High Water Table - A Problem?

[SlideRuleEra]

I would like for you folks to help educate me on some of the questions that routine come up in the foundation, geotechnical, etc. forums. I have worked virtually all of my career in areas (coastal South Carolina) where the water table is normally at, or just below the ground surface. To us (local Engineers & once-upon-a-time as a heavy construction Contractor) - this is no big deal. The foundations (spread footings, piling, drilled pier, etc. "work" properly when located above, below and at the water table. Constructing them (controlling groundwater) takes a certain amount of understanding, but is not "rocket science". This is just every day business - not a problem.

Yet I routinely see questions where the member "freaks out" because they have encountered ground water in an excavation - and don't know what to do about it (either for design & construction).

Do soils / ground water in other parts of the world cause more problems than I am aware of? Or are high water tables an unusual situation that seems "normal" to me?

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[dgillette]

You got me. I can think of a few problems with an exc below the water table, but they are the obvious ones that you already know: dewatering, heave and instability, sloppy working conditions, uplift and leakage into basements when the water table rises unexpectedly, floating of empty pipes below the WT.

(I know enough not to build a frozen storage facility in a basement at or below the WT. When I was in Florida, our company ran into the only permafrost problem in the history of the state. There was no capillary breaker below the floor slab of a wholesale grocery freezer with a high water table. The floor kept on heaving relative to the structure and they kept sawing off the bottoms of the doors until things got too bad.)

Aside from that, I think it's just a matter of sigma'=sigma-u, so I don't know why it should be a big deal. Like you said, it's something you anticipate and work around. If the concerned persons hit water when not expected, maybe they need to have a talk with whoever did their foundation investigation.

 

[Mccoy]

It's usually the less "geotechnical-savvy" guys who seem to freak out at the idea of water proximity. Understandably enough, may be, if they expected just dry, reassuring soil.

Here in Italy, along coastal areas and everywhere GWT is shallow, construction goes on as usual.
When suitable they use a lot wellpoint systems, I saw some pretty deep excavations below WT.
Some times not without settlements problems in nearby buildings, usually when it's not the usual medium-coarse sand but also soft silty layers. And with lack of a proper geotechnical survey.

 

[BigH]

SRE - I'll drop in my few cents worth later; I have the wifey looking over the shoulder now as we are supposed to be on vacation in Bangkok. ) . I'll smile at all the gals for all my FBT mates!! - but only if my little lady doesn't see! The old expression of "liku-liku laki-laki" (that's the meandering way of a man) doesn't work as well as it once did!!

 

[oldestguy]

More two cents:

My experience has been mostly in north-east and midwest northern states and high water tables are becoming more and more present on jobs due to the good sites being already built on. So, I'd expect more questions on these points from non-geotech types as time goes on.

I saw one that stumped me however. A 35 foot deep wet well (some 20 x 30 ft. in area) at a WWTP was to go on 13 feet of loose silt, but below which was loose clean sand. Test borings showed GWT at the ground surface, but no unusual condition. They stopped in that sand since the structure would basically be an empty container, no net load on subsoils. Resistance against uplift was considered more important.

No one knew (in design office or at testing firm) of an artesian well flowing nearby. That sand zone apparently was under artesian pressure.

Poor contractor, with the usual dewatering wells and well points, plus deeply penetrating sheet piles had a real quick sand type condition with "boils" galore. This flow was not controlled at all until compaction grouting was done to help tighten loosened ground. With that effort, the upward flow was reduced, but not controlled. Even so, the completed structure settled unevenly.

It seemed not possible for a nationally recognized dewatering subcontractor on site to control that artesian pressure using all the best techniques available these days.

Why the test borings didn't show artesian pressure was a mystery.

A big extra on that job

I wish I knew of this group at that time and we could have had a fun time with ideas.

 

[BigH]

The test borings might have shown the situation but it (maybe) wasn't recognized in the office. There were many times that I, while supervising drill rigs, ran into "live" sand as the Quebeckers would say. Hollow stem augers (that when you pulled the plug (given slow pull-out and trying to keep full of water) or casing, you found that the 'sand' had come up inside. This would be noted on the field logs. (If not, then the field engineer/technician wasn't trained - nor was he thinking). But, giving the logs to the draftsman (oops, CAD operator) this relevant information might have been missed - it doesn't have a "natural" place on the log. Unless the office engineer reviews the original field log, he may very well not know!
Another thing is that even in all soils, in my view, even when sands (and especially if lower permeable material lies on top) are involved, standpipes should be sealed into the stratum to confirm, over the long term, the groundwater/piezometric level. This would be especially accute when you know that you will have a deep excavation.
In the end - quite often it is either poor field training and or observations and or poor office attention to the field notes that can lead to this.
I had a job of similar nature - the ground surface was a bit damp - not bad, though, and our initial drilling was into a silt layer - not clayey silt but cohesionless silt. We then ran into sand and scheise, water started gushing out the hole like a locomotive coming out of a tunnel. I installed a standpipe (with a pressure guage) and sealed the standpipe in the silt layer - but it was a real bear of a time to try to get it plugged - the pressure was quite a bit. And, the previous boring some 20 ft away hit hard clayey silt till the whole way down! Go figure.

 

[eric1037]

It's a bit easier to detect an artesian condition if the groundwater level observed during drilling is a decent distance below the ground surface. That way if you observe the groundwater at a higher level upon completion of drilling it gives an indication of an artesian condition.

However, in the type of situation oldestguy is describing, it is likely that a good driller would use drilling fluid or wash-rotary drilling to advance the boring to reduce blow-in and groundwater levels during drilling are misleading. Fluid may have been flowing out of the hole just because of the drilling technique.

In hindsight, a groundwater observation well should have been installed and the groundwater levels observed over time.

This is a case when it would have been prudent to review nearby water well records. I'm not sure about other states, but in Michigan and Indiana, all well records are available online in the public domain. Sometimes you get lucky and find a well very near to where you are drilling.

Live and learn.

 

[oldestguy]

Good comments. I was not involved with the drilling, but called in later, when the boils showed up.

Oh for the good old days with "wash borings". Lots of labor, but good information. Thirty feet a day was good progress.

 

[BigH]

. . . and 20 ft was also considered a plus!