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?
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Determining adequate depth of soil boring 1
- Thread starter blaz0033
- Start date
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?
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?
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. . . .
f-d
¡papá gordo ain’t no madre flaca!
f-d
¡papá gordo ain’t no madre flaca!
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.
bucknell06
Civil/Environmental
I stumbled across this and thought it was interesting:
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.
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.
DarthSoilsGuy
Geotechnical
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
Ham
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
Hamhodgeman:
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?
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?
DarthSoilsGuy
Geotechnical
Oldguy,
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
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
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