×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • 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!
  • Students Click Here

*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.

Students Click Here

Jobs

Settlement

Settlement

Settlement

(OP)
Hello, i am first time poster.

I have a site where I need to calculate settlement of a pad (17' x 12') on clay layer over a sand layer.

Clay extends to 15' with sand to 40' plus.

I am calculating settlement down to 24' which is 2B. I have used boussinesq stress distribution to calculate delta P.

I have consolidation data and calculated the settlement of the clay layer using Mv of 0.046 (calculated from consolidation testing). I have calculated settlement in the sand layer using the following formulae based on elastic theory - Layer thickness * Delta P / modulas of elasticity of sand for each layer.

Is the above approach reasonable. Any geotech book i find only details settlement of a clay layer or settlement of sand layer. Never a combination of the two.

I welcome your advice. Thank you very much

RE: Settlement

I would just sum the elastic settlement of your sand plus the consolidation settlement of your clay for the total amount of expected settlement. You may also need to check the time for your calculated consolidation settlement to occur.

RE: Settlement

A common rule for situations like this is extend the depth of calculation to the point where the added pressure is 10% of existing pressure.

RE: Settlement

the total extent of settlement is informed by soil modulus and the consolidation index (Cc) or consolidation ratio (Cec). If the clay is overconsolidated, then it'd be Cr, or Cer.

Mv relates to permeability and informs the time frame for the total consolidation to occur. Not sure how Mv would be related to the ultimate amount of consolidation.

I agree that the stresses should go to the point where delta sigma v is less than 10 percent of the overburden stress.

f-d

ípapß gordo ainÆt no madre flaca!

RE: Settlement

Mv is the constrained modulus; mv is the inverse of this. Both of these parameters vary with applied stress, so any settlement calculation should reflect this.

Personally I prefer to use the bilinear (Cc and Cr) approach (these parameters relate to void ratio, whereas mv relates to strain), but if applied correctly, both approaches will give you the same result.

cv is the parameter which defines time to end of primary consolidation.

Total settlement is equal to: elastic (undrained) settlement + primary consolidation (drained) + secondary consolidation (creep). I've seen several people state that primary consolidation is included in the undrained elastic settlement, though I've never actually seen a definitive reference explaining this.

As noted by others above, you should calculate the settlement of each layer separately (e.g. divide your soil profile into ~0.5m layers). It seems you already has a grasp of how to model varying stress distribution versus depth. You can typically neglect layers where the stress is less than 10% of the applied stress at surface (again, I haven't got a reference for this, but it is standard practice). After calculating these settlement components for each layer, sum them to obtain the total settlement.

RE: Settlement

(OP)
Thanks you for responding.

I have a reasonable grasp on calculating settlement. I was thought that the Mv settlement method is generally suitable for soft to firm clay. For very soft cohesive soils it is better to use the Cc method.

Do you think the method of calculating settlement of granular using the change in stress x layer thickness / E is acceptable. I know there are other methods likes schemertmann for granular but i dont think these can be applied to mulitlayered soil?

RE: Settlement

The Mv (or mv) approach is no better or worse than the Cc/Cr approach for different clay soils providing that it is implemented correctly. Mv is derived from oedometer stress-strain plots whereas Cc and Cr are derived from oedometer stress-void ratio plots; strain and void ratio are related, so whether you prefer to calculate settlement in terms of strain or change in void ratio, you should come out with the same answer whatever approach you use. Whenever I have seen a difference in results it has always been due to an assumption that Mv is constant with stress/strain, which is totally incorrect. At least with the Cc/Cr approach the engineer has explicitly considered pre-yield and post-yield behaviour separately.

In sand soils, the Mv approach is likely the better option - purely because oedometer laboratory tests on sand will likely be recompacted, so Cc and Cr may not be representative of in situ conditions (unless you are compacting sand on site, I guess?). However, once again, changes in Mv with stress/strain need to be considered explicitly to produce meaningful results.

The Mv approach is consistent with your proposed approach for granular soils ('E' being oedometer Young's modulus (Eoed), which is the same thing as constrained modulus (Mv)).

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

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! Already a Member? Login


Resources

eBook - 10 Reasons to Choose CATIA on the Cloud
To compete in today’s fast-paced and competitive market, smaller and newer firms need a powerful platform that will enable them to compete with bigger players, without the heavy investments needed in computer hardware, software and personnel. Download Now
White Paper - Smart Manufacturing for Electronics
This white paper describes a transformative approach to electronics manufacturing made possible by the addition of Mentor Graphics to the Siemens family. It describes a completely digitalized strategy that supports both printed circuit board (PCB) and mechanical design and manufacturing, uniting the entire product lifecycle – from idea and production to customers and back. Download Now

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