DESIGN OF FLEXIBLE PAVEMENTS
Welcome
to Mbedule design centre
This is a specialized blog in Tanzania aimed at
generating various construction contents in engineering perspectives.
This post
it is basically aimed at giving guidelines to civil and highway engineering
students in the design of flexible pavements.
Why pavements should be designed?
The following are objectives of flexible pavement
designing. These are as follows
a). To provide a strong road which can overcome the
expected loading.
b). To minimize stresses which can affect the subgrade
in a particular area.
c). To ensure pavement layers do not fail seriously.
Pavement design is
the process of generating economical pavement layers which can work together
with natural soil to carry expected traffic in particular area. Flexible
pavement has various layers which are Subgrade, subbase, granular base, and
asphalt concrete surface.
Consider a following picture.
The image showing flexible pavement layers.
Pavement
design process
The following are main procedures or steps to
considered in designing flexible pavements. These are as follows.
i). Determining the amount of traffic in particular
area and the cumulative number of equivalent standard axles that expected to
pass to that road.
ii). Assessing subgrade strength where the road will
pass.
iii). Selecting suitable combination of pavement
layers.
How
t o determines Commulative number of Equivalent Standard Axle Loading (CESAL)
Before determining CESAL we need first to know what is
Vehicle Equivalent Factors. Equivalent factors are used to determining damaging
effect of Vehicles in comparison to standard axle (E80) load of 8 tonnes,
Equivalent factor is given by the following formula
Equivalent factor = (axle load in kg/8160)
^4
After
determining E80 of various vehicles we need to find proportion of E80 for
vehicle of 13 tonnes and above to the vehicles of less than 13 tonnes. Then if
the proportion of E80 of vehicle greater than 13 tonnes is equal or greater
than 50% to the total E80 of vehicles, then a road should be designed for heavy
vehicles otherwise should be designed for light vehicles.
How E80 can be described in damaging effect of
vehicles.
1. For example, an axle of 17 tonnes. By finding its
Equivalent factor in damaging effect.
Since an axle have 17 tonnes then,
Damaging effect by an axle=
(17/8.2) ^4 =18.47
This means that an axle of 17 tonnes when passes once
(1 pass) on a road have same effects as 18 passes of standard axle of 8.2
tonnes.
2. For example, an axle of 6.5 tonnes. Its equivalent
damaging effect will be as follows.
Since an axle have 6.5 tonnes then,
Damaging effect = (6.5/8.2) ^4 = 0.3948
Since is less than one, then take 1/0.3948=2.53
This means that 2.53 passes of 6.5 tonnes is equivalent to 8.2 tonnes of standard axle.
Commulative
number of Equivalent Standard Axle Load (CESAL)
This is typically E80 of number of years, or total E80
of years. It is determined by the following formula,
Where, t1
= average day to day number of standard axles in the year of traffic survey.
r
= Annul growth rate.
n
= calculated period in years.
For example, given design life is 20 years, growth
rate is 4.5% and average day to day number of standard axle is 760. Determine
CESAL.
Solution,
T1= 760
R=4.5%
N= 20
E80=
