Getting Geotech. to verify bearing capacity? 3

[abusementpark]

I've noticed that a significant number of structural engineers put a general note on their plans for shallow foundations like such: "Owner's geotechnical engineer shall verify bearing capacity and suitability of subgrade prior to placing foundations."

Does your firm put a statement like this on their plans? If so, what is the specific intent of the statement? Like the example I gave above, many times it isn't clear to me what specifically needs to be done (maybe the specs clarify, IDK). I'm not sure if this is just reiterating that the geotechnical engineer needs to be on site for proof-rolling and all the general site preparation, as I would imagine most earthwork specs already cover that. Or is it indicating that the geotechnical engineer needs to be there to verify the subgrade just prior to the shallow foundation pour?

My firm doesn't normally make the latter a requirement. However, there have been some instances where I been out on rebar inspections for a grade beam or footing pour and noticed that the superficial subgrade looked poor (i.e. very soft, mud-like). But I have been hesitant to question it with too much scrutiny, other than making them scrape off some really bad areas, since I think the geotechnical engineer is the one who should be making that call.

Thoughts??

 

[Ron]

Abusementpark...this is fairly standard. It is a means to shift liability for the soil conditions from the structural engineer to the geotechnical engineer, where it should be. It is one of those statements that is intended to keep you from being invited to the post-construction litigation party, although I would probably word the statement a bit differently...something like...."Owner's geotechnical engineer shall verify suitability of bearing soils and subgrade prior to placing foundations and floor slabs". I realize this is a slight semantic change, but the other statement implies that the structural engineer considers only the bearing capacity to be relevant to his design, when in fact, there are other soil conditions that can impact the structural design as well.

Depending on the complexity of the project, it might or might not have significant implications. For instance, there are several levels of geotechnical site preparation, each on leading to a specific result. Assuming the site is suitable for shallow foundations and it is reasonably large, the entire site is typically treated to some level of proofrolling. Considering that individual column and strip footings would be excavated separately, the primary concern would be disturbance of the soils at the bearing level, since the geotechnical investigation has already determined that the soils have a suitable bearing capacity with appropriate site preparation.

While in-place density does not give the bearing capacity of a soil, if taken along with the other parameters already determined by the Geotechnical engineer, it can be a relevant indicator. Further, it is intended to validate the uniformity of the footing bottom preparation.

 

[ash060]

I agree with Ron.

On some jobs, especially the smaller ones, I may not have a geotech report, and in that case I get very specific in my notes regarding the foundation. I will use an allowable bearing pressure out of the building code, but have several notes stating that the foundation preparation is the responsibilty of the contractor and owner. Also, that they are responible for any problems arrising from bad soil, and several other notes as well.

I don't like being in those situations, but I can not force the owner to get soil testing done before or during construction. I just have to cover myself as best as possible

 

[Lion06]

This is a required special inspection. The footing subgrade is supposed to be inspected to ensure that the bearing capacity of the in-situ soil is in agreement with what you've used in your design (and what you put on the drawings).

 

[Lion06]

Yes, we have a similar General Note.

 

[JedClampett]

We put this note in our specifications (and our drawings when we remember). But it's not to shift liability. The Geotechnical Reports very often request this involvement. If they ask for it, we pass it on. Don't forget, they're making a pretty awesome judgement based on very limited information (specifically borings) that we're basing our whole design on. If there's some kind of garbage or dead bodies that they missed, this is the best way to discover them.

 

[Ron]

JC..you made me think a bit more. When there's not geotech involved, the statement is to wake up the owner and to shift liability from the structural. If there's a geotech involved, it is just a clarification of responsibility and to reiterate that the structural is relying on others for such data.

 

[rscassar]

I also have a similar request in my specification. I also put a note on the drawings as well as confirming on-site that the geotech has confirmed bearing capacities while I am inspecting reinforcement. I prefer to be given a geotechnical engineers site inspection report with a license number.

 

[abusementpark]

This is a required special inspection. The footing subgrade is supposed to be inspected to ensure that the bearing capacity of the in-situ soil is in agreement with what you've used in your design (and what you put on the drawings).

