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Determining adequate depth of soil boring

Determining adequate depth of soil boring

Determining adequate depth of soil boring

What is the standard or rule of thumb for deteriming the overall depth of a soil boring? I have a project were a underground concrete lift station was to be built at 29 feet below ground. The soil testing firm drilled borings to 31 feet and did not find bad soil. During construction very poor soils were found just below the depth of the boings feet. Should the borings have been run deeper, if so, how much?

RE: Determining adequate depth of soil boring

A good rule of thumb is usually 10 feet below the footing elevation. This could change from prior knowledge of the site or encountered problamatic soils.

RE: Determining adequate depth of soil boring

Another consideration would be the weight and footprint of what you are planning to build and the depth of influence into the soil you are expecting.  

RE: Determining adequate depth of soil boring

Is there actually a ASTM (or similar) standard for this?

RE: Determining adequate depth of soil boring

Field exploration program should extend through the complete depth of the "seat of settlement".  The seat of settlement is a function of the anticipated foundation width and includes the soil profile that experiences increased stresses from the new construction loads.

For the case of below-grade construction there may be a net "unloading" (i.e., the removal of the soil loading is greater than the newly-applied foundation loads.  In this case you would likely still need to go at least 10 or 15 ft below the anticipated lower grade just to consider the point load reaction of any foundation elements or to properly address dewatering concerns.

I don't think a target depth that terminates within a few feet of the design grade sounds adequate (unless the exploration terminated in granite).


¡papá gordo ain’t no madre flaca!

RE: Determining adequate depth of soil boring

There is no ASTM for determining soil boring depth, and I hope to god there never is.  Some building codes require borings to a certain depth below the bottom of the anticipated foundation level.

In my practice, I plan the exploration to extend at least 2 footing widths below the bottom elevation of the planned footings with a minimum of at least 5 to 10 feet depending on loads, etc.  Of course this requires some guess work, but you can usually make a close enough estimation.  

RE: Determining adequate depth of soil boring

thanks everyone

RE: Determining adequate depth of soil boring

I 've been always wondered that there are no rules or straightforward guidelines on  very common and conventional geotechnical things that engineers come across daily /or at least weekly .
For example..
every project we come across the technical question (keep the budget aside): How many boreholes you should drill and how deep you should go. The answer of this frequent question  is  based on guess or on hit and miss rule.
Is not strange..?

RE: Determining adequate depth of soil boring

The answer to the how many question is rooted in the geologic setting and the nature of the project.  The answer to the depth is often the same.  It's not based on guess, but rather experience and service to client.

I'd like to offer a supplement to GeoTrafficPave's comment on the depth being two times the foundation width.  This is an oft-cited guideline; however for spread footing loads, four times is the more likley range to clear the seat of settlement and neither of these take into account how earthworks can affect the seat of settlement.  Consider a MSE retaining wall (i.e., no footing, just a leveling pad), the appropriate boring depth must take in account other factors.  Also consider a site with 20 ft of relief and 10 ft cuts and fills.  In the fill area (i.e., prior to earthwork) what is the appropriate depth for the foundation exploration?  Some areal fill loading assessment may contribute to the exploration depth (depending on the local geology).  Again, if you are on granite it may be different.

How could an ASTM standard address these factors?  The absence of considering these factors would constitute guesswork.  Thankfully (I hope) the profession has advanced.


¡papá gordo ain’t no madre flaca!

RE: Determining adequate depth of soil boring

i'll throw something out there...we can drill as many as you want. the more info you have, the better evaluation can be made. several things will change the number of borings i'd propose (i.e. fill, residual, big/small loads, big/small area loads, sensitivity of the structure, geology, history with the client, thickness of compressible layer, etc). sometimes, we have not choice but to guess on the front end and try to expand the work scope to catch most everything on the back end. here is a pretty good very general representation of a subsurface exploration. keep in mind that this is not the bible and cannot possibly address all aspects. from region to region, geology to geology, or soil to soil, this thing will not completely address every possible scenario. in many aspects, it prescribed an excessive amount of sampling/tests. again, it's a good "ball park" estimate.
the file is 32mb in size
here is the parent page: http://www.fhwa.dot.gov/engineering/geotech/library_listing.cfm

RE: Determining adequate depth of soil boring

A fun subject.

How many borings?   Well, after you are done with them you probably know if you need more.  Clients don't like that answer.  I commonly answer the question at the start with: "At least one".

How deep?

My old prof at Cornell U., B.K. Hough, (long dead now) may have it in his text, but this one has worked well for me.

Estimate the increased vertical stress distribution with depth in the subsoil due to the foundation or other load.  The depth where that increase of stress is ten percent of the existing stress is where you quit.  Take into account unloading for basement excavations.

