Exception for footing subgrade inspection

[Lion06]

I have read chapter 17 of IBC pretty throroughly, but can't find any exceptions that allow you to get out of footing subgrade inspections.

There are exceptions for concrete footing inspections, and I thought there might be something similar for footing subgrade if you used some very low value for allowable bearing. That doesn't seem to be true, though.

Am I right, or is there something located someplace other than IBC chapter 17 that would allow this?

 

[conceng]

So far as I know there is no responsible reason for eliminating bearing soil inspections. I doesn't really matter how conservative you design your footings, if you have 10-feet of soft swamp muck as a bearing soil, lawyers will eventually be involved.

Greg

 

[BAretired]

One would hope that the contractor would not pour footings on 10 feet of soft swamp muck.

For single family residential footings, it is not likely that the engineer will make a special trip to inspect the subsoil unless the contractor indicates concerns about the soil.

For larger commercial projects or for any project with doubtful soil conditions, the geotechnical firm should inspect to confirm that the subsoil is in accordance with recommendations made in the original soil report.

BA

 

[Lion06]

We've been getting some smaller work from architects lately, that require a small number of footings and the loads are VERY small.

We've been getting a lot of push back from contractors about the inspections. They've been saying that they've never had to do these types of inspections before for the type of work they're doing.

I thought there might be an out somewhere if you use certain conservative assumptions.

 

[BAretired]

SEIT,

Why should the contractors be concerned? It isn't costing them anything to have you visit the site to inspect the soil. In my experience, it has never been an issue.

On the other hand, if you are asking the contractors to retain engineers to inspect the subsoil, I could understand their reluctance to comply, particularly on smaller projects.

BA

 

[msquared48]

SEIT:

Table 1804.2 applies here, and, yes, it can be done without an on-site inspection of an engineer. I say that tongue in cheek.

The table implies the use of local soil classification maps to determine the general soil type, but also includes the local building official in the decision making mix. The latter gets into a plethora of previous documentation, some of which are other soils reports, that may reflect soils poorer than the soil classification map for the project site.

The problem here, in my opinion, is that soils maps are not very reliable, as local anomalys do frequently occur. However, on a broader scale, they may apply. Consequently, I still rely on site inspections and/or test pits to see what's there. There is nothing wrong with testing the bearing level of soil yourself with a piece of rebar and making a professional judgement as to it's capacity based on your experience, so long as you do not go beyond your level of expertise.

That being said, never, but never, allow a contractor dictate what the soil bearing should be, or whether an inspection shoulod be done when your reputation is on the line, plus the safety of the public.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[beton1]

If you have a geotechnical engineer/firm that you work closely with, it may be a good idea to contact them. Explain the situation (small jobs, low loads). Likely they are familiar with most soils in the area and can warn you of problem areas/locations. Educate yourself, have lunch and do a drive by if possible. I had a soils engineer who
gave me some basic insight. If there is no soil report, they will do a walkthrough, 'heel' test or probe with steel rod. As BA mentioned, usual cases of residential do not have a soils report/investigation. Some day you can return the favour to your friend.

 

[Lion06]

This is a very small project with an even smaller budget. It is the special inspections as required by Chapter 17 of IBC. It requires that the footing subgrade be inspected to verify the bearing capacity is as designed for.

There is no geotech report (and there won't be), but we have information from an adjacent building. We're being conservative in using 3ksf.

The problem is that chapter 17 of IBC seems to REQUIRE a footing subgrade inspection by a special inspector. There are two problems that I see with my firm going out to do this inspection. First, I don't know if we technically are allowed to be a special inspector for our own job. IBC says that the special inspector should be hired by the owner (but I believe this often gets passed on to the contractor). Second, I have no idea what 3ksf soil looks like, feels like, or anything else like.

 

[Ltwine]

In using presumptive bearing values from the code for small projects, it is better to exert extra conservatism to account for the unexpect conditions. I often increase the safety factor to the extent that the cost (of footing) may see 10%-20% increase.

Since no lab was involved, the inspection is just to have a general feel of the uniformity of the excavated soil and effort of compaction. Make sure there is no sitting water, and debris. The "rebar method" mentioned by MS48 is a good way of measurement (check penetration depth from free fall at a fixed distance).

The contractor has no obligation to "inspect" the excavated material, but to maintain correct dimensions. It is the engineer's/inspector's job to identify the problems and come up with corrective measures, though some of the contractors are willing to do that, but the ultimate responsibility is still at the engineer's/inspector's hands.

 

[msquared48]

If it is a small project, as in small retail or residential, why would you even need 3000 psf soil? 1500 psf should work just fine.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[Lion06]

There is virtually no DL, and reasonable overturning and uplift.

Even if 1500psf would work, and I used it, doesn't 1704.7 require footing subgrade inspection to ensure that the actual soil is capable of supporting 1500psf?

 

[SirAl]

Sometimes it is the small add-ons or similar modest projects that can open up unforseen adverse conditions. Former excavations, in-fill, landscaping, old utility trenches, drainage issues, etc. Dry, crusty, poorly compacted fill will settle when moisture softened, no matter how conservative the recommended bearing values are.

 

[beton1]

I would venture to say that you or someone in your office has to make a visit to the site and make a judgement on the soil capacity. Is it good, bad or ugly... If ugly, get a geotech out there! Local building inspectors often comment on residential sites and their input may put your mind at ease. 1500 psf is a threshold. Thereafter, you are looking at alternates to shallow foundations.

 

[msquared48]

I will look at Chapter 17 harder, but in the 30 years or so that I have been practicing structural engineering, bearing verification was not needed below a certain value. For years, that was 2000 psf.

Currently here, most jurisdictions use 1500 psf as the threshhold, and I believe that that value stems from the chapter 18 table I previously qouted, but what do I know?

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[Lion06]

Mike,

I don't doubt your knowledge in this area being greater than for a second. I just want to be able to follow the logic of the code to reach the same conclusion.

I see the tables for presumptive bearing for different soil types. I don't remember off the top of my head where it is, but you say chapter 18, so I believe that and it makes sense.

When I read chapter 17, though, (the chapter on special inspections), it gives some out for concrete construction, but doesn't give any outs (that are jumping out at me) for soil bearing.

The way I read it is that those presumptive values can be used for bearing capacities if you don't want to have a geotech study done, but that the special inspection is still required to verify that the presumptive value used is valid.

Do you read it differently?

 

[msquared48]

SEIT:

I looked at Chapter 18 again. Look at section 1802, specifically sections 1801.2 and 1802.2. The latter specifically states that soils investigations are required in certain circumstances as listed in section 1802.2.1 through 1802.2.6. Outside of those areas, as stated in the exception, the local BO can deem that an investigation is not needed and the values in the table I previously mentioned used.

I will now look at Chapter 17...

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[msquared48]

SEIT:

OK, I looked at Chapter 17.

Look at the three exceptions for section 1704.1, particularly 1 and 3. Then look at Section and Table 1704.7, noting the link in the text to Section 1802.2.

The way I still read everything, if the local BO says that the soils are OK to be designed to 1500 psf or whatever, then special inspection, such as a soils report, is not required. It would be prudent, however, to do an on-site inspection during excavation to verify the conditions, which would comply with Table 1704.7.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[conceng]

The way things have been working in our market lately is that if the building inspector shows up on the site and has any concern whatsoever regarding the bearing soil conditions, he requires the contractor/owner enlist a geotechnical engineer to evaluate the soils.


Greg