AS1170 vs AS4678 Soil Reduction Factors - Passive Pressures?

[Tomfh]

AS4678 Earth Retaining Structures requires you factor the soil material properties (eg friction angle) when calculating earth pressures. In turn you only need to factor the active Earth Pressures by 1.25 instead of 1.5

AS4678 comments that this results in equivalent Active earth pressure factors in the order of 1.3-1.7, which is consistent with AS1170's previous approach of factoring by 1.5. I.e. you get similar results both ways.

That's all well and good for Active Earth pressures, but for walls that rely largely on passive pressures to resist overturning the AS4678 approach appears to result in completely different designs, because it punishes your Kp passive pressure factor, but doesn't compensate in any way.

Am I right about this? Am I missing something?

 

[Agent666]

Isn't it because the lateral load you are resisting has already been factored by 1.25 instead of 1.5, this is where you are still getting your benefit/equivalence in achieved safety factors, so the reduction is still there in the load being applied.

As opposed to doing it using the normal Ka and 1.5 factor being resisted by normal Kp and 1.5 factor. You have increased Ka and 1.25 factor being resisted by lower Kp and lower 1.25 factor.

So its still hidden away in the calculation of safety factors. Its not like you are taking the overturning load with 1.5 applied and a reduced capacity derived from Kp.

With the increased Ka (higher load) you can accept a lower load factor on the soil (1.25 vs the normal 1.5).

With a decreased Kp (lower capacity), the loads are still derived using the lower load factor (1.25 vs the normal 1.5).

Hopefully that makes some sense!

 

[Tomfh]

No, You are resisting approximately the same factored load (which you are correct about), but the factored resistance is now much lower.

1.5 x Old Active Earth Pressures = 1.25 New Active Earth Pressures (approximately)

But

0.8 x Old Passive earth Pressures > 0.8 x New Passive Earth Pressures

 

[Agent666]

Yeah, I see what you mean throwing a few numbers at it this end.

This might just be an unintended consequence of the way things are being factored before even working out soil parameters. I'm sure someone else has also come across the same issue, and you would expect that the standard might of addressed this aspect but there is very little discussion regarding earth pressures, and then only active pressures.

I'm in NZ so don't use AS4678 much, just a passing familiarity with the way it does things. Because we don't have a retaining structures code in NZ, we also have lots of arguments around the best way to approach things (everyone's doing something different)!

The way the standard goes about factoring the soil properties always seemed a bit weird to me when compared to the usual factor up the loads, decrease the final capacity type Load Factored Resistance Design approaches which are generally taught in universities here. The way AS4678 goes about things is counter-intuitive to the way limit state design of retaining structures is taught in NZ.

Can you perhaps contact standards Australia and ask for clarification of this point (be sure to post back here). Or ask in the geotechnical sections of the forum.

 

[Tomfh]

I discussed it more with a number of different engineers. A few experienced voices told me that it's (one of) the reasons engineers tend to ignore AS4678.

 

[TLHS]

Honestly, this is just the result of switching to factored loads for geotech stuff. It's tricky to calibrate, especially when a lot of the balancing relates to service level deflection work. Generally stuff gets calibrated to some sort of central typical loading condition and everything comes out as it may. It's not wrong, but it's different than historical numbers.

I'm conflicted on what to do about it. The issue isn't so much that treating geotech items as factored resistances is wrong, but that we don't have consensus as an industry on whether we're doing it or not, and what appropriate capacities are.

 

[Settingsun]

AS 4678 is a pretty typical example of how Standards Australia operates from what I can see. See note 9 to clause 4.2: there was an intention to revise 4678 to be consistent with AS 1170.0. 16 years later... 4678 & 1170.0 were published in the same year anyway, so Standards Australia could have just held back 4678 by a couple of months and gotten it right the first time around.

But to the question at hand, the 1.5 factor in 1170.0 has been downgraded to "being proposed with some hesitation" in the commentary. It seems that the position of Standards Australia is that AS 4678 is preferred for calculating earth pressures so it probably wasn't the code committee's priority to match past practice.

Are you looking at gravity walls, cantilever sheet pile walls, anchored walls or something else? For gravity walls, most guides and books that I can recall either recommend against using passive resistance entirely, or recommend downgrading it. An additional (partial) factor of 0.5 on passive pressure is common. This partial factor could alternatively be considered as requiring a larger global factor of safety for walls that had heavy reliance on passive pressure. That's the same as what you're finding with AS 4678 so it might not be unintended; it might be codifying what the committee considered to be good practice. We can't know for sure without a commentary though.