Of course for the case mentioned you can't quit at footing grade using this rule and there your past experience and knowledge of the local geology is needed.

 Oh yes, you may have to cheat a little when you hit rock.

RE: Determining adequate depth of soil boring

Another good idea will be to do a preliminary site geology prior to proposal writing. This will help determine cost and boring depth along with anticipated settlement and loads. Taking 15 to 30 minutes can save a lot of headaches in the long run.

RE: Determining adequate depth of soil boring

There is some quantitative methods based upon structure recognition, that is, if you drill by an n X n grid, you're able to know by probability theory which are the dimensions of a structure that may escape your investigation.
The structure may be a soft soil lens, an ore level and so on...

Pretty specialized and rarely, if ever, applied in practice....

RE: Determining adequate depth of soil boring

   There are several guidelines out there as many above have attested to.  You most definitely should go at least 2 footing widths if on spread footings and perhaps deeper if on piles (subject to what you hit - i.e., rock) as the zone of influence of a pile group is greater than the "size" of the pile.  But, I trust that you are working for a reputable soils firm - if you are, then you should have senior engineers who can explain this - much is really based on experience and judgment and of course on the nature of soils encountered. I agree, I DO NOT WANT ASTM TO SPECIFY SUCH.  They have damaged geotechical engineering enough in my view.
    As for the original post, it appears that you have dug a big pit - even though you found "poor" soils below the founding level, to me it appears that you have removed a lot more load in your excavation than the weight of the structure you are putting in - this, then, if you use a rigid raft and walls becomes a bouyancy raft or compensated foundation - if the loads are such that the load removed is more than that applied, you shouldn't get any additional settlement.  Foundations on such foundations have been founded within poor soils successfully.

RE: Determining adequate depth of soil boring

BigH    ASTM doing damage??? Say it ain't so!!

The above replies generally reflect my experience.
A quite analysis of the proposed construction, combined with an analysis of the geology, with a big dose of experience determines how I propose a subsurface exploration. Includes number of, types and depths of borings, anticipated sampling methods and zones of specific interest. I also try to have an experienced or well tutored geologist as logger. Finally, my phone number is available, with the understanding that even a poor question is better than having to go back on the job.

RE: Determining adequate depth of soil boring

Amen to having a geologist on the drill to log the samples.  A good geologist will also be able to determine if the boring should be advanced further than originally planned.

Good comments and discussion.  fattdad, I agree that 2 footings is not always enough, but it is a good starting point for the area under the building, as for areas with significant cut and fill, well that is a whole different ball of wax.

Lastly, let us all hope and pray that ASTM never tries to set a "standard" for number or depth of borings.

RE: Determining adequate depth of soil boring

Amen to keeping ASTM out of this!

At the risk of repeating what's already been said, 2x the footing width is appropriate for column footings and 4x for strip footings.

Regarding the target depth where stress increase is less than 10 percent of the pre-construction effective stress, this can be a great goal, but unattainable for areal fills.  If you have an areal fill of any size you have to go to quite a depth to get to where the change in stress is less than 10 percent.  Not to worry; however as in elastic compression the modulus increases with effective confining stress and the practical seat of settlement may not extend throughout the entire soil profile with increased stress.  Then again, if you are dealing with normally consolidated soils and the need for a preload, you may have no choice but to fully penetrate the soft layer.

Regarding friction pile groups, you have to look at the depth of penetration and distribute the full group load on a plane that's acting 2/3rds of the way down the pile group.  From there it's just a Boussinesq solution to see where the increased load attenuates (i.e., within 10 percent?).

Hopefully my comments and the words of others are helpful.


¡papá gordo ain’t no madre flaca!

RE: Determining adequate depth of soil boring

A couple of points- 1.) I often see borings that go down to the level of the footin or slightly below, but the footing is in a sheeted hole where the sheeting may extend 10 to 15 feet below the footing. With no data the design of the sheeting is generally very conservative. If you anticipate sheeting, be sure to extend the boring below the possible sheeting toe. 2.) If you find questions from the borings, you should be able to do a second round of borings.Its not that you missed something in the first round, its that the first round gave you a better idea of where to look. Additional borings are a lot cheaper than a pile crew on delay due to unforseen conditions. 3.) Be sure to leave the boring open long enough for the atertable to stabilize. I would say 90% of the borings I see the single water level is taken with in 5 minutes of completeing the boring. Finally 4.) Be sure to have the borings surveyed in. It does no good to have a great profile if you don't know where it was taken.

RE: Determining adequate depth of soil boring

I agree that a geologist or geotechnical engineer should always be assigned on a drill rig to log the holes.  Drilling and programmes of such should have the flexibility to reflect the conditions encountered - and the data desired.  Experienced loggers/supervisors can get that "extra" thin-walled tube at the appropriate location that might not be if following set rules!  Good points all.

