Grameen Sampark Final April 0... - pmgsy

20
A maximum of 50mm rut depth was recommended for
the unpaved roads. The reliability adopted is 50%.
Asphalt Institute [5] adopted the following subgrade
strain criteria
Where,
Grameen Sampark
-2 0.223
= 1.05*10 *[1/N] (5)
N = No of 80 kN equivalent single axle loads;
Z
Z
= Vertical subgrade strain ( micro strain) at
top of the subgrade layer
TRRL [6] considers the following relationship to
complete allowable subgrade strains for 85 percent
probability of survival to a design life of N repetitions of
o
80kN axle and a pavement temperature of 20 C.
Log (N) = -7.21-3.95* log ( ) (6)
Z
The failure condition considered for the above criteria is
rutting of 20 mm
The Shell [7] design method uses the following
subgrade strain for different confidence levels:
-2 -0.25
50% confidence = 2.8*10 *N (7)
Z
-2 -0.25
85% confidence = 2.1*10 *N (8)
Z
-2 -0.25
95% confidence = 1.8*10 *N (9)
Z
A terminal value of percent serviceability (PSI) of 2.5
was taken as failure condition.
Indian Road Congress (IRC 37:2001) [8] adopted the
following performance criteria for high volume roads.
N R =
-8 4.5337
4.1656*10 [1/ Z ] (10)
N R = Number of cumulative standard axels to
produce rutting of 20 mm.
Z
= Vertical subgrade strain (micro strain).
Development of an average rut depth of about 20 mm is
considered to be failure in rutting mode.
IRC:SP:20:2002 [9] is for design of low-volume rural
roads in India. The criterion for determining the
thickness of a flexible pavement with a thin bituminous
surfacing is the vertical compressive strain on top of the
subgrade imposed by a standard axle load (80kN). The
maximum rutting allowed is 50mm before any
rehabilitation work is taken up. The design charts have
reportedly been prepared as per Road Note 29 of TRL,
IRC: 37 and other related experiences in India. No
mechanistic rutting criterion has been proposed
correlating the design life with subgrade strain or any
other parameter.
Mohanty et al [10] studied the performance of 59 village
road sections in Orissa and proposed some design
charts for construction of new low-volume rural roads.
The pavement condition data collected on these
sections were correlated with mechanistic response of
the pavement to develop a performance criteria based
on limiting rutting value.