4.4PAVEMENT DESIGNThe pavement design process is the technique
of developing a combination of top layers of different materials to
cater for the total axle load over the design life of a road. In
other words this is an art through which the stresses as induced in
to layers of a road due to movement of heavy wheel load is
disseminated and minimized to a safe level through selection of
different type and appropriate thickness of pavement layers.In
order to carryout pavement design, following parameters in terms of
axle loading and soil strength are required:
4.4.1Equivalent Axle LoadsThe damage caused by vehicles to a
road depends on the axle loads and wheel configuration of the
vehicles. It is therefore important to determine the axle loads of
heavy commercial vehicles in the projected traffic mix (Refer
Traffic data) that is likely to use proposed roads.
In order to determine the cumulative axle load damage that a
pavement will sustain during its design life, it is necessary to
express the total number of heavy vehicles that will use the road
during the design period in terms of the cumulative number of
Equivalent Single Axles Load (ESALs).
I. Design LifeDesign life is the number of years reckoned from
the completion of pavement construction and application of traffic
load until the time when major maintenance is required so that it
can continue to carry traffic satisfactorily for further period.A
design period of 20 years has been adopted. However for asphalt
layers, stage construction is suggested; the asphalt requirement
for ten years design life is ascertained and shall be placed so
that the pavement can perform satisfactorily for 10years. After 10
years, the fresh traffic count will be taken and pavement condition
survey will be conducted to ascertain distress in the pavement.
Accordingly asphaltic overlay will be placed without adding in the
granular layers.II. Cumulative Equivalent Single Axle LoadsTraffic
load is converted into Equivalent Single Axle Load (ESALs). ESALs
is related to a standard axle of 8.16 tones (18000 lbs), using
equivalence factors, which have been derived from empirical
studies. The ESALs as worked out for roads with different ROWs
within Design Life (Refer Annexure-A) are as under;Table 4.5
Equivalent Single Axle LoadsSr. No..R.O.W
ESALs
10 Years20 Years
161m 10.536
236m 3.3210
325m 2.37
4.4.2 Design CBRThe design of pavement is based on the Subgrade
Soaked CBR 14 at 95% Modified AASHTO material for the project. The
detail is provided in the Geotechnical Investigations Report
attached as Annexure C.4.4.3 Design MethodologyThe pavement design
has been carried out as per AASHTO guide (1993) based on the
following main parameters.
4.4.3.1 AASHTO Procedure for Pavement Design
The AASHTO Guide for Pavement Design 1993 outlines this
procedure for determination of flexible pavement thickness by
solving AASHTO equations manually, by using different nomographs or
by using the computer software. For accuracy the computer program
is preferred. In all options basically the Structure Number (SN)
required to be assigned to the proposed pavement structure for a
given set of conditions is determined by solving the following
numerical equation:
The estimated future traffic in terms of ESALs for the design
period, W18The reliability level, R Standard normal deviate Value,
ZRThe overall standard deviation, SoThe roadbed soil resilient
modulus, MRThe design serviceability loss, PSI = Po Pt
The ESALs have been provided at Table 4.5. The other general
design variables have been discussed in the following
paragraphs.
4.4.3.2 Reliability (R)
Design reliability refers to the degree of certainty that a
given design alternative will last for the entire design period. A
design reliability level of 90% has been adopted for pavement
design of the Project Road.
4.4.3.3 Standard Deviation (SO)
The reliability factor is a function of the overall standard
deviation that accounts for standard variation in materials and
construction, the probable variation in the traffic prediction and
the normal variation in pavement performance for a given design
traffic application. The recommended value of standard deviation
for total variation in material properties and in traffic
estimation for flexible pavement is 0.45 and has been adopted for
pavement design of project road.
4.4.3.4 Standard Normal Deviation (ZR)
The value corresponding to reliability (R) of 90% is -1.282
which has been adopted in the design based on the recommended
values of standard normal deviation (ZR) by AASHTO Guide 1993.
4.4.3.5 Performance Criteria
The serviceability of a pavement is defined as its ability to
serve the type of traffic that uses the facility. Initial and
terminal serviceability indices have been established to compute
the total change in serviceability that will be used in the design
equations.
I. Initial Serviceability Index (Po)
The initial serviceability index is a function of pavement
design and construction quality. For flexible pavement design
typical value as recommended by AASHTO Road Test is 4.2 which has
been adopted.
II. Terminal Serviceability Index (Pt)
The terminal serviceability index is the lowest index that will
be tolerated before rehabilitation, resurfacing or reconstruction
becomes necessary and it generally varies with the importance or
functional classification of the pavement. Recommended value of
terminal serviceability index is 1.7 for the project road.
4.4.3.6 Resilient Modulus MR
The basis for material characterization in the AASHTO Guide 1993
is Elastic or Resilient Modulus (MR). In the absence of necessary
equipment required to determine resilient modulus of subgrade,
following correlation between CBR and MR has been used.
