Estimating Min. Dry unit weight

[jakeh76]

I'm trying estimate the min dry unit weight of a cohesionless backfill we will be using. I am in need of this value to estimate (Ko)for a wall with compacted backfill. I have:

Max Dry Density
D85
D30
Cu
D60
D15
D10
Classification

Any help would be appreciated. Thanks

 

[BigH]

Why do you want the dry unit weight? You should be using the bulk unit weight which includes the dry weight of the soil plus the "natural" water content in the soil. Do you want ka or ko?? ko is the at rest lateral earth pressure typically take as 0.5. ka is the active earth pressure - means the wall will be able to "move" a bit. Doesn't take much to go to active. ka is typically in the order of 0.3 to 0.33 for horizontal backfills. This is in most elementary soils books.

I hope that you won't put the following on your drawing:

"Backfill to consist of sand to sand and gravel with a unit weight of 18kN/m3 having a "delta" of 15degrees and a phi of 33 degrees" - then specify 98% MDD Modified Proctor!

 

[MRM]

You could simply run a minimum unit weight test if you want to know that soil property. ASTM has standards for the minimum unit weight test. It's simple to run.

As an aside though, I'm not sure how knowing the minimum unit weight will help you estimate earth pressures coefficients. Earth pressure coefficients are more dependent on friction angle than unit weights. For example, two different soils with different minimum units weights can possible have the same, or very similar earth pressure coefficients if they have a similar friction angle.

 

[UcfSE]

MRM is right. Active earth coefficients are not based on unit weight, but are more dependent on the soil friction angle. You will see this if you look in some typical texts on geotechnical and foundation engineering. I suggest you contact your geotech guy for the project for the information for which you are looking , that is what I always do.

 

[Focht3]

Hmmm,

I'm going to conditionally disagree with [blue]MRM[/blue] and [blue]UcfSE[/blue]. Since the effective friction angle is closely tied to the soil's density, one cannot separate the two. There is no such thing as "the" angle of internal friction. So the earth pressure coefficients are affected by the soil density...

While I understand [blue]jakeh76[/blue]'s question, I, too find it hard to see a practical application of this information. Please describe your problem in detail, [blue]jakeh76[/blue]. Perhaps we can help you yet -



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[BigH]

Hey, all - I think that if you read my initial post, you will see what he wants to do - what I hope he didn't/doesn't want to do. ka is directly proportional to unit weight and tan(friction angle) - so not sure that "one" is more dependent than the other. But I've seen too many structural drawings using the parameters to define the backfill, but call for a relative compaction that is too much than they use - they just want to cover themselves in the "design" earth pressure. At least this is my take of his question.

 

[BigH]

- forgot to add to my previous post that ka (or kp) is proportional to the square of the height - this is really the most critical aspect of the earth pressure.

 

[jakeh76]

The reason I was asking this is in Das "Principles of Geotechnical Engineering" 3rd edition page 384 he talks about the how Jaky's equation under esimates the Ko if the back fill is dense sand. He reccomends using the folowing equation:

Ko=(1-sin(phi))+(((dry unit weight/min. unit weight in loosest state)-1)*5.5)

 

[MRM]

Hi jakeh76,
I see why you’re looking for minimum density now. I've never seen the equation form you show, but I think it's similar to one that provides an additional term onto the Ko=(1-sin(phi) term, which involves the OCR. The Ko of a compacted sand, at shallow depths, will usually be greater than simply (1-sin(phi)) due to the overconsolidation effect. This is assuming we are talking at rest. It appears that your equation provides a different way to estimate Ko of a soil that has been compacted, or one that has an OCR greater than unity. I still stand by my original recommendation and that is to run a minimum density test using ASTM D4254 (check on that # to be certain). The results of that test can be expressed in terms of dry density or moist density if required if you know the moisture content you're dealing with.

Since Focht3 has conditionally disagreed with what I said, I'll take a stab at explaining myself and clarifying what I said. I’m up for a challenge, I guess! Besides, it’s been a while since I was involved in a discussion! I think something just came out wrong when I said what I said, or the lines got cross somehow.

I think the problem with my original post was that instead of saying, "earth pressure coefficients are more dependent on friction angle than unit weights," I should have said, "earth pressure coefficients are more DIRECTLY dependant on friction angle than unit weights." And by “directly dependant” what I really mean is that “the unit weight is one of many variables required to arrive at a friction angle estimate for use in theoretical earth pressure relationships.” It is true that unit weights influence friction angle and earth pressures, but here’s why I think it’s confusing when you start talking about unit weight as a basis for earth pressures, or any other soil parameter. Here are three things to consider and probably none of them are new to us…

Soil density (unit weight, take your pick) alone explains nothing about the soil strength or compressibility characteristics of a soil. Two different soils can be at the same in place density, yet have different soil characteristics. This is because the soil characteristics depend on many other variables than simply "density." Example, “that soil has an in place density of 15kN/m^3.” Is that good? What are the friction angle, compressibility, and permeability? (Please note: I’ll catch myself now and state that friction angle itself is also dependant of type of failure, strain rate, plane strain vs. triax conditions, etc. before anyone else does!)

Relative density is a step forward toward providing a basis for soil characteristics and earth pressure estimated. If we know relative density, then at least we know where the soil's density falls between the min and max theoretical densities, right? However, talking about earth pressure with respect to relative density, solely, is also erroneous since, again, friction angle is dependent on many things-relative density being only one of them. What other things? Gradation, particle shape, mineral composition, etc. We know that.

