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A structural design guide
for low volume secondary.
and feeder roads in
C S Gourley, P A K Greening and
Transport Research Laboratory
Berkshire RG45 6AU
United Kingdom PA3395198 PA3395/98 GOURLEY, C S, P A K GREENING and U BRUDEFORS.
A structural design guide for low volume secondary and feeder roads in Zimbabwe,
Secondary Feeder Roads Development Project (SFRDP) Workshop, Harare,
Zimb~abwe, June 1998. A STRUCTURAL DESIGN GUIDE FOR LOW VOLUME SECONDARY
AND FEEDER ROADS IN ZIMBABWE
C S Gourley', P A K Greening 2and U Brudefors'
I. Transport Research Laboratory, Old Wokingham Road, Crowthorne, Berks, RG45 6EA, UK
2. Transport Research Laboratory, P 0 Box CY38, Causeway, Harare, Zimbabwe
3. SweRoad, P 0 Box CY263, Causeway, Harare, Zimbabwe
Transport and its associated infrastructure is a key component in the development process. In rural
areas, provision of roads, especially surfaced roads, is always high on the list of the priorities requested
by local communities. One of the benefits of road provision for rural communities is that operators
more readily provide public and commercial transport services. This in turn offers the communities
greater mobility, better access to education and medical services, generates greater economic activity,
improves employment opportunities and ultimately offers local communities a better quality of life. This
improvement in access is extremely important as a high proportion of the agricultural capacity of the
country is carried out in these areas by small scale farmers. Transport provision is also needed to
enable movement and exploitation of the countries natural resources such as mining, timber, etc as well
as opening .up improved access to, attract tourist traffic. At present just over 70 percent of the MOT
secondary and feeder road network is unpaved. There is therefore considerable scope for improving
the well-being of the rural communities if ways can be found of surfacing these roads at an economic
In recognition of this the Department of Roads in Zimbabwe has in recent years been involved in a
number of research programmes specifically aimed at addressing the needs of the secondary and feeder
road network. The Sida funded Secondary and Feeder Road Development Programme and the DFID-
Sida funded TRL-SADC Regional Research Programme both comprised sizable components 'aimed at
deriving solutions to particular pavement materials, design and construction problems so that costs for
provision of these lower volume roads could be reduced.
In some countries, there has been a tendency to construct many secondary and feeder roads to the
structural design standards which emanated from studies in developed countries or to design standards
which are similar to the trunk road network. Zimbabwe, which has a long history of development
behind its current standard pavement and materials design procedures and has always recognised that
low volume rural roads need not be designed and constructed to the standards demanded of the trunk
road network. Examples of this include the use of strip roads and narrow mats, provision of design
classes at very low traffic levels e.g. 50,000 esa's, stage construction techniques, and the use of relaxed
materials specifications as traffic levels reduce. In recent years further innovation has been shown by
adoption of low cost sealing techniques, flexibility in geometric designs and the use of experimental
surfacings, such as the Otta seal.
The evidence of the recent research programmes shows that a further general relaxation in the
specifications for rural roads is achievable and that there are still further very large savings that can
be made on rural road projects.
1 2. RESEARCH PROGRAMMES
The strategy to develop and test revised design standards is usually achieved by construction of full-
scale trials where the number of variables is strictly controlled. Measurements of the performance of
these sections are made over a number of years through the design life by assessing the deterioration
that occurs in the road as a result of traffic or environment. Trials constructed at different locations
enables a range of typical environmental conditions to be included. This approach was adopted on the
SFRDP programme in Zimbabwe where 15 experimental trial sections, shown in Table 1, were
established on short sections of four secondary roads. All of these roads comprised natural gravel bases
with thin surfacings and were monitored regularly to evaluate the pavement deterioration. The
performance of these sections are described more fully in the Secondary and Feeder Road Development
Programme final report (SFRDP 1995).
An increasingly used and alternative method to the construction of individual trials is the collection of
data from existing roads. This approach was adopted on the recent DFID-SIDA funded TRL-SADC
Regional Research Programme on natural gravel road bases. Although the variables cannot be as
strictly controlled as with a full-scale trial, this approach has the advantage that data can be assimilated
quickly, providing roads are selected which cover a range of age, deterioration, structure, material and
subgrade type and environment. This recently completed study in southern Africa comprised a number
*network surveys in three countries in the region to determine the range of traffic, materials and
*collection of construction records for preliminary investigation of the pavement designs and materials
* field reconnaissance surveys, complemented by materials testing for final selection of short sections
of the road
*site marking, equipment installation and initial monitoring, comprising measurements of:
* in situ strength using Dynamic Cone Penetrometer (DCP)
* roughness using Merlin
* in situ moisture content and density of pavement layers using nuclear methods
* Deflection using a falling weight deflectometer
* Visual condition
* Geometry, drainage and levels
* Regular monitoring carried out twice a year at end of the wet and dry seasons.
