benching trench backfill on road crossing??

[swazimatt]

every time i go over a depressed road crossing repair (where the water company has excavated a trench to re lay a pipe)i wonder if it would not be better practice to bench the trench so that there is more friction on the sides. Basically you would make the excavation a bit wider for the top 150mm then step it in for the next 150mm and then maybe have the same width for the rest of the trench depth.

when you backfill your top two layers will be supported on the existing road layerworks at the edges, but i am not sure if they would have any tensile strength across the trench if there is settlement in the trench bckfill

I never see this happening and was just wondering if it would reduce the effects of settlement/poor workmanship?

 

[civilman72]

I'm not sure I see the benefits. Just seems like more trench area to backfill and properly compact, which is obviously the main reason for trench settling/failures.

 

[Ron]

Agree with civilman72....roadway crossing trenches are usually done quickly and with less care than the original. Part of the reason is that roadway use demands short closure times, resulting a trench that is no larger than absolutely necessary. Benching would be counterproductive.

The municipality needs to be more diligent in their trench repair requirements and testing. In my area, up until about 20 years ago, all roadway crossings were required to be compacted in lifts and tested at various levels. The base material under the asphalt had to be compacted to 100% of the Modifed Proctor maximum dry density....tight requirement. Few of those crossings were ever an issue. Now those requirements have been relaxed and we have bumpy lumpy roads!

 

[swazimatt]

Thanks for the replies. I agree that properly compacted trench will work. My query is more hypothetical and would a benched trench mean that the backfill could actually work as well with less compaction, and would it be significantly less to the point of catering for poor workmanship?

With regards to the local municipality, please don't get me started!!

 

[cvg]

if you really want to reduce the amount of work in compaction of pipe trenches, then backfill with 1/2 sack CLSM requiring no compaction and probably will not settle at all. some cities require it

 

[Ron]

To your second question for clarification.....

No. Benching with lower compaction will not reduce the amount of depression you get in a trench from subsequent traffic. The vertical deflection that causes such a depression is caused by increased compaction of the soil. Keep in mind that compaction from the vibration of traffic will occur before significant increases in shear strength of the soil and before such shear strength is mobilized by loading.

 

[darthsoilsguy2]

i think it would help but not for the reasons you pose.... the bigger cut would let you get bigger and heavier equipment in to compact or will allow the compaction to be done better with the smaller equipment you were going to use.... trench compaction is rarely homogenous. the excavated trench wall confines the backfill soils to some extent for compaction, but the center of the trenchline is usually tighter. trench compaction is hardly ever tested near the sidewall condition.... considering the compaction varies and declines off-centerline, it would make sense that staggering the trench wall zone would improve settlement.... i have a hard time imagining a way to argue that the benefit would outweigh the cost

 

[BigInch]

We don't really open cut anything that actually has pavement on it.
What about sloping out the trench sides just a bit, rather than benching.

 

[cvg]

Really? who is this "We"? Open cut trenching is commonplace in city streets. sloping and benching is generally not done for several reasons. a) more work b) more cost c) limited room with both lane restrictions and other underground utilities close by. d) doesn't really make a difference in the amount of future settlement.

 

[BigInch]

You know, "us", "we".
Benching it seems like a whole lotta' more work and cost.

 

[swazimatt]

thanks for the comments. I guess down here i will just have to continue riding over the bumps - i suppose it keeps the speeds down ;-) (even if it does mean those speeds are on the oncoming traffic side!)