Bearing capacity of Till Materials 1

[ONENGINEER]

The excavated test pits in till materials (Vashon Drift - calyey,silty sand and gravel/cobbles/boulders) to a depth of 4 m showed that the ground was hard and with 90 degree excavation wall. The pocket hand penetromer readings on the excavated walls were consistantly over 3.5 tsf. What would be a typical allowable bearing pressure for these materials.

 

[fattdad]

I'd say if the footing was less than 5 ft square you could use 3.5 tsf. Then again, it's not my stamp and I'd never trust such advice from an internet forum.

Concern #1 - the pocket penetrometer is rather empirical.
Concern #2 - the pocket penetrometer is an attempt to depict the undrained shear strength, which is not meaningful to granular "till" like soils.
Concern #3 - what's below the depth of 12 ft?
Concern #4 - what's with the mixed units, i.e., tsf and meters?
Concern #5 - what's the anticipated column loads?
Concern #6 - what you building?

I could go on, but it's Friday. . .

f-d

¡papá gordo ain’t no madre flaca!

 

[ONENGINEER]

The TP depth was 14 ft and till materials are assumed to continue to about 10 m before bedrock.

A raft foundation is planned.

Fattad just curious that where that 5ft criteria is coming from. Thank you.

 

[dgillette]

f-d: There is an awful lot of good information to be found on the internet. Some of if it is actually even true!

 

[fattdad]

d.g., Ha!

to the OP, the only reason I said anything about 5 ft is because you have only depicted the soils in the upper 12 ft or so. No knowing what's below that depth, I figured it'd be correct to limit the seat of settlement to the known interval. Typically the seat of settlement extends 2x the square footing dimension or 4x the strip footing width.

f-d

¡papá gordo ain’t no madre flaca!

 

[ONENGINEER]

Hi Fattadd

Actually the structure is founded at a depth of 10 ft on a raft foundation. But we know that from local experience that the soil is expeted to be similar until it gets to the bedrook. Therefore for a soil with pocke penetrometer readings of 3.5 to > 4.5 tsf what would be a conservative bearing pressure.

 

[BigH]

Question - if you know, from local experience, that the till extends all the way to bedrock in the same vein, then what have you used before for bearing? Another question is, since it is a mat foundation - what load bearing pressures do you require? If it is only 200 kPa or so, then you have no worries. Sometimes, think of the question in a backwards way . . . what do I need? is the soil/bedrock a green light or not. I have been on some jobs where a raft foundation put such little pressure on the soil that all one had to say was that the founding pressures were of no concern from a bearing or settlement standpoint.