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

 

[DBNodurf]

Just for fun, let me up the ante; the models of Ka, Kp, and Ko (at least the ones we learn in intro soils) assume the entire backfill suddenly appears (or shall we say the wall suddenly appears against the backfill) and the backfill is then "allowed" to rotate (or not, in the case of Ko).

This doesn't happen in construction; as backfill is placed from the bottom, the wall deflects and Ka results. As more backfill is placed, it is compacted against the deflected wall, which then must deflect some more, but as the wall is required to deflect more, the reinforcement is more thoroughly mobilized and begins to resist movement more, so the pressure is somewhat greater than Ka, but still less than Ko. As the backfill is installed, the pressure grows closer and closer to the Ko value, but likely never quite reaching it. This would make the actual pressure envelope nonlinear.

When the wall movement is restrained by tiebacks or by sttel strips as in the case of RECO walls, the K value can actually be greater than Ko.

D. Bruce Nothdurft, MSCE, PE, PG, M.ASCE, etc, etc,...
Principal Engineer/Geologist
Atlantic Geoscience & Engineering
Charlotte, NC

 

[Focht3]

Bruce -

Absolutely correct. Very few engineers (regardless of flavor) seem to understand this.

In a similar vein, what pressure (K_P, K₀, K_A) would you use for the following: A bulkhead is constructed in a port facility. Mudline is at El. -22' MSL, sheet piling driven to about -45'. Cast iron tie rods installed to dead men anchors (pipe pile A-frames) and adjusted for "final" length. Then a "beach sand" (typical angle of internal friction of about 32&[ignore]deg[/ignore] is hydraulically placed behind the bulkhead...

The designer assumed that the sand fill would achieve fully active pressures. They did - when the bulkhead failed!



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

 

[DBNodurf]

I presume that was a drained angle of internal friction; the sand being placed hydraulically would never be able to achieve this angle (or an active case) until all pore pressure had dissipated (the drained condition). Depending on how much of the backfill was below the adjacent water level, it might never reach this condition. A hydraulically placed backfill is the perfect use of the equivalent fluid pressure concept structural engineers have long loved.

D. Bruce Nothdurft, MSCE, PE, PG, M.ASCE, etc, etc,...
Principal Engineer/Geologist
Atlantic Geoscience & Engineering
Charlotte, NC

 

[Focht3]

Yeah, the "typical angle of internal friction of 32&[ignore]deg[/ignore];" is based in an in place dry density of about 115 pcf - NO WAY the hydraulic fill met that density! It was a pretty screwed up design.



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

 

[BigH]

Soooooooooooo, what we might be trying to say is that to use ka for design may be unsafe and it would be better to use a value of k closer to ko than to ka. In many structures, the extra steel and concrete would probably not be all that prohibitively costly to do so. In some ways, we do this anyway in that most, but certainly not all, geotechs likely use values of phi < actually achieved with proper compaction (not hydraulically placed). Even though direct shear tests show phi values for medium to mainly fine sand of 36 degrees, we would likely temper this to a value of 33 degrees or less. This would increase the ka value. (let's not get started about LRFD). One other reason to, perhaps, use the higher k value (and hence perhaps more conservative) is that vibratory compaction plants, even smaller ones, impart a somewhat higher horizontal pressure than ko values due to the overloading of the vibratory impacts. This, then, again will take us up closer to ko.

So, in the end, judgment does prevail - strengths are tempered, unit weights a bit lower, likely k values are the &quot;book&quot; value of ka but perhaps we should be a bit higher in the value.

As in my first comment, we should hope that structural engineers don't write on the drawings something like: The backfill material is to bave . . . (including the adhesion factor). The specifier should indicate which soil he wants to use (fine to medium sand, sand and gravel, etc., and then give a juddged proper level of compaction to meet his design requirements - for few of us would ever do direct shear tests, adhesion tests, etc. on a backfill material for &quot;most&quot; jobs.

 

[Focht3]

No LRFD &quot;discussion&quot;? Aw, you're no fun!

[blue]BigH[/blue] is right - a little conservatism is warranted. The small additional cost of the extra steel and concrete is well worth every penny. Inadequate basement walls are damn hard - and expensive - to fix.



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