Recommended Found for Short Span Bridge on Poor Soils
Recommended Found for Short Span Bridge on Poor Soils
(OP)
I am currently working on a design of the replacement of a 28ft span bridge on a 25deg skew where soil borings completed to 100ft show an N value of 6 to WOH for the entire depth. The soils are classified as interbedded silts, clayey silts and silty sands to 18ft then silty clays to 100ft.
If this was a shorter span, I would consider designing a 4-sided RCB, however 28ft exceeds what the local plants can manuf. Other considerations I have made were a 3-sided RCB on a CIP floor spread footing or installation of monotube piles/sheet piles with CIP beam seat and prestressed conc voided adjacent slabs.
Any recommendations for a suitable foundation with cost considerations is appreciated? Thanks.
If this was a shorter span, I would consider designing a 4-sided RCB, however 28ft exceeds what the local plants can manuf. Other considerations I have made were a 3-sided RCB on a CIP floor spread footing or installation of monotube piles/sheet piles with CIP beam seat and prestressed conc voided adjacent slabs.
Any recommendations for a suitable foundation with cost considerations is appreciated? Thanks.
PAUL S SNYDER, P.E.





RE: Recommended Found for Short Span Bridge on Poor Soils
RE: Recommended Found for Short Span Bridge on Poor Soils
PAUL S SNYDER, P.E.
RE: Recommended Found for Short Span Bridge on Poor Soils
RE: Recommended Found for Short Span Bridge on Poor Soils
PAUL S SNYDER, P.E.
RE: Recommended Found for Short Span Bridge on Poor Soils
RE: Recommended Found for Short Span Bridge on Poor Soils
RE: Recommended Found for Short Span Bridge on Poor Soils
I've never heard of burying a culvert floor four feet below the streambed. Most scour protection for culverts is a toe wall both upstream and downstream. The upstream toewall being about 3' deep and the upstream being about 2'. When culverts are buried it's usually not for scour its for stream continuity and a minor sump is provided, 12 to 18" this ensure water always runs through the box and doesn't pool at the ends.
The bigger surprise is a recommendation for a 28' four sided box. Since the moment is proportional to the square of the length, this is going to be a very deep slab. I presume there is only minimal fill so live load will be a larger factor that fill on top. Sides will be governed by the soil and surcharge. A bottom slab every bit as deep as the top slab will be required. Slab thickness will be on the order of 3' for a single span. Figure about 3,000 psi concrete due to field operations (ie not precast factory).
A double box, 2-14' will provide a decent slab thickness but the wall will provide an impediment to driftwood which will become a maintenance problem. If walls aren't a problem you might consider a triple box culvert and use a shallower slab thickness.
Without the four-sided culvert you'll wind up driving pile for the foundation. It doesn't read as if you're set up for that operation.
Most State DOT's have single, double and triple box standards and many provide assistance to local municipalities to aid with engineering. Try working with them.
Regards,
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RE: Recommended Found for Short Span Bridge on Poor Soils
Foundations are a problem - I agree. If this were Asia, they would probably drive a whole lot of bamboo piles. What can be done to make the soils stronger in the upper reaches? If you can "stabilize" a box that would be under the footing, you might be able to look at a rather small footing. How can we do this? A regime of deep soil mixing - chemical stabilization - cement or lime - might be considered to provide a strong base onto which to place the footing. Electro-hardening (see T&P for discussion - the 1967 edition in particular) but this might be expensive - but has been used successfully since the 40s. One could also consider placing lots of small pin piles. Stone columns might also be considered.
