How to calculate roller compactor load on top of box culvert 1

[bratty]

Hi

Just a question about compaction on top of culvert. After completion of a precast box culvert, how is the soil on top of the box culvert going to be compacted? If the vibrating roller is too heavy, the compactive force may break the top slab if the soil layer is too thin.


Is there a formula to calculate the force or pressure exerted on top of the culvert given the layer of backfill on top of the culvert changes? I can think the easy way is to assume the roller exert a line load and is distributed 45 deg down the depth of the soil.


Any helper?


Thanks

 

[SlideRuleEra]

MTNClimber has provided a good answer to the question about the Engineer requiring specific equipment. That is not done... and if it is, the Engineer / Owner have opened themselves up for high initial bids and endless, costly and valid change orders. Don't interfere with a Contractor's means and methods... review them, yes. Hold the Contractor's feet to the fire if the means and methods are questionable. Even require a Contractor the hire an independent PE to get involved with the means and methods as needed... but stop there.

Concerning the designer not thinking through the project; IMHO that is very likely:

Probably (to "save money" on permanent materials) selected the minimum precast culvert design for "driveway traffic load"; no serious consideration to high labor / equipment costs need to backfill/compact above the culvert. An experienced Contractor (not the case in this thread) would have picked up on this and bid accordingly... either include money for excess installation costs or Contractor (voluntarily) pays for a more robust culvert design, that will withstand larger, faster compaction equipment. As a Bridge Contractor, we used both of these techniques from time to time in similar (but not identical) situations.

Concerning the "fragile" nature of box culverts, especially precast box culverts, in a word... geometry. The relatively "flat" top does not lend itself to structural strength until the compacted soil is in place. This is unlike rigid circular or even elliptical pipe or culvert geometry which can take quite a bit of abuse during backfill placement and compaction.

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[r13]

A quick research find the following information.

Vertical Rammers - lift height 6" - 12" (max), best for clayey backfill.
Vibratory Plate Compactors - lift height 4" - 6" (max), best for granular backfills.

 

[IRstuff]

Did the designer specific the amount of compaction? If they did, then it seems to me that it's on them, if there were no special instructions or caveats about compaction loading.

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[r13]

It depends how the specification was constructed.

Compaction specifications

A word about meeting job site specifications. Generally, compaction performance parameters are given on a construction project in one of two ways:

■ Method Specification—detailed instructions specify machine type, lift depths, number of passes, machine speed and moisture content. A “recipe” is given as part of the job specs to accomplish the compaction needed. This method is outdated, as machine technology has far outpaced common method specification requirements.

■ End-Result Specification—engineers indicate final compaction requirements, thus giving the contractor much more flexibility in determining the best, most economical method of meeting the required specs. Fortunately, this is the trend, allowing the contractor to take advantage of the latest technology available.
Normally

 

[MTNClimber]

Vertical Rammers - lift height 6" - 12" (max), best for clayey backfill.
Vibratory Plate Compactors - lift height 4" - 6" (max), best for granular backfills.

I would separate vibratory plate compactors into different sizes. Those lift heights would be fine for the 170lb plate but large vibratory plate compactors can easily get away with 12" lifts. I've seen them obtain the minimum dry density while using a nuke gauge with a 12" probe.

 

[r13]

Yes, I agree. However, given the low load capacity of the culvert, I think the lighter machine combined with a thinner lift has the advantages that 1) not to exert too much pressure on the culvert, and 2) the compaction could be achieved with fewer passes, thus speed up the operation. Once beyond the influence zone, larger, heaver equipment with thicker lift should be considered.

 

[BridgeSmith]

Especially when we're talking about smaller compaction equipment, the span of the top slab makes a big difference in whether it overloads the slab or not. On a 3' span, the force effect of the a small plate compactor is almost a uniform load on the slab. However, on a 10' span, the loading effect of the plate compactor is far less than a uniform load. For example, an 18" x 18" plate compactor exerting 18 psi on 6" of soil produces only about 2/3 of bending moment on the 10' slab as the 6.2psi design uniform load on that slab, without accounting for lateral distribution of the load in the concrete, beyond the 2' wide distribution width of the plate + soil (18" wide plate + spread through soil at 1:1).

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

 

[r13]

Rod,

Excellent point.

 

[MTNClimber]

I was just talking about lift size, not how to approach this situation. I don't have enough project information to give compactor size recommendations, only my experience with small compactors.

 

[r13]

MTNClimber,

BridgeSmith has confirmed your point that the culvert should be able to sustain a larger compactor with larger lift height, as the local activity does not likely to produce a stress in exceeding the design. The OP can try 1' uniform load, and place a concentrate load in the mid-span, then set the moment equal to the maximum design moment to back check the allowable concentrate load. From that, substract operator's weight, the remaining is the allowable for weight of the equipment and operation force. I didn't give it a deeper thought from the beginning.

ADD: On top of producing maximum moment, the concentrate load should be placed at location that produces maximum shear too.