Engineering Behavior of the Bridge Lift

[fattdad]

I'm open to discussing the bridge lift. Working for a highway agency, we are all involved in the preliminary engineering for new projects. For the case of an embankment, we may test the strength of foundation soils and the proposed embankment fill. We may assume ground water table and run slope stability.

Then construction begins. Somebody authorizes a bridge lift on wet subgrade. To what extent would this affect the slope stability parameters considered in preliminary engineering. 2 percent? 10 percent? Would it make a critical circular failure evolve to a critical wedge failure?

Just what uncertainty does the bridge lift add to the engineering analyses?

I just don't like them, but. . .

f-d

ípapß gordo ainÆt no madre flaca!

 

[oldestguy]

From a former geotech employee of a DOT what is a "bridge lift"? Approach fill? Abutment fill?

 

[oldestguy]

Sounds like there is the typical waterproof wall between the designers and the construction departments. This question should have been resolved in the design stage, not during construction. I'd suggest a meeting between the deportment heads of Materials, design, and construction with the problem discussed and resolved. One has to expect rain and if that affects the job, that should have been allowed for. These sort of problems get worse when there is lack of cooperation between folks. It takes tact sometimes and allowance for changing or not following the plans (if there is one) in order to proceed with the job. I've been there and it is not easy.

 

[EireChch]

You’ll have to explain what a bridge lift is exactly?

 

[fattdad]

Sorry. . .

Contractors often claim the subgrade too wet for fill placement. Fill specifications typically want 8 to 10-in thick lifts compacted to some reference density. When the subgrade is wet (and the contractor doesn't control dewatering), they often want to broadcast a first lift that is 3 to 4 ft thick. Just tailgate it onto subgrade, maybe get some slop displacement? Now that you have the initial lift in place, you can compact the top and get on with business. You'll likely get some good measure of compaction on the top surface of the, "Bridge lift." What's going on between the wet subgrade and the compacted surface is something different. Not to spec, not reflective of strength considered in design. Something else?

f-d

ípapß gordo ainÆt no madre flaca!

 

[cvg]

around here a "bridging lift" (which might be aggregate base material or gravel) is not typically allowed. wait till subgrade is stable before venturing on it with any equipment. otherwise, the lift could be placed to allow construction traffic "in the dry", but would later be removed and start again.

 

[oldestguy]

The term "subgrade" can mean a lot of different things depending on your area and local terms. If your specs call for a certain number as to acceptable density, that acceptable density might be needed mainly up near the base course, again depending on your specs. If the specs arn't clear, time to review for next edition. Also, if the specs only require a number to be met, layer thickness may not be something to complain about. I've seen compaction obtained with very thick layers, of course depending on several things. That thick dumped layer may be the best way to do the job for what is finally needed.

How's come before percent compaction came along, roads were built and all came out OK?

However, we can't solve your predicament here.

 

[geomane]

I used to see bridge lifts or lifts “affected by water” all the time on FDOT projects with embankments. I see bridge lifts used on USACE projects also. I have recommended bridge lifts when over-excavation and replacement or moisture conditioning was just not economical. I had to make engineering assumptions on the materials behavior but I never really recommended it in areas that were highly sensitive to settlement or under high fill embankments.

I don’t know if the FDOT checked stability analysis on those projects when bridge lifts were used or not. Maybe they thought the maintenance of adding pavement layers down the road would be cheaper than mitigating the issue during construction? The bridge lift usually is granular material, so settlements should occur rapidly. But there could be potential for noncircular failure planes. What about seismic conditions? You raise a good question.

 

[Ron]

In some cases the only way to get compaction without pumping is to use a bridge lift. Depending on the soil type, that can be anywhere from 1 to 3 feet, also depending on compaction equipment being used. If using vibratory equipment with a static at drum weight of 10 kips or more, and a drum diameter of 4 feet or more in sands, 3 or 4 feet of bridge lift should be able to be compacted full depth. That should result in no weakened shear plane for slope stability.

 

[Mad Mike]

Bridge lifts can add to the stability of your embankment if the material used is coarser than the bulk embankment fill, which it generally is. The effect of it would be to drive the critical failure circle backward, resulting in a deeper failure circle with a higher factor of safety.

With that said, if the bridge lift is placed over a deep clay soil profile, its effect becomes minimal in terms of stability, since the critical failure plane takes on a steeper angle at its intercept with the bridge lift and simply shears through it into the underlying soft clay.

All the best,
Mike

 

[fattdad]

I can tell that several of us are familiar with the term, that's for sure!

I wish I'd know that the bridge-lift fill material was granular! Maybe that's how to handle it, by specifying, "Select type 1" fill materials, which would have minimal fines, but still be fairly-well graded. Just claiming a, "Coarse-grained" fill material in the word of ASTM could return a clayey sand with 49 percent CH fines?

