Bridge Abutment Piles and adjacent Surcharged Embankment

[DirtMD]

I am curious to see where this thread will go. This is not a case history but I think the following question can be raised more and more as accelerated bridge construction techniques are being utilized. Here are the possible project circumstances:

A row of 18 driven bridge abutment pile foundations are installed prior to construction of a 2-stage MSE wall embankment. The piles are 100 feet long through a fine-grained soil profile. The tips are established in a moderate end-bearing layer. The MSE embankment (abutment) is then surcharged with 10 feet of material in order to minimize the bridge approach “Bump”.

Question: In an effort to minimize the potential for differential settlement (or differential stresses) within the pile cap, When (and why) should the foundation contractor be allowed to construct and cast the abutment pile cap and thereby tying the pile group together? A) While the abutment is under surcharge conditions; B) While under surcharge but at or beyond 95% of primary consolidation; or C) After surcharge has been removed.

Thanks for your time.

 

[BigH]

How long will the surcharge be on? (i.e., are you using wick drains or sand drains to speed up the consolidation settlement?) Is the pile cap going to be near the top of the embankment? - i.e., piles driven in such a way as they extend to near the embankment top - because if you want to pile, then build the MSE wall - you will need to maintain excavation (shoring) - would you not - to build the pile cap if at a later date? One may have to worry (at least consider) about dragdown forces on the piles due to the embankment being built after the piles are driven. Given the scenario you propose, I would probably construct the pile cap after pile completion - and make it quite rigid. I would then build the embankment, let it settle, etc. and then put on the bridge superstructure - i.e., build out the settlement due to the embankment prior to the main structural loads coming on. These are my initial thoughts, anyway . . .

 

[sfr529]

We have just discussed this same situation as contractor changes from drilled to driven (new overpass with abutment at ex grade). Now the pile #s increased, cap footprint increased and therefore the variance in distance from embankment increases. The argument was that different settlement (with distance) would make varying downdrag magnitudes.
However, since skin friction (downdrag) mobilizes with very small settlement, all the pile should experience the same magnitude of downdrag (full magnitude of the soil column). No pile cap timing is needed if the capacity at tip elevation is designed for the downdrag magnitude (i.e., the 30' of soil can settle forever, but the total downdrag will remain the same). Agree?

 

[DRC1]

sfr529: I agree that down drag is not a function of settlement magnitude, but rather soil strength relative to the soil profile. All piles experience downdrag. Further I am not sure why there is an issue. Properly designed piles accounting for down drag should not settle excessively.(Of course a proper pile design is predicated on a proper soil investigation.)The MSE wall will only effect the soil to a given depth. If the embankment should settle during bridge construction, that could be accommodated in the design. The settlement should be complete by the time the approaches are graded and paved, or they will need to be redone in the future

 

[fattdad]

I'd look at the stress profile from the fill acting at various depths along the piles. It's very easy to think of these earth loads as areal, but depending on the geometry of the MSE wall and surcharge the loads on the piles may not be too great. Don't know, would want to know that though.

Other posts seem on topic too. . .

f-d

¡papá gordo ain’t no madre flaca!

 

[BigH]

The question remains - if he has built a large MSE wall fill, how will the pile cap be constructed for the abutment afterwards unless there is a short span from a wall mounted abutment to the main abutment? To reach the hammerhead of the abutment, the wall would have to be built adjacent to the hammerhead - like touching.

 

[fattdad]

Yes, I'd like to know about that too!

f-d

¡papá gordo ain’t no madre flaca!