Perched Groundwater Table Over Bedrock 3

[cap4000]

I did 6 test holes last summer on a Church project and in every test hole the groundwater almost overflowed out of the top of the hole after a rainstorm. The bedrock is a solid diabase rock with 6 feet of clay above it. Now that all the soil has been removed the groundwater flow seems to be very small. Does anyone know why the test holes indicated such a drastic water problem and when fully excavated almost none appeared. Any tips will be greatly appreciated. Perhaps it had do with atmospheric pressure differences in the holes.

 

[escrowe]

Where was groundwater found in the borings?
Were the bore holes covered after drilling?
What was the position of the bore holes in the landscape (i.e concave, convex, level areas)?

It could be that the open bores filled with rain water/runoff.

 

[cap4000]

The perched groundwater was found right at the bedrock surface in all cases. The boring holes were not covered. Level Ground Surface. Over the course of 7 days with the boring open the groundwater elevations varied by 18 to 24 inches. At all times there was water in holes.

 

[escrowe]

I normally recommend a grouted, slotted PVC piezometer to measure groundwater levels over time;
I have seen open bores fill with runoff, and I would expect isolated ponding on a level clay soil.
I expect that, once filled, the low permeability of the clay soils would keep the holes filled for some time.

 

[cap4000]

I think the term is "artesian water" which is when the clay overburden in this case creates a gravity pressure on the groundwater with the bedrock just below, the groundwater has to come up the boring due to the confining pressures.

 

[escrowe]

An artesian condition is possible,
but I'm not sure you have enough data to confirm it.
For confirmation you would need piezometer data,
or direct observation of groundwater levels in an open test pit.

Is there an aquifer layer (sand or porous rock) beneath the clay?
Is the geologic formation known for artesian conditions?
Are there springs or artesian wells in the vicinity?
Is your site down-gradient from a known aquifer recharge area?

If the clay soil layer is functioning as an aquitard,
then expect a permenant increase in water level
at the borehole or test pit when the underlying aquifer is breached.

Also, if the groundwater is confined in the diabase,
then the pore pressure is more likely a function of aquifer elevation,
not soil overburden pressure.
Unless the aquifer is rather limited in extent,
I would expect to see elevated water levels
even after local excavation of the clay.

Answering your question difinitively
could require a somewhat detailed study.

 

[cap4000]

Yes it would require a very detailed study. Originally I thought it was a spring creating the rise in groundwater. Now that the excavation has started I am changing my mind. Its a very thin layer of water running on top of the rock but when you expose it to the air the borehole immediately fills up. I think, now that the soil has been excavated the confining pressure has been removed resulting in very little water. The diabase is so solid I doubt any water can even come thru it. The RQD was over 90%.

 

[oldestguy]

Did your clay layer have shrinkage cracks? It is surprising how much shrinkage cracks affect water situations in clay. The soil scientists usually refer to these situations as "structure". Even a site with lots of worm holes can give these kind of situations. This also explains why spray irrigation as a way to dispose of waste water on some clay areas can work quite well, almost like dumping the water on sand.

I suspect that you merely were providing a relief for water in those cracks, if that was the case.

This does not seem to be an artesian situation, since you don't have a water flow of importance once the clay has been removed.

I don't know how many times I've seen bore holes with plenty of water, but once an excavation is done, there is little if any. In none of these situations was there any artesian water pressure, but the overburden was somewhat porous and saturated. It can fool one when he decides to make plans for lots of ground water pumping in an excavation and it turns out to be no problem.

While not the same, when excavating peat deposits, located below the water table, the low permeability of the peat makes excavation quite easy, but of course, later that hole fills up with water.

 

[cap4000]

oldestguy

You are spot on. Do you know what the engineering formulas and principles are used to explain a saturated lean clay filling a borehole and then when excavated very little groundwater appears. I have over 30 soil books and none of them mention this unique situation. Thanks for the great tip.

 

[fattdad]

This is a hydraulic conductivity problem. When you have clays below the water table, you can remove soil in a boring and see in the next day or so the infill of water. If you take a widespread excavation, you can remove the soil MUCH faster than the water table can inflow and replace the soil volume. Calculate how many acre-feet of soil was removed from below the water table and then convert that to the amount of water that would have to flow in from the perimeter of the excavation. You will quickly see that for low permeability clays and the like it can be a matter of months!

Just one idea, that is. . .

f-d

¡papá gordo ain’t no madre flaca!

 

[cap4000]

fattdad,

The water seems to run right along the bedrock surface. Its a perched water table and when the borhole is dug and hits the rock it immediately fills up. When it rains it almost overflows out the top. Thanks for the hydraulics tip.

 

[escrowe]

Adding one more layer to the molehill--

A natural moisture content survey of the clay layer, top to bottom,
would answer some questions regarding saturation.

 

[fattdad]

A natural moisture content survey of the clay layer, top to bottom, would answer some questions regarding saturation.

I'm not too sure about this statement. A moisture profile would show the variation in moisture content as it may be affected by descication, change in void ratio, change in Atterberg limits, etc. Without knowing the in-situ density and specific gravity of solids, moisture content alone will not provide saturation (even if you know you are below the phreatic surface).

f-d

¡papá gordo ain’t no madre flaca!

 

[escrowe]

Absolutely right. Moisture content must be placed in context.

 

[oldestguy]

CAP4000

I trust the excavation is open by now.

Aside from all the ideas given above, now you might think about keeping the basement dry.

There are numerous threads about perimeter drainage, etc. in these rooms. However, keep in mind the principles of filtration. You certainly don[t want your drainage system plugged up with clays that migrate along with the water.

I'd look to references by US Corps of Engineers or the Navy NAVFAC via the Internet as guidance. There are a FEW texts on the subject also.

Also be prepared for numerous different ideas here on what to do. Some work good and some are expensive.

I'll wait for your question in the Soil Groundwater section and do my preachng then.