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How to calculate roller compactor load on top of box culvert

How to calculate roller compactor load on top of box culvert

How to calculate roller compactor load on top of box culvert

(OP)
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

RE: How to calculate roller compactor load on top of box culvert

Quote:

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.


or


www.SlideRuleEra.net idea

RE: How to calculate roller compactor load on top of box culvert

The linked paper maybe helpful. Link

RE: How to calculate roller compactor load on top of box culvert

(OP)
Hi

What I mean is the compaction of the back fill to the top of culvert. It happen that the culvert is directly under a road 1.8m below for 150mm in length. Structurally the culvert is designed for 1.8m backill and driveway traffic load. But during backfill, the builder may use a heavy roller for a quick job. To avoid damaging the culvert, what is compactor to be used initially to prevent the damage? The compactor is dynamically adding extra force onto the culvert

RE: How to calculate roller compactor load on top of box culvert

All quotes below were copied from the linked paper.

Quote:

The diesel powered Ramex P/33 Trench Compactor is hand-operated and used in trenches and difficult access areas.  These walk-behind and remote controlled compactors weigh about 3000 lbs and were developed for compacting backfill in pipeline trenches more than 27 inches wide  They typically exert between 10 and 18 psi contact pressures at frequencies around 62 cycles per second (Hz), necessitating lift thicknesses of no more than 4 or 5 inches.

Quote:

Pogo Sticks”  Hand-operated tampers, like this Wacker BS 700, typically exert compaction contact pressures between 7 and 18 psi  Tampers are only useful for compacting soils in lifts 2 to 3 inches thick at near-optimum moisture content, if trying to achieve 90% of the ASTM D 1557 compaction standard

Quote:

Vibratory Plate Compactors  Above left - This Wacker VP1340A Plate Compactor only weighs 170 lbs, but only exerts a dynamic contact force of 5 to 7 psi, using 63 Hz frequency  Lower left - This Bomag plate compactor weighs 726 lbs and exerts a compactive force of 13 psi, at 62 Hz frequency.

RE: How to calculate roller compactor load on top of box culvert

Quote (bratty)

To avoid damaging the culvert, what is compactor to be used initially to prevent the damage?

The two images that I posted are small dynamic compactors operated by one worker, on foot. The first is sometimes referred to as a "tamper rammer" or "jumping jack". The second is a "plate compactor". As you noted, the Contractor has to start with no soil cover at all. The first lift (layer) of loose soil is, say, 15 cm thick. To get compaction, small equipment must be used. Either of the above pieces of equipment are the smallest available that can accomplish needed compaction. After a few lifts have been compacted (with this small equipment) the Contractor can switch to heavier compactors.

Here is some information about rammers and plate compactors.

www.SlideRuleEra.net idea

RE: How to calculate roller compactor load on top of box culvert

I agree that lighter compaction equipment should be used if the culvert wasn't design for heavy rollers.

Jumping jacks are useless in my opinion. So are plate compactors that one worker can lift and put back into their truck. I'd get a plate compactor that requires mechanical equipment to lift on and off a truck bed.

RE: How to calculate roller compactor load on top of box culvert

(OP)
Thanks for the info.

The design load on top of the culvert is 43kpa or 6.2 psi. But the hand operated compactors are compacting at 7 to 18 psi. It means for the first layer, the hand compactors may damaged the culvert slab below as it can take 6.2 psi. The builder may like to go for a quicker way using light weight roller due to the width is 6m x 150m. So at what soil depth he can start using a moving light weight roller?

RE: How to calculate roller compactor load on top of box culvert

Quote (bratty)

So at what soil depth he can start using a moving light weight roller?

That will depend on soil properties, spec requirement for compaction, the details for the roller (and any equipment moving it), and to large extent the Contractor's skill using the equipment.

www.SlideRuleEra.net idea

RE: How to calculate roller compactor load on top of box culvert

(OP)
But some builders are not specialist and for a small job, they want to backfill fast to create a platform to carry out his building work. How to advise the builder when or what equipment to use?

RE: How to calculate roller compactor load on top of box culvert

Quote (bratty)

1) But some builders are not specialist and for a small job, they want to backfill fast to create a platform to carry out his building work.
2) How to advise the builder when or what equipment to use?

1) In other words, the builder does not know what he is doing, but wants to do the work quick. Ask the builder is he is prepared to replace (for free) a culvert that he damaged because he did the work "fast".

2) All equipment (large and small) is available in a range of sizes. Either you or the builder need to look into the exact specs of available equipment and pick a specific make and model of compactor that is suitable for the initial compaction on this project.

My comments are not intended to offend you, but you need to know what you are doing to provide the level of advice the builder has hired you to provide.

www.SlideRuleEra.net idea

RE: How to calculate roller compactor load on top of box culvert

Bratty, like SRE is saying, if you want to advise this contractor well, you need to look into a soil stress distribution analysis (such as Bousinessq), taking into account the specific machinery the contractor plans to use and the specific soil properties at your site. Many construction engineers or geotechnical engineers can solve this problem for you.

If the contractor does not want to hire an engineer to help them, they should either budget to replace the culvert or to use the small equipment.

