Concreting of Friction Caissons

[DocDirt]

How long should the hole be left open after drilling and before concreting?
The job involves a friction caisson with a socket in Shale rock. Shale tends to slake and weather on soaking. I was told that the practice in some states dictates that an open excavation should not be allowed to stand for more than 4 days or so.In this project, the hole is filled with water to balance the hydrostatic pressures in the hole. We sre going to run some durability tests on Shale.
DocDirt

 

[BigHarvey]

Good sense is to concrete as soon as possible after finishing the excavation.However, knowing that you have to install reinforcement and tremie pipe and that you might have a week-end between end of excavation and equipment, 4 days seems a reasonable practical limit.
In practice the bottom should be checked bewore lowering the reinforcement and cleaned if necessary.

 

[DocDirt]

Thank you BigHarvey
DocDirt

 

[Zambo]

For a pile using bentonite as a support fluid most specifications will limit the period to 24 hours. This is sufficient to excavate, install the rebar cage and cast the concrete.

 

[DocDirt]

Based on slake durability tests, we found that there is minimal slaking. However, we have stipulated to the contractor that the holes have to be concreted as soon after drilling. If the hole is left open for more than 12 hours, contractor has to pull ot the rebars, roughen the sides of the socket and clean the bottom again.
DocDirt

 

[dik]

Are you relying on the socket for load? Stiffness may be that the socket takes 90% and the soil friction is 10%. I've not often combined the two.

Dik

 

[DocDirt]

Thank you dik for your posting.
For determining the load taken in friction and in baring, I use Figure 9.18 b, Page 371, from the text on Rock Mechanics by Goodman.
By the way, I did find some reference from Ohio DOT specifications regarding my question. It states that especially for caissons constructed by the wet method, if the hole is left open for more than 24 hours, hole has to be drilled an additional 6 inches and cleaned before concreting. In a job, I am involved now, where we are using friction caissons, we stipulated that if a hole is left open for more than 24 hours, the sides have to be roughened and the bottom cleaned before concreting.
DocDirt