A total of 57 sections on the road networks in Malawi, Botswana and Zimbabwe were established of
which 33 were on the Zimbabwe network, as shown in Table 1. A wide range of pavement designs and
material types were incorporated in the study with particular emphasis being made on the selection of
sections with sub-standard or `marginal` base materials. The base materials included lateritic gravels,
calcretes, weathered rocks (basic and acidic), quartz gravels, alluvial gravels and aeolian sands, in
addition to crushed stone and stabilised base control sections.
The data collected during the programme has been used to develop a series of structural design charts
to promote better use of natural gravel bases for low to medium volume rural roads throughout the
southern African region and were fuirther adapted to Zimbabwe conditions for the recently published
DoR-SFRDP "Guidelines to road design of low volume roads` (DoR 1997).
2 3. STRUCTURAL DESIGN CHARTS
Two thickness design charts are available, their use depending on the climatic area and whether a
sealed shoulder design is selected. The criteria for selecting the design chart to use are given in Figure
1. The pavement thickness designs given in Tables 2 and 3 are appropriate in the range 0.01 to 1
million cumulative esa's. The 3 million esa design class follows the recommendations given in Road
Note 31 (Transport Research Laboratory 1993) and is similar to that offered in the DoR pavement
design manual (DoR 1983). Table 4 outlines the materials requirements for the base materials with the
grading envelopes given in Table 5.
4. THE DEVELOPMENT OF STRUCTURAL DESIGN CHARTS
It is an important principle of developing a design chart that the performance of each structure is
associated with the actual field conditions that have been experienced by that structure. The behaviour
of road pavements is controlled very largely by the most adverse conditions that prevail, even if such
conditions occur for a relatively short time. Thus it is important to associate the performance of
individual roads with the most adverse conditions experienced by that road. In practice, this means
the weakest conditions experienced by the subgrade and, by implication, all the other pavement layers.
The spread of sites in the region which were used to develop the charts covered a reasonably wide
range of climates, which during the period of the study, experienced both very dry and very wet
Design charts can be presented to the user as a single design chart based on in-situ subgrade strength,
with the onus on the user to predict the in-situ strength based on soil type, climate, drainage conditions
and any other risk factors. This approach has advantages but the complexity and demand of the
procedures involved are usually beyond the scope of the facilities available to engineers. An. alternative
is to develop several design charts based on a subgrade classification test but varied to suit the different
climate, drainage conditions and so on. The large number of sites incorporated within the study
provided sufficient information for this approach to be feasible.
The laboratory test procedure used by most authorities in the region is the soaked CBR test which is
carried out on subgrade samples prepared under a standard set of conditions. Adopting this test
approach reduces the task to estimating a relationship between the soaked values and the in-situ CBR
of the subgrades. Examination of the data collected in southern Africa showed that in wet climates with
poor drainage, the most adverse in-situ conditions gave in-situ CBR values no worse than the laboratory
soaked values. Furthermore, sufficient examples were found in the region to tie down the performance
of low volume roads at various points in the inference space. In arid areas the in-situ CBR was found
to be about twice the value in wet areas. Structures that are known to work well under wet. and poorly
drained conditions (i.e. when the in-situ subgrade strength at the seasonally worst condition is similar
to that obtained in the standard laboratory soaked CBR test) will behave in a similar way on a subgrade
of half this strength in arid conditions. Therefore, as a first approximation, the design chart for arid
climates can be identical to that for wet climates except that the subgrade strength values in the standard
soaked CBR 'classification' test will be halved. This is equivalent to a shift of one subgrade category
in the chart because each category covers a CBR range of a factor of two (ie. 2-3, 4-7, 8-15, etc).
After climate, the next most important factor to influence road performance was found to be the
drainage conditions as measured by the height of the crown of the road above the invert of the drainage
3 ditch, and the distance of the outer wheel track from the edge of the sealed area. Until fairly recently,
the provision of sealed shoulders on low-volume roads would have been considered to be both
expensive and unnecessary. However, there is also a structural benefit from maintaining a drier
environment under the running surface. The resulting high strengths derived from the relatively dry
condition results in a stronger pavement. It also allows weaker materials to be used in the upper
pavement layers in situations where materials which satisfy conventional specifications are unavailable.
There is a whole-life benefit from the reduced maintenance alone, as well as a safety benefit from the
sealing of shoulders.
It was apparent from the data collected in the region that the subgrade strengths in the wheel tracks are
affected only marginally by the addition of sealed shoulders of 0.5 metres width whereas with sealed
shoulders of 1 metre or more, the subgrade strength is increased to about 2.5 times the worst case (wet
and poorly drained conditions) in arid and moderately wet conditions.
