Differential Settlement Design

[OzEng80]

Hi

I was after some guidance for dealing with large total and differential settlements.
I have a couple of two story residential houses (one of reinforced masonry and concrete suspended floor (& basement),& one steel framed) that are to be constructed on reclaimed land comprising of about 10m of fill. The geotech has provided a shrink-swell range (20-40mm) which is ‘moderate’ by my design standards and is easily accommodated. 100kPa allowable bearing has been provided which is also standard. The issue I have is that the total settlement for the site has been estimated as 130mm with a differential settlement equal to 2/3 of this. I am not sure what to do with these numbers....

I contacted the geotech who provided me with the attached document which provides a formula for determining the relative stiffnesses of the raft and the founding material (clayey sand, Es = 6 MPa). Provided this is greater than 0.5 then the raft can be treated as rigid and differential settlements ignored.

How are differential settlements accounted for in footing design (is the attached document the standard procedure)? Are there any other references to stiffness requirements that I should be following?

Thanks for your assistance!

 

[ishvaaag]

I would introduce a word of caution here for these formulations contain the B that usually is related to bulb of pressures and then settlement. I have seen repeated this error of concept many many times and so I will state it once more.

The thing is that settlement under an overall foundation and for homogeneous soils depends mainly (80 to 90% most of the times) from the magnitude and position of the column loads, and NOT of the kind of foundation used, be it footings, piles or mats. The effects of the placed loads in the underground soil is integrated and produces this effect.
Exceptions to these are foundations extremely stiff (in their whole dimension) respect the soil and unhomogeneous soils of almost nil settlement where loads are passed through piles to rock.

So it is an error to say that a foundation made of a set of footings will have less settlement than a mat "because" the bulb of pressures will affect in full force less depth; the effect of the set of bulb of pressures of every of the footings is integrated in a great bulb of pressures and you may turn having MORE settlement using the footings just because lesser foundation stiffness is present.

The caution here as related to your problem is that the B you should use to ascertain the stiffness of your foundation must be that one overall mat extending wholly under the building (since low you may still have nice stiffness as related to a weak soil), but you'd never should rely in one B of one individual footing to make the check.

As to how most would study this, I think that now would adjudicate a modulus of ballast to be converted in Winkler's springs and let the program find elastic settlement; and then use several sometimes much different modulus of subgrade reaction or ballast to bracket for almost sure the interaction response. Whilst the difference in stiffness favour much the foundation respect some weak soil, a relationship somewhat related to that in the book will stand, it is just like sinking something rigid enough in butter, we do not expect then much differential settlement from the butter reaction.