RE: Determining adequate depth of soil boring

A point that I forget.
The project is a Subsurface Exploration.
you are exploring.  It is nice when the borings confirmed what you thought you knew.  The exploration usually shows how limited your knowledge is.

RE: Determining adequate depth of soil boring

Here is one I learned the hard way.

Never drill your boring at the foot-print of deep excavations, generally lift station types, single pole caison foundations, etc.   That otherwise is likely to be a "pipe" from which can come lots of water up into the excavation.

Why caue the contractor this grief when drilling nearby usually is sufficient?

RE: Determining adequate depth of soil boring

Interesting oldestguy!  I'll be drilling for a pumping station in the next few weeks and I'll take that into consideration.  We will be below the ground water table and your experience may become relavent.  On a related note, it is truely amazing how long borings can stay open below the ground.  During a recent excavation for a grade beam along a caisson supported perimeter wall, I saw the hole of a boring that I had drilled about 2 years prior.  Who'd a thought. . . .


¡papá gordo ain’t no madre flaca!

RE: Determining adequate depth of soil boring

here's a suggestion: grout the hole upon completion. i've got to drill for a couple shafts and a pump station coming up but the geology is rather erratic, so i don't plan to be too far off my mark. 5 borings at each location with all going into rock and one going 200+ feet deep. in certain situations, it's better to insist on fixing the holes.

RE: Determining adequate depth of soil boring

I stumbled across this and thought it was interesting: http://www.geoforum.com/knowledge/texts/broms/viewpage.asp?ID=71

I would argue that you need to go deeper than 1.5B for strip footings, but it is an interesting perspective.

In my experience, having the surficial soils mapping for the area you intend to work in and having a database of previous exploration locations performed by your firm helps immensely in determining number and depth of borings needed. A site with very dense till over rock will likely not require the same number or depth of borings as a site with soft clay. It helps to know what you are getting into so you have the correct number of days scheduled for exploration.

It also helps to understand the proposed construction and to state what you are assuming (or have been provided) in the proposal.  Sometimes my firm includes a proposed exploration location plan in the proposal showing what we understand to be the proposed construction.  This makes it a lot easier to go back to the client and request additional borings when the project changes (i.e. location of building, FFE, size, etc.).

As mentioned above, the proper depth of a boring is generally site specific and there is nothing better than having qualified people to make field decisions.    

RE: Determining adequate depth of soil boring

I may be off-based with what I'm imagining is going at your site.  I'll just throw in a story and you can see if it applies.

A geotech company i worked for did a site where they would take a hill off the site to make way for a industrial bldg addition in western NC.  We had done the drilling and geotech engineering.  We had extended soil borings deeper than proposed grade (15' maybe) and one deep one for our seismic classification.  The site was drilled before I had worked there.

When construction started, we got the soil & concrete testing.  We tested each pier footing and all along the strip ftgs to about 8' after excavation (this is generally overkill but it was for a reason here)

The blow counts were less than we would have accepted for the bearing pressure if it had been a normal balanced site job.  We ran the loads for settlement and it was still w/n tolerance if any immediate settlement occurred.

But, we expected that the looseness of the soils is from the rebound of the soil from the removal of the overburden since we were absolutely confident in our drilling results and the during field construction testing demonstrated consistency all over the site and matching materials w/ the boring logs.

My question is: If the geotech exploration had extended 10' to 15' more and found no red flags would the site change? Further drilling around the site and away from the excavation can "indicate" this if the problem is big enough that you think the flags are there.

I can imagine some other possibilities which would mess it up but it's just imagination since i don't know where you are, how big your excavation is, how long it is open, does the excavation get disturbed, what the soils are like there, where the soils came from, and groundwater elevation.

long post,
back to work

RE: Determining adequate depth of soil boring


Sorry, but I don't get the point. Knocking off the top of a hill versus digging a hole seem like different situations in my view, other than both being an unlaoding situation.

Were there arguments as to "changed conditions" or similar?  If so, on what basis?

RE: Determining adequate depth of soil boring


I wasn't really answering his question. it's been well answered.

I don't know how big underground concrete lift stations are if their typically the same size.  Never worked on one.  

the way i read the question, it sounds like a problem has come up during construction and blaz is looking to see if this was something that should have been discovered during preliminary design.  i think the borings should have been deeper and blaz is absolutely right to question it. it just seems too inconvenient and too unlucky that the geotech stopped feet short on deep borings from hitting these "poor soils."

my point is, construction can affect the geotech design in many ways which are easy to overlook, and especially so when a problem comes up and there's a goat in the room.  I'm talking out of my #@! more than usual cause we don't have any info besides the depth and a brief description of the structure.  I threw my story out there for blaz to consider. you're right, we have no way of knowing if it's applicable. blaz is holding the cards.

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