[i] Permissible Undulations of Roadway Surface [/i] 3

[BigH]

Lasses and Gents!
I would be greatful if one and all could provide information on the permissible settlements and undulations for high-speed roadways resulting from settlements of soft embankment foundations soils. I am not particularly interested in the "just after pavement" limits for constrution monitoring, but what your area would permit for long-term settlements and undulations when you build significant embankments on soft soils (without ground improvement).
Thanks - Merci - Muchas Gracias and Terimah Khasih

 

[Ron]

BigH....to my knowledge there are no "permissible variations". Everything in pavement management is generally based on its overall contribution to the Pavement Condition Index. Rutting (into which settlement is lumped), generally has a high influence on the condition index. A rut depth of 3/4" can be serious and cause problems. Conversely, corrugation of the pavement surface is a nuisance factor that contributes to the overall impression of ride quality.

 

[sc]

Comfort is what it is all about. In this case comfort is governed by vertical acceleration. Larger changes in grade will generally result in a vehicle rising or lowering too fast.

I'm not sure where I've seen it but there is a guide available to put limits on changes in grade for comfort reasons. Most road authorities probably have their own limits.

Also for the short length undulations that I think you may be referring too many road authorities run with their own limits. One that I can recall is no more than 25mm variation in surface level over 3m for a 100km/hr zone.

Regards
sc

 

[BigH]

Thanks - I remember somewhere that some authority tries to keep embankment settlement down to 100mm max after construction. In building on a soft soil foundation with moderately high embankment, you can imagine that we will be getting quite variable consolidation settlements down the line with time - perhaps, at k25+500 it is 50mm and k25+503 it is 30mm (due to variations of consolidating thickness,embankment thickness, how much of the poor surface soils were removed, natural variation of the soil with respect to pre-consolidation pressure (perhaps local stock-piles during original road construction), variations of Compression and Recompression ratios, etc. We are finding on 2 years after DBM is on (base course asphalt) that in the shoulder we have variable settlements of 60 to 90mm and in the median side about 10 to 20mm (widening of an existing 2-lane carriageway to make it two 2-lane carriageways). Our after construction bump integrator readings and even the 2 year later readings show we are under the 2000mm/km limit of "ups and downs" - but you can surely notice the ups and downs when you drive. The existing carriageway, as you would expect (having been there for 20 yrs) is behaving a lot better. Anyway, it is just something I wanted to explore - what do others' embankments do after two to 5 years after construction - still smooth or roller-coaster!

 

[KRSServices]

BigH,

You ask a very good question, and one that lawyers love to ask to get larger settlements for thier clients in injury accidents. Been on the end of one of those enquiries.

The correct response is that there is no standard, and it would be impossible to formulate a reasonable one to boot. The reason is that a single undulation will affect the operation of vehicles differently. Factors such as vehicle type (car or heavy truck), suspension type (spring, air, performance), posted speed, weight of vehicle, number of undulations (single or washboard), rate of depression (dip) versus incline, curve data (of road) and the general use of road (light traffic versus heavy truck traffic).

Having had the pleasure of operating a truck, a 1" dip in a road is quite noticable in a loaded unit versus a little car. Each circumstance is going to be different.

I your case specifically, the more severe dips and bumps will be firstly pointed out by the truckers. If a passenger car finds the rideability objectionable, then it may be consideration for some remedial work to be done, from a safety point of view. Just my thoughts, good luck.



KRS Services

http://www.krs-services.com

 

[VAD]

Hello BigH:

The problem you are facing is very pronounced when grade widenings are undertaken. Unfortunately this is hardly ever taken into consideration and we just widen sometimes without examining the sub-soils. If we do, not many really address the concern that we are placing a new load adjacent to an existing foundation and that the foundation for the new load will undergo movement.

Some have advocated removing the pavement structure from the existing and effect a new structure across the existing and widening in an attempt to minimize the differential settlement problem. Further, unfortunately this type of design is not very often regnized as requiring geotechnical input.

I am not saying that this problem can be readily solved but treatment of the foundation can be undertaken to ensure that the settlement of the widening is not a problem. Since treatments can be expensive, many highway agencies live with the problem and treat by overlays etc.

The problem can hower be addressed by putting in perspective the existing and widened embankment and determining the zones where settlement is likely to occur and determining what approach is required to limit the settlement to a tolerable amount. For cross slope differential settlement the tolerable value would be an inch or less. For settlement long the profile at transition locations where the problem is manifested this should not be graeater than about four inches. Longitudinal or profile grade differences are readily addressed by maintenance.
using overlays at intervals when the ride becomes rough. Signs may be oposted as well to warn motorists to reduce their speed.

There is no firm answer but many have given ideas. The OCED publication on strengthening of existing roadways provide some tolerances for roughness etc. Some information may be contained in the HDM model on road deterioration. Road roughness limits are provided for varying speeds of travel etc. The problem is one of recognizing that you are widening a roadway and that behaviour of the widened portion would be decidedly different than the existing when soft ground foundation prevails. The time to address the issues are prior to construction.

I trust that this helps.

Regards

 

[BigH]

Thanks for all the help from all - hats off to VAD and KRS for good discussion. It does show that many engineers/owners don't really address the issue of foundation settlement. You build a new road embankment (not just a widened one) over soft to firm soils - no problems with shear distress but you will get a lot of variations in the settlements across and down the road. Many designers likely don't even point this out to the owners. As you've indicated, in most cases, all you can really do is do some levelling courses when necessary from time to time until the consolidation period (4, 5 8 years??) is over - then, you'll face VAD's widening sceario. VAD's 4 inches in longitudinal direction is similar to what I remember reading once - but forgot the reference. Anyway to all who gave their comments!