If the shoulders of the road are sealed to a sufficient width such that the outer wheel track is more than
1.5 metres from the edge of the sealed area, and the drainage is ensured by maintaining the crown
height greater than 0.75 metres above the ditch, a further improvement on the performance is possible.
For arid areas this is equivalent to a reduction in the subgrade strength requirement by a factor of 3,
in moderate climates the factor is 2.5 and for wet climates it is around 1.5 to 2. Where the factor is
a whole number, a simple shift in subgrade strength by one category is reasonable if sealed shoulders
are provided. If the shift is not a whole number, thickness interpolations become necessary.
Traditional design principles for the traffic factor rely on two assumptions:
a) the thickness design is sufficient to protect the subgrade from traffic 'fatigue' type failure such that
greater traffic means thicker structures.
b) the strength of the base is sufficient to prevent failures of any sort, i.e. the base is a zero risk design.
The evidence from the studies in Zimbabwe and the rest of the region indicates that marginal quality
base coarse materials have performed satisfactorily for low volume rural roads carrying typical rural
road traffic. As the traffic level increases, the specification for road bases should approach those of the
traditional design charts. Experience in the region indicates that this change of function occurs when
the traffic levels reach 0.5 million cumulative esa's and therefore the local design specifications or
Road Note 31 should be used at these higher traffic levels. The traffic category 0.5 million cumulative
esa's is a suitable intermediate level at which intermediate material specifications are used. If for any
reason, the functional use of roads at lower traffic levels differs from the basic assumption, eg. roads
serving a specific "heavy" but small industry such as mining or timber then either the road base
specifications can be tightened or, more simply, the next higher traffic category can be used for design
to reduce risks. At lower traffic levels, traffic induced failure is most unlikely and all of the
deterioration observed on the low volume roads in Zimbabwe was controlled mainly. by the
environment. Thus the thickness designs and material specifications have been devised to mitigate this
deterioration mechanism and the increases in thickness as traffic increases have been kept to a
minimum commensurate with gradual transitions to the thickness required at the higher traffic levels
where road function essentially changes.
4 5. GUIDELINES FOR BASE MATERIAL SELECTION
The materials design characteristics recommended for use with the design chart have been developed
using the laboratory and field data collected from the road sections in Zimbabwe, Malawi and Botswana
in addition to other information available from other pavement studies, including the SFRDP, and
published sources such as the CIRIA report on laterites (CIRIA 1988). The materials design follows
the format of the design chart and upper limits on material properties assigned to reflect traffic level,
poorer subgrades and the influence of climate.
As mentioned, very little evidence of traffic related distress has been found on the secondary roads in
Zimbabwe and in the road base layers studied. This enabled revisions to the current road base
specifications to be made.
The principles of the materials design are:-
i) The strength, plasticity and grading requirement varies depending on the traffic level and
ii) The soaked CBR test has been used to specify the minimum base material strength. The
compaction requirement for the test is 98 % Mod.A.ASHTO and the soaking time is a minimumn
of 4 days or until zero swell is recorded.
iii) Four grading envelopes (A,B,C and D) are used which depend on the traffic and subgrade
iv) The maximum plasticity index of the base also depends on the tr'ffic and subgrade design
class. A maximum value of 6 for the plasticity has been retained for higher traffic levels and
The wider grading envelopes allow the use of a much wider range of natural gravels including the more
commonly gap-graded materials such as laterites, ferricretes etc. Hlowever, to prevent excessive loss
in stability a requirement of 5 to 10% retained on successive sieves has been specified. Envelope C
applies only to dry climates and extends the upper limit of B to allow the use of calcareous materials
and Kalahari sands, common in the western and southern provinces.. Envelope D for very low traffic
volumes is essentially a gravei wearing course specification which is given in terms of a grading
modulus range. A number of roads in Zimbabwe and the rest of the region, which were constructed
to gravel wearing course standards were sealed after construction and have performed well.
6. ADAPTION TO ZIMBABWE CONDITIONS
The general principles of the structural design and pavement materials guidelines outlined above have
been further adapted for Zimbabwe by DoR-SFRDP in the "Guidelines to road design for low volume
roads". These guidelines generally follow the DoR notation (DoR 1983) for subgrade design, subgrade
treatment and traffic class but do include a number of modifications. The pavement design guidelines
(DoR 1997) provide guidance on the design approach for low volume roads and should not be
construed as being a prescriptive manual or specification. The message throughout is for the engineer
to be more flexible in his approach to the design standards used for low volume roads and to move
away from the rigidity of design manuals. Considerations for relaxing standards need not apply to the
whole road length but there may be sections where such changes in approach are justified.
5 6.1 Two new class traffic classes are introduced at