Perched Groundwater Table Over Bedrock
Perched Groundwater Table Over Bedrock
(OP)
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.





RE: Perched Groundwater Table Over Bedrock
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.
RE: Perched Groundwater Table Over Bedrock
RE: Perched Groundwater Table Over Bedrock
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.
RE: Perched Groundwater Table Over Bedrock
RE: Perched Groundwater Table Over Bedrock
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.
RE: Perched Groundwater Table Over Bedrock
RE: Perched Groundwater Table Over Bedrock
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.
RE: Perched Groundwater Table Over Bedrock
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.
RE: Perched Groundwater Table Over Bedrock
Just one idea, that is. . .
f-d
¡papá gordo ain't no madre flaca!
RE: Perched Groundwater Table Over Bedrock
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.
RE: Perched Groundwater Table Over Bedrock
A natural moisture content survey of the clay layer, top to bottom,
would answer some questions regarding saturation.
RE: Perched Groundwater Table Over Bedrock
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!
RE: Perched Groundwater Table Over Bedrock
RE: Perched Groundwater Table Over Bedrock
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.