Construction Joins in large raft - Overlapping stress zones

[UP830]

We have a large raft 160 x 80m. We have a borehole to 30m only. Medium dense Sand to 9m, clay to 13m medium dense to dense sand / gravel below.

I argued that to do a settlement calculation, you need information to 2B, which is 160m. She argued that if you install construction joints, the slab essentially acts independently. There will be overlapping stress joins and in sand, these areas will have higher stiffness due to increase confinement. I am not sure on that part tbh.

I argued that it is a flexible foundation still and acts as one. For example if you construct a 20m wide embankment x 100m long. You have a ZOI of 40m 2B...it doesnt matter what the surface it is applied to, the soil still feels the load in the same way.

I understand the difference between flexible and rigid foundations, that some loads gets shed to the edges or middle etc. But in this case i think I am right that the ZOI is still 2B

Refer below - I say A is right, shes says B. Let me here your thoughts and evidence to back it up

 

[GeoEnvGuy]

160 m by 80 m raft on 9 m of sand, why? Conventional thinking says footings.

Also where are the loads going to be, i doubt it will be evenly distributed along the entire area. Typically I am familiar with the edges loaded and support columns in the middle. This is where we would determine the ultimate and serviceable bearing capacity of a thickened edge slab of less than 2 m width.

 

[SlideRuleEra]

IMHO, you are right about the raft being considered one foundation (160m x 80m), but don't go claiming that you are "right" about boring depth:

From "Soil Mechanics Fundamentals", by Muni Budhu

It is not unusual to have incomplete information... learn how to do the best you can with what you have. In this case, you have a lot, soil boring 30m deep.
Stop "arguing" and get to work:

One approach is to create your own "evidence" (as you call it). "Bound" the settlement. Make two calculations:

1) Best Case - Use available data to 30m depth and assume "very good" material to whatever depth you need for the calculation.

2) Worse Case - Use available data to 30m depth and assume "poor" soil to whatever depth you need for the calculation.

Depending on loads on the raft, you may find that what you are arguing about does not make a dime's worth of difference. If the two assumptions do result in significant differences in settlement calculations, you have taken the first step toward demonstrating the need for additional data.

http://www.SlideRuleEra.net

 

[EireChch]

i also agree that it will act as one applied load over the area. as for evidence, i am not sure but the idea of construction joints making the stress bulbs depth halved....not buying that.

I also think he is right SRE about 2B, however in an ideal world. in my experience its not common to drill a BH to 50m plus for shallow foundations no matter what size raft. hopefully you would hit rock within 50m and prove it is continuous for 10m at least. you could then assume it extends to depth. you obviously need to have a good grasp of the local geology to know that that assumption is valid!

if youre still in mediocre material at 50m then you may have a settlement issue. however your field geologist should be relaying information and a call should be made to go 10m more and reassess

the above all depends on the applied load, if its lightly loaded then assumptions such as SRE stated could be employed IMHO. if its heavily loaded then you need to have some "evidence" to back kt up