Chapter - 8 Highway Ii R 4
Chapter - 8 Highway Ii R 4
Chapter - 8 Highway Ii R 4
STRUCTURAL
DESIGN OF
PAVEMENTS
All the charts provide alternate pavement structures for all subgrade
classes (S1 through S6). They are not however suitable for all classes of
traffic, as some structures would be neither technically appropriate nor
economically justified.
HIGHWAY II - Structural Design of Pavements
STRUCTURAL DESIGN PAVEMENTS
Method of Flexible Pavement Design
4
Design Example
An example of traffic calculations was given in Chapter 3 for a
particular section of a trunk road. In the example, a traffic
class T8 has been derived (with a total of ESAs on the order of
20millions over the design period).
From Table 8-1 given above, for that class of traffic, it is
readily apparent that the use of the design charts in the
catalog of structures is narrowed down to Charts 4 through 7.
From the same table, without further information regarding
the subgrade and the materials, it would also appear that any
type of surfacing is possible, as well as several types of
roadbase.
The subgrade strength has been represented by CBRs of 5 to 7
and class to be assigned to this project is therefore S3.
HIGHWAY II - Structural Design of Pavements
STRUCTURAL DESIGN PAVEMENTS
Method of Flexible Pavement Design
8
Design Principles
STEPS TO BE CONSIDERED IN THE DESIGN PROCESS
1. Traffic (Baseline flow and forecast)
2. Material and geotechnical information (Field survey and
material properties)
3. Subgrade (Classification, foundation for expansive soils
and material strength)
4. Thickness design (Gravel wearing coarse thickness)
5. Materials design
ALL-WEATHER ACCESS
An essential consideration in the design of gravel
roads is to ensure all-weather access.
This requirement places particular emphasis on the
need for
sufficient bearing capacity of the pavement structure and
provision of drainage and sufficient earthworks in flood
or problem soil areas (e.g. black cotton).
SURFACE PERFORMANCE
The performance of the gravel surface mainly depends on
material quality,
the location of the road, and
The volume of traffic using the road.
MAINTENANCE
The material requirements for the gravel wearing course
include provision of a gravel surface that is effectively
maintainable.
Adherence to the limits on oversize particles in the material
is of particular importance in this regard and will normally
necessitate the use of crushing or screening equipment
during material production activities.
Design Method
The required gravel thickness shall be determined as
follows:
Determine the minimum thickness necessary to avoid
excessive compressive strain in the subgrade (D1).
Determine the extra thickness needed to compensate for
the gravel loss under traffic during the period between
regravelling operations (D2).
Determine the total gravel thickness required by adding
the above two thicknesses (D1+ D2).
GRAVEL LOSS
According to TRL Laboratory Report 673, an estimate of the
annual gravel loss is given by the following equation:
GL = f T2 / (T2 + 50) (4.2 + 0.092 T + 3.50 R2 + 1.88V)
Where
GL = the annual gravel loss measured in mm
T = the total traffic volume in the first year in both directions, measured in
thousands of vehicles
R = the average annual rainfall measured in m
V = the total (rise + fall ) as a percentage of the length of the road
f = 0.94 to 1.29 for lateritic gravels
= 1.1 to 1.51 for quartizitic gravels
= 0.7 to 0.96 for volcanic gravels (weathered lava or tuff)
= 1.5 for coral gravels
= 1.38 for sandstone gravels
Material Requirements
EXPERIENCE WITH LOCALMATERIALS
Knowledge of past performance of locally occurring materials
for gravel roads is essential.
Material standards may be altered to take advantage of
available gravel sources provided they have proved to give
satisfactory performance under similar conditions.
MARGINALMATERIALS
Figure 8-1 illustrates the performance characteristics to be
expected of materials that do not meet the requirements for
gravel wearing course.
Refinements and amendments of the standard material
specification may be necessary to overcome problem areas
such as towns (dust nuisance) or steep hills (slipperiness)
HIGHWAY II - Structural Design of Pavements
STRUCTURAL DESIGN PAVEMENTS
DESIGN OF GRAVEL AND LOW STANDARD ROADS
29
MATERIAL CHARACTERISTICS
Soils used for improved subgrade layers shall be non-expansive,
non-dispersive and free from any deleterious matter.
