Measuring groundwater height with gauge in stiff clay 4

[Plantec]

Hello,

I have a problem with the groundwater level in a stiff clay. We have to design a hospital, with a large basement. There is also an entrance for ambulances, at -3 m, with a kind of car park for the ambulances.

To design this car park, we had to know the maximum groundwater level, so we could calculate the upward water pressure. We decided to place 3 gauges over the site. The water level was measured once a week during 7 months. From these measurements, we derived that the average groundwater level was about -2 m.

The construction of this building has started in january. The contractor started digging and digging, till he was at about 4 m. Still there has been no water in the pit, and the pit has been digged for about 1.5 months now.

I know water can show up a bit in clays, but not that it would take this long, certainly if you pay in mind we are already 2 m deeper than the average water level.

The architect of the building is mad at me, because he says I "dream of concrete", and is constantly suggesting I said the water level was so high to get a more expensive solution (as we are paid in % of the cost price of the concrete).

Now for my question: Are there problems with gauges in clays? Or is it possible that the groundwater will come?

Thanks

 

[Focht3]

Stick with your original opinion for now - unless you know you were wrong. I've seen this happen (dry excavations in clay soils just below the GWT) many times!

Where is your site? Is a shallow groundwater typical for the area?

What type of piezometers were you using? How did you verify that they were functioning? Did you see seasonal variations in your readings? How often were they monitored, and over what period of time?

Will the parking area be enclosed? Did the architect include a subfloor and wall drain system? Even if the water is below the lowest floor level, capillary rise is a problem in clays. And microbial growth (mostly mold) can be a serious problem in an "enclosed" space unless a the drain system was put in.

Let us hear from you -

 

[BigH]

You might have a dry excavation - have you noticed any "softening" of the excavation sides or bottom. I've had sites that were "dry" yet below the water table - but I have noted such softening - usually for about 5 to 10mm. Such softening would be indicative of being below the groundwater level. Unless you have fissures, you might find evaporation > inflow. - hey Focht3, like your knew one!!

 

[Plantec]

Thank you Focht3 and BigH for your remarks.

Focht3:
Shallow groundwater is typical for the region. The piezometers were measured during 7 months, each week. There were seasonal variations - we measured from July untill January - the highest water level was in November (fall).
The parking area will be enclosed. There will be a drain system

BigH:
I don't really know if the clay has "softened", I will take a look on my next visit to the site (Tuesday).

Greetings

 

[VAD]

Hello Plantec:

Here is where you get your hands dirty and use your eyes at the same time. You learn alot about water movement by looking carefully at the material excavated. Very often there is no evidence of free water but you can invoke that water movement is present if you see rust stains, gypsum salts, and thin films of water when you break pieces of the material or examine the fissures, if clay is fissured, and look closely. Your piezometers are probably working okay. Ofen it is good to use a standpipe piezometer as well.

This is where you begin to examine the patient carefully, lots of fun. Take your architect and let him get his hands dirty as well. Every one wants to see free water. However this is not the proof that water exists in clay soils. Do not be surprised at your Architect's statement, many geotec engineers make the same mistake as well.

In any case the use of perimeter drains etc is sound since apart from the ground water there would be the need to take care of water infiltrating from above ground etc. Good comments from Focht3 and BigH.

Good luck

 

[MRM]

If you get rain in that area, it sounds like the excavation might become a bathtub. That is unless arrangements have been made to remove the water. Precipitation might complicate the observation because you won't know if the water's coming from the ground or the sky. Keep an eye on the weather.

I think Focht3, BigH, and VAD are right though...you may not see groundwater given the nature of the clay if there are no substantial fissures or sand/silt seams.

The relatively long term GW monitoring you did is probably reliable, unless rain somehow found its way down the hole (perhaps through sand pack) and collected. I'm assuming you used small diameter monitor wells. Were the areas of your monitoring protected by placing bentonite around the area to prevent water from entering down the hole?

 

[Plantec]

Thanks to everyone for their responses.

I don't really know which kind of piezometer was used, as it was the engineer who worked here before me who ordered them. I will ask what kind of piezometer was used on Tuesday.

If the pit stays dry, do I have to take the upward water pressure in account in my calculations? Maybe the clay has such a low water transmitivity (don't know if I use the correct English term), that one can look at it as if it is a block of impermeable material. So I can take the weight of the clay in account against the upward water pressure.

I wouldn't do it like that, because you never know if there is a sand seam or something, but suppose there isn't, am I right in thinking so? Or is a little bit of transmittivity enough to get the upward water pressure.

I used Atlas piles (screw piles with double ground compaction). Suppose there is a sand seam at about 8m, then the water pressure in this sand seam is 6 m.w.c., wouldn't the concrete of the piles be affected?

P.S.: When I said the highest water level was in November, I meant the lowest .

Greetings.

 

[BigH]

A good point has been brought out - if water level is higher than floor level and no means is provided to relieve the water pressure (i.e., no drainage blanket below floor level connected to positive outlet - either gravity or sumps), then you might have to worry about uplift pressures on concrete floor - and floor would have to be designed for such. Even if the pit is "dry" visually but you are under the gwl (groundwater level), there will be groundwater pressures that have to be handled even for seemingly very very low flows. Water flow (or not) is not a measure of the hydrostatic pressures.

We had a situation during construction of the at-grade section of Vancouver's Skytrain in early 1980s. Our concrete pad for the rails was sitting on great material of low permeability but about 1 to 1.5m down was a sand layer under 4 to 5m of artesian pressure. Provided french drain with pipe down into the artesian sand draining positively "downhill" to keep water pressures at bottom of low permeable material low enough to prevent any uplift deformation which would have been detrimental to operations.

For your situation, always, in my opinion, better to have the subfloor drainage - whether you have or not the groundwater levels. If you have, it can handle the situation; if you don't it is "insurance". Such drainage blanket can be natural coarse sand (or coarse) or of other design. Or you can use one of the geotextile covered platic ribbed drainage panels and lay floor atop it. Always have the outlet be positive by gravity or sumps.

Earlier response was for verifying that you were below the gwl.

 

[SirAl]

Plantec:

Your observations with this site are not uncommon in a clay of very low permeability. It is good to see 7 months of monitoring! That would give me a higher degree of confidence in the groundwater level measurements (not with- standing that very dry or very wet years could skew the results somewhat). Stick to your guns on this issue. I like the sub floor drainage and peripheral drains. Although precautionary measures may consume money otherwise available for architectural enhancements, the building foundation is a critical element. You'll seldom be praised for a working foundation design but surely criticized for one that is problematic.

 

[BigH]

Snaps to SirAl ! ! ! He hit it on the head about praise and criticism.