind of confused. You say you are concerned that the posters are giving advice outside their area of expertise, but as I review the posts all of the "regulars" (Ron, TAC, DRC1, dhooper) have all expressed their view that footing inspections SHOULD certainly be done by a qualified soils engineer in a specific area. Please explain what you mean because it sounds like there is something that I'm missing. Thanks.

 

[cslaude]

I apologize if some take my posting personal.

1. If you are a licensed engineer in my town, I will hold to the the codes of professional conduct and ethics. Do not practise geotechnical engineering if you are not qualified.

2. If you are not a professional engineer, please do not provide ANY engineering services. It is unlawful and can be dangerous.

To the orignial poster, building inspection is not a field to "get into" because you have a few nuclear density gages and a dynamic cone penetrometer (see 1. and 2. above).

Having said that, here are my apologies:

Ron has intellegent and insightful comments. He sounds like he might be a good structural engineer. He is not a geotechnical engineer and should not address getechnical engineering issues, which he did not. I aplogize profusely for including Ron in my rant.

TAC says he uses a pocket penetrometer on clay soils, which will test the upper 1/4-inch of soil. This may give the causal reader and the original poster the erroneous impression that a pocket penetrometer may be used to evaluate footing excavations. As he points out, "The primary purpose of the inspection is to verify that the near surface soils in the excavations are consistant with those encountered in the geotechnical exploration." I meant no disrespect to TAC and applaud the "Knowing how to interpret the readings from the instuments and understanding their limitations is the real key."

DRC1 did not address the original posters question. I Apologize to DRC1 for not excluting him from my rant.

dhooper is right on the mark and I apologize profusely. I should have applauded him.


Christopher S. Laude, P.E.
Carolina Geotechnical Services, P.C.

 

[PSlem]

The residential building code does allow blow count in lieu of actual density tests. The DCP is my method of choice as my SCP gives erroneous readings when there is rock in the ground. Durham Geo makes both, but instead of the $25 probe rod they have one fitted with a hydraulic cylinder and pressure gauge so every user will report similar numbers. You will still need some local knowledge to relate either to bearing strengths. For Atlanta Piedmont soils, clayey silts, 250N=S Mainly I look for buried topsoil and trash or excessive fill so I tend to drive through neighborhoods before footing grading begins to assess the situation better. Checking of the bottom of the footing rarely gives you a good estimate as the grading contractor will put extra effort in the last few feet.

 

[MRM]

Hi slaude,
I think I see now. There are a few things that I've learned when participating in these discussions.

Focht3 mentioned or warned first that there is no way of knowing if a poster is a PE and what their field of expertise really is. For better or worse, that is just part of the territory of these fora in that anyone is welcome to post, engineer or not. I know what you mean in that this complicates things greatly in some cases. In other cases though, even the comments of a non-engineer or non-PE may be enough to help you see things in a different way. Because of that, it's never really bothered me much that non-engineers or non-PE’s may be posting to threads. I'm making my own mind up in the end anyway. I'm routinely given advice on how something should be done by an excavator operator in the field. That doesn't mean I'm going to take it, but I may listen anyway.

FYI: Within the "Corporate Survival" and other Ethics-type threads, there have been interesting discussions related to whether or not non-PE, non-engineer posters should be allowed to participate in the threads, what the ramifications of posting are, and what constitutes giving "professional advice." There have been some interesting viewpoints. I've enjoyed reading these discussions and you may too! Just search around and you'll come across some. I thought you might have an opinion to add to those threads too... there's quite a variety of threads!

The third thing that I've learned is that sometimes the title, "structural, geotechnical, mechanical" that one gives themselves is not a true indication of their abilities. For example, I've found that Ron, a "structural," usually has very good comments related to soils. This leads me to believe that he practices in this area of engineering (soils), perhaps along with structural, or alone. After reading many of his posts, I know that I can put some stock in what he is saying. After a while you can usually see who knows about what. Many posters will also warn you when they believe you should look further than just their comments. This gives you a valuable tool, and a fun way to bounce ideas off others, using this site.

