Pavement Lugs & Pavement Slippage

[GeoPaveTraffic]

I work in the St. Louis, Missouri (USA) area which is comprised of rolling hills. Our maximum street grade is 12% and most of our pavements are concrete. Soils vary from silt with some clay (ML) to lean clay with some silt (CL) to some fat clays (CH). Bedrock has not been an issue with respect to this topic. The pavement section generally consists of 6 or 7 inches of concrete either installed directly on the soil subgrade (old design standard) or installed on 4-inches of crushed limestone over a non-woven geotextile fabric (new design standard).

General practice in this area has been to install pavement lugs, really a grade beam extending 2 to 5 feet below the top of pavement. The exact installation details have varied over time but they are generally lightly to moderately reinforced and tied to the pavement for 5 to 10 feet up and down hill of the lug. In some instances there has been no observed pavement movement, in others pavement slippage/migration/creek down hill of half a foot or more is evident. Data about the exact installation methods used and the soil conditions is not available. However, there does not seem to be a higher incidence of movement with either the old or new design standard.

The city that I work for is trying to evaluate this situation and determine if a change to the design standard should be made for “steep slopes”. This has caused real problems, from houses pushed off the foundation due to the street movement pushing on a driveway to the total destruction of trough drains running across the street.

It would seem to me that loading comes from two main sources. First traffic loads; since most of these streets are residential in nature the worst loading is garbage trucks. However, a truck on a 12% slope imparts about 7,200 pounds static load and between 4,000 and 20,000 additional pounds while braking to a stop.

I am in the process of attempting to model the interface forces between the different layers to see if the predicted behavior matches the observed at all. Any comments, experiences or ideas that any of you can contribute would be appreciated.

 

[Focht3]

Are any of these "slippage" areas associated with shallow seepage? I have a hard time seeing a pavement slip downhill in spite of the installation of fairly deep lugs (or "key", depending on the local terminology) unless other problems are also occurring.



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[GeoPaveTraffic]

No there is no evidence of groundwater at these locations.

Generally, in this area groundwater would not be near the surface, but would be 10 plus feet below.

And yes your comment about "key" would be correct. The lugs effectively act like a key on a retaining wall footing.

 

[Focht3]

Are the lugs shearing off, or moving with the soil mass?



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[GeoPaveTraffic]

The lugs are moving with the pavement.

 

[Focht3]

Hmmm,

It sounds like a shallow slip surface. Is the pavement "moving away" from sidewalks and driveways that are perpendicular to the slipping road?

Why do you think the road is slipping?



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[GeoPaveTraffic]

My opinion is that the pavement is slipping due to traffic loads and possibly due to thermal expansion. While the pavement lugs can develop passive resistance, with time that resistance is going to decrease with out further movement. The decrease would be due to consolidation of the soil. With time the pavement would migrate.

The observed movement is down hill, i.e. perpendicular to driveways. It has also been observed in curves, particularly on downhill stretches of roads.

 

[Focht3]

Are you seeing the movement at driveways and intersections?

If this is a consolidation problem, I don't see any easy answers. What are the soil properties? (I re-read your original post describing the subsurface conditions.)

Have the lugs helped reduce the problem, or is your research triggered by the failure of lugs to improve the situation?



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[Focht3]

Another question -

Have you talked to any of the "good" local geologists about this? What is their "take" on the problem?



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[GeoPaveTraffic]

Up until 2 years ago, I was one of the "good" local geotechnical engineers in the area. The local opinion varies from "It can't be moving." to "It is moving, but we have no idea why." . The local market is very fragmented and price competitive, therefore, all the good engineers are just treading water and trying to keep their head above water. That doesn't leave any time for extraneous research projects. Most of the municipalities in the area just live with the problem, as does mine. However, we would like to make the situation better not worse as our design standards continue to evolve.

 

[Focht3]

I understand the local business climate - I'm there myself. (Maybe things will pick up during the election cycle...)

Nothing you have told us jumps out as an, "Aha!" I suspect a mass movement rather than slippage between the paving and subgrade. Why? Primarily because the lug is moving with the paving; if the pavement were slipping, then the pavement/lug connections would be heavily damaged. And the heavy truck loadings are transient in nature - so the appropriate soil strengths would be undrained and fairly stiff. Again, the load is too quick to cause consolidation of the soils at the downslope face of the lug.

Have you talked to any geologists about this? Perhaps they could shed some light on this problem. University types might have time for "extraneous research projects", so it's a fairly cheap avenue to pursue. You might even suggest it as a part of a senior research project - to be graded jointly by you and the professor. Anyway, it's worth a shot -



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[GeoPaveTraffic]

There is nothing to indicate that any type of mass movement is occurring. Driveways perpendicular to the pavement are not damaged and show no evidence of movement.

We have considered and are working on the idea of getting one of the local Universities to look at the problem. That was one of the reasons for posting the problem here, to judge the extent of the problem and to see what ideas might be out there to explore.

 

[cdh61]

GeoPaveTraffic,

Have you or any one else surveyed the street elevations in the area of the movements? I am wondering if the paved surface increased in elevation (upward bulge) due to a shallow slip plane developing on the down side of the key. Since the street (pavement) is moving, clearly the soil has to move since the key is still in place and large verticle displacements in the street are not present. This may occur in shallow keys (2 ft) but deep ones without large verticle movements??

regards

 

[GeoPaveTraffic]

Measurement have been few and far between and are hampered by the lack of acurate as-builts. All I can really say is that there is not observed bulge.

 

[cdh61]

GeoPaveTraffic,

This project is interesting. I have a few more comments/questions:

- how well packed is the limestone base, I do not suspect, but it is possible to have some of the ML soils migrate (pushed into) the base combined with sliding of a soil mass (failure of clayey soils). This could account for less pronounced vertical movements of the pavement structure. But 6in horizontal movements of the pavement at the surface is quit a bit and I do not think this would account for that much movement. I agree with Focth3 that consolidation (horizontal) of the native soils is not likely given the type of loading and sudden failure/slidding of pavement.

- If bulging is not well pronounced now, it is possible that it may have been immediately after failure, then normal traffic compacted the soils to reduce the amount of bulging. What do the records say regarding compaction of the roadway during construction? Mostlikely the subgrade and maybe the limestone base were only compacted to 95% of Standard Proctor.

- Construction of the key, is the concrete poured into an open excavation or are the keys formed? If formed, the backfill may not have been compacted between the concrete key and native soils, and larger than usual horizontal movements of the key (and pavement) could occur (consolidation of fill). This could also account for the keys being intack.

- Also something else to think about, if the keys were formed and granular was used as fill between the concrete and native soils, moisture could soften the native soils and the granular could be pushed into the native soils, due mainly to the impact loads from heavy trucks. This is much the same as seperation of granular base under roadways, but in your case horizontal.

Please post you findings regarding your modelling. Are you using a program like SigmaW?

Regards

 

[GeoPaveTraffic]

cdh61,

I must have given the wrong impression, the movement has not occured suddenly. Instead the movement occurs over time. Some of the streets where this has been observed are 30 years old others are only 4 or 5.

The keys are generally constructed using bank pour, i.e. no forming or backfill.

The standard compaction of the subgrade and base rock is 90% modified.

On all pavements constructed in the last 10 years, a non-woven geotextile has been placed between the base rock and the subgrade. Also, most of the soils are CL (modified loess) in nature.

The modeling is just an idea at this point. Right now I'm just doing some freebody diagram checks.

If I missed any other questions you have, just let me know.