How to estimate staged construction pore pressure factor.

[soil1999]

Dear all,

B bar (pore pressure coefficient) equals to the pore pressure divided by the vertical overburden pressure. It ranges from 0 to 1. It is unity when one assumes that the soil is under undrained state ( e.g, soft soil under staged loading). I am doing staged construction, but I have a soil that has acceptable permeability (partially drainage). How can I assign B bar value of this layer or any other layer that is not clayey soft ...?. Are there any guidelines for assigning B bar value based on the any other soil properties..?

Thank a lot 4 help

 

[ishvaaag]

Just some papers I find (no specialist in geotech matters)...

http://users.ntua.gr/gbouck/downfil...uced_Pore_Ppressure_in_Sand-Clay_and_Silt.pdf

http://www.ces.clemson.edu/UsTaiwanWorkshop/Paper_SDEE/SDEE_Shamsher Prakash.pdf

http://scholar.lib.vt.edu/theses/available/etd-08132001-170900/unrestricted/12_Chapter_4.pdf

http://www.springerlink.com/content/u862r2pu688246m5/

Just have perused the first 4 pages of a google search, deeper probing may give something closer to your intent.

 

[dgillette]

Why not a plain old consolidation analysis with oedometer tests? Just beware that the vertical permeability of the foundation may be governed by the thin clay interbeds, and not by the typical material.

If the soil is much more pervious than clay and not very thick, you may find that the excess PWP is more or less gone by the next morning after each placement (meaning staged construction is not necessary).

It's not obvious to me how any of ishvaaag's links are relevant to this question.

 

[oldestguy]

Long ago I gave up trying to estimate settlement rates using the results of consolidation tests. The tests are OK for generally guessing at the total amount outcome only.

These usually are then used for surcharge pre-loading of building sites. If the settlement total estimates are slight, no pre-loading or other work is done.

For pre-loading, I've only seen two jobs where estimating rate of settlement came close to the actual, with one having a total predicted settlement of 8 feet in 20 years. Otherwise those intervening sand layers result in more rapid settlements on the job than estimated.

Instead I use the results of settlement platforms, tying those into the height of fill on site at the time. Then, projections into the future can be made with those results to help decide when it is Ok to remove the surcharge. That becomes a matter of experience in certain areas and I am afraid to say it is not easy. Plotting time as the square route seems to be best for me.

 

[soil1999]

Thanks alot guys.
Sorry about confusion I am talking about performing effective stress analysis for ende of construction. To do so one should have the pore pressure produced by loading.
This pore pressure is usually estimated from B bar.
B bar = 1 means pore pressure is equal to the vertical stress applied. This is good for soft saturated clay.
WHat if one have partially saturated clay with moderate permeability. B bar in this case should be less than 1. it can be 0.95 o 0.5. But it is reallly not clear in literature how one estimates B bar correctly.

 

[soil1999]

Thanks alot guys.
Sorry about confusion I am talking about performing effective stress analysis for ende of construction. To do so one should have the pore pressure produced by loading.
This pore pressure is usually estimated from B bar.
B bar = 1 means pore pressure is equal to the vertical stress applied. This is good for soft saturated clay.
WHat if one have partially saturated clay with moderate permeability. B bar in this case should be less than 1. it can be 0.95 o 0.5. But it is reallly not clear in literature how one estimates B bar correctly.

 

[dgillette]

"This pore pressure is usually estimated from B bar."

Is that true? I've never seen it done that way with explicit B bar (>25 years of practice). Far more common is to follow a SHANSEP process per C.C. Ladd's Terzaghi 1989 lecture published in the August 1991 ASCE JGE, "Stability Evaluation During Staged Construction," or to simply find residual pore pressure from consolidation tests, often backed up by piezometers in the foundation.