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Differential Settlement Design

Differential Settlement Design


I was after some guidance/confirmation 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 controlled fill obviously flags the horrid 'P' classification from the geotech but the shrink swell range provided (20-40mm) is indicative of a class 'M' site – 100kPa bearing has been confirmed. This is pretty straight forward except that the total settlement for the site has been estimated as 130mm with a differential of 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.

I was going to take a standard 'M' raft from AS2870 and assess the stiffness to see if this is adequate then deepen the beams as required. Given there is no 'strength' requirement I was going to use the same reinforcing as per AS2870 (for the determined beam depth). Is this the correct procedure for designing the raft? Are there any specific Australian code/commentary/guidelines that I should be looking at? What is the threshold after which this type of analysis should occur?

Thanks for your assistance!

RE: Differential Settlement Design

I think that the potential differential settlement will over-ride the M classification. Is a separation distance between max and min settlement given?

My suggestion would be to use Section 4 of AS2870, specifically Cl.4.5.2 & fig.4.1.
'ys' could be taken as your potential  differential settlement and 'delta' will come from Table 4.1. The deep seated movement profile is probably the relationship to use.

It should be recognised that the potential differential of 87mm is equivalent to the surface movement for a class E site which is not covered by the standard raft designs. Also, these are my initial thoughts and there is a degree of uncertainty attached.


RE: Differential Settlement Design

My spider senses tell me something isn't quite right with using the AS2870 code for this analysis.

a few things concern me, this is complete differential settlement, 130mm is a lot of movement. The AS2870 code is based on an assumed soil profile/movement. They assume that you will get movement of this expected amount of 40-60mm over the last 4m-6m (or something like this) of the edge of the building for a H class site, but the internal part of the building will remain close to the same. take a look at the appendix.

your soil profile would not look anything like this. you could get 130mm settlement at one end and 43mm at the other end of the building. hope they don't want a pool table in there house.

I am conservative by nature and I would put in piles (timber) to the 10m depth.

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that them like it

RE: Differential Settlement Design

Thanks for your responses.

I think AS2870 deals primarily with the shrink swell (& potentially cyclic) loadings which I believe would be more severe than settlements. My understanding of differential settlements is that a typical raft will rotate rather then attempt to cantilever/span due any 'mound' actions as the settlement is at a deeper level - ie 43 at one end 130 at the other but in a straight line with minimal stresses developed in the raft.

According to the document that I attached (provided by the geotech) if the stiffness of the raft is large enough than the differential settlements can be treated as 0 - ie it all goes down together. A quick look at the raft stiffness (which includes the stiffness of the slab, beams and walls) revealed that a typical 'M' raft should be stiff enough to achieve this – Es = 6MPa. Unfortunately there is no correlation with this formula to actual settlements, or to a level of tolerance suitable for the type of construction (table 4.1).

I have more or less been instructed to proceed using this document (provide a class M raft) unless I can come up with another reference. I was going to specify some sort of heavy rolling and compaction to 98% but it is impractical to provide internal articulation to the buildings – they are quite small <16m.

Any other ideas?

Thanks again for you time.


RE: Differential Settlement Design

I'm not an expert, but it seems to me that your attachment assumes a homogenous foundation soil with a modulus of E'.

From what you've written I assume that significant  differential settlement of the fill could occur due to the fill self weight, implying that it is not a homogenous soil. If that is the case the guidelines in the attachment may not be valid.

It may be worth posting the question in 'structural eng other tech topics'.

An M class raft may be appropriate provided the separation distance between max and min settlement is large enough.

Compacting a decent basecourse of crushed rock under the raft will tend to iron out the differential settlement.

RE: Differential Settlement Design

Thanks apsix.

I was hoping there would be some Australian reference or guideline...

Why would a basecourse effect the differential? It is my understanding that this is caused by 'deeper level effects' rather than high level soft spots.

RE: Differential Settlement Design

The crushed rock if deep enough can act as a bridging layer.

I am reminded of a story once told to me.

Apparently the BSA had to buy a few homes a while back from the owners. The subdivision was built on soft clay at a deep level. It varied across  30m strip of the subdivision near the base of a hill. The soft clay depth changed from 10m-30m over the strip. Thus all the buildings along this 30m strip had a nice tilt; this was enough for the owners who had just brought the house to complain.

