ROAD DESIGN MANUAL FOR ROADS
AND BRIDGES
Part III: Materials and Pavement Design
for New Roads
April 2016
,ROAD DESIGN MANUAL PART 3 – Materials and Pavement Design for New Roads
1 GENERAL ........................................................................................................ 1-1
1.1 Introduction ................................................................................................ 1-1
1.2 Design principles ...................................................................................... 1-2
1.3 Definitions and abbreviations ................................................................. 1-2
1.3.1 Pavement............................................................................................ 1-2
1.3.2 General materials .............................................................................. 1-4
1.3.3 Bituminous materials ........................................................................ 1-5
1.3.4 Traffic .................................................................................................. 1-6
Figure 1-1: Cross section terminology ..................................................................... 1-3
Figure 1-2: Pavement terminology ........................................................................... 1-3
1-1
Ministry of Transport and Infrastructure, Republic of Kenya April 2016
,ROAD DESIGN MANUAL PART 3 – Materials and Pavement Design for New Roads
1 GENERAL
1.1 Introduction
The Road Design Manual sets forth the policy and standards to be adopted for the
design of roads in Kenya. The manual is divided into six parts:
Part 1 (a) Geometric Design
Part 1 (b) Road Safety Audits
Part 1 (c) Traffic Surveys
Part 2 – Drainage
Part 3 – Materials and Pavement Design for New Roads
Part 4 – Pavement Rehabilitation and Overlay Design
Part 5 (a) Road Signs
Part 5 (b) Road Markings
Part 5 (c) Traffic Signals
Part 6 Bridge and Culvert Design
This manual makes particular reference to the conditions and materials in Kenya.
It adopts an integrated approach in that full details of possible materials for the
pavement layers are given as well as the details as to how these may be combined,
and in what thicknesses to form a satisfactory pavement for a given traffic volume.
The pavement designs are given in the form of catalogues.
This manual gives recommendations for the structural design of ‘flexible’ pavements
in Kenya. The definition of a flexible pavement is simply a pavement that does not
include a layer of high strength concrete. Thus ‘flexible pavements’ include
pavements with unbound granular aggregate layers and pavements with aggregate
layers that are bound together with bitumen. It also includes pavements that may
contain layers of aggregate that are bound together (or stabilised) with hydraulic
binders such as cement and lime, but with relatively low levels of binder.
Pavements which include a layer of high strength Portland cement concrete are
called ‘rigid’ pavements and are designed on different principles. The design of rigid
pavements is also covered in this manual.
Gravel or ‘unpaved’ roads are also a form of flexible construction. Their design is
similar to that of other flexible structures but the gravel itself wears away, depending
on traffic, rainfall and terrain, hence additional material is required to make sure that
the gravel is always thick enough. The design of gravel roads is also dealt in this
manual.
A chapter on the design of low volume sealed roads is also included in the present
manual.
The contents of this Manual are partly guidelines and recommendations to be
considered, and partly standards which as a general rule should be adhered to. In
some instances special conditions may demand modifications to these standards.
Such instances will usually be brought to the attention of the designer by the Chief
Engineer Roads. However, it is also the responsibility of the designer to put forward
1-1
Ministry of Transport and Infrastructure, Republic of Kenya April 2016
, ROAD DESIGN MANUAL PART 3 – Materials and Pavement Design for New Roads
any proposals for modifications to the standards which he considers will result in a
better and more economical design.
The design engineer should also realise that the adoption of the guidelines and
standards given in this Manual does not automatically ensure a serviceable and
economic road design. This can only be achieved through a careful consideration
and balancing of the various controls, criteria and elements involved.
The Road Des1gn Manual forms a Part of the aforementioned set of manuals, some
of which have a bearing on design. These are:
Part 1 (b) Road Safety Audits
Part 1 (c) Traffic Surveys
Part 5 (a) Road Signs
Part 5 (b) Road Markings
Part 5 (c) Traffic Signals
1.2 Design principles
Road pavements are designed to limit the vertical strain imposed at the subgrade
level as a result of traffic that is travelling on the pavement surface thus preventing
significant deformations of the subgrade. The pavement spreads the concentrated
loads of the vehicle wheels over a sufficiently large area at subgrade level. At the
same time, the pavement materials themselves should not deteriorate to any serious
extent within a specified period of time.
However, it is inevitable that road pavements will deteriorate with time and traffic,
therefore, the goal of pavement design is to limit, during the period considered, the
deterioration which affects the riding quality of the road, such as rutting, cracking,
potholes and other such surface distresses, to acceptable levels.
At the end of the design period, a strengthening overlay would normally be required
but other remedial treatments, such as major rehabilitation or reconstruction, may be
needed (please refer to Part 4 - Pavement Rehabilitation and Overlay Design). The
design method aims at producing a pavement which will reach a relatively low level
of deterioration at the end of the design period, assuming that routine and periodic
maintenance are performed during that period.
An ‘acceptable’ riding quality depends on a match between what the users expect
and what the highway authority (and hence the government) is prepared to provide.
For roads with high traffic levels designed with high geometric standards (and higher
vehicle speeds as a consequence), less