They shall comply with the requirements shown in Table 8-5.
STRUCTURAL DESIGN PAVEMENTS
DESIGN OF GRAVEL AND LOW STANDARD ROADS
35
Type 2 & 3
These materials may be more rounded particles fulfilling:
The Plasticity Index lies in a range of 5-12% in wet areas, and in any case
less than 16% in other areas
The materials have the sanction of local experience
Use of more rounded particles may allow the use of river gravel.
Trials should nevertheless be conducted to verify whether
crushing occurs under traffic or whether crushing should be
considered prior to use.
Subject to trials, a minimum percentage by weight of particles
with at least one fractured face of 40% may be considered. This
requirement may also be expressed in terms of crushing ratio.
Except for very low traffic, CBR should be in excess of 20 after 4
days of soaking at 95% of max. dry density under Heavy
Compaction.
For very low traffic, it may be relaxed to a CBR of 15.
HIGHWAY II - Structural Design of Pavements
STRUCTURAL DESIGN PAVEMENTS
DESIGN OF GRAVEL AND LOW STANDARD ROADS
38
Type 4
This material gradation allows for larger size material
and corresponds to the gradation of a base course
material.
The use of this gradation of materials is subject to the
local experience and shall be used with PIs in a range of
10-20.
Type 5 & 6
These materials gradations are recommended for smaller
size particles.
They may be used if sanctioned by experience with
plasticity characteristics as for material Type 1.
HIGHWAY II - Structural Design of Pavements
STRUCTURAL DESIGN PAVEMENTS
DESIGN OF GRAVEL AND LOW STANDARD ROADS
39
Table 8-6
STRUCTURAL DESIGN PAVEMENTS
DESIGN OF GRAVEL AND LOW STANDARD ROADS
40
HOMOGENOUS SECTIONS
Identification of sections deemed to have homogenous
subgrade conditions is carried out by desk studies of
appropriate documents such as geological maps, followed
by site reconnaissance that includes excavation of
inspection pits and initial indicator testing for
confirmation of the site observations.
Due regard for localized areas that require individual
treatment is an essential part of the site reconnaissance.
Demarcation of homogenous sections shall be reviewed
and changed as required when the CBR test results of the
centerline soil survey are available.
STATISTICAL ANALYSIS
The flow chart in Figure 8-2 shows the procedure to
determine CBRdesign.
The CBRdesign for cuttings is the lowest CBR value encountered
for the homogenous section.
The CBRdesign for sections that do not require special
assessment or are not within cuttings are determined by the
90%-ile value of the CBR test results.
The 90%-ile value for a section of this type is the CBR value
which 10% of the test results fall below.
The following example shows how this is calculated.
CBR values are plotted in ascending order (number of tests on the "x
axis" and the CBR test result values on the "y axis");
Calculate d = 0.1 x (n-1), where n = number of tests;
d is measured along the "x axis" and the CBRdesign is determined from the
"y axis".
HIGHWAY II - Structural Design of Pavements
STRUCTURAL DESIGN PAVEMENTS
DESIGN OF GRAVEL AND LOW STANDARD ROADS
45
The following example shows how this is calculated.
CBR values are plotted in ascending order (number of tests on the "x
axis" and the CBR test result values on the "y axis");
Calculate d = 0.1 x (n-1), where n = number of tests;
d is measured along the "x axis" and the CBRdesign is determined from
the "y axis".
STRUCTURAL DESIGN PAVEMENTS
DESIGN OF GRAVEL AND LOW STANDARD ROADS
46
LABORATORY TESTING
Each CBR value shall be determined in laboratory carried out for
a minimum of 3 density values to give a CBR - Density
relationship at the normal field density specified for the
respective operation.
Figure 8-3: Pavement and Improved Subgrade for Gravel Roads for ADDTs < 200
THANK YOU