When we break it down, simplify it, and just talk about friction angle (say drained friction angle for argument sake) we have taken a giant step forward in expressing a soil characteristic that can be used to solve theoretical problems, like estimating the earth pressure coefficients. A friction angle of 35 degrees, is a friction angle of 35 degrees regardless of the in-place density. It no longer matters. We’ve temporarily removed the “middleman” (intermediate variables required to develop the friction angle) in order to use a more direct parameter to solve the problem. The friction angle now represents the consideration of numerous variables involved in developing the friction angle parameter for use. That’s all I meant.

 

[Focht3]

[blue]MRM[/blue]:
Well done. Your previous post didn't quite "sound like you" - if you know what I mean. Hence my "conditional disagreement." I guess that I could have done a ,"What he meant to say was...", but I didn't have time. Been very busy lately. (Yeah!)

[blue]jakeh76[/blue]:
I have a copy of Das' book - will look at that section later today. If I see anything that deserves a comment, I'll post it.



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[BigH]

I'm on vacation now - so my brain cells are a bit "loose" - but why does this thread keep pointing to ko for earth pressures against a retaining wall rather than ka?? Unless the wall is a restrained basement wall, it will deflect to some degree and ka should be used - I would think.

The points made on the factors affecting the ka (or ko) value are valid. I would suggest that the difference is that the at-rest ko value (before wall tilts) is from the OCR as MRM states. Sure, but, again, for the active - or at-rest - earth pressures (the loads), the height is obviously much more important than whether the unit weight is 18 or 19kN/m3, or (phi) is 33 or 35 - don't think any of us would use phi = 40 even if we densified a lot. The difference between k (active) of 0.3 to even 0.35 is, say 15%, but the diffence of H=7 to H=9 is 81/49; a 60% increase.

 

[MRM]

The height is certainly a big factor. Actually bigger than phi or unit weight as BigH said, especially since when computing the resultant earth pressure, the height appears as the only "squared" term...in that respect it is quite a bit more sensitive than phi or unit weight really. Phi and unit weight can't be considered moot points necessarily, but height is probably the most sensitive variable.

As far as ka and ko go, I'm not sure what kind of wall we're talking about either. Ka may be the more appropriate coefficient to use-depending on the situation and whether the wall is allowed to deflect to the point of creating active conditions without lots of damage to the wall. It seems like choice of coefficient comes down to allowable deflection (and failure load) of the wall; if some movement is tolerable and won’t hurt the overall integrity of the structure, design using ka. If the structure is very sensitive, design for ko conditions (which would result in a "beefier" wall design, and would lower the chance for distress if movement is not allowed). Although, not much lateral movement is really necessary to reduce the pressure a great deal…in fact that movement required probably reduces quite a bit as the fill gets more compact too.

I think geotechs will continue to be the biggest reason clients or structural engineer claim their wall design is too conservative no matter how we justify our reasoning!! Sometimes I wonder if we should even fight it anymore! Ha ha!

 

[egirijesh]

For normal conditions and structures the very basic coeff. of Ka is sufficient for design purpose. You should clearly see the drainage conditions of soil so that realistic estimates of unit weight, cohesion and friction coefficients are made.

 

[egirijesh]

if you are not academically interested in finding ko, normal ka coeff. shall do for design purpose. You should be clear about the drainage conditions of soil to estimate accurately bulk unit wt., coeff. of friction and cohesion. I think this will be sufficient.

gr

 

[MRM]

egirljesh,
I want to make sure I understand you correctly; Assuming good drainage conditions, etc, the amount a wall of "normal" or "typical" height would need to move laterally in order to develop active conditions, would not generally cause undue stress to the wall itself (excessive cracking, high internal structural stress, etc.) or damage to buildings or structures near the top of the wall as a result of the movement? This is good practical info to know. In your experience, have there been any instances where the wall was tied in with an especially sensitive structure of some kind and you had to design for very little lateral deflection? In those cases did you use Ko instead of Ka, or something in between the two, maybe? Just curious.

 

[Focht3]

[blue]jakeh76[/blue]:

My copies of Das' books don't have that on page 384. What's the section heading?



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[solutioninc]

When one talks about the earthpressure coefficients (Ko or Ka) and the displacements needed to mobilize active conditions, one also needs to look into the factor of safety the designers put in. With this factor of safety a wall designed for active conditions may not move at all and active conditions may not ever be realized. So why worry about the distress? If the factor of safety is not sufficient for whatever reason, the wall could move but at the same time the pressure will be reducing to active conditions and this will prevent distress in the wall.
Problems arise when one wants to estimate actual pressures existing behind the wall or when elemets of the wall are designed with very small factors of safety and the whole wall with a large factor of safety

 

[MRM]

Solutioninc, Good point regarding the F.S.

 

[Focht3]

Yes, but...

One also has to be aware of any elements connected to, or affected by, the wall. Particularly when dealing with braced or anchored walls. In these circumstances, movement compatibility is the key issue, so the wall can still "fail" when it hasn't cracked -



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[Focht3]

As a side note, it is possible to design walls and incorporate movement criteria, much like p-y criteria for laterally loaded piles and drilled piers. The soil reaction - soil movement curves are called q-w curves. It takes some work, but is worth the effort on "major" structures -

The basic idea is to develop realistic unfactored values of passive, at rest and active earth pressure at various points along the profile of the structure. NAVFAC DM-7 contains approximate displacement values (as a function of wall height) of the movement needed to achieve fully active and fully passive soil pressures. I normally use a parabolic curve (similar to Matlock's soft clay p-y curve criteria) to model the transition from passive to at rest, and at rest to active. Using this approach, you can see the effects of using various tie-back types, anchor levels, etc. in a very consistent manner - using expected performance as your final design standard. This is also a great way to model the construction sequence.



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.