Ret wall reduction in passive pressure for sliding. 3

[koodi]

Does anyone know where the origin of the recommendation for 1/3 reduction in passive pressure? The 1997 UBC Table 18-I-A explicitely states that the two may be combined without reduction.

These office standards are funny. I had the soils engineer ask me where the reduction came from--he was the one that prepared the report and made the specification!

 

[PEinc]

A 1/3 reduction in passive pressure will give a 1.5 safety factor on the passive resistance.

If Kp = 3.0, a 1/3 reduction means Kp' = 3.0 - (3.0/3) = 2.0

3.0 / 2.0 = SF = 1.5

However, you will not have a sliding SF of 1.5 unless you also factor down your frictional sliding resistance similarly.

 

[koodi]

The reduction is in addition to the 1.5 factor of safety for stability analysis (sliding and moment). And this is not the first time I have seen an instance of this recommendation.

I have also had soils engineers provide the stability safety factor within their design values, as PEinc suggests above--this is a nuissance. Software programs, at least the ones I use, apply the safety factor--no way to bypass. So when the soils engineer has already put the FS in the value, I have to confirm with him what factor he used then unfactor the value and then input it into the program.

 

[koodi]

Ooops...need to clarify.
The 1/3 reduction in passive is for sliding when friction and passive are combined. I have also seen it where it is specified the reduction may be taken in either friction or passive when they are combined.

 

[GeoPaveTraffic]

I would expect that the geotechnical engineer (and/or their company) has adopted a policy of recommending a 1/3 reduction to account for minor elevation differences soil property changes, etc. Passive pressures increase very dramatically if the depth of soil increases, they also decrease very dramatically.

An elevation change made in the field during construction that would normally be no big deal could drastically reduce the amount of passive pressure available. Or someone could make a small excavation in a passive zone, again normally no big deal; however, it could lead to movement of a wall.

Counting on the full available passive pressure to meet minimum factors of safety is just not worth the risk. That said, all reductions in soil strengths and determinations of appropriate factors of safety should be made in light of the applied loadings, i.e. you don't need a reduction in strength AND a factor of safety AND a factored load for a load condition that can only happen during a flood that occurs during an earthquake.

And Koodi, The geotechnical engineer should not be expected to make recommendations that they do not believe in, just because the software program that you are using is limited. Maybe you should do the calculations a different way instead of expecting the engineer to say something they don't believe is appropriate.

 

[cap4000]

There are two basic situations for sliding resistance. 1) If you only use the total dead weight of the retaining wall,footing,soil,etc times a friction factor of say 0.45 its 1.5 as the safety factor against the Horizontal Thrust. And 2)If you need the Passive Resisitance Force in addition to the above dead weights it then becomes 2.0 as the Safety Factor. The main difference has to do with the Sliding Wedge & Block Analysis Theory. The Basic Formula with Passive Resistance is Pp+SL/Pa=2.0. Good Luck

 

[koodi]

Yeah cap4000 that sounds about right thanks for the reply. Typically for passive the first foot to six inches of soil is neglected, our office policy. I was more or less entertaining the idea of whether it was necessary seeing that the soils engineer may already be including this contingency in his design values.

I don't know what GeoPaveTraffic was talking about in his last paragraph, and don't care to. I use whatever software the reviewing agency uses--for obvious reasons. Including the stability factor in the design values is a nuissance. I can multiply or divide by 1.5 just as well as anyone else.

 

[Focht3]

Hmmm,

There's more to [blue]GeoPaveTraffic[/blue]'s comment than meets the eye.

Including the stability factor in the design values is a nuissance. I can multiply or divide by 1.5 just as well as anyone else.

The geotechnical engineer provides allowable values rather than ultimate values because some structural engineers have failed to apply the appropriate factor of safety to the design parameters provided by the geotechnical engineer. It was - and remains - a response to carelessness by a few. While you may be annoyed by this practice, don't expect it to change anytime soon.

I don't know what GeoPaveTraffic was talking about in his last paragraph, and don't care to. I use whatever software the reviewing agency uses--for obvious reasons.

