Thank you.You have explained the issue in a proper way, however, I am going to add we should calculate the stresses for 25 tons axle load instead of 8.2 tons and for tire pressure of 150 PSI and then check the resistance of materials right up to one meter below subgrade top then subgrade followed by sub base and unbound base. Maybe we need to improve the testing mechanism, for example, the modified proctor test will be needing further improvement on the same analogy as it had been upgraded from standard proctor to modified proctor. It will make the pavement layer stronger to resist the overloading. Similarly the capacity of equipment may also need upgradation. Because the compaction effort will be more. Similarly, the strength test of asphalt concrete may also need to be reviewed for withstanding to the excess loading conditions. I think you may have got my points.
I am again thankful to you for taking so much interest in my country's issue. I am really glad to see the interest of great pavement experts across the globe or resolving this important issue of developing countries.
Thanks and with best regards,
Ejaz Ahmad Khan
------Original Message------
Ejaz,
In following the comments and suggestions made by all in the group regarding this issue, I found the comments interesting, and very valuable, and most of all, encouraging that so many pavement engineers were willing to participate. However, coming back to your original question of can we as design engineers, and subsequently, how should we, account for effects of overloading, and also the concomitant trend of significantly higher tire pressures, in order to mitigate the detrimental effects? Now one can debate the fact that it should be a law enforcement responsibility to curtail overloading, and should not be the responsibility of us as design engineers, but that in my opinion, is a matter for a separate discussion all together. To keep in mind, the primary purpose of asphalt wearing courses, and subsequent pavement layers such as base course, and/or subbase layers, is to protect the subgrade, by sufficiently reducing the stresses imposed by traffic loads. While we can only "improve" the load bearing capacity of the in situ subgrade to some extent for practical, and for economical reasons, the most effective way is to "add" layers. Hence, we should be designing the pavement as a "system," and consider each layer as a component within this system. The first, and most important input parameter is imposed traffic loading. All expected traffic loads needs to be converted to ESALs, i.e. 8.2 ton (80 kN/18kip) single axle, dual wheel tires, and future traffic loading in terms of expected number of these loads that the pavement needs to withstand, needs to be calculated. Expected "overloaded," or axle loads in excess of the standard, will also need to be expressed in terms of ESALs. In addition, the effect of such a wheel load should also be evaluated in terms of the stresses and strains it will cause in the respective pavement layers, in order to ensure that it will not exceed the allowable limits for that material, within that particular layer, i.e. evaluate the point load effects. Then, pavement structure needs to be evaluated and analyzed to determine appropriate material type selection, and thicknesses.
------------------------------
Gerhard du Toit, PE (TX), Pr-Eng (SA)
Pavement Engineer, Transportation, Northeast
D +1-978-905-2496
M +1-512-915-2332
Gerhard.DuToit@...AECOM
250 Apollo Drive
Chelmsford, MA, USA
T +1-978-905-2100
aecom.com
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Original Message:
Sent: 01-20-2019 14:05
From: Sajid Raza
Subject: Designing Sustainable Pavement
Dear Ejaz Khan,
Yes, you are right that we should not only focus on subgrade. If you want to analyze the stresses and depth of stress in subgrade, you may use any pavement design/analysis software and work this out. Then you can proceed to the next steps. But another suggestion is that the construction of pavement is also one of the main components. In Pakistan, some contractors may not be following the mix design protocols very strictly, i.e. asphalt content, and mixing and paving temperature etc. so also you should think about that. Thank you for asking the international community regarding this issue in Pakistan. I hope you would get valuable input from international pavement experts in the community.
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Sajid Raza, Registered Engineer
Graduate Research Assistant
University of Alaska Anchorage
Anchorage, Alaska
<maskemail>sraza@...</maskemail>
907-223-5720
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Original Message:
Sent: 01-20-2019 02:48
From: Ejaz Khan
Subject: Designing Sustainable Pavement
Dear Sajid Raza,
You are right Sir, but It's not only the subgrade we need to work on tests protocol of various layers to sustain the overloading of axle above 8.2 tons or in Pakistan we called it legal weight per axle which is 12 tons. I am of the opinion that we should work out the stress on top of subgrade for overloaded axle and also work out the depth below subgrade to which the stress propagates for replacing the weak soil from that depth. Then we need to replace or improve the soil for taking the real or actual axle load which the subgrade will experience. Moreover we also need to check the state of compaction and strength of pavement layers like sub-base ' base and bound layers to resist the overloaded stresses that will be experiencing by the structure layers. I will give you one example. Take the case of standard and modified proctor compaction, AASHTO has switched over to modified proctor because the compaction of standard proctor was become insufficient to resist. Based on this analogy we need to move forward.
Thanks and regards,
------------------------------
Ejaz Khan S.M.ASCE
Director
Pakhtunkhawa Highways Authority
Islamabad
Original Message:
Sent: 01-14-2019 14:49
From: Sajid Raza
Subject: Designing Sustainable Pavement
Here, the main issue is overloading which is about 4 times the standard load, and it would be uneconomical to increase the thickness. I would suggest that first focus on sub-grade stabilization, a professor at NUST Islamabad has conducted tests on sub-grade stabilization using sugarcane Bagasse Ash. You may contact him in this regard. Also, I have observed that sometimes the <g class="gr_ gr_704 gr-alert gr_spell gr_inline_cards gr_run_anim ContextualSpelling ins-del" id="704" data-gr-id="704">rutting</g> is due to weather conditions especially hot summer in Pakistan. On some sections of highways in the mountain areas (high grades), you can use rigid pavement on outer lanes for trucks (also paved on M-2 near Salt range).
