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No Geotech Report

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peconsultant

peconsultant

Structural
Joined
Apr 4, 2011
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US
I have two questions:

1. What would be a good value to assume for the shear modulus of a sandy clay?

2. Would you ever design a foundation without a geotech report?

Thanks in advance for your help.
 
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1. I would ask a geotech that has done work in the area before.
2. IBC (if that is what you use) permits you to assume values without a report. I have done it but don't like it and think it is bizarre we have to jump through so many hoops for other calculations then it allows you to just assume a soil bearing capacity for a site.
 
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Let me clarify #2; IBC has values you can assume, don't just use whatever you want to assume.
 
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We have designed foundations for minor structures without the soils report (party shelters, gazebos and such). I typically used 1500 psf and put a note on the drawings that this is an assumed value and the owner is responsible for verifying before construction. This would be communicated via email to the owner as well so that he understood the risks of not having a soils report. A couple of times the city reviewer came back and asked for us to assume 1000 psf.
 
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Technically, you should not state the value as assumed. I have been called on that in the past by reviewers.

You USE the value in your design, so in your notes, state it as the DESIGN value based on Table XXXX in the IBC for example. Although not always possible, a personal visit to the site helps too...

Mike McCann
MMC Engineering
 
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If you need the shear modulus of a soil in order to design a foundation, you should insist on a geotech report.

BA
 
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It largely depends on the nature of your foundation. A simple spread footing carrying a minor structure then I would go with the IBC assumptions. Anything supporting a heavier structure, subject to uncertain conditions, or in a high seismic area (liquefaction) then I would insist on the report.

Explain to the owner he can either pay for the $2K-$5K report now or pay for it in construction since you will be putting it on your drawing that the contractor must confirm it before construction. The downside in waiting is that you may also incur charges for redesigning foundations if the existing conditions do not meet your assumptions.



PE, SE
Eastern United States

"If a builder builds a house for someone, and does not construct it properly, and the house which he built falls in and kills its owner, then that builder shall be put to death!"
~Code of Hammurabi
 
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If I had to wait until construction to get the report then I would not redesign the foundations for free if I requested a report be done and warned of a possible redesign required.
 
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Thank you, everyone, for your comments. I'm actually investigating a foundation that has already been designed and constructed. The engineer designed the foundation without a geotech report and assumed a value for the shear modulus. I am being critical of the original engineer and wanted to see if I was alone; based on some of the comments, it doesn't sound like I am the only one who would have handled the situation differently. I still welcome any further comments.
 
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Hmm, that changes the situation in my mind. Be careful if you're going to challenge the original designer. It is one thing to say "I would have done it this way" or "the way he did it is not the best way." It is quite another to say that what the original designer did was not acceptable practice. The IBC clearly allows a designer to assume a bearing value. There are many ways to skin a cat, and designers are not required to choose the best one, just an acceptable one.
 
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nutte, I agree with you. I am not attacking the engineer; the engineering design is only one issue of many in a project where excessive problems and failures with a foundation have occurred. The major problems have been with the construction practices. My intent has been to say there were some decisions made by the engineer that I would not have made, but I wouldn't go as far as to say the engineer was wrong in his decisions or negligent in his design. I appreciate your comment.
 
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My partial general notes for this and part notes for PWF footings are (I have similar notes for other foundations/piling, etc.):

GEOTECHNICAL

A GEOTECHNICAL REPORT HAS NOT BEEN PREPARED FOR THIS SITE. THE CONTRACTOR, THROUGH THE CLIENT IS RESPONSIBLE FOR OBTAINING A GEOTECHNICAL REPORT TO CONFIRM DESIGN ASSUMPTIONS. THE [CONSULTANT | ENGINEER] ASSUMES NO LIABILITY FOR THESE DESIGN ASSUMPTIONS OR FOR ANY FOUNDATION REDESIGN NECESSITATED BY DIFFERING SOIL CONDITIONS

FOUNDATION DESIGN IS IN ACCORDANCE WITH THE GEOTECHNICAL REPORT PREPARED BY XXX, HAVING A FILE No.xxx AND DATED XXX. THE CONTRACTOR SHALL REFER TO THE INFORMATION CONTAINED IN THIS GEOTECHNICAL REPORT AND SUPPLEMENT IT AS REQUIRED. THE CONTRACTOR SHALL REVIEW THE GEOTECHNICAL REPORT AND FOLLOW THE RECOMMENDATIONS WITHIN THE REPORT PRIOR TO PROCEEDING WITH ANY FOUNDATION WORK

FOUNDATION WORK SHALL BE INSPECTED BY A QUALIFIED [CONSULTANT | ENGINEER] TO CONFIRM THE DESIGN ASSUMPTIONS PRIOR TO PLACING CONC


