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# Raft Foundation on Rock

## Raft Foundation on Rock

(OP)
Hello!

I am designing a raft foundation (4m x 7.65m)(~600mm thk.) sitting on Grade III or better rock (bearing capacity ~ 5000kPa). I try to design it as a rigid foundation (ignore the subgrade interaction and flexural deflection).

I commonly use the equations in the book "engineering in Rock Mass" to estimate the settlement in my rock socket pile, but it seems that the equation only work for circular or nearly square foundation.

So, to estimate the settlement of my raft foundation, are there any equations that I can refer to?

PS: In my local Cod eof Practice, it states

"The settlement of a shallow foundation resting on rock can be estimated using theelastic theory (Poulos & Davis, 1974). Kulhawy (1978) proposed a geomechanical model for estimating the settlement of foundations on rock. This model provides a means for
accounting for the presence of discontinuities and can be used to estimate settlement for
foundations on isotropic, transversely isotopic or orthogonally jointed rock masses. The
formulation can also be found in Kulhawy & Carter (1992a). Alternatively, the rock mass
modulus can be determined from the rock mass rating (see Section 6.5.3.2). "

### RE: Raft Foundation on Rock

(OP)
Anyone can help me ?

### RE: Raft Foundation on Rock

I have not heard of a raft foundation on competent rock being analysed for settlement.

The need to analyse pile settlements in rock arises from the considerable stress exerted on the pile shaft...there is no such issue associated with a raft foundation, unless it is to be loaded monumentally, which you have not mentioned.

MM

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