------------------------------
Sajid Raza, Registered Engineer
Graduate Research Assistant
University of Alaska Anchorage
Anchorage, Alaska
<maskemail>sraza@...</maskemail>
907-223-5720
Original Message:
Sent: 01-10-2019 11:19
From: Ejaz Khan
Subject: Designing Sustainable Pavement
In Pakistan, we are designing pavement structure by using AASHTO empirical method of pavement thicknesses design. First we worked out the traffic rather classified traffic then convert it to ESAL by multiplying it with the equivalency number as you all are aware that each classified vehicle has its specific equivalency no. Then we worked out commutative ESAL for 10 years. These are then the design ESALs which the pavement will be experiencing through the design life of 10 years. After this we explore the soil properties specially the sub-grade and it's classification plasticity index CBR and Modulus of resilient are worked out. Then according to the AASHTO equation by sitting the reliability serviceability and other parameter the structure no is worked out and then the respective thicknesses. The usual design are as follows:
Soil is usually A4 with passing no 200 usually up to 85 percent and CBR is 8 to 15 percent PI is usually 8 to 10 percent.
Sub-base 25 CM. Waterbound macadam is 30 CM asphaltic base course is 8 cm and ACWC is 5 cm.
Still rutting specially pavement rutting and fatigue cracks are the usual phenomenon in the wheel track after 2 to 3 years.
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Ejaz Khan S.M.ASCE
Director
Pakhtunkhawa Highways Authority
Islamabad
Original Message:
Sent: 01-09-2019 11:23
From: Christian Steinbrecher
Subject: Designing Sustainable Pavement
This is as much a social engineering as a civil engineering challenge. The social engineering element will be solved when the population as a whole comes to grips with the need for sustainable and affordable roadways - to which a bit of enforcement for limiting weights is the encouragement. Taxation could be a partial solution as could the prohibition of the importation or manufacturing of tires beyond certain inflation pressures. Other solutions could include truck lanes that are reinforced to handle the loads. Perhaps geotechnical conditions in certain limited areas could handle these high loads - such as areas where the sub-grade is composed of sufficiently stout material.
Putting aside flagrant violators, subsistence living standards may be pushing these owners & drivers to load up their vehicles to these extents.
------------------------------
Christian Steinbrecher P.E., M.ASCE
President
Ukiah Engineering, Inc.
PORTLAND OR
(503)292-2165
Original Message:
Sent: 01-08-2019 02:35
From: Peter Singh
Subject: Designing Sustainable Pavement
I find this discussion very interesting and technically informative.
I understand now how significant are tire pressure, axle load and overloading factors are for pavement design! Thanks Mr. Gerhard Du Toit.
------------------------------
Peter Singh M.ASCE
Managing Director
Kingdom Designers
Birganj
9779855020043
Original Message:
Sent: 01-07-2019 08:26
From: Gerhard Du Toit
Subject: Designing Sustainable Pavement
Ejaz,
Overloading, and subsequently the detrimental effects that has on pavement life, is a common problem in many developing countries. From experience in many African countries, excessive overloading is a reality one has to take cognizance of. Yes, the ultimate solution is effective law enforcement, but again, from my experience, not always a practical solution. As you know, high tire pressures induce substantial surface stresses in AC surface layers, and based on the information given in your example, an axle load of 30 tons, as opposed to the standard 8.2 tons, will effectively result in 179.15 ESAL loads. With simple linear elastic based pavement modeling software, the combined effect (in terms of equivalent single axle loads) can be quantified. So yes, while one can to some extent "design" for these conditions, the effect on costs to build roads to withstand these conditions will also be astronomical.
Factors to take into account to model higher loads and tire pressures , will need to include:
- Axle load and axle configuration (single, dual, tandem, etc.)
- Wheel configuration (single wheel, dual wheels, super singles, etc.) as well as spacing
- Tire pressures
This should then allow you to calculate the "design load equivalency factor" to in turn, calculate design traffic.
PS. I used ELSYM 5, or CHEV 15 in the past for calculating load equivalency factors, but there are many similar software programs available on the market that can be used.
Regards
Gerhard du Toit, PE (TX), Pr-Eng (SA), M.ASCE
Pavement Engineering Group Manager, Transportation, Northeast, USA
<maskemail>Gerhard.DuToit@...</maskemail>
------------------------------
Gerhard du Toit, PE (TX), Pr-Eng (SA)
Pavement Engineering Group Manager, Transportation, Northeast
D +1-978-905-2496
M +1-512-915-2332
<maskemail>Gerhard.DuToit@...</maskemail>
AECOM
250 Apollo Drive
Chelmsford, MA, USA
T +1-978-905-2100
aecom.com
Original Message:
Sent: 01-05-2019 10:54
From: Ejaz Khan
Subject: Designing Sustainable Pavement
In Pakistan, we are suffering from axle overloading of commercial vehicles which are the main cause of premature failure of highway pavements. The usual axle loadings experienced on highways are up to 30 tons against the standard axle loading of 8.2 tons also termed as ESAL in pavement design terminology. The tire pressure is up to 150 PSI.
Can some expert guide me what factors should be to be taken into account while designing a sustainable pavement structure for the above mentioned loading and tire pressure conditions?
------------------------------
Ejaz Khan S.M.ASCE
Director
Pakhtunkhawa Highways Authority
Islamabad
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