Compaction Loads on Retaining Walls 4

[KootK]

At my old company in the US, our policy was to basically use the CRSI manual for retaining wall design whenever possible. As far as I know, the tabulated designs in the CRSI manual do not include provisions for compaction loads.

At my new company in Canada, EVERY soils report that comes in says that retaining walls should be designed for compaction loads. They're serious loads too (12 KPa lateral etc.). It resutls in beefy retaining walls.

This issue has me confused. I've been treating compaction loads as if they are permanent. i.e. I design the wall for soil pressure, surcharge loads, AND compaction loads concurrently (and other stuff too as required).

Is this approach correct? Or can compaction loads be treated as temporary construction loads that should NOT be applied concurrently with live loads?

I guess the over arching question is: are compaction loads permanent?

Thanks,

KK

 

[bridgebuster]

I generally don't use the compaction load, which is a function of the roller weight and width, and I've never seen it done as a standard practice. I typically use Ka +surcharge; I go a little conservative on the soil weight.

Although on a recent project involving rehabilitation of 2000 LF of gravity walls, the geotech recommended using an earth pressure coefficient that included compaction effects. It was much higher than the at-rest pressure (I can look it up when I go back to the office.) His reason was that after 75 years the soil is now over-compacted. I disagree with that logic - then every wall we're designing is under-designed.

The lateral forces from compaction diminish with depth - there's an illustration in the old US Navy Manual, D7.02. It could be argued that neglecting the increase in rebar stress near the top of the wall due to compaction is no big deal due to minimum reinforcing requirements. So far, I haven't come across any walls that overturned while being backfilled.

 

[KootK]

I agree that, for larger retaining walls, the importance of the compaction loads is small. And, like you, I've designed a lot of walls ignoring compaction that fared just fine.

However, for little dinky walls (<8' say), the impact can be significant. It also doesn't help that:

a) Geotech gives me a range of compaction equipment from banging the dirt with you fist up to gargantuan industrial equipment.

b) Contractor is either not involved when I perform the design or doesn't want to commit to NOT using certain equipment.

Based on your rehab project, it sounds as though compaction loads actually increase over time rather than decreasing. Crap..

 

[miecz]

We don't treat compaction loads as permanent loads. We include a note on the drawings requiring the contractor to shore the retaining wall during backfill and compaction.

 

[KootK]

That's a slick way to handle it Miecz. Do you get any bitching about the need for shoring?

 

[miecz]

My answer would be: Did you read the notes before you bid the job?. Honestly, my guess is the contractors don't shore, unless the inspector enforces the note (not likely). But if there is ever a failure during construction(even less likely), we have the note to wave at them. So, hopefully, they'll be a little more careful. The real advantage is, we don't have to overdesign a wall for a temporary condition that, in reality, should have a lower factor of safety than the permanent condtion.

 

[KootK]

Your point about the lower factor of safety is a good one. If I could design the wall for two seperate cases...

1) Compaction loads x 1.25 and;

2) Live loads x 1.5

...it probably wouldn't have much effect on my design at all. I'd just like some confirmation that the compaction loads are in fact temporary.

 

[msquared48]

Bridgebuster:

"So far, I haven't come across any walls that overturned while being backfilled."

Neither have I, but I have come across a long retaining wall that failed in diagonal tension at the ends from the stem rotating from too much heavy equipment load during the compaction process. It was a classic failure.

As a consequence, I do restrain heavy compaction equipment from a horizontal distance "H", the retained earth height, from the stem wall to lessen the chance of the failure observed above. Small compaction equipment only allowed inside "H" - bobcats or very small dozers.

As an additional comment here, the wall may not have come up to full strength before it was compacted. I was not privy to the construction records, and it was not my project. However, I have not had any failures with the restriction.


Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[miecz]

We also include a note that reads:

After backfilling and compaction, the Contractor shall keep heavy construction equipment and all temporary fill away from foundation walls at a distance equal to the wall height.

 

[NS4U]

Check out this paper publishe in the Journal of Geotechnical Engineering. I beieve that this is a very real load and I include it in design. I don't believe it significantly increases the lateral load on my wall like you suggest, may be a US/Canadian code thing.

ESTIMATION EARTH PRESSURES DUE TO COMPACTION
By J. M. Duncan, Fellow ASCE, G. W. Williams, Associate Member, ASCE, A. L. Sehn, 3 Associate Member, ASCE, and R. B. Seed, Member, ASCE

ABSTRACT: When soil is compacted in layers by rollers, vibrating plates, or rammers, the horizontal earth pressures within the compacted soil mass are increased. Charts are presented that can be used to estimate compaction-induced earth pressures quickly and reliably. These charts are developed using the hysteretic theory developed by Duncan and Seed, with a computer program called EPCOMP2. The charts offer the advantages of being easy to use, and providing very rapid results, while retaining most of the accuracy of the detailed computer analyses. Because earth pressures can be estimated quickly, it is easy to evaluate the effects of changes in the factors that influence the magnitudes of the compaction-induced earth pressures. Comparisons of earth pressures calculated using these charts with the results of computer analyses indicate that the values calculated using the charts are sufficiently accurate for practical purposes. Field measurements of compaction-induced earth pressures indicate that the horizontal earth pressures induced by compaction of sand do not change appreciably with time unless the wall on which they act moves toward or away from the backfill. In clay backfills, the high horizontal pressures induced by compaction tend to decrease over time to normal at-rest values.

 

[KootK]

Thanks NS4U. I'll try to get my hands on that paper. I'm quite curious. The abstract makes it sound as though the estimation of the pressure is quite complex to work out. Typically, I get it as either a lateral pressure or a simple equation from the geotechs. Perhaps a bunch of conservatism is workinf it's way in there.

 

[NS4U]

it's just reading charts-- actually it's quite simple...

 

[NS4U]

keep in mind too that there is also formal discussion on paper and some of the equations and charts were revised after the initial publication.

 

[KootK]

Damn, I just bought the paper from ASCE. Can you point me to the revisions?

 

[NS4U]

http://cedb.asce.org/cgi/

http://WWWdisplay.cgi?9105640

I would think that the errata would come with it, no?

 

[KootK]

Sadly, it did not come with the errata. I'm not sure if I want to spend another $35 at the Linda Hall library for a darn errata..