Determining adequate depth of soil boring
Determining adequate depth of soil boring
(OP)
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
RE: Determining adequate depth of soil boring
RE: Determining adequate depth of soil boring
RE: Determining adequate depth of soil boring
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).
f-d
¡papá gordo ain’t no madre flaca!
RE: Determining adequate depth of soil boring
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
RE: Determining adequate depth of soil boring
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
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.
f-d
¡papá gordo ain’t no madre flaca!
RE: Determining adequate depth of soil boring
the file is 32mb in size
http://isddc.dot.gov/OLPFiles/FHWA/012546.pdf
here is the parent page: htt
RE: Determining adequate depth of soil boring
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
RE: Determining adequate depth of soil boring
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
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.
Cheers
RE: Determining adequate depth of soil boring
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
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
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.
f-d
¡papá gordo ain’t no madre flaca!
RE: Determining adequate depth of soil boring
RE: Determining adequate depth of soil boring
RE: Determining adequate depth of soil boring
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
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
f-d
¡papá gordo ain’t no madre flaca!
RE: Determining adequate depth of soil boring
RE: Determining adequate depth of soil boring
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
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
Ham
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.
-Ham