Interpreting a Geotech Report 9

[GalileoG]

I am curious if there is a "Geotech Reports for Structural Engineers 101" reference out there that can help me better interpret and understand geotech report jargon so that I choose an appropriate foundation system. Typically the geotech reports that we get recommend a foundation system, but I would like to understand why. What should I be looking for in a geotech report to help me choose how deep my strip footings go below grade? Or, what considerations should I take for water table? Or, when looking at the bore hole log, what difference does it make to my foundations (strip footings or piles) whether there is silt, clay or sand.

I would have posted this in the Foundation Engineering forum, but I would prefer advise from fellow structural folks. Any help is appreciated. Thanks.

Clansman

If a builder has built a house for a man and has not made his work sound, and the house which he has built has fallen down and so caused the death of the householder, that builder shall be put to death." Code of Hammurabi, c.2040 B.C.

 

[ishvaaag]

Normally an intelligent geotechnical report should give good information and sound advice. In Spain is typical the structural designer makes a list of things he or she wants to see (about) in the report. Since making a report has become mandatory the project author is responsible of ensuring that the standing standards about geotechnical reports (that exist within the CTE, Technical Code for Buildings) are met and previous specification plus revision make at least for a start in what to talk about in the report.

Then typically those doing the report, based on field and laboratory tests, use to give advice in quite practical terms to structural designers; the reason for that is that till relatively recently the most informed guys in the matter have been the Ingenieros de Caminos, Canales y Puertos, that more or less have been representing the highest knowledge here in practical terms about structural and civil engineering, so they more or less still stay high in the geotechnical firms, and even if they are surely being replaced by geotechnical professionals of more recent apparition will get at least settled this practice even if by example. Hence it is not uncommon to see reports that if not guide you on the structural design, at least give useful data for such.

The worse things along the years I have seen in the reports, and this is precisely a thing of the last years have been some bias towards excessive conservatism, particularly some cases of ridiculous overstatements of potential bad behaviour or settlement. The scarce old ones that were 3 decades ago in practice were wiser to that. Now the placement of potential liability on geotechnicals and also excessive statements on how to proceed about everything is making the younger less experienced and informed guys to make some of such overstatements.

The morale of that is that one can't expect sound engineering advice in a geotechnical report if those making it are not knowledgeable enough in the field; yet the same can be said contrarily, structural designers will find hard to correctly interpret some well targeted aspects of geotechnical reports without enough knowledge about. And the daunt task is how in an evergrowing complexity of the knowledge standing keep apace of what constitutes the main set of knowledge requirements of own's practice. This of course is made at any level by proper placement of resources upon sound coordination, but nor proper resources and I would say even less proper coordination are the luxuries most of us use to enjoy.

 

[Ron]

Clansman...a good quality geotechnical report should be written such that you would not have significant questions. For instance,if clayey soils were encountered at the anticipated footing bearing level, the geotechnical report should outline how to deal with those, including compaction, accumulated water and settlement.

As for the depth of your foundations, the report should give some general guidelines such as " to reduce the potential for scour and undermining the foundations by surface water drainage, the footings should bear at least 18 inches below existing grade".

As for water table, that affects several things. One...is it at or near the seasonal high groundwater level at the time of boring? Does the report give an estimate of the seasonal high? Second, if the seasonal fluctuation is significant and you place your foundation loads on the soil when the groundwater is at its highest, then when the groundwater goes down, you can experience settlement as a result of the reduced groundwater level.

Another significant groundwater level issue is constructability. Will dewatering be necessary? Will water get trapped in the footings during construction? All of these things should be covered in a geotechnical report. If they are not, go back and make them give you your money's worth (or the owner's money's worth!)

 

[msquared48]

Clansman:

Different foundation systems are more economical and function better in certain soil types and conditions.

The deeper you go into the soil strata, usually, unless there is fill down deep, the higher the allowable soil bearing. This depth affects the economy of a strip or spread footing system. That's another reason for soil borings - not only to see the soil types, but also to determine any poor bearing strata at deeper levels, p[lus determind the depth of the max water table.

As to the water table, the higher the water table, the more effect on the building to float or have water intrusion through any basement wall, uplift on the lowest slab which could drive it to be structural instead of a SOG, and the possibility of needing an underslab gravity or sump pump drainage system.

Just a few of my thoughts.

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

 

[msucog]

Here's the best advice I can offer: TALK TO THE GEOTECH BEFORE, DURING AND AFTER THE EXPLORATION. Ask questions. Also, don't hire a geotech based on lowball prices. If you want a cheap price, I'll give you a cheap report with "cheaper" recommendations (that could cost the project bigtime). I can default (so to speak) to deep foundations without even thinking about it. The geotech could be very cheap but then the foundation system cost you many times what it would've cost had you hired a good geotech and paid a little more. But ultimately, communication breakdown causes the geotech to do a lot of assuming without clear direction and less specific recommendations in the long run. If you're not sure about something they recommend, ask them to explain. If they can't generally explain where their recommendations came from or why they did something, then you hired the wrong geotech. At least if they have an explanation for doing something (right, wrong or otherwise), then they are at least paying attention to what they've done and have the capabilitiy to explain how/why they got there.

As far as good references that thoroughly explain geotechnical explorations, try UFC (formerly NAVFAC), USACE, ASTM and FHWA. All except ASTM are free. You can find old threads of mine to locate the links. Ask your project geotech which guidelines they follow.

