Rock Subgrade Modulus 1

[transmissioneng]

Does anyone know of a good reference for typical values of Rock Subgrade Modulus of Deformation, Ks? I am designing some deep caisson foundations that are socketed into Rock and I need the Subgrade Modulus for the Rock.

Thanks.

 

[muuddfun]

What kind of rock do you have? What exploration methods were used to evaluate the rock? Are you looking for end bearing in the rock socket, or are you looking for lateral capacity for your caissons? What are you constructing the caissons for?

 

[fattdad]

transmissioneng: Maybe what you really need is rock mass modulus (along the lines of Young's modulus) - something to describe the elastic properties of rock under normal loading. For a drilled shaft load (vertical) you will find that there is stress increase in the depth below the bearing surface. This stress increase will result in deformation of the rock mass, based on the elastic properties of the rock mass. This is a different mechanic from subgrade modulus (i.e., k), which is related to a spring constant in the structural design of slabs on grade.

I present to you my thesis:

http://home.comcast.net/~fatt-dad/thesis.pdf

This has some reference to intact rock and rock mass moduli values and the factors that affect the modulus of rock mass.

Hope this helps.

f-d

¡papá gordo ain’t no madre flaca!

 

[transmissioneng]

I am designing a 30-foot deep by 8 foot diameter reinforced concrete caisson. The value that I need is the horizontal subgrade modulus for the rock in order to determine what deflection the foundation will see when bearing against the rock.

Bowles has some empirical formulas for estimating Ks from the Es. I calculated the Es of the rock mass from the RMR.

 

[fattdad]

In light of that last post, it sounds like you are trying to model a "P-Y" curve for the portion of the pile that's embedded in rock. It's more likely that the movement at the top of the pile will be based on the "P-Y" curve of the soils in the near surface.

f-d

¡papá gordo ain’t no madre flaca!

 

[BigH]

You might wish to check with Chapter 8 of Poulos and Davis' book "Pile Foundation Analysis and Design." They have a good section on laterally loaded piles - both mod of subgrade reaction and elastic solutions. I would surmise that Tomlinson's piling book also covers this - but don't have my copy with me.

 

[mudman54]

my vote goes to fatddad's post; you don't want a spring constant because it's not constant and far too often gets applied wrong. i've used the k recommended for stiff clay in LPILE and it usually gets me where i need to go. if anyone has a good p-y curve for rock, please let me know.