Is it specific on when the subgrade inspection is supposed to occur? The finished building pad could be inspected at the completion of compaction prior to formation of footing excavations. But the condition at the base of the footing excavations just prior to placement of concrete could be inspected as well.

 

[abusementpark]

Also, since based on the responses, most on here give a general note similar to the one stated in the original post:

To what level do the geotechnical engineers generally document their involvement in verifying that adequate bearing capacity is being achieved in construction?

For example, do you normally get a field report from the geotechnical engineer documenting their inspection of the footing excavation subgrade?

 

[Ron]

abusementpark...again, depending on the complexity of the project, you will likely get a geotech report of either a footing inspection, density testing or both. For small projects, light loads and good soil conditions you surmise the validity of the geotech report by density testing or lack of warning from the geotech. For more complex projects, you will likely have the geotech perform footing inspections and report such.

 

[Lion06]

It needs to happen after the footing excavation, after all, that's the footing subgrade. Our General Note also says that if the subgrade is found to be unacceptable that an additional 3' of soil should be removed down to acceptable subgrade with the overexcavation bein filled with lean concrete.

The geotech for the job has never been the special inspector in any job hat I've been involved in.

We typically get a report from the inspection agency (similar to a steel or rwbat special inspection) that gives the date of the inspection and what the special inspector found. It typically says that it was determined to be able to support _ psf in accordance with our drawings - it never goes anything higher.

 

[BigH]

So far (don't take exception Ron), a geotech has not responded. Here is my take - and I don't do structural drawings. (I apologise ahead of time for, perhaps, a winded response.) I am assuming spread footings for this particular discussion.

1. A geotechnical report is prepared. In the report, the geotechnical engineer will provide descriptions of the subsurface conditions that were encountered at the time of the investigation - areal and depth extents. The geotechnical engineer will identify the type of foundations that would be suitable to support the planned structure(s) and the bearing stratum that is appropriate for doing so. The bearing "value(s)" he provides will probably be net allowable bearing pressures (NOT bearing capacity) as he will likely know or show that settlement (serviceability) issues govern. He will estimate the settlements that will occur under the bearing "value". He will also provide details about adjacent footings and the effects that such footings (existing) or to be constructed concurrently will have on the allowable bearing pressures. In our practice in Canada we used to give allowable bearing pressures vs size of footing as standard - especially for Sarnia. I see that many states are now suggesting a similar practice - this gives the structural engineer the data he needs to design for similar settlements when different sized footings are adjacently situated.

2. The report goes to the structural engineer who will provide the design of the footing to handle the loading conditions that he anticipates. Hopefully, he will not deviate from the geotechnical report as to the bearing stratum chosen or exceed the allowable bearing values (unless permitted to for short term transient loadings). As the structural engineers point out, a disclaimer is put on the structural drawings that the founding conditions for bearing "capacity" must be confirmed in the field by the geotechnical engineer.

3. This is where I would strongly deviate to the conventional practice noted above by my eminent structural colleagues. Unless the structural engineer positively states that the bearing stratum is that which has been identified in the geotechnical report (and described accordingly so that there are no doubts) and that the bearing "values" used are in accordance with the geotechnical report, I think that such a statement is spurious at best. The structural engineer is asking the geotechncial engineer to "verify" but doesn't tell the geotechnical engineer what he is to verify specifically - i.e., that the design has not been 'changed' with respect to bearing stratum, bearing pressures and the like. The notes should also cover the design loadings/pressures should be so indicated; deviation for transient loadings should be identified.

3. When the work goes to construction, the verification process should only be to
a) confirm that the exposed bearing soils are
i) in accordance with the geotechnical report
ii) are in accordance with the structural design which is based on the geotechnical report
b) if the soil conditions encountered in the footing locations differ from that determined by the geotechnical report, the geotechnical engineer will immediately inform the EOR of the discrepancy and the potential problems that might arise if not addressed - can the construction continue?; are there any changes required due to the unexpected conditions?; is additional investigation required? The EOR must then act on the findings.
c) the contractor has not "screwed" up the founding stratum by disturbing the underlying soil due to excavation procedures and the like; and if this has been done, then the approved (signed off by the EOR) method for bringing the damaged bearing stratum into compliance with the geotechnical and structural requirements as stipulated under "undisturbed" conditions.