Side note: The BSA has been keeping records on the building since and the settlement is always increasing. The new building owners, who got a great discount on the home, say they don't really notice the tilt.

so while you footing maybe strong enough whats happens if the building tilts?

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that them like it

RE: Differential Settlement Design

Thanks - I have been watching that thread. Unfortunately any interpretation of differential settlement requires the distance over which it might occur (taking it between two supporting elements (in most cases only 4-5m apart) is a bit severe!! I am still trying to get more out of the geotech, but as the numbers are 'theoretical' I don't like my chances.

Fortunately the builder has suggested a waffle slab as a cost saving measure which is much stiffer than the raft (will have 600 deep beams for one building). Did have a question of waffles though... For beam depths greater than the 385mm provided in the table is additional reinforcing or ties/ligs required? Also why is the type of construction limited to 'clad frame and articulated masonry veneer'?

Thanks again

RE: Differential Settlement Design

Waffle slabs don't perform as well as normal raft slabs apparently, at least that's true for the standard pod dimensions.

To maintain ductility an increase in reinforcement is probably required for deeper than standard beams. This is why additional bars are required if the beams are sufficiently wider than standard, see note 1, p. 25. As it says in note 6; modifications shall not be undertaken without engineering design to section 4.

I can't see that lig. type reinf't is required unless the beams are getting too slender. You could then look at using a light mesh, but your builder won't thank you for that.

RE: Differential Settlement Design

Thanks - any idea why they don't perform as well? They go to H-D for clad frame and masonry veneer... Going to 600 deep beams increases the stiffness by 240%. So i think that is something to point at come the court case.

I think the beams will be OK 1N16 covers minimum steel, don't need ligs unless D>750mm and slenderness is fine (don't know how practical though) due to the return beams at 1100crs.


RE: Differential Settlement Design

I'd say the main reason why they don't perform as well because the standard maximum depth is 385mm.

How will you achieve 600 deep, are bigger pods now available?

I would use Cl.4.4 (f) to ensure ductility, although it's written for a conventional raft.

Good point about the intersecting beams, slenderness won't be an issue for your beams.

RE: Differential Settlement Design

I am still seeing the learning tower of Pisa.

AS2870 uses the tensile properties of concrete for the design thus the N16 bar is there for shrinkage. Be very careful about how you design your waffle once you leave the comfort of AS2870. There are many articles that say AS2870 isn't conservative and for your proprieties i doubt it would stand up in court.

There are programs that help you design using ultimate limit state floating around, have never used these myself, but worth a look. apparently they give heavier reo and less concrete.

Why don't waffles perform well, it is to do with the direct stiffness under the external wall, I believe you can use single story full masonry if you want, but you have to have a 300min wide footing under the walls. take a look at the notes, if i remember correctly.

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that them like it

RE: Differential Settlement Design

My problem with settlemenmts like this is

-  as rowingengineer (RE) said, uneven settlement will result in a leaning tower of somewhere!

-  at the edge of the slab it connects to something else, paving, gardens, driveways, something else. What happens at the interface if the raft has settled 130mm?
EG at my house (which I did not design and we moved into when it was 8 years old) with a concrete raft and verandah all around. Some idiot finished of the edge of the verandah with some nice bricks to match the house. Then tiled over the lot as you would to provide a nice verandah surface. We only found out 20 years later when the roots from the palm trees beside this have grown under the brickwork and raised it about 10mm. With 130mm differential between the raft and the surrounds, they had better be prepared to add steps periodically over the years.

RE: Differential Settlement Design

It has been a few years since I used AS2870 but I do not think it applies to areas of deep fill.

My wife once looked into buying some property on fill on the banks of the Marybyrnong (yes I know I misspelt that) but I convinced her not to because the place would never be perfect.

First step CYA
Second Step CYA
Third step be a little stiffer with thew foundation.
Fourth Step specify articulation joints in walls and pavement to allow for the (inevitable) movement.
Fifth Step CYA

As for waffle slas versus raft slabs, I would say that that really depends on the distribution of your loads e.t.c. I have used waffles in poor soil before.


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