Hello everyone,

A topic come up the other day in our laboratory about why there is no requirement as to the preparation of subgrade for box culverts. Our specifications do not dictate to perform any type of compaction, only excavating to the proper grade and then spraying the base if it is dry prior to placement of concrete.

Why is this done for certain building pads, but not done for concrete culverts for roadways?

When a box culvert is excavated and constructed below original grade, there is a reduction in vertical load over the footprint of the culvert. An empty culvert and a water filled culvert both weigh less than the soil that was revoved to build the culvert. Therefore, culvert settlement should not be an issue.

www.PeirceEngineering.com

most specs do require subgrade prep for box culverts

PEinc,

We were told that and that does seem to make sense. Thank you

cvg,

We have yet to find a specific spec from any municipality or agency that specifically states that you should do any type of subgrade prep, if you find one I would like a link to it.

Contact your local DOT. I bet they have a spec. If not contact Virginia DOT.

Richard A. Cornelius, P.E.
WWW.amlinereast.com

Perhaps you are being too specific in looking for the specification. Usually the spec for this would be under something "Structural Excavation," and would apply to a wide range of structures that may be excavated for, including a box culvert. Could also be under a spec for excavating trenches, drainage, or even the general excavation and embankment spec.

Actually I just googled this and I don't have time to sort out all the Virginia DOT references to other sections of their spec that got me to this section, but it appears this falls under the requirements of section 305.03, which requires scarification of the top 6" of the existing material and recompaction to a given specified density that varies according to the percent of the material retained on a #4 sieve.

What if the foundation soil is very weak. How much of it needs to be removed to provide adequate support. As an example, a box culvert under say 30' embankment. The pressure due to the embankment on top of the culvert is 3,600 psf. Is there a spec anywhere that tells you how much of this soft material needs to be taken out

ask your geotech to do a settlement analysis

Why worry about a 30' embankment if it was not part of the original poster's problem? Based on the original question, it seems to me that this thread has drifted off topic.

www.PeirceEngineering.com

During and after excavation for box culvert a determination is made as to the need for additional "undercutting" and replacement of the existing material. It is not uncommon to place a seal slab (conc. slab) or stab. material if need.
The S T A i am familiar with spec.s out the allowable backfill as well. In extreme conditions dewatering is required. Certainly some support is achieved after proper compactive effort is utilized. Lots of spec.s on box culvert placement.

Dear seniors, i hav same question regarding box culvert as my embankement height is nearly 11 m. over it...my senior's told me that they never experience and seen such kinda problem with box culvert, but im sure dat these culverts are goin towards DAM...