Bridge Abutment Hydrostatic Pressure

[haynewp]

I have a bridge abutment that is retaining about 8 feet of soil. The bridge is over a small creek and I am providing weep holes at 4 ft on center in the wall. The area is very wooded and the project is going in to the DOT for approval before clearing the woods. After clearing, a geotech is going to go in and drill to determine the allowable pile capacities to support the abutment.

I am not sure what lateral pressure to design the abutment for. I was going to assume 60lb/ft3 for lateral earth pressure, but what about hydrostatic pressure? Are weep holes normally considered sufficient to eliminate water buildup behind the wall on a creek bank? There could be seepage of water from the site to behind the wall. I imagine most the runoff in the area is leading toward the creek, so I expect it to be moist at the banks. I don't know about the water table at the site.

 

[JAE]

I haven't worked with DOT's for some time, but when I did bridges (for Iowa, Nebraska, Indiana, and Illinois)they all seemed to have standard earth pressures that were mandatory on their bridge designs.

As far as weeps - I would make sure you backfill with a clean granular material against the wall to facilitate drainage TO the weeps.

 

[haynewp]

I have used the standard pressure I got from their website before for a wall adjacent to a road. And I provided the weeps and granular blanket behind this wall.

I am just a little nervous using their dry lateral soil pressure in this wet wooded area, adjacent to a creek. It is a decent sized pedestrian bridge the abutment will be supporting.

My question really comes down to this; can you assume hydrostatic pressure is not an issue behind any wall as long as you provide adequate number of weep holes and proper granular fill? Couldn't the soil hold water but yet not be 100% "saturated" thus still affecting the lateral earth pressure?

 

[JAE]

First off your concern of the water is a good one and you are correct in re-assessing what to do vs. just using the std. pressure.

I guess that in this situation, I would not use a higher pressure, but go the extra mile in ensuring that the water gets out. That might mean using more gravel, more weeps, or even a secondary system in addition to the weeps such as a perforated pipe with drain fabric that draws the water away from the structure.

We structural dudes love redundancy, right?

 

[haynewp]

Dual system!

 

[Qshake]

I agree with JAE, it is very common in the midwest and I hope elsewhere to use geotexile drain fabric and peforated pipe drains. 45 EFP is the AASHTO minimum I believe.



Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[DRC1]

Weep holes tend to clog, even with good backfill. If it is not soil in the back, then lots of little critters love to make homes in them. I would favor a drain pipe wraped in filter fabric with good freedraining backfil. This does two things, it controls water and controls lateral pressure. Hydrostatic does not occur until100% saturation. However what does happen is that as water content increases, the unit weight increases. So you can have a bone dry material with a unit weight of 95 pcf to the same material just shy of 100% saturation may have a unit wight of 125 pcf ora315 increase. Tis would also increase the lateral pressure 31%. So even though the Hydrostatic effect has not kicked in the increasing saturation has had an effect. However with proper controls hydrostatic pressure can be controled.