Recommended Found for Short Span Bridge on Poor Soils

[psnyder]

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.

PAUL S SNYDER, P.E.

 

[cvg]

why can't you cast in place a 4-sided RCB? Does it have to be pre-cast?

 

[psnyder]

cvg-I have considered that option, however, my feeling is that it would be more labor-intensive for my crews thus taking more time. I am a designer for a small county highway dept, and have two crews which are spread kind of thin. That option is still open and I will speak to my foreman to see what his feelings are for constructing a CIP 4-sided RCB. Thanks.

PAUL S SNYDER, P.E.

 

[cvg]

not much different than your 3-sided RCB on a CIP floor spread footing option - or was that pre-cast box set on a CIP floor? Note that leakage and alignment issues at joints for pre-cast box sections (3- or 4- sided) is an issue that needs to be addressed if you go with that option. Another option would be to pre-cast the box on site (if you have room to do it) using your own crew, hire a crane and set it in place on a mud slab.

 

[psnyder]

We definitely do not have the room to precast onsite. It was a 3-sided RCB on a either a CIP or precast floor (depending on cost). If I go with the floor option, what do you suggest be placed under the floor for stability-geotext over poor soils with gravel base? We also have to consider scour protection and may just bury the bottom of the floor 4 ft. Thanks.

PAUL S SNYDER, P.E.

 

[BigH]

Do you know what the foundation for the existing bridge is? What is the unit weight of the soil, say, in the upper 3 m or so (oops - 10 ft) of the stratum? You say that you have "N" values of 6 to WOH - is the higher values near surface - i.e., a somewhat desiccated crust? I'd like to see a copy of the borehole log, if possible. I've a few ideas floating around among the grey cells . . .

 

[cvg]

you could bury the bottom of the floor 4 feet for scour protection, but 4 foot thick slab would be a lost of excavation, lot of concrete and what about de-watering? better to dig a 4 foot trench at each end of the box culvert and pour a concrete cutoff wall. suggest you run some hydrology / hydraulics / scour calcs to determine if 4 foot scour depth is enough. By the way, what was your plan for the headwall and wings? you can't precast those, they will need to be cast in place. This sounds like an ambitious project for a small county crew. Might be worth having a contractor come in and help out, especially with forming and steel placement.

 

[Qshake]

Whoa, hold on now.

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,
Qshake

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

 

[BigH]

Maybe we should be thinking things a bit differently? Just throwing out some thoughts for consideration. This is not a big span bridge - 9m. This sounds like it is a small rural bridge. They don't have to be that heavy. Why no look at ways to minimize the loading - thin abutment walls and steel I-beams with a thin concrete deck. Use of lightweight aggregate? Perhaps abutment columns with soil nailed shotcrete walls behind - maybe an arch bridge where some of the weight of the bridge would be, in effect, lateral loads on the foundations which could be handled in the approach fill 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.

 

[Qshake]

BigH has a good point, there is room for the proverbial out-side the box thinking here.

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,
Qshake

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

 

[cvg]

if we are going to think outside of the box, a corrugated steel multiplate arch could be built. I have done one and it had a soft bottom with concrete footings, below grade on each side to support the ends of the arch. We used gabions for headwalls and wingwalls.

 

[Zambo]

100 foot isn't deep for your BHs, once the N Values were found to be so low I would have asked them to go deeper to about 60m. You will know the cost of the original work so can decide if it would be worthwhile bringing them back to site. Can you find any other BHs from nearby that can give an idea of the change in soil conditions at greater depths.

I suggest piling is an obvious solution but more information would help you to get an economical design.

 

[Compositepro]

How about a composite bridge?

http://www.compositeadvantage.com/products/frp_bridge.html

 

[Qshake]

Compositepro - Good idea. But until some specifications come out that govern composites in bridges these types of bridges will need to be load tested to ascertain capacity etc.

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,
Qshake

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

 

[Compositepro]

Here is a concept that uses inflated braided tubes to make forms and reinforcement for concrete filled bridge arches.

http://www.braider.com/?a=44

 

[psnyder]

I would have responded earlier, but the holidays are upon us. Some interesting suggestions have been mentioned above. Here is some more info to give a little more detail to the scenario at hand:
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.

 

[psnyder]

A couple of other little details I should mention is that soil borings for another bridge a little further downstream show that the soils are poor as well and for at least 50ft deeper (150 ft) versus what we got for this bridge. Also, the existing bridge (1928) has a CIP gravity abutment without piles and with somewhat of a small footprint. It is amazing that there wasn't very much settlement over the years.

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.

 

[FixedEarth]

Given the large thickness of the compressible layer, have you considered a composite pier-Say a 30 to 36 inch diameter drilled pier with a W shaped section put in the middle? This composite section would assist in lateral load capacity and in opposing negative skin friction.

 

[psnyder]

We have certainly considered that option, but do not have the equipment, and it would require the use of a contractor to construct it. Therefore, the cost would exceed what we are willing to spend on the replacement structure.

PAUL S SNYDER, P.E.