17 Comparative Analysis of Flexible Pavement Design Methods …
177
factors considered are (i) Strength of construction materials (ii) Strength of subgrade
material (iii) Traffic load.
The traffic load S Initially calculated as the ESAL-that is, the total number of
18,000-lb axle loads in one direction-as discussed earlier, and then converted to a
traffic index (TI), where
T I = 9.0x(
E S AL
10
6
)
0.119
(17.4)
The design objective is to determine the total thickness of material required above
the subgrade to carry the projected traffic load. This thickness is determined in terms
of gravel equivalent (GE) in feet, which is given as
G E = 0.0032(TI)(100 − R)
(17.5)
where TI = traffic index, GE = material thickness needed on a given layer in terms
of gravel equivalent (feet), R = resistance value of the supporting layer material,
mostly determined at a discharge pressure of 300 Ib/in
2 . Thickness of each of the
layer is obtained by dividing GE for that layer by the GE factor G f for the material
used in the layer. A check must be done to ensure that the requirements for expansion
pressure is adequate for the layer thickness obtained.
17.2.6 Hd26/01
The United Kingdom department of transport developed this standard procedure in
2001. It is the modification of the LR 1132 based on availability of recent construction
materials and innovative research. The most important and primary factor of this
method of design is the traffic assessment (Chidozie and Joshua 2016). The weighted
yearly traffic is computed using Eq. 6 and the corresponding design traffic is obtained
using Eq. 17.7.
T i = 365 × Y × F × W × G × P × 10
−6 msa
(17.6)
Design Traffic (T ) =
T i
(17.7)
where: Y = Design Period in Years, F = Traffic flow for each class of traffic at
opening, W = Wear Factor for each traffic class (W N in case of new design or W M in
case of Maintenance), G = Growth Factor, P = Percentage of vehicles in the heaviest
loaded lane (Highways Agency 2006).
177
factors considered are (i) Strength of construction materials (ii) Strength of subgrade
material (iii) Traffic load.
The traffic load S Initially calculated as the ESAL-that is, the total number of
18,000-lb axle loads in one direction-as discussed earlier, and then converted to a
traffic index (TI), where
T I = 9.0x(
E S AL
10
6
)
0.119
(17.4)
The design objective is to determine the total thickness of material required above
the subgrade to carry the projected traffic load. This thickness is determined in terms
of gravel equivalent (GE) in feet, which is given as
G E = 0.0032(TI)(100 − R)
(17.5)
where TI = traffic index, GE = material thickness needed on a given layer in terms
of gravel equivalent (feet), R = resistance value of the supporting layer material,
mostly determined at a discharge pressure of 300 Ib/in
2 . Thickness of each of the
layer is obtained by dividing GE for that layer by the GE factor G f for the material
used in the layer. A check must be done to ensure that the requirements for expansion
pressure is adequate for the layer thickness obtained.
17.2.6 Hd26/01
The United Kingdom department of transport developed this standard procedure in
2001. It is the modification of the LR 1132 based on availability of recent construction
materials and innovative research. The most important and primary factor of this
method of design is the traffic assessment (Chidozie and Joshua 2016). The weighted
yearly traffic is computed using Eq. 6 and the corresponding design traffic is obtained
using Eq. 17.7.
T i = 365 × Y × F × W × G × P × 10
−6 msa
(17.6)
Design Traffic (T ) =
T i
(17.7)
where: Y = Design Period in Years, F = Traffic flow for each class of traffic at
opening, W = Wear Factor for each traffic class (W N in case of new design or W M in
case of Maintenance), G = Growth Factor, P = Percentage of vehicles in the heaviest
loaded lane (Highways Agency 2006).