MR = 2555 (CBR) 0.64Where MR is resilient modulus in psi.
4.4.3.7 Computation of Required Pavement Thickness
The structural number (SN) requirement as determined through
adoption of design parameters as discussed above is balanced by
providing adequate pavement structure. Under AASHTO design
procedure the following equation provides the means for converting
the structural number into actual thicknesses of surfacing, base
and subbase materials:SN = a1 D1 + a2 D2 m2 + a3 D3 m3where:
a1, a2, a3=layer coefficients representative of surface, base
and subbase courses respectively
D1, D2, D3=actual thicknesses (in inches) of surface, base and
subbase courses respectively
m2, m3=drainage coefficients for base and subbase layers
respectively
4.4.3.8 Recommended Values of Layer Coefficients
Asphaltic Wearing Course, a1= 0.40 / inch(0.157 / cm)
Asphaltic Base Course, a1= 0.40 / inch (0.157 / cm)
Aggregate Base Course, a2= 0.13 / inch (0.051 / cm)
Granular Subbase, a3= 0.125 / inch (0.049 / cm)
4.4.3.9 Pavement Thickness
The pavement thicknesses thus worked out exploiting AASHTO
approach for pavement design are as under, subject to enforcement
of Load restrictions:
Table 4.6 Pavement Thickness for Road (ROW : 61 m)
LAYERLAYER THICKNESS (cm)
Asphaltic Concrete Wearing Course5
Asphaltic Concrete Base Course 12
Aggregate Base Course25
Subbase Course15
Subgrade CBR 14% at 95% MDD
Table 4.7 Pavement Thickness for Road (ROW : 36 m)
LAYERLAYER THICKNESS (cm)
Asphaltic Concrete Wearing Course5
Asphaltic Concrete Base Course 9
Aggregate Base Course25
Subbase Course15
Subgrade CBR 14% at 95% MDD
Table 4.8 Pavement Thickness for Road (ROW : 25 m and 18m)
LAYERLAYER THICKNESS (cm)
Asphaltic Concrete Wearing Course5
Asphaltic Concrete Base Course 8
Aggregate Base Course25
Subbase Course20
Subgrade CBR 14% at 95% MDD
GENERAL RECOMMENDATIONSThe geotechnical investigations revealed
that soil consist of Lean Clay, Sandy Lean Clay & Sandy Silty
Clay. For the construction of the sub grade soil with CBR value of
14 and 95% Modified AASHTO should be used. Whereas it would be
desirable to use materials with minimum CBR values of 50% and 80%
for sub-base and water bound macadam, respectively. For a roadway
to perform well, it is imperative that the subgrade for the roadway
should be competent to support the anticipated traffic loads. It
is, therefore recommended that the subgrade should be properly
prepared to meet the design CBR. In order to meet this requirement,
all the areas that will support roadway, should be properly cleared
and grubbed by removing any top soil. Any wet, soft or loose soils
pockets should also be replaced with improved soil, as the result
of proof-rolling.For layer thickness and compaction following
levels are recommended for various pavement elements.Material
TypeMaximum Compacted Layer Thickness (cm)Recommended Modified
AASHTO Compaction (%)
Water Bound Macadam10100
Sub-base1098
Sab-base & general fillUpper 30 cm (subgrade)30 cm-70 cm
(fill)Below 70 cm (fill)151515959390
4 - 2
i) Approach RoadsThe project site is adjacent to Motorway
M-2.Two approach roads have been proposed to provide access to the
apparel park. The one is proposed from existing bridge after 200m
from Upper Chenab (UC) canal and second from Sheikhupura
Interchange. Each approach road is about 2.5 km long. Approach
roads are shown in Figure. 4.2
Figure 4.2 Approach Road
Design of interchange and approach road are being carried out by
some other departments (C& W Punjab, NHA)
ii)Typical Cross SectionsRoad Cross-sections are shown in Figure
4.3 to Figure 4.5. Widths allocated to different components of the
roads along with the cross-section are provided in the Table
4.5.
Table 4.5 Cross Sectional Details of Roads
ROADTYPER.O.WWIDTH OF CARRIAGE WAYLANE WIDTHWIDTH OF
SHOULDERSWIDTH OF MEDIANWIDTH OFFOOTPATHWIDTH OF SERVICE ROADWIDTH
OF UTILITY CORRIDOR
(m)(m)(m)(m)(m)(m)(m)(m)
Approach Roads/Main Roads6110.83.627.4275
Secondary367.23.624.62-4.5
Tertiary253.63.62-2-4.9
150mm
Figure 4.3 Typical X-Section of 25m R.O.W150mm
150mm
Figure 4.4 Typical X-Section of 36m R.O.W
Figure 4.5 Typical X-Section of 61m R.O.W