Foundations on varying strata

[Vinny7]

I am seeking advice on foundation solutions for a domestic structure on varying strata. At one side the structure is on rock and at the other side at a depth of approx. 2.5m is a stiff gravel. Would the differential settlement be significant? I would expect the gravel to have a larger initial settlement during construction with only a small long term settlement (this assumes strip and trenchfill foundations). Any thoughts on this or other foundation solutions, pile etc.?

 

[Mccoy]

Vinny,
if the gravel is real stiff, with say, 100-200 MPa of elastic modulus, the elastic deformation of a 2.5 m layer would be usually not significant. unless loads are (very) large. Time-dependent deformation should be zero. Unless there is a clay matrix, and the rock is not clast-supported.
Hard rock will not settle at all unless very fractured with clay seams and large loads. Soft rock will settle in minimal amounts.

In seismic conditions, that soil profile might give problems (not granted), you mightt want to add a layer of costipated gravelly subgrade on the rock so you create lateral continuity with the in situ gravel at the foundation level.

 

[BigH]

Gravel is not stiff. Gravel is dense, very dense, loose but never stiff, soft or hard.

You might want to consider overexcavating the rock for a foot or so and replacing with a sand and gravel nominally compacted. This should even out the immediate settlements to a bit although as Mccoy says, "stiff" gravel really won't have that much behavioural difference if normal tolerances to settlements are permitted. You might want to see details as given in transitions from concrete to asphalt road pavements.

 

[csd72]

How far down to the rock on the other side? Have you considered piers/caissons down to rok on the far side to avoid the issue altogether?

csd

 

[Mccoy]

Gravel is not stiff. Gravel is dense, very dense, loose but never stiff, soft or hard.


Unless embedded in significant cohesive fraction

 

[oldestguy]

Ok guys: No comment on what stiff means.

On what I've done for these conditions is over excavate the rock, if needed but create a tapered thickness of gravel below the footings at the junction between all soil and all rock. For a house I'd make this tapered section at least 10 feet long, with gravel thickness at the "fat" end of twice the footing width.

Then throw in some extra steel in the wall, top and bottom, for good measure.

Buy the way, if the rock has been blasted, be careful about what you set the foooting on. Blasters can make mistakes and over shoot. If you see blasting wires in the rock, that is likely what happened.

For a fussy structure I call for hand cleaning out of all loose rock.

I also have seen excavators over excavate and shove back in loose rock. Watch for that also.

 

[BigH]

Mccoy - then it wouldn't be called gravel, but silty clay or clayey silt with gravel!! Why we need proper descriptions of strata

 

[Mccoy]

BigH,
we may also call it matrix-supported gravel, when gravel, although making up a significant quota of the rock's volume, does not govern much since grains do not touch each other.

But, above all, please note I was 'poking' you: in jest of course, I felt particularly jocular last day,
I hope to the annoyance of none.
As a matter of fact, you were right, propriety of tech language among tech people should be respected.


oldestguy,
since I work in an higly seismic area, your solution interests me, sure the excavation thickness ain't small. Also, tapering the artificial layer is an interesting trick. Is that results-based experience, tech intuition, tech common sense?
By the way, structurals in my area don't like the idea of drilling piles in such situations, they say in case of an earthquake such a type of mixed foundations may do some damage. I do not know if that's true in general, or just speculation...
Would pretty much like to know the board's opinion about that

 

[BigH]

Mccoy - I know a poke when I read one. As old colleagues - we do understand each other!! We should all be more jocular!