Bearing capacity for box culvert 2

[pelelo]

Engineers,

I am working on a report in which i need to provide the bearing capacity for a proposed box culvert system.

The intent is to install the curvert in sections. The total length of the culvert is around 200 FT. The individual sections are going to be 6 FT long x 3 ft wide x 10 ft high. For bearing capacity purposes, would you compute the bearing capacity assuming a typical shallow footing (B = 3 ft) or would you consider it as a strip footing (L = 200, L/B >>>10)?

 

[geotechguy1]

The soil doesn't know the culvert is made up of individual sections it just knows it has 200ft of 3ft wide 10ft high culvert on it; I'd go strip footing.

 

[HTURKAK]

In this case, the culvert composed of PC segments.. the bearing capacity for (6 ft long x 3 ft widh) spread footing would be better approach.

I will suggest you to look AASHTO LRFD BRIDGE DESIGN ( SECTION 12. BURIED STRUCTURES )..

 

[pelelo]

Thanks,

AASHTO LRFD BRIDGE DESIGN doesn't provide any info about bearing capacity for culverts.

 

[PEinc]

Remember, a water filled culvert usually weighs less than the original soil that it replaced. Therefore there may be a reduction in the stress in the soil below the culvert.

http://www.PeirceEngineering.com

 

[pelelo]

Thanks PEinc,

But what do you think?, use an isolated section for bearing capacity or the whole system (L/B >>10)?

 

[geotechguy1]

If the culvert is more rigid than the surrounding soil it will tend to pick up additional load and concentrate it beneath the culvert which may end up fully compensating - or more - the lighter weight of the culvert vs the surrounding soil.

Imagine sticking a pencil through a sponge vs sticking a straw through a sponge, and then trying to put an object on the sponge. For the sponge with pencil, it should be easy to visualize that the pencil is going to end up carrying all the load, whereas for the sponge with straw, the sponge will have to work for it's living (and probably compress alot in the process)

 

[PEinc]

I guess you need to do a FEA to see what actually happens.

http://www.PeirceEngineering.com

 

[TigerGuy]

Is this a new alignment, or are you perpetuating an existing channel beneath new paved areas? What load will the culvert and underlying material need to support?

Your contact pressure at the base of the culvert will only be 1,500 psf (72 kPa) if it is solid concrete through the entire volume (less even when full of water). Unless the supporting soils are very compressible, your settlement will be almost nothing.

 

[pelelo]

I agree settlement will be almost nothing, however the original question was what is the best approach for bearing capacity computations: assuming just 1 simple section (isolated footing)? or assuming the whole culvert system (L/B>>>10) (strip footing)?.

 

[BridgeSmith]

To the original question of 6ft or 200 ft, I would say it depends on the amount of soil cover over the culvert, and therefore the distribution length of vehicle wheel loads. If the soil depth is enough to distribute the vehicle wheel loads across more than one 6 ft section, then using the full length would be appropriate, but if the cover is shallow enough to only affect one section and not the adjacent sections, then the 6 ft is more appropriate.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[PEinc]

Also consider whether the precast culvert sections are securely connected. It seems to me that there will always be load distribution between the 6' sections.

http://www.PeirceEngineering.com

 

[TigerGuy]

Overall, it will behave as a strip footing. As PEinc points out, they will move independent of each other to some extent. To account for that, you would model as individual elements. Then, you have to account for the overlapping stress distribution zones from one to the next.

 

[PEinc]

Assuming there are 8" thick walls and slabs, the concrete would weigh about 2400# and full water inside would weigh about 900# for a total weight of 3300#. For a soil weight of 120 pcf, the culvert would displace about 3600#. The new culvert should not cause any additional load or settlement to the soil below unless, as BridgeSmith mentioned, there is insufficient soil cover to distribute any ground surface wheel loads. Even then, the culvert sections probably are designed to resist shear between 6' long sections. So, wheel point loads would be distributed over more than the 6' x 3' bearing area of a single section.

http://www.PeirceEngineering.com

 

[BridgeSmith]

The sections are typically connected with gasketed joints, which allow for some movement (< 1/4"). That would only occur if one section was loaded considerably more than the adjacent section, though, i.e. a truck wheel load with less than 3' of fill.

Rod Smith, P.E., The artist formerly known as HotRod10