Buoyant or saturated weight to stabilize cantilever ret. wall?

[VoyageofDiscovery]

I have a bridge abutment providing also as an open hydraulic channel wall. The geotechnical report mentions to take hydrostatic forces into account wrt applied loads to the wall. All this being done, my question is what do I use for my stabilizing weights on the heel? Buoyant or saturated weight of concrete and soil?

Thanks

VOD

 

[RWF7437]

Unless you know for certain that the backfill will never be saturated (unlikely), use which ever is most conservative. You can probably ignore the hydrostatic pressure on the channel side of the wall unless there is a possibility that the channel will be full and no backfill in place at the same time. This is only likely during construction or during subsequent repairs to the abutment.

good luck

 

[gandersen]

VOD:

Could you give us some idea of what you mean by using the bouyant unit weight of the concrete? Are you refering to accounting for the uplift force from the water that would act on the bottom of the wall? I agree with RWF7437, you need to look at combinations of backfill water conditions and channel water conditions and select the most critical. The most critical for sliding or overturning might be a rapid lowering of the channel water level with a saturated backfill.

Could you give us some idea of what design cases you are considering so that we can comment on what forces you may be using in the analysis?

 

[Daryoush]

For the portion of soil and concrete assumed to be below a design hydrostatic/groundwater level, you should consider buoyant weights in calculations. For the portion above such a level, you should assume full weight of the materials.

 

[VoyageofDiscovery]

Thanks all for you input

By the way gandersen, I am using typical combinations found in bridge codes such as Dead + Earth + Water + Live, etc.

I will be using special draining material behind the wall.

Regards

VOD

 

[BigH]

Regarding Daryoush - He's correct but he forgot to add that you have to add, for the lateral pressure, the hydrostatic pressure, too.

What kind of special drain are you using? I've seen some good specs where the drainage layer against the wall is given, then you place necessary filters to the embankment - still, I have also seen the embankment soil be the "base" and you put filters against the wall - haven't figured this out yet! If you have weepholes, make sure that you have the hole "plugged" to prevent loss of fines/gravel. Try placing the weepholes going "up" to the outside rather than "down". Aggregate, even under water pressure, don't roll up hill well!! Make sure outlet for base drain - sometimes, I've seen it put too deep - it needs to be just above, in my view, normal water level.

 

[Focht3]

Depending on the weep hole details, it is possible that channel flows will penetrate into the wall drain backfill, adding to the piezometric level behind the wall - in spite of good drainage, impervious cover, etc. This could saturate the "semi-pervious" soils behind the wall drain backfill, increasing the wall pressures under some circumstances. But that isn't the problem you asked about -

To me, there are only one circumstance where buoyant weights matter: for all soils (and concrete) below the level of the flow in the channel, or the groundwater level - whichever is highest.

I don't see another circumstance where buoyant weights would apply. Anyone else have an idea?



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

 

[VoyageofDiscovery]

Hi BigH,

The special drain I refer to is 600mm thk of graded stones, no less than 3/8" diameter, from the interior of the wall for the entire height of the wall and wingwalls with filter fabric between the backfill and 600mm thk drain.

I cannot see the build up of hydrostatic pressure with this setup as the water will drain as the water level drops in the channel. The abutment sits on sandy soil.

Regards

VOD

 

[ishvaaag]

One should use always consistent hypotheses and so, for non submerged situations, to imagine a push affected by the water presence without the water load be useful for stabilization may (but may not) turn unnecesarily conservative. Normally the first case will accommodate most of the situations. In case the foundation of the wall comes as far from the wall that the water affected pressure is there, but part of the soil far from it (yet still on the stabilizing foundation) may have dried, one may choose a drier soil weight.

Of course for submerged situations, only the buoyant weight is standing and is the one than must be used.

 

[VoyageofDiscovery]

Actually, I am detailing the free draining layer to go in under the wingwalls, thus allowing the water to go out (or in) from under the wingwalls.

VOD