×
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

Subgrade modulus for multiple layers

Subgrade modulus for multiple layers

Subgrade modulus for multiple layers

(OP)
Typically the recommended subgrade modulus values are for one type of soils, say compacted gravel, stiff clay, etc. I happen to have a case that I will have 2 feet of compacted gravel sitting on medium-stiff clay. What is the best way to estimate a good subgrade modulus for this case? Maybe weighed average for soil within 5B range?

Thank you.

RE: Subgrade modulus for multiple layers

If this is for a rigid pavement, likely the shallowest is used. For AASHTO Ottawa Road test design method it doesn't make a much difference in required thicknesses if the number varies by a lot.

RE: Subgrade modulus for multiple layers

If for any pavement section, layering is common. Each has a different strength and influence. Treat them separately, whether you are using AASHTO methods or elastic layer analysis.

RE: Subgrade modulus for multiple layers

(OP)
Thanks everyone. This was actually for a concrete pad (16'x40').

Ron said "treat them separately". I am not sure what this means. My structure guy would only ask for one value for his analysis.

RE: Subgrade modulus for multiple layers

If you have distinct layers, give your structural consultant a value for the layer immediately beneath the slab. The lower layer would be considered subgrade and would not be counted on to mitigate loading, but would be a "responsive" support layer only.

You have mentioned a "subgrade modulus", which is different than a "modulus of subgrade reaction". A subgrade modulus would be given in psi; while a modulus of subgrade reaction (K), has units odd units of psi per inch of deflection.

If you are required to give the K value, then keep in mind that ultimately, it takes a relatively large variation in the K value to greatly affect the thickness of the overlying slab.

RE: Subgrade modulus for multiple layers

rainandcm,
if I understand well you need a Winkler Modulus, or maybe a spring stiffnes, for structural design.

You have first to assign an active depth. You hint at a 5B value but that may be too large. Opinions are not all the same and there are good reasons for differing opinions.

I would follow the suggestions of the GCR 12-917-21 NIST publication, available on line, which are specific for thsi kind of problem.
Relations 2.18b to 2.18d give the effective depth, zp, for different impedances (translational, rocking-torsional).
Relation 2.18a is an harmonic mean, weighted to the layer's thickness, although this is not written there.

The procedure would be the following:

1)Assign a zp depth by the 2.18 relationships
2)Assign elastic moduli, E or G, to each layer
3)Calculate the harmonic average of E or G by formula 2.18a (just substitute G or E to Vs)
4)Apply a formula where the Winkler modulus or the spring stiffness is correlated to E or G, using the above average (like the Vesic formula, or other similar ones).

Pls note that the publications also carries the Pais and Kausel formulas to calculate the spring stiffnes from G.

www.mccoy.it

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!


Resources