We are scoping a new road and the whole thing will go through wet swampy land. We'll have high embankments to go over railroad and another high embankment to cross a braided stream. I can already see the writing on the wall - minimal undercuts and a bridge lift.

Regarding settlement - I agree that would not likely be on the critical path if it's a fairly clean coarse-grained soil.

Regarding shear strength? Just not sure how to model a bridge lift phi reduction, that is if one is appropriate.

Thanks for the feedback.

f-d

p.s., I use the term drainage blanket for an open-graded aggregate surrounded by filter fabric in the first lift.

ípapß gordo ainÆt no madre flaca!

 

[maxim22]

Seems multiple issues.

The nearest example I have is agricultural reservoirs in chalk up to 100 million gallons. The first lift only is deeper by necessity and is acceptable because it is compacted part by surcharge from later lifts. The embankment overall would get stronger as it rises (not too quickly time establish by trials), compaction/settlement increases density and friction between particles. The slope gets trimmed to finish.

If there was suspect slope stability specified initial partial coarse granular blanket pre-lift and a toe wedge to send the critical failure plane away (and give downstream drainage).

Ground bearing/settlement under the embankment wasn't a problem in chalk like it would be in soft clay.

 

[fattdad]

Hi Maxim,

I would only allow for strength gain for the case of a fine-grained fill material. Then I'd imagine some Su/P' that would provide strength with successive lifts of fill. Seen way too many granular deposits under load at or above their critical void ratio.

Thanks for the comment!

f-d

ípapß gordo ainÆt no madre flaca!

 

[GeoEnvGuy]

If your bridge lift is made up of crushed rock your loss in strength due to compaction is a lot less than that from increasing confining pressure as illustrated below. When you mention wet swampy land I envision thick organic deposits and normally consolidated or slightly over consolidated clays, you are going to be doing preloading which would fail due to excess pore pressures during construction in the low permeability clay before gaining strength.

 

[MTNClimber]

I've seen contractors try to use bridging lifts and have it go wrong with granular material. Even though the bridging material wasn't saturated, pumping was still observed after the bridging material was placed and compacted. I'm not a fan of bridging material unless you are putting down filter fabric with crushed stone or geogrid with clean granular material. I've heard of crowding crushed stone into the layer that is pumping but haven't tried it in critical areas. 49 out of 50 times, the contractor would rather just let the material dry but that's probably not an option in a swamp unless you have some insane dewatering system. Obviously a contractor will only opt for the more expensive route if the owner is paying for it.

Maybe consider reinforcing the interface of the natural saturated soil and the beginning of the embankment fill? Its expensive but maybe that's the safer way?

 

[fattdad]

to GEG: I'm referring to ordinary fill materials placed in a 3 to 4 ft initial lift. I'm not considering open-graded aggregate (i.e., drainage blanket). I'm also mindful of the native soil properties and understand the difference between compression and consolidation and effective ways to use ground improvement, surcharges and preloads (i.e., via Su/P'). My inquiry relates to a perception or reality of a low-strength zone within the actual newly-placed fill.

To MTNClimber and others: I think we'll have to make a technical requirement to use drainage blankets in those areas with wet subgrades. Site visit is soon and we'll do some peat probing. Our consultant will be drilling, sampling and testing and I'm still building a box around their scope of work.

Thanks to all!

f-d

ípapß gordo ainÆt no madre flaca!

 

[GeoEnvGuy]

In my experience working with tailings dams which have a lot of loose saturated silty sand with non-plastic fines. The latest guidance I have seen is to determine if the loose sands will behave in a contractive or dilative manner, which is correlated to determining the relative density. If your bridging layer is so loose that it will behave in a contractive manner you would need to consider peak undrained strengths at the base of your model, for dams we consider the peak or yield strength proposed by Olsen and Stark 2003 which is back calculated from previous failures using the pore pressure conditions prior to failure.

 

[Ron]

F-D...I've done bridge lifts over swamps (tidal in coastal Georgia and Florida) and over soft mucky soils in other areas. The lifts have ranged from specified clean sands to cellular concrete fill so as to reduce the overburden load for settlement. As for the phi angle change, you are more likely to have an increase in phi than a decrease.

 

[fattdad]

Hi Ron, Yes, that makes sense and I'd agree. I think I'll solve the problem via the d-b technical requirements. I just have my suspicions that if left unsaid, they'll take the embankment fill (not open-graded highly-frictional materials) and dump it for the first few feet. I just don't want that!

I'll call out locations for drainage blankets on the concept plans.

To GEG, I understand critical state soil mechanics and went to graduate school with Tim Stark. I understand what you are talking about, recognize it as correct. I'm just talking about something else.

f-d



ípapß gordo ainÆt no madre flaca!