[b]mat foundation

[charlieroc]

I am working on a project that has a 5 million pound load that we can spread over a 30 foot to 60 foot diameter area. We have SPT values on the upper SP/GP soil which rest on moderately hard ROCK that ranges in depth from 12' to 22' below grade. We have some soil samples in the lab that we will perform classification test on and CBR. Any other lab test we should perform to provide information to the project design team for design of a mat foundation?? What will they need from me to design this mat foundation.

 

[UcfSE]

Modulus of subgrade reaction if it's a mat that supports multiple columns.

 

[dmoler]

Rock modulus would be very helpful. For settlement estimates, I would use a combined modulus of the rock and sand for mat foundations. Poulous has good methods summarized in his book 'Elastic solutions for Soil and Rock Mechanics', pg 164.

 

[charlieroc]

Thanx...the poulos book is sold out but still looking.

 

[dmoler]

The Navfac DM 7.02 (pg. 158 or so) and Navac DM 7.01 chapter 5 have methods that are ok also. You could use a simplified method for the subgrade modulus. Something like:
upper 12 feet: (200 pci*(12/60)
next layer rock 12 - 22': (determine rock modulus * 10/60)
then lower layer (sand?) to 60'200 * (8/60))

then add all three together for combined modulus.
The value of 60 is depth. Of course, use modulus numbers that you think are right and depth of influence.

 

[dmoler]

Actually last would be (200 *(48/60) for last depth

 

[BigH]

It is a hard book to find - so I've been told (which is why I am happy to have two copies - one here in India and the other at home). Anyway - DeBarros (as given on pg 164 of Polous and Davis) suggests combining two E layers as below. This was for a three layer system so that you could use a two-layer solution chart. They note that there is no reason this wouldn't work for multi-layers - so I would say that you could combine all layers similarly and use single layer charts.

Let A = h1 x cuberoot of E1 + h2 x cuberoot E2.
Let B = h1 + h2

Then E(equivalent) modulus = (A/B)^^3

You will many times find these equations in pavement design books as well where they are looking at determining stresses in a pavement section.