Smart questions
Smart answers
Smart people
Join Eng-Tips Forums

Member Login

Remember Me
Forgot Password?
Join Us!

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips now!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

Join Eng-Tips
*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.
Jobs from Indeed

Link To This Forum!

Partner Button
Add Stickiness To Your Site By Linking To This Professionally Managed Technical Forum.
Just copy and paste the
code below into your site.

boulders backfill with concrete binder for overcut foundation?

redshot (Civil/Environmental) (OP)
19 Oct 11 2:48
Our hospital project has deep foundation.Overcut of more less 1.8 m below pile cap was made due to space problem. Depth of water is about 80 cm. I want to backfill first with boulders mixed with concrete binder up to the water level. But I doubt the stability of subgrade foundation of the pile cap. Please any expert can give me opinion on this, highly appreciated.  
hokie66 (Structural)
19 Oct 11 7:06
The sketch you attached has been seen here before...from another engineer, presumably the design engineer on your project.  Why not ask him?

Stability of the material directly under the pile cap should not matter, as the vertical load is supported by the piles and the lateral load is braced by the slab.  However, I don't understand why the undercut was made.  What does a space problem have to do with it?  I trust this was for one column only.
redshot (Civil/Environmental) (OP)
19 Oct 11 10:11

tnx to your reply, greatly appreciated.Our foundation is clustered piles in different pile caps, it is located the edge of building with adjacent existing structure,on one side and a deeper pile cap on the other so overcut was made. In this case, I want to place boulders first with concrete binder up to the level of water, then backfil with selected fill materials. However, because of deep overcut, I doubt if stability of subgrade level below the cap might be affected. Hope you can explain further.tnx a lot.
cvg (Civil/Environmental)
19 Oct 11 14:15
If you are concerned with stability of the adjacent existing structure, you should have built a retaining wall to prevent movement of the subgrade into your overexcavation.

TXStructural (Structural)
21 Oct 11 1:40
Why "boulders with concrete binder"? Sounds like a prime situation for flowable fill (controlled, low-strength concrete).  However, using large rubble or rocks with no intermediate size aggregates sound like a recipe for expensive disaster.  If you are looking to conserve on the amount of fill by adding very large rocks, the difficulty will be in getting consolidation so water does not accumulate in voids and promote collapse of the fill above into the voids.  If you decide to go with your idea, be sure the concrete used to fill the voids is modified (HRWA, viscosity modifier) so that you get the result you need.
grabens (Geotechnical)
22 Oct 11 10:03
I agree with the flowable fill recommendation.  Boulder fill under an addition to an existing hosptial building sounds like a bad idea.  Usually expensive medical equipment is sensative to settlement.  The only one who wins will be the contractor and/or owner that pocket some money while you take on unnecessary risk.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!

Back To Forum

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close