 

[BigH]

Another point to be made is that in the various threads, they might start off on a specific question but are unlikely to stay exactly there - the threads seem to have a life of their own such that previous comment is then commented on and - well, liku liku thread (liku liku is Indonesian for meandering). But, such threads can be fun. As I had indicated in another, it is always good, though, to have someone come in a bit late and refocus on the original question.

Another point is the "area" of the poster. I always respect Ron's and JHeidt's comments ragardless of their "area". Having read hundreds of their posts, they have more knowledge of geotechnical engineering than most geotechs have. On the other hand, I have run into "geotechnical" who have more practical knowlege of road design than most of the "highway engineers". Areas of expertise cannot always be categorized in a simple one word term. to MRM on this point.

Now getting back to the original post - all projects should have quality investigations done. The footing founding levels have been chosen (presumably) based on the soil stratigraphy encountered and the local knowledge of the site. At the time of inspection, the basic focus should be is whether I am in the same stratigraphy or not - and does the strength properties of the stratigraphy reasonably match those given in the soils report. If my observations are that the soils report is correctly confirmed in the field, the footing subject to no-disturbance, etc. can be poured. As for the use of pocket penetrometers, etc. - these are tools to give the geotechnical inspector an "indication" of the soils met - in clays, it is almost always settlement that governs so, in the end, what of the footing level strength? This won't give us any information as to the consolidation characteristics (in the short time frame of opening, cleaning, pouring the footing). This is why qualified experienced inspectors are needed (whether a junior geotechnical or a senior technician). Unless you have tricky local soil conditions, most residential footings - or light commercial footings - are overdesigned for the most part anyway because, in most cases, geotechnical engineers recommend (suggest) minimum footing widths and structural engineers use minimum footing widths - so the actual bearing pressures are less than the theoretical ones.

Now as to the DCP. Be careful - all. This term is used for a variety of equipment. They range from what I grew up on (2inch cones driven by 140 pound hammer dropping 30 inches - standard Canadian practice - called pentests) to the TRRL miniature cones - 1/2 cone driven with small 5kg (I believe) hammer. Then you have the others mentioned in other threads. They all have their correlations - and are different.

So, cslaude - welcome to the site but read a lot of the past threads – find out who gives the good advice, then join in a lot of interesting and lively discussions.

 

[SirAl]

Great post BigH

You more than covered the points that I would have contributed to the topic of footing inspections. I would not put too much stock in density values or penetration results. Verification of bearing certainly should not be compared with breaking a concrete test cylinder to verify compressive strength. I have seen dry crusty fill (seemingly hard and strong) become soft and settle when moistened. Conversely, I have seen wet silts (SPT < 5) perform adequately.

A good topic for discussion.

 

[BigH]

SirAl: Right on. Too, I have seen soft clays appear &quot;hard&quot; when exposed for a long time - you think that it is a good stratum and dig a small pit - no way Jose! That's one thing for a good inspector/engineer to know at the time of inspections. How long was the clay excavation open? If they don't have a clue about the soil report findings, they need to exercise extreme caution - under most circumstances.

 

[Focht3]

Good comments, all. If you read the older threads, [blue]cslaude[/blue], you will see that [blue]BigH[/blue], [blue]SirAl[/blue], [blue]MRM[/blue], [blue]DBNodurf[/blue], [blue]DRC1[/blue] and I have participated in many threads related to DCP, plate load tests, CBR tests, and attempts to correlate the results of those tests with &quot;allowable bearing capacity&quot;, modulus of subgrade reaction values, etc. While we are not all of a like mind, I find the discussions useful - and enjoyable.

And I have had a few good discussions with some non-engineers on this site. Those individuals can also provide useful comments and suggestions.

But you have to decide which comments are most useful for your particular situation. As you have said, we are not responsible for your design decisions - you are.



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