We don't know how high the approach spans are - if high at all? Clearly, the approach spans will cause the supporting soils to settle - and will lead to lateral pressures on the abutment wall - although perhaps one could use columns instead of a wall with soil nails back into the embankment so support the embankment soil. This would minimize the lateral pressures on the abutment and hence the foundation. One could use geofoam for the embankment - virtually no loading on the founding soils so no settlement problems and no embankment influence of settlement below abutment foundations.If groundwater is a problem, you could use geo-tubes (can't remember off hand the trade name) - they are basically pvc tubes standing vertically, tied together - allowing water to go "up and down". One might be able to excavate a couple of meters of existing soil and replace with the geofoam - to get no added pressure at all. One could also use shredded tyres for lightweight fill.
We don't know what the "scour" potential of the river/stream is. Is scour a problem? Does the original bridge have a problem? If so, one might consider placing a rip-rap blanket up and down stream large enough that if scour occurs by the time it reaches the foundation, a wall of rip-rap will have developed.
This is an interesting problem and sounds like a lot of fun. There are ways out there that might be done relatively cheap. I hope that some of the thoughts above might get one to think that there are other "toys" out there rather than box "culverts", etc. Good luck.
RE: Recommended Found for Short Span Bridge on Poor Soils
Some states, NY for example, have low volume road bridge standards that make use of sheet pile for the abutment walls and the primary vertical load resistance for the endbent. THe detail is similar to a sheet pile bulkhead with a concrete beam cast atop the sheet pile and precast concrete box beams supported by the sheet pile and cap.
And at 28' it's entirely likely that a shallow precast panel decking can used instead of actual box beams with voids.
It's probably worth looking into a reputable geotech firm for advise on the soil support and scour conditions.
On the other hand and inside the box, CIP concrete is pretty cheap as a box culvert.
Regards,
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RE: Recommended Found for Short Span Bridge on Poor Soils
RE: Recommended Found for Short Span Bridge on Poor Soils
I suggest piling is an obvious solution but more information would help you to get an economical design.
RE: Recommended Found for Short Span Bridge on Poor Soils
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RE: Recommended Found for Short Span Bridge on Poor Soils
I'm a big fan of using composites in infrastructure and await more code material to help push this along. Just recently there has been a code developed for using composite structural sections such as those Strongwell manufacturers but that is largely extrusions with particles or short fibers.
I'd like to see more "real" composite materials for infrastructure.
Some companies, Martin Marietta, for example, have dropped their bridge products due to lack of interest.
Regards,
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RE: Recommended Found for Short Span Bridge on Poor Soils
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RE: Recommended Found for Short Span Bridge on Poor Soils
1) I am actually in a rural NY county and we design our structures (all rural roads) for HS25 load rating.
2) We routinely use either precast voided slabs or box beams (depending on span) where the abutments consist of short concrete beam seat (depth of the beam + 2ft), steel plate cap over steel sheet pile and H-piles that are driven to refusal or deep enough for friction resistance. This small rural road is not worthy of such a costly pile project since soils are so poor.
3) There is a minor scour problem. I have done a hydraulic analysis and the stream has somewhat of a flat slope with low velocities, but I think the skew combined with the poor soils of the stream bed lead to the scour. We are improving the skew and relocating the new structure to counteract this as much as possible.
4) There is only about 6.5 ft of freeboard between bottom of beams and stream bed and historical information that shows that debris and ice has been an issue, which is why I am not considering double and triple boxes.
Thanks for all that have contributed. Your suggestions thus far and to come are welcomed.
PAUL S SNYDER, P.E.
RE: Recommended Found for Short Span Bridge on Poor Soils
The reason for only doing the soil borings to just over 100 ft for these little bridges, is that it would be cost prohibitive for us to do an abutment on piles that is driven any deeper. Usually, we strive for no deeper than 80-100 ft as the maximum depth for the small bridges, although, we can usually find refusal or rock at depths much less than that. There are a few glacial valleys though where the soils are very poor for great depths. I may post a copy of the soil logs tomorrow if I get a chance.
PAUL S SNYDER, P.E.
RE: Recommended Found for Short Span Bridge on Poor Soils
RE: Recommended Found for Short Span Bridge on Poor Soils
PAUL S SNYDER, P.E.