 

[BigH]

Yours is a "BIG" question - why does the geotechnical engineer choose the foundation support sysem that he does. (sorry for the length)
There are many reasons; #1- the geotechnical ground conditions encountered; #2- perceived understanding of project requirements (some projects have varying levels of serviceability, e.g., maxiumum total settlement of 10 mm, maximum differential settlement difference of 0.04%); #3- modifications to the ground conditions at time of construction or subsequently; #4- level of risk he is willing to take.
#1(a) Soil conditions consisting of cohesionless materials at surface - one chooses spread foundations unless the soil is very loose to loose. Foundations will rarely "fail" in bearing capacity and it is settlement that rules and high bearing pressures are the norm. Material, unless it contains significant fines content, will not typically be affected by frost; it will not swell during seasonal changes (wet/dry). If groundwater is very low (say for fine to medium sand) or the soils are very clean, you can typically found within the normal frost depths as the soil is non-frost susceptible. Building Departments might not like the idea as they would want to go below frost depths as a matter of course.
#1(b) Soil conditions consisting of cohesive soils. If soft to firm, spread foundations are possible from a strength point of view but settlements may be a problem. Soft soils are rarely "suitable"; firms soils are for light structures. Heavy structures will require more robust support needs and it is invariable that one would go with piles to "hard" bottom - an underlying layer of sand/gravel, rock, glacial till; stiff to very stiff clay. If the cohesive soil is stiff to very stiff, spread foundations are usually possible unless theire is complexity in the need of support. However, clayey soils are susceptible to frost heave - which is why you want to take your foundations deeper than the depth of frost. Clayey soils are also subject to wet/dry cycles - heave susceptiable soils such as found in many parts of Texas are a problem and tensioned ground slabs are many times necessary unless other methods of handling the shrink/swell are undertaken. Trees are also problematic (google Bozozuk for a good paper on the effects of trees on the shrinking and swelling of clay soils).
#1(c) Many times one has clay soils that have a desiccated crust - groundwater is at depth and the upper zones are 'stronger' than lower (see Sarnia Ontario soils as an example). Here if you can keep your spread footings small, you can take high loads (bearing pressures) but if the footings need to be large, the soil below the desiccated crust come to play and the allowable bearing capacity (shear) and/or the allowable bearing pressure (settlement) will be affected - a case that is outside the "usual" that was put forth by msquared48 when he said you would get higher bearing the deeper you go.
#1(d) The soils consultant may recommend ground treatment methods to improve the upper soils so that you can bear on them with spread footings (or higher embankments).
#1(e) Sometimes you can permit very large settlements to occur and although the soils may be poor, deep foundations are not necessary. For an example a large storage tank - may undergo settlements of 200 mm or more (total or differential - as the sides would be less than the centre)without problems (other than requiring flexible connections). For spread concrete foundations this wouldn't be possible. RE walls can undergo large settlements and can be founded on poor soils with the proper engineering and ground improvement techniques - not necessarily on piles.
#2 Some projects have stringent settlement requirements. For instance I was involved in one project where the maximum settlement could be only 1/4 inch - much less than the typical 1 to 1.5 inches normally associated with structures on spread foundations. Another had a differential settlement permissible of something like 10 mm over 125 m (high speed paper machine). These types of restrictions will dictate one type of foundation over another.
#3 Have indicated abover that the geotechnical consultant might have some "toys" up his sleeve that will permit one to use less than ideal soil conditions. Wick drains in soft clays and building with stage loading for embankments (as an example).
#4 Risks must always be evaluated. If you were to hire me to do a soils investigation and prepare a report for $1500 (not unheard of in Toronto in the mid 1990s - even though such investigations cost $3000 in the 1960s), I will be risk adverse. I will give you what I know will work and not worry so much about construction cost (I understand that this might go against our "desire" to give you the most cost effective support mechanism). Even if you give me more money, the risk is too great to "cut it thin." In the end, on this it is much harder and costly to fix a foundation problem than to fix an above ground strutural problem.
And there are many other factors as well: MSUCOG was correct in advice on choosing a geotechnical consultant based on his experience, his reputation and being able to work with his clients as a team rather than just a "cost." Working closely with your GC will permit both of you to select the most suitable and cost effective support system.

 

[GalileoG]

Thank you all for the great responses!

Clansman

If a builder has built a house for a man and has not made his work sound, and the house which he has built has fallen down and so caused the death of the householder, that builder shall be put to death." Code of Hammurabi, c.2040 B.C.

 

[BigH]

Further to #3 - forgot to include that adding fill on a site of compressible soils will induce downdrag forces (negative skin friction) - this is an example where site conditions might change as a result of the project or subsequently. Downdrag forces must be subtracted from the allowable bearing of piles. At the time of an investigation, it is many times unforeseen that a large areal fill will be placed 10 years hence.

 

[msucog]

this thread is a perfect example of how much better a project can be with a little communication between the structural and geotechnical.

everyday i'm amazed at how much of an 'art' geotechnical engineering is and it excites me EVERYDAY even when i sit in this God-awful traffic here to drive in to work...and am still EXCITED! moral of the story: hire a 'good' geotech that uses logic and true, sound engineering practice even if it costs a little more on the front end.

Good luck!

 

[Ron]

If we could only clone BigH, oldestguy, oldfieldguy and a few others here!!