I do not see any other verification that can be done other than confirmatory geotechncial investigation (which 99% of the time would be outlandish). I hear of geotechs going to the site and sticking in a pocket penetrometer that measures the upper 8 mm or so of the soil - what does this have to do with the soil that is 1 m deeper? I hear of geotechnical engineers (and I have done it myself) pushing "sticks" into the ground - again, this can only tell one of the "surface" conditions - it doesn't cover situations like a footing to be founded in a desiccated crust - and the thickness of the desiccation below the footing might be the most important situation - and how would one verify this at a site visit?

In the end, there has to be belief that the geotechnical engineer and structural engineer have acted in good faith, that the bearing conditions at the site are what the geotechnical report so indicates (and this is the verification noted in 3. above) and that the contractor has not 'screwed' up the founding level with inappropriate construction activities - either directly or by, perhaps in some cases, not properly handling groundwater issues.

I hope that this helps to clarify what BigH, as a geotechnical engineer, sees the process and what in my view, a structural drawing must contain to clearly identify what the geotechncial engineer is to verify.

 

[msquared48]

BigH:

"In our practice in Canada we used to give allowable bearing pressures vs size of footing as standard - especially for Sarnia." Never heard of "Sarnia". At first I thought you were referring to one of the Balkan States. Then, based 0on muy level of experience on the subject, I got to thinking... No, they don't have Geotechs in Sarnia. So...

I have to assume that Sarnia is a particular type of bearing stratum that you have dealt with that has special characteristics, as in expansive clays?

Sometimes, for larger projects, the Geotech will ask the structural for maximum footing reactions from the anticipated structural system, but this is usually not done from my experience (it is a luxury for Geotechs) as I, as the Structural, am the last to be contractually engaged, and the Geotech report already exists.

So goes the prior planning phase...

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

 

[BigH]

Sorry Mike - Sarnia is a city in Ontario on Lake Huron - north of Detroit. It is the "home" of most of the country's refinery capacity (or was, at least) - most of the biggies are there. Our company did a lot of work there and the site is characterized by a desiccated crust of some 8 to 12 ft in thickness and then a firm to stiff clay (Su in the 900 psf range) down to basal till or bedrock. The charts of allowable bearing pressure vs footing size were especially important due to the large refinery footing loadings - and the "heavy" civil aspects in general. Of course, we didn't do this on smaller more routine projects.

 

[msquared48]

Mutually sorry... I guess I need to check my Geography a little here...

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

 

[BigH]

Mike - probably few outside of Ontario and oil people even heard of Sarnia. No probs.

 

[Ron]

BigH...nice job!

 

[abusementpark]

The geotech for the job has never been the special inspector in any job hat I've been involved in.

In my part of the country it is generally the norm. They tend to stipulate in the geotech report that they should be hired for inspection services during construction.

 

[abusementpark]

Also, delving into this issue a little further, I noticed that it is listed as a verification and inspection task in Table 1704.7 of the 2006 IBC for required special inspections of soils. The task is listed as "Verify materials below footings are adequate to achieve the design bearing capacity." So to meet code, it should be done on every job, unless you can get an exception from the local building official. The statement is still pretty vague with regards to how this should be specifically achieved. I guess it is best left to the judgement of the geotechnical engineer.

Coincidently, today I came across a geotechnical report in which this issue was discussed. He basically calls for his firm to be engaged in observation and documentation of the footing excavations prior to steel or concrete placement to verify that the foundation materials are consistent with the requirements given in the report and are able to support the design loads. Any soils with equivalent N-values of less than 8 blows per foot as indicated by blows with a DCP are considered soft/inadequate and are removed and replaced with compacted fill. This is first time I had seen someone put this in a report.

Thoughts?