Heavy Duty Pavement Design (Flexible) Over Soft Subgrade
Heavy Duty Pavement Design (Flexible) Over Soft Subgrade
(OP)
I am looking for advice on design methods for heavy duty pavement over weak, compressible subgrade conditions. Due to the likelihood of ongoing differential settlement at the site, I am considering a flexible pavement design (asphalt over granular base). The subgrade consists of sand fill overlying peat and organic silt.
MAKP
MAKP





RE: Heavy Duty Pavement Design (Flexible) Over Soft Subgrade
RE: Heavy Duty Pavement Design (Flexible) Over Soft Subgrade
Heavy duty pavements have been built over soft subgrades and have performed satisfactorily for the conditions you have described. Settlement/differential settlement would perhaps be the most worrysome problem. Preloading, if feasible, and delayed construction of the pavement structure have been used with good success. The depth of soft ground would be important to know in this regard.
While a rigid pavement structure is assumed to bridge softer ground I would be concerned about whether settlement is an issue at this time since this would also influence the behaviour of the rigid pavement. I presume that your choice of a flexible pavement has therefore to do with deepseated settlement. Generally, it is often easier and cheaper to remedy a settling asphalt pavement than a broken up concrete pavement to provide satisfactory rideability
Geotextiles, geogrids etc do not stop vertical settlement in deep settling deposits but are somewhat advantageous in minimizing lateral spread.
What is the experience in the area. You may wish to research this as well.
RE: Heavy Duty Pavement Design (Flexible) Over Soft Subgrade
Answering some of the other questions - the site has a varied loading history and has undergone considerable preloading in some locations and little if any at other locations (it is a very large site). To futher complicate matters, the thickness of existing fill over the site varies from less than 1 m to more than 5 m. The peat thickness varies from < 0.3 m to as much as 3 m (variation is both natural and due to compression/consolidation). Underlying the peat is organic clayey silt to clayey silt. The highly compressible peat and silts extend to depths ranging from 10 to 15 m. As expected, the moisture content in the peat is lower in locations where the overlying fill is thicker.
RE: Heavy Duty Pavement Design (Flexible) Over Soft Subgrade
The desigh approach to your problem does not lie with any of the pavement design models you have mentioned since most if not all design methods assume that the underlying ground below the pavement subgrade is "well behaved" in the sense that there are no serious or complicating geotechnical issues. As such the KENLAYER or Ports method etc would not be of assistance except for the determination of the layers etc above the subgrade.
The problem that you face is best resolved by understanding your foundation conditions in relation to settlement and differential settlement.
You have to look at fundamental aspects of foundation behaviour which will occur when you load the site with a pavement structure. Look at stress distribution that will be invoked by pavement and load conditions. What will be the likely depth of influence of the superimposed stress due to traffic loading etc?. Does it still matter if I have 10m of soft ground. Is my ground still consolidating? What is the likely impact of the increased stress of further ground consolidation and what is the zone of influence and so on etc.
Fundamental ground investigation information needs to be mulled over to determine based on experience and judgement what is likely to happen. Is it posssible to still preload the areas that were not preloaded?. This would remove some of the undesirable features of differential settlement.
Is it possible to excavate. This is often shied away in peat areas but has been used with success. Your silty subsoil my cause you a problem in this respect, but can be done if needed.
You will probably end up floating the pavement on this ground and could expect to have maintenance problems. Can this be lived with. This may have to be lived with as other approaches may be cost prohibitive. Such is life in dealing with these deposits and ground conditions.
The decision for a job of this sort brings together a series of knowledge and generally can only be best resolved by someone being in intimate contact(hands on) with the site and conditions, proposed development re parking areas etc etc.
This is a job for someone who is grounded with both geotechnical as well as pavement design experience. Unfortunately, in today's practice these are often relegated to separate camps and hence the uncertainity of what can happen and hence indecisions.
You have a challenging problem and I wish you the best.
RE: Heavy Duty Pavement Design (Flexible) Over Soft Subgrade
For something like a container terminal where the loading from container lift trucks can be substantially higher, a thicker subgrade would be required. A simple approach is to look at the increase in stress at the base of the fill from the wheel loading and compare it to the strength of the underlying material. Geogrid can be used to reduce the total thickness of fill required to protect the natural subgrade from overstressing but I haven't found a method of analysis that I am comfortable with yet so our tendancy is to stick with thick mineral fills over the peat.
Depending upon the requirements of the pavement (ie storage of containers) a preload of the order of 5 feet for vehicle travel is usually sufficient. If you are storing heavy loads you should use a preload of about 1.5 times the anticipated load on the pavement. For a peat thickness of about 10 feet a 5 foot preload can cause settlement of the order of 2 to 3 feet which will become settlement fill. You have to add the anticipated settlement fill to the preload fill so that the required preload can be removed. This would mean that for the 5 foot preload and 2.5 feet of settlement you would have to put on 7.5 feet of fill which would cause more settlement. You have to iterate through the calculations to find the ultimate fill height required to remove 5 feet of preload. You need to have good information on the distribution of compressible settlements to do this properly because thicker fills will be required over more compressible sections and thiner under less. Otherwise you end up taking off too much fill in some areas and not enough in others resulting in a non-uniform preload of the underlying soils.
Secondary consolidation of high moisture content soils and peat is also a concern since preloading will not eliminate post construction, secondary consolidation, it will only reduce it.
RE: Heavy Duty Pavement Design (Flexible) Over Soft Subgrade
If you are looking at container storage, you likely need to get your settlement taken care of before paving or anticipate recommendeding funds for maintenence.
Zdinak
RE: Heavy Duty Pavement Design (Flexible) Over Soft Subgrade
Now if you have such major settlement issues, you won't need to overcompact the embankments. The small settlements that would occur if compacted to 90% MDD vs 95%MDD (standard or modified) is not likely significant compared to the foundation settlement.
RE: Heavy Duty Pavement Design (Flexible) Over Soft Subgrade
Preloading, wick drains, prediction of settlement and bearing capacity these are all geotechnical issues I deal with on a daily basis and have a considerable amount of experience. I am less experienced with heavy duty pavement design.
I have done a lot of reading about pavement design but most of the methods are based on standard 18 kip axle loads. Rutting models look at a limiting compressive strain at the top of the subgrade; however, the stress distribution with depth for a standard axle loading is considerably different than from the front axle of a reach stacker.
Again I ask - are there any design methods out there based on heavy equipment that might be applicable?
Re: Geogrid
I have used geogrids in unpaved roads such as mine haul roads and log sort yards but I am not considering them for this project because I expect the pavement would rut long before the strength of the geogrid was mobilized.
I would use a geotextile as a separator in areas where there is little or no sand fill overlying the peat.
RE: Heavy Duty Pavement Design (Flexible) Over Soft Subgrade
I think the Asphalt Institute has a publication on Heavy Duty Pavement Design in one of their series. You may wish to check their web site. If I remember correctly they have examples with loaders etc.
RE: Heavy Duty Pavement Design (Flexible) Over Soft Subgrade
You can optimize the material to reduce costs and you can utilize some soil improvement techniques in conjunction to give better results.
As for analyzing your issues, I would suggest you go beyond the AASHTO methods and look at elastic layer analysis or a finite element approach.
RE: Heavy Duty Pavement Design (Flexible) Over Soft Subgrade
Before I forget - Happy Diwali to all our Indian colleagues!
RE: Heavy Duty Pavement Design (Flexible) Over Soft Subgrade
Hi - this use of heavy duty pavements over "soft" and/or compressible soils is difficult and challenging. You might look at Geofoam, a lightweight polystyrene material as an underlay for a road [pavement] section. This allows for overexcavation and removal of up to several feet of in-situ material, a depth determined by the ultimate traffic loading, and its' replacement with a lightweight but structurally competent material. Geofoam weighs between about 1/2 and 1 pcf, considerably lighter even than [pure] peat. "Geofam Geosynthetic" by John Horvath might be a useful text, as can be the Syracuse University [New York] geofoam web page.
Also, "Guidelines for use of Fabrics in COnstruction and Maintenance of Low-Volume Roads by Steward, Williamson & Mohney prepared by the U.S. Forest Service and the U.S. Department of Transportaion may be of assistance.
Good Luck!