Yes, either way the compaction of the first few lifts may be challenging. What is the total height of fill? Is it acceptable for the first few lifts to have a lower level of compaction, making it up with the later lifts?

----
just call me Lo.

RE: How to calculate roller compactor load on top of box culvert

Bratty,

Quote:

Vibratory Plate Compactors  Above left - This Wacker VP1340A Plate Compactor only weighs 170 lbs, but only exerts a dynamic contact force of 5 to 7 psi, using 63 Hz frequency

RE: How to calculate roller compactor load on top of box culvert

Don’t bother with the 170 lb asphalt compactor unless you need to compact the soil to 90% MPD.

Pick a heavier compactor and follow Lomarandil’s advice. Pressure will dissipate if the first lift is thick enough or add shoring.

Are culverts usually designed this low?

RE: How to calculate roller compactor load on top of box culvert

MTNClimber,

I was pointing to the OP that he has missed there is equipment available in the linked paper if the compaction impact is the concern. Since I am not practicing in this area, I won't be able to pinpoint and offer my opinion on which compactor/method is better than the other, but I doubt that one type of equipment can always satisfy all project needs, and be the only one to be used.

RE: How to calculate roller compactor load on top of box culvert

Fair enough, but two people here are saying to use relatively light compaction equipment. My experience with relatively light compaction equipment is that it's OK to use in areas that are not important. It is very hard to obtain compaction test results in the 92-95% MPD range unless thin lifts are used and water is readily available. It will also take a very long time to install fill if its more than 1 foot thick. When I was a young field engineer, every single time a contractor tried to use a jumping jack or light plate compactor when installing several feet of structural fill or filling a large area I would roll my eyes and tell them to be prepared to call the rental company for a real soil compactor. I actually can't think of one time when they were able to obtain compaction without pissing off the foreman or superintendent for slow progress. So unless its just a landscaped area on top of the culvert, I would stay clear of small compactors.

Now onto the contractor's means and methods. I don't think it should be 100% on the contractor to figure out how to place the fill, unless they want it to be, since the designer did not account for normal construction loads in their design. The engineer and contractor should work together on this. The engineer should warn the contractor that there is a possibility that they could collapse the culvert if they are not careful. The engineer should ask what compaction equipment the contractor has and see if they can use any of it with destroying the culvert. As mentioned in prior responses, you can see if you can get away with less compaction in the lower portions of the fill layer, check the pressure reduction with the first lift of fill, shore the culvert, or maybe something else.

Just my two cents.

Edit: You could also look into the possibility of using crushed stone with fabric on top of the stone layer. That's something a small plate compactor can actually compact.

RE: How to calculate roller compactor load on top of box culvert

Damage to the structure while under construction is the contractor's responsibility under our (the DOT) contracts. If we design the structure, we tell them the loading capacity, and it's up to them not to damage it, or fix it if they do.

I have reviewed a bridge for compactor loading, though. The impact loads exerted by the roller compactor the contractor wanted to use were readily available in the equipment specifications. The AASHTO specs have provisions for distribution of loads through earth fills.

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

RE: How to calculate roller compactor load on top of box culvert

Sure but it seems like this culvert doesn’t have much structural capacity to it if a 170 lb plate compactor could damage it. I would say that the designer didn’t fully think this through... no offense.

RE: How to calculate roller compactor load on top of box culvert

My impression from reading responses above is that the engineer only specifies the optimum compaction requirement, then the lift height, passes, equipment are left to the contractors judgement. Am I correct? Please advise, thanks.

RE: How to calculate roller compactor load on top of box culvert

Engineers are routinely involved in constructability but not necessarily picking compaction equipment. Some specifications do require the contractor to submit equipment for the engineer's approval though. If an engineer designs something that cannot be installed with whatever the standard of practice is in that area, then they should make an effort to clearly notify the bidding contractors that they will need to follow non-standard installation procedures. In my opinion, it should be clearly stated on the plans that the contractor needs to take certain precauctions when building something that can be easily damaged. It doesn't need to specify certain equipment but the plans can certainly provide with a maximum load with some emphasis written in the notes about not damaging it while compacting fill on top. But in the end, I would just design for the anticipated construction loads I'm not slowing down progress which is typically valued more than loosing some reinforcement and concrete in a PCC structure. This topic is all open to each person's interpretation though. Best to just make sure you cover yourself in the design and/or in the plan's text.

RE: How to calculate roller compactor load on top of box culvert

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.

www.SlideRuleEra.net idea

RE: How to calculate roller compactor load on top of box culvert

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.

RE: How to calculate roller compactor load on top of box culvert

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.

TTFN (ta ta for now)
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RE: How to calculate roller compactor load on top of box culvert

It depends how the specification was constructed.

Quote:

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

RE: How to calculate roller compactor load on top of box culvert

Quote (retired13)

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.

RE: How to calculate roller compactor load on top of box culvert

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.

RE: How to calculate roller compactor load on top of box culvert

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

RE: How to calculate roller compactor load on top of box culvert

Rod,

Excellent point.

RE: How to calculate roller compactor load on top of box culvert

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.

RE: How to calculate roller compactor load on top of box culvert

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.

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