Settlement within a road 1

[aaron252]

Hi

Can anyone offer me some advice on how to calculate settlement in a road from cyclic loading? The road is proposed to be built on made ground with clay occuring at around 1.5m depth. The risk of settlement has been identified but we have no idea how to calculate the expected settlement figures...any tips? We want to calculate it based on cyclic loading, not point loads - typically it will be used by HGV's.

Cheers

 

[Ron]

Assuming you have an adequate structural section to withstand the traffic loads, roadway settlement has little or nothing to do with the traffic loading. It is dependent on the additional overburden required to achieve the desired grades relative to the type of underlying soils. If the soils are granular, you can get some minor "settlement" from the vibration of traffic in the upper 2 to 5 feet of subgrade, but that is usually mitigated by appropriate compaction of the subgrade prior to building the pavement section. Traffic vibrations are usually attenuated by the pavement section and do not transmit much beyond the base.

In short, roadway settlement is generally related to the static conditions, not the dynamic. If you have soft underlying soils in the upper 10 to 15 feet of soil and you add fill, you'll likely get some consolidation of the soft layer, resulting in settlement. A geotechnical investigation of the proposed route would tell you what you need to know along those lines.

 

[oldestguy]

Hello Aaron252:

Sounds like a job in G.B.

Over here in US I'm using the word "Fill" instead of"made ground".

Ron is right in saying traffic has little or no effect.

Instead, how you build this fill is important. If the fill was just dumped (your word is "tipping" I think), then you can expect settlement in time; more rapid to start and decreasing with time, depending on the material used. It is unlikely you will find anyone willing to tell you how to predict the settlement of dumped fill.

Your best bet for a new job is to use contractors familiar with placement in layers, each of which is compacted. To get fancy you can also test to see how good a job you are getting. This procedure is pretty stndardized these days.

Done right and it won't settle within its body, but ground below may compress (See Ron's comment).

 

[VAD]

I think what aaron252 is getting at is deformation under repeated traffic loadings. Sometimes people refer to it as settlement but in cyclic loading or repeated loading terminology it is referred to as permanent or non recoverable deformation.

Yes, while not in most design manuals one can determine a measure of deformation likely under repetitive loading. A layered model is assumed and deformation of the various layers can be determined from laboratory testing where in deformation for various layers can be determined and accumulated.

While the typical designs do not apply this approach, the concept of stage construction is often applied by allowing a first layer of asphalt for a couple of years and then applying the final layer. This allows for conditioning of the layers as aresult of repeared traffic loadings. The same applies to railroad ballast.

Despite compaction to 100 % proctor etc, movement of particles say in a granular mass still occurs under the traffic loading. Some people refer to it as shakedown etc. I look at it as the mass getting accustomed to the imposed loading characteristics. In other waords if I push you against a wall, you will articulate your body until you become comfortable to the effects of my push. Soils behave in similar fashion and if the loading regime increases then further adjustment takes place.

Sometimes the final rolling of pavement structures is best done by equipment that closely simulates the type of equipment that will use the road finally. Pneumatic tired wobblies for base and pneumatic finishing rollers are two such. However, these days to attain densities we use vibratory compactors. I am not too sure that roads compacted with vibratory rollers achieving high densities perform as well as those that are compacted with equipment that mimic equipment that finally use the roadway. There is much more to compaction in my mind than just achieving a global density. Particulate orientation is a factor to be considered and which is not taken into consideration and I am not sure how this can be done in design.

The aformentioned are my personal concepts and beliefs and these are not enshrined in common practice but rather in research.

Much of the concepts of repeated loading etc can be obtained from a search of the journal articles under repetitive and cyclic loading.

Our concept is to overlay if we rut- permanent deformation. Why we rut/deform brings out numerous reasons despite on paper we had sound designs.

So much for now

 

[Ron]

Rutting is controlled by the vertical strain at the top of the subgrade. The key is to make the sections sufficiently stiff so as to mitigate this parameter and decrease the rutting potential. The repetitive elastic strain at the top of the subgrade eventually causes enough lateral displacement (as related to Poisson's ratio for the subgrade) to allow a vertical drop (rutting).

 

[BigH]

Good points by all - in the respective point of view. In summary, Settlement of roadways consist of:
1> Foundation settlement due to overlying loads. This has least control but is for all intents and purposes a static load situation (viz., soft clays overlain by moderate embankment). This occurs whether the embankment is "well compacted" or poorly compacted other than the influence of the density of the well vs poor placement
2> Embankment settlement due to self consolidation and/or compaction due to transient loadings leaving residual deformations on release. Obviously it happens more with clayey soils that are poorly compacted than with granular soils even if poorly compacted. Better compaction usually means less self-consolidation - unless you build up too much porewater pressure in compaction of clayey soils - then you can get more even if you think that you have densified quite a bit.
3> structural pavement layers undergoing repetitive loading. I found that base course and wearing course asphaltic pavements 'densify' some 2% during secondary compaction - i.e., traffic. If we placed it at 97% Marshall, then later it would end up (say in a couple of years) as 99%. This translates into a settlement although it is minor. It could be calculated but need to consider 3-d effects - not just 1-d loading. Well compacted base courses will settle very little if properly compacted. Subgrades a bit more - as less attention is paid to them.
4> pavement material deterioration. Can be due to rutting - as per Ron's note above; can be due to shoving due to softening at high ambient temperatures (poor choice of binder). As years go on - it is lateral differential settlements (channelization) in my view that manifest themselves.
What is hard is to find out what is acceptable settlement for each condition - especially with foundation settlement where you have, in practical terms, little real control.

 

[aaron252]

Many thanks for your responses guys, you have helped me develop a much better understanding of the problem now