Grameen Sampark Final April 0... - pmgsy

18
Subgrade Failure Criterion
The basic design thickness in the case of low-volume
roads is the thickness of unbound granular material,
which will limit the vertical compressive strain on top of
the subgrade to an acceptable level. The strain induced
in the subgrade by moving wheel load is mostly elastic
(recoverable). However, the accumulation of the
irrecoverable part of strain leads to permanent
deformation in subgrade. The permanent deformation
manifests at the surface of the pavement as rutting in the
wheel paths, although due to inherent variability of the
subgrade and pavement materials and the construction
techniques, surface roughness develops along with
rutting.
Performance Criteria Addopted By Different
Organizations
AustRoads Design for granular pavements with thin
bituminous surfacing [2 3]
The limiting strain criterion for the subgrade is given by
Eq. 1. It was derived by applying mechanistic procedure
to a range of pavements.
Where,
7.14
N = [8511/ µ ] (1)
µ
Grameen Sampark
= vertical compressive strain (microstrain)
at the top of the subgrade
N = allowable number of repetitions of the
strain before unacceptable level of
rutting develops.
Implicit in the design procedure for granular pavements
with thin bituminous surfacing, is a terminal condition
which is considered to be unacceptable and hence
signifies the end of life of the pavement. The terminal
condition is considered to be an average rut depth of
about 20mm, and terminal roughness about three times
the initial roughness.
AASHTO Low Volume Road Design Procedure [1]
The low-volume road design procedure used by
AASHTO is based on lower level of reliability (50%)
because of their low usage and associated low level of
risk. Predicted future traffic, seasonal resilient moduli of
roadbed soil, elastic modulus of aggregate base and
subbase layer, design serviceability loss, allowable
rutting and aggregate loss (GL) of surface course are the
main input for the design of low volume roads.
Common values for terminal serviceability index are
Pt=2.0. For minor highways like aggregate surfaced
roads, where funds or economy is the main factor, the
design is done by reducing the traffic or design life rather
than reducing the terminal serviceability to a number
lower than 2.00. Aggregate loss due to traffic and
erosion should be considered in the design and this may
be calculated using any of the following equations 2, 3
and 4.
According to a study by University of Texas at Austin:
Where,
GL = 0.12 + 0.1223 (LT) (2)
GL = Total Aggregate loss in inches;
LT = Number of loaded trucks in
thousands
Another study in Brazil formulated the following
equation:
GL = (B/25.4)/ (0.0045LADT + 3380.6/R +
0.467G) (3)