Slotted piezometers vs Non slotted piezometers 6

[pelelo]

Engineer,

I have a big confusion.

I plan to install a pzm in a sandy layer. The way I plan to install it is by having the bottom portion of the pzm resting on the layer where I want to get permeability value from.

The piezometer will have 1 foot of silica sand at the bottom.

My question is: is there any difference if the pzm is slotted on the sides and open at the bottom vs NOT slotted on the sides and open at the bottom? will my permeability tests (either falling head or constant head) be affected?

Please let me know,

 

[GeoEnvGuy]

Yes read hvorslev paper

 

[oldestguy]

I'd use a zone of porous pipe (or tube). Check various geotecn equipment dealers such as what used to be Soiltest in Chicgo.

 

[fattdad]

Irrespective of your method of running the test, you need to have control on the boundary conditions. A slotted screen provides a defined entry point for the flowing water, primarily in the horizontal direction, albeit.

In our agency, I'd require a slotted length (i.e., less than 5 ft), placed on the bottom of the hole. I'd require a bentonite plug within 12 inches of the top of screen and an appropriate gravel pack.

What's the screen length? It's the length of the gravel pack (i.e., 6 ft or less).

Please be mindful of running falling-head or rising-head tests. If the phreatic surface is below the top of the gravel pack, you'd have to run a rising head test and use the saturated length (i.e., less than the screen length) in your calculation.

good luck!

f-d

ípapß gordo ainÆt no madre flaca!

 

[Okiryu]

Something related to f-d response - I heard from a senior geotech that he preferred to avoid having the piezometer open only at the bottom because the test became only one direction test... for permeability tests he suggested to do it in test pits using a large diameter borehole...

 

[fattdad]

The problem with the open-only-at-the-bottom standpipe relates to the boundary conditions. If it's above the water table, the flow (i.e., even after a soak) is not truly vertical. If it's below the water table, there are similar concerns on the boundary conditions for the flow lines. My mind more clearly understands the flows from a slotted interval.

f-d

ípapß gordo ainÆt no madre flaca!

 

[TDAA]

Open only at the bottom does not make it a one-directional test. As the water enters the soil, it will move in "all directions", absent any confining situations. Two-stage borehole testing is designed in that way, with only being open at the bottom. The first stage will give you a permeability, but no concept of where the water is going. The second stage changes the geometry of the hole below the pipe, allowing a calculation of horizontal and vertical permeabilities. Gordon Boutwell wrote a paper for this, and it did become an ASTM standard.

As i interpret what the OP asked, I would likely set things up as fattdad described.

 

[fattdad]

Yes, Gordon Boutwell's approach is great! It's hard to do in a borehole that's very deep; however.

I also like the Bower and Rice approach.

Not to take away from Hvorslev, that is. . .

f-d

ípapß gordo ainÆt no madre flaca!

 

[Okiryu]

Good to know this. Can you share the title of Boutwell’s paper?

 

[fattdad]

Here is one paper that references the Boutwell test:

https://ascelibrary.org/doi/pdf/10.1061/(ASCE)0733-9410(1989)115:9(1205)

You'll find the citation in the publication.

f-d

ípapß gordo ainÆt no madre flaca!

 

[Okiryu]

Thanks fattdad...

 

[TDAA]

I did not download that paper; however, I saw it was published in '89. I do believe Boutwell published a paper "THE STEI TWO-STAGE BOREHOLE FIELD PERMEABILITY TEST" in '92, as well. Per my discussion with Boutwell, this was the basis of the ASTM standard (D6391). It was interesting that the first iteration of the ASTM standard contained an error; he recommended following the paper. This was fixed.

 

[BigH]

An issue of "sealing" a piezometer in sand is that a bentonite seal will have no effect on the measured groundwater level as the stratum is connected. for the permeability test - follow Horzlev as OG said. When we did permeability tests during drilling, the casing sealed (although perhaps not 100%) the soil and all the water existed the bottom.