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  • 1.  Granular subbase drainage

    Posted 10-23-2017 09:43 AM
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    Roads in my locality have been constructed without proper GSB and the pavements are failing very fast... Is there a way in which such roads can be provided drainage facilities. I have also attached the GSB gradations used in my country below. Can someone please check it because roads with gsb are also failing due to permeability problems of GSB layer.

    Rahul Sreedhar S.M.ASCE

  • 2.  RE: Granular subbase drainage

    Posted 10-24-2017 08:34 AM
    ​Do you use a subdrain under the granular subbase to drain the pavement? Any time we have a drainable base under our pavement, we use a perforated subdrain in a trench with porous backfill to give the water a place to get out from under the pavement. Otherwise you are just collecting the water under the pavement and letting it  deteriorate your pavement from the bottom up. See the following link for more details https://www.iowadot.gov/erl/current/RS/content_eng/dr303.pdf

    Jenifer Bates P.E., M.ASCE
    Project Manager
    Indianola IA

  • 3.  RE: Granular subbase drainage

    Posted 10-24-2017 12:32 PM

    I don't doubt that subbase and drainage may be at play here. However, in order to provide meaningful input and help determine the cause(s) of failure, it is important to begin, not with a conclusion ("Roads in my locality have been constructed without proper GSB"), but with some questions:

    • What kinds of failures have occurred and when do the failures first manifest themselves? Photos of some typical failures would be very helpful in diagnosing the problem.
    • What are the environmental factors in your area (rainfall, temperature ranges, etc.)?
    • What kind of traffic are these pavements subjected to?
    • What are the typical overall pavement cross-sections, including the material and range of thickness of each pavement layer?
    • What are the subgrade characteristics?
    • How is the subgrade graded?
    • What provisions have been made both for surface and subsurface drainage? Jennifer's comments are right on point.

    Fernando Souto P.E., M.ASCE
    Senior Project Manager
    Cadillac MI

  • 4.  RE: Granular subbase drainage

    Posted 10-26-2017 12:21 PM
    Jennifer and Fernando are giving you good advice.  My experience with pavements is that the sub-grade is the weakest part of the system in most designs.  Failure to drain the sub-grade properly is a major cause of failure.  As Jennifer is hinting at, the problem is likely to be not the gradation but the failure to drain it.  If your area is subject to monsoon conditions that probability goes way up.  Do your pavements have shoulders or curb and gutter? If shoulders, how wide are they?  Does your area have a program to detect and fine overweight vehicles?

    James Anderson P.E., M.ASCE
    Project Engineer
    North Richland Hills TX