FOUNDATIONS (GENERAL)

SOIL CLASSIFICATION (CASSAGRANDE) IS SIMILAR TO xxx

REPORT ANY INCONSISTENCIES IN THE FOUNDATION SOIL MATRIX TO THE [CONSULTANT | ENGINEER] AN PROCEED ONLY ON HIS INSTRUCTIONS

IF UNSUITABLE MATERIAL OR GROUNDWATER IS ENCOUNTERED DURING FOUNDATION EXCAVATION, CONTRACTOR SHALL NOTIFY THE [CONSULTANT | ENGINEER] IMMEDIATELY BEFORE CONTINUING WITH CONSTRUCTION. OVER EXCAVATE AS REQUIRED AND INSTALL A PERIMETER DITCH AND PUMP AS REQD

PROTECT FOUNDATIONS AND ADJACENT SOIL AGAINST FREEZING AND FROST ACTION AT ALL TIMES DURING CONSTRUCTION

FOUNDATIONS SHALL BE PLACED:
NOT MORE THAN [3 | 75] OFF CENTRE
NOT MORE THAN 2% OF THEIR LENGTH OUT OF PLUMB

FOUNDATIONS SHALL BE CENTRED UNDER COLS AND WALLS UNLESS SHOWN OTHERWISE

THE CONTRACTOR SHALL MECHANICALLY VIBRATE ALL FOUNDATION CONC TO CONSOLIDATE IT AND TO CREATE AN UNINTERRUPTED BEARING SURFACE

FOUNDATIONS SHALL NOT BE CONSTRUCTED ON/IN FROZEN SOIL. THE CONTRACTOR SHALL ADEQUATELY PROTECT FOUNDATIONS AND FILL MATERIALS AGAINST FREEZING

ROCK OBSTRUCTIONS MAY BE PRESENT ON SITE. CONTRACTOR SHALL INCLUDE FOR THE REMOVAL OF ALL ROCK OBSTRUCTIONS ENCOUNTERED HAVING A MAXIMUM VOLUME OF 0.5 CUBIC METRES (17.7 CUBIC FEET)

PROVIDE [6FT | 1,9M] MIN OF BACKFILL FOR FROST PROTECTION U/N

WHERE REQD, THE CONTRACTOR WILL MAKE ALL NECESSARY PROVISIONS TO ALLOW DIRECT INSPECTION OF THE BEARING STRATA

THE GENERAL CONTRACTOR IS RESPONSIBLE FOR FOUNDING ELEVS FOR FOOTINGS AND CUT-OFF ELEVS FOR ALL PILES


FOUNDATIONS AND DRAINAGE (FOR PWF FOUNDATIONS)

SOIL BEARING PRESSURE IS 1500 PSF EXCEPT AS NOTED ON PLAN

MODULUS OF SUBGRADE REACTION IS 150 PCI EXCEPT AS NOTED ON PLAN

FOUNDATION BEARING SHALL BE ON UNDISTURBED NATIVE SOIL AND SHALL BE HAND EXCAVATED FOR THE FINAL [6 | 150]
 
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Why do most engineers shout on their drawings? What's wrong with using lower case printing?

BA
 
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"THE [CONSULTANT | ENGINEER] ASSUMES NO LIABILITY FOR THESE DESIGN ASSUMPTIONS..."

Actually, if you stamp and sign the plan you are definitely assuming liability for all your assumptions.
 
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BA... These are from drawing notes... and all notes are Upper Case...

cvg... depends on the matter and the level of sophistication of the client/contractor. At worst, it clouds the issue and at best it indemnifies you from claim. You have clearly stipulated that the information requires confirmation by Others.

Dik
 
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For high end Residences, Retail, Commercial, and land that is suspect for bearing as high water table, clays, landfills, etc. I would DEMAND a soils report. If the owner refused, I would politely tell him why I needed it, and him, and if he still refused, say I could not do the work without it.

I have done foundations for small residences and remodels for years with the 2000, now 1500 and sometimes 1000 psf values mentioned with no problems.

Mike McCann
MMC Engineering
 
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Geotechnical reports are so inexpensive that I can't imagine why, unless it's a really small project, the Owner would just procure one.

It seems to me that our geotechnical bretheren work way to cheaply!
 
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Ugh. That "too cheaply".
 
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I went round and round with a owner on a footing size, he couldnt understand why it was so big, the soil was 'hard and clay like' according to him. I eventually got off the project and told them to hire another engineer. That, and his contractor was advising that a 12' cantilevered masonry wall didnt need to be 12" thick on a 3' wide footing, he could do '8" down to the footer and it would be good'

Always insist on a report, I dont take jobs without them anymore. Not work the headache of a cheap owner.
 
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