Water Pressure in 'Rock'

[dcarr82775]

We are looking at a 40ft braced cut. Water is about 10ft down, and claystone bedrock ranging 25-30 ft down. The bedrock is relatively impervious. In the overburden there is clearly a head of water that the wall must resist. Once you get into the claystone, do you think you continue to gain water head with respect to the wall or does the increase in head stop because the claystone is impervious?

As always, thanks

 

[GeoPaveTraffic]

Depends on the geology. You need a good geologist and geotechnical engineering team.

You are talking about a very complicated situation.

Mike Lambert

 

[hokie66]

I can't see how you could prevent hydrostatic pressure between the wall and the rock. What would prevent the water from invading the interface between the two?

 

[dcarr82775]

True Hokie66. That was easy. Water pressure it is.

 

[GeoPaveTraffic]

While I agree with Hokie66, be carefull of water pressure in rock.

Water in the joints of rock can transmit pressure a long way and the resulting pressure can be much higher than measured in the soil above.

Mike Lambert

 

[fattdad]

I'm always suspecious of assumptions. Is the claystone, "impervious?" It would simplify things, but if it's not true, you could have a failure. The permeability may be a few orders of magnitude less (seemingly impervious), but it could also have a phreatic surface, which you'd only know if you had an isolated well in that layer.

rock fractures can also convey water, as already stated.

f-d

¡papá gordo ain’t no madre flaca!

 

[hokie66]

GeoPaveTraffic,
Water pressure is controlled by the height of water. Why does being in rock make a difference?

 

[cvg]

because the cracks in the rock layer may transmit pressure from a long distance where the elevation may be greater. but cracks of this kind in claystone seem to be very unlikely

 

[GeoPaveTraffic]

hokie66 - as cvg stated, hydrology in rock can be complicated and surpising.

cvg - if it is a true claystone and massive, I agree completely. However, most of the claystones that I've worked in have limestone or sandstone strata at varing intervals. Limestone in particular often has very complex hydrology.

Mike Lambert

 

[hokie66]

Well, I interpreted dcarr's question as asking whether hydrostatic pressure at depth against the wall might no longer be proportional to height, due to the presence of impervious material. No matter the material, the answer is that hydrostatic pressure is always proportional to height of the water column.

I understand that water can move a long way laterally, so in design of basement walls, we normally make conservative assumptions about the water level, rather than just use the level at the moment.