## Settlement vs Foundation Size

## Settlement vs Foundation Size

(OP)

Hey everyone,

In the CFEM, there is an equation by Burland and Burbidge (1985) for foundations on normally consolidated coarse-grained soils:

\[ S = B^{0.75} \times \frac{1.6}{N_{60}^{1.4} \times q} \]

I believe that this equation is based on observed data, but there are a couple of things I'm not sure I understand:

1. The equation seems to suggest that as the foundation width \( B \) increases, the settlement \( S \) also increases. I understand that a larger stress bulb covers a larger volume of soil with a larger footing, but isn't the stress lower, assuming the same load on both smaller and larger footings? Isn't this similar to having wider tracks on heavy machinery to distribute loads and reduce settlement?

2. Taking the equation to an extreme, if we have a very small foundation width (close to zero) and a massive load, the equation suggests almost zero settlement. This seems unlikely in a real-world context.

I'm curious if there are specific limitations or conditions under which this equation should be applied that I might be missing. Maybe there's a range of values for \( B \) where this equation is most relevant?

Thank you

In the CFEM, there is an equation by Burland and Burbidge (1985) for foundations on normally consolidated coarse-grained soils:

\[ S = B^{0.75} \times \frac{1.6}{N_{60}^{1.4} \times q} \]

I believe that this equation is based on observed data, but there are a couple of things I'm not sure I understand:

1. The equation seems to suggest that as the foundation width \( B \) increases, the settlement \( S \) also increases. I understand that a larger stress bulb covers a larger volume of soil with a larger footing, but isn't the stress lower, assuming the same load on both smaller and larger footings? Isn't this similar to having wider tracks on heavy machinery to distribute loads and reduce settlement?

2. Taking the equation to an extreme, if we have a very small foundation width (close to zero) and a massive load, the equation suggests almost zero settlement. This seems unlikely in a real-world context.

I'm curious if there are specific limitations or conditions under which this equation should be applied that I might be missing. Maybe there's a range of values for \( B \) where this equation is most relevant?

Thank you

## RE: Settlement vs Foundation Size

Little q is a unit load in kpa not the load in kN, so if you increase B and apply the same q (which would result in a larger Q) then yes, settlement would increase. However, if you apply the same Q (reuslting in a smaller q) then a bigger footing would reduce settlement.

## RE: Settlement vs Foundation Size

## RE: Settlement vs Foundation Size

In a review of other references to the method, the equation you posted is incorrect.

The standard form I find is that q is in the numerator vs the denominator, the coefficient is 1.706 vs 1.6, and the exponent is 0.7 vs 0.75.

See eqn #15 in their paper:

https://www.researchgate.net/profile/John-Burland/...

## RE: Settlement vs Foundation Size

## RE: Settlement vs Foundation Size

In Eqn 4 in the reference paper q' is in the numerator.

## RE: Settlement vs Foundation Size

## RE: Settlement vs Foundation Size