 

[IDS]

This is one of the many examples where standards and common practice which treat loads and resistance as being clearly differentiated become confusing when we have loads that are also part of the resistance mechanism.

It is not clear to me how AS 4678 is inconsistent with AS 1170.0 (which is very general), or AS 1170.1. Appendix J of AS 4678 details how the provisions of the 1170 series should be applied in relation to retaining walls.

In the case of passive resistance it seems to me entirely consistent with the limit state approach to factor down both the density of the soil and the soil strength properties. Not to do so, when applying a factor close to 1 on the density, would be highly unconservative.

For bridge related structures the AS 5100 allows "soil supporting structures" to be designed using unfactored soil properties (both density and strength), then applying a factor of 1.5 to the design actions. This is much simpler than the pure ULS approach, but as noted previously gives similar results.

Also note that AS 4678 is currently under revision, so there will be an opportunity to comment on the public draft in the near future.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[Tomfh]

It's not wrong, but it's different than historical numbers.

That's what it seems to boil down to.

If you refer to the CMAA design guide for block gravity walls you can see that many of the old designs don't work any more. The new cookie cutter designs have much wider bases - often much wider than the height of the wall, when traditionally the height is 2/3 the height of the wall or thereabouts. And they have much deeper toes for sliding resistance. All of this derives from AS4678 slashing the soil's ablity to resist bearing/sliding/etc.

A bog standard 1500 high wall that needs a 2000 wide base? Not sure I really believe that...

 

[Settingsun]

I've re-read AS1170 in the context of retaining walls - my recent walls have fallen under AS5100. I'm not convinced it actually gives a value for the reduction factor for passive resistance. Earth pressure isn't dead load under 1170 and passive pressure really isn't dead load; it's a (varying) reaction to the applied loads. I also wouldn't say it fits the guidance for dead loads given in the 1989 version of 1170.1 (known within 10%).

I'm thinking the available passive resistance has to be determined outside 1170, in a similar way that (eg) you go to AS4100 to design bolts that hold down a cantilever backspan. Unlike steel though, there was no standard that gave a definite reduction factor. So 1170's 0.8 (now 0.9) by default.

Thoughts?

 

[IDS]

Thoughts?

I don't follow what you are saying. You start off saying AS 1170 gives load factors, not material reduction factors, which are found in other codes, then you say you'll use the load factor from AS 1170 anyway.

What is wrong with AS 4678 as the source for the capacity reduction factor on passive pressure?

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[Settingsun]

I'm typing on a phone and taking a shortcut has made my last post confusing.

I was trying to say that the AS1170 beneficial dead load factor was probably adopted by many for passive pressure because there was only one factor given for beneficial effects and no other standard for guidance. But I think that is/was engineers making that decision (or misreading) rather than 1170 actually saying it applied to passive resistance. This would have been supported by 1.5/0.8 being about the same as a traditional FOS.

More succinctly, this thread started out comparing 4678 vs 1170 for passive resistance, but I now think the latter doesn't actually exist. I was after other people's thoughts on whether AS1170's factor applies to passive pressure.

(I would probably use AS4678's 'anything goes' clause to justify the AS5100.3 method out of familiarity if I thought I could get away with it. With a partial factor on passive in addition to the global factor, per my first post.)

 

[IDS]

More succinctly, this thread started out comparing 4678 vs 1170 for passive resistance, but I now think the latter doesn't actually exist

OK, I agree with that.

I should add that I'm not that familiar with AS 4678. Projects I work on are invariably AS 5100 and/or the applicable State authority requirements for reinforced soil retaining walls.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[Tomfh]

But I think that is/was engineers making that decision (or misreading) rather than 1170 actually saying it applied to passive resistance. This would have been supported by 1.5/0.8 being about the same as a traditional FOS.

Yes good point about it being what engineers have traditionally done, as opposed to something spelled out in AS1170

 

[Tomfh]

What is wrong with AS 4678 as the source for the capacity reduction factor on passive pressure?

It results in much more conservative designs. We already get bagged out enough for our "over engineered" retaining walls. Now we have to make them heavier again...