Shallow vs. deep foundations

[Mccoy]

I recently came across the following link:

http://www.dot.state.oh.us/research/2006/Structures/14747-FR.pdf

apparently, DOT Ohio and FHWA are finding advantageous to adopt shallow foundations for HW bridges instead of the traditional deep ones.
An interesting analysis of settlements and economics.

I too was considering the possibility of suggesting shallow foundations in places where traditionally large-diameter piles are drilled. Sure geotech investigation is more expensive when considering the shallow foundations option(of course I would support my ideas by probabilistic analyses).

What would you think of this "heretic" view?

Does mere economic advantage justify the "heresy" of going upstream (if something goes wrong you'll have everyone against), and maybe a little restless sleep at night?

It's interesting that FHWA and DOT Ohio are willing to experiment.
I was also just wondering about the design earthquake intensity in Ohio.

 

[jdonville]

McCoy,

As far as I know the seismic design criteria in Ohio are modest. According to the US-based "International Building Code", the highest MCE ground motions are maybe 0.35g (0.2 second spectral response acceleration, 5% of critical damping) and 0.075g (1.0 second spectral response acceleration, 5% of critical damping), both assuming that the site-specific stratigraphy is classified as rock.

For areas where the soils are soft (i.e. su over the top 100 ft < 1000 psf), these values can increase to 0.88g and 0.26g, respectively. Design values are taken as 2/3 of these values which are then used to develop the full synthetic design response spectrum curve.

For most of Ohio, the 0.2s base acceleration is on the order of 0.17g (as opposed to 0.35g max), and the 1s base acceleration averages about 0.055g (as compared to 0.075g max).

The IBC has tables that present the seismic design category in terms of the short and 1-second period design accelerations and the occupancy type of the structure. The siesmic design category feeds into structural-specific portions of the IBC such as load combinations, restrictions on zoning and minimum design loads.

Jeff

 

[Panars]

Mccoy-

This is a "research" project in Ohio. Don't assume that we are putting alot of bridges on spread footings. Right now, about the only bridges for Ohio DOT on spread footings are those on rock. We are doing some spread footings at the abutments when the abutment is an MSE wall. The use of spread footings anywhere else is a rare thing.

Seismic design requirements are modest. We don't usually consider the seismic when reviewing calcs and plans, because it never controls.

 

[Panars]

Sorry, forgot to mention that I am the Foundation Engineering Coordinator for Ohio DOT. You can find my contact info on the website at

http://www.dot.state.oh.us/se/bridgecontacts.htm

It may help if you know my handle is composed of initials and the beginning of my last name.
P.A. Nars__

 

[SlideRuleEra]

As a bridge contractor, in 1977, we built a spread footing supported highway bridge for the South Carolina DOT over a railroad. As Panars says, the footings were on rock. The following year we did a pair of spread footing supported Interstate Highway bridges (also over a railroad). These were on very stiff soil, but not rock. I don't recall either design being considered unusual - just "everyday" business.

http://www.SlideRuleEra.net

 

[BigH]

I did a railway bridge for spread footings many years ago - why put, on piles, a foundation when we have hard glacial till. In Ontario they put a lot of spread abutment foundations in embankment fills - of course the embankments are made out of crushed stone for the last 25 ft or so - but the abutment is resting fine using something like 350kPa (which is low in my view for the material). I don't see the sense of putting bridge foundations - whether piers or abutments on piles unless there is a good valid reason.

 

[Mccoy]

Panars
(or Peter... Here we are out of the public eye!)
your words make the situation much more usual.
The report is a darn good one anyway. Interesting settlements studies. I'll take the time to read more in detail.

My situation is that of an alluvial plan, silty clay sequence, up to 30-40 m of depth where you have a 3-4 m uncemented conglomerate layer on very OC blue clays. Water table is shallow. Qc values from mechanical CPT are reasonable, in the region of 1 to 2 MPa in the shallower 10 m (30 ft), with a decreasing trend proceeding from 2 m to deeper layers, with occasional Qc values lesser than 1 MPa. I have a four-CPTs survey done in a site, a rigorous probabilistic analysis gave positive results for a 7-levels building on a slab, differential settlements were also acceptable (of course everything needs to be verified by lab samples and site-specific correlations. The fact is that a very powerful (politically and money-wise) contractor builds in the area, who also owns his foundation firm, with lots of linkbelt rigs eager to drill large-diameter shafts. Used to drill down to the blue clays with no site investigation at all (now the municipality requires them).
Piles are tipically drilled to a lenght of 32 to 40 m. Didn't run an economic analysis but savings should be substantial.
However, in the areas (closer to the coastline) where you run across the peat layers, I would forget about the heresy and join the congregation.

 

[eric1037]

I have provided recommendations for construction of spread footing for several bridges in Michigan that are under MDOt jurisdiction. I normally wouldn't recommend it for stream or river crossings due to scour, but for overpasses I don't see a problem at all.

 

[jheidt2543]

eric1037 beat me to the point I always considered the reason for using piles at bridge foundations -- to avoid undermining by scour. Even so, I've seen a number of very old highway bridges (1880's to 1900) that were erected on stone piers, no piles at all and they are still in use.

 

[Mccoy]

Even so, I've seen a number of very old highway bridges (1880's to 1900) that were erected on stone piers, no piles at all and they are still in use.

Ah well, ,
then I suppose Ohio DOT + FHWA realized that having a look back at the origins may turn out to be advantageous after all!