I am bothered by this comment; it reminds me of the computer expression "garbage in - garbage out." It is unrealistic to expect the geotechnical engineer - any engineer, in fact - to provide parameters for use in a computer program that s/he is not familiar with. Geotechnical parameters don't come from code books or cookbooks. Again, the geotechnical engineer's cautious response is a response to the carelessness of others.



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See faq158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"

 

[koodi]

I disagree. Providing "soil property" values that already contain the stability factors leads to confusion. It is quite possible that an inexperienced designer becoming accustomed to this practice may assume the stability factors are included when they're not.

 

[BigH]

In many cases, I am surprised that the passive resistance (above the base of the foundation) is not ignored - even in sliding. There is reason to ignore the passive resistance - first, the formulation of Kp is exponential and for a small increase in phi - the Kp increases quite a bit. (Typically, I limit Kp to 3). Also, if there is any chance that the soil in front of the wall will be lost (erosion/scour) or excavated (sewer pipe installation), then you've lost your passive - will your wall slide? Some items to ponder when you consider passive resistance.

 

[Focht3]

Both [blue]PEinc[/blue]'s and [blue]BigH[/blue]'s comments answered your original question. The reduction addresses the risk of partial loss of passive pressure - or misjudging the passive pressure available - by applying a separate (partial) factor of safety to that soil factor.

Regarding the following comment:

I disagree. Providing "soil property" values that already contain the stability factors leads to confusion. It is quite possible that an inexperienced designer becoming accustomed to this practice may assume the stability factors are included when they're not. Thus, it is a very unintelligent practice.

I guess it's a function of perspective. There are two approaches to this issue: provide the soil parameter to the structural (or general civil) engineer as an ultimate value, and expect him to apply an appropriate factor of safety to the soil parameter as a part of the overall design. The downside to this approach is that if s/he chooses too low a value - or fails to factor it at all - then the structure could suffer a catastrophic failure. And someone could die.


If, on the other hand, the geotechnical engineer provides an allowable value and the structural/civil engineer doesn't factor it - nothing bad happens. If the structural/civil engineer factors it inadvertently, the design is too conservative. But this is usually caught because of the higher-than-expected construction cost. Regardless of what happens, there is no risk of danger to the public.

Providing allowable soil values has a very practical basis. It is rooted in a desire to protect the public health, safety and welfare. Every seasoned geotechnical engineer that I know provides allowable values, and I have heard only a handful of complaints about the practice over the 22+ years of my professional career. The complaints usually occur when the structural engineer wants to use unfactored values with USD/LRFD design methods.

The real answer to your concern over allowable vs ultimate values is simple: communication. It's the very kind of problem you have described that ASFE has been working to prevent for almost 30 years. Remember that the geotechnical engineer should be a valued part of the design team. After all, your design literally rests on his work -



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See faq158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"

 

[Baldie]

I have been providing ultimate passive pressure values in my reports and explicitly stating that appropriate factors of safety should be applied. I then discuss that relatively large movements are needed to mobilize the ultimate passive pressure and that an allowable passive pressure equal to 1/3 of ultimate is recommended.

Focht3 has got me thinking that only allowable pressures should be presented because of the potential for misunderstanding, even though everthing is clearly stated.

 

[DirtDr]

I think Baldie is right in that the displacement necessary to mobilize full passive resistence can sometimes be excessive or at least larger than desired. This is a very good reason to reduce or ignore passive resistence in retaining wall design. In addition, the fill compaction in a narrow trench is often less than desired and difficult to inspect properly.

As to the other issue of providing allowable or ultimate values, I usually provide allowable values and then state that they are allowable values. In some instances, such as pile foundations, I will provide the allowable design capacity per pile with the recommended FS. It can get complicated if there is downdrag or other issues that will require that the driving capacity be other than twice the allowable load. If that is the case, then I will recommend several values for different conditions and then be clear at towhich value pertains to which condition. This usually generates some conversations before the report or memo is issued. Since there usually isn't enough commo in most projects anyway additional conversations are not a bad thing.