Pavements Drainage Layer

[Okiryu]

Hi, for flexible and rigid pavements what could be the main reasons to install a drainage layer? My project is located at a non-frost area, with the water table very deep (at GL-15m). Storm intensity is 142 mm/hr for 10-year design period. Traffic is low and at a low velocity (for aircraft and ground vehicles), so pumping may be low. I am thinking that my base (150 mm) can serve as a drainage layer as well. My main question is what are the parameters that determine the use of a drainage layer. Thanks !!

 

[Ron]

Any time the base has the potential to be saturated and lose stability, even for short periods, drainage is necessary.

 

[Okiryu]

Ron, thanks for the comment. My base is 150 mm GW compacted to 100% modified proctor which I will try to use as a drainage layer also. My subgrade has very low permiability. I will check if the base course can act also as a drainage layer... Hope it can. Thanks again.

 

[Ron]

Okiryu....I would not depend on the base course to be a drainage layer. The drainage layer should be somewhat lower than the base course, preferably by several feet.

 

[BigH]

Might I suggest that you find a copy of Cedergren's book on "Seepage, Drainage and Flow Nets". He has an excellent chapter on road drainage. At that compaction level and if your "GW" material has 5 to 7% fines (I am assuming it is a well graded material and is likely crushed rock) you might find out that it has a very low coefficient of permeability (you might like to run a test on it to prove that it really isn't "permeable") . I once saw water ponding on a well graded crushed rock Road base material to be placed under a paper machine - and it sat thee for 3 days before it disappeared.

 

[Okiryu]

Hi Ron, BigH, as always, thanks for your advice. While I am looking for Cedergreen's book, my base has a permeability coefficient of approx. 2.5 x 10^-3 m/s which I was considering as a permeable material. At least, my USACE manual for pavement design shows drainage layers considered as "Rapid Drainage Material" (RDM) with k values of 3.5 x 10^-3 m/s. Since my base material was close to the USACE value, I was thinking that the base could be used also as drainage layer.

Anyway, I will double check my numbers. The existing pavement that I have to replace does not present signs of drainage problems and it used similar base course as it is a standard material here in Japan. I still have to do more field investigations to check if it has a drainage layer but I think it does not have it as I did not noted any daylight outlet along the pavement (The subgrade has very low permeability so if there is a drainage layer (open grade material layer), I think that it should be a daylight outlet).

Thanks again !!

 

[cvg]

if you provide adequate cross slope, keep the joints sealed and provide good cross drainage, keeping the 10-year or maybe 25-year, high water level below the level of the aggregate base, than I would tend to agree that a granular drainage layer is questionable

 

[Okiryu]

cvg thanks for your comments. I was going to ask about the rainfall design period. Do you normally use 10-year or 25-year? The USACE manual for pavement design calls for 2-year rainfall. Anyway, will check for 10 and 25-years as well..

 

[cvg]

return interval for pavement drainage varies by agency and region. most US DOT's use 10-year, FAA has long used 5-year, one local city uses 2-year. These generally limit the spread to one lane wide. For critical facilities, makes sense to go higher, maybe 25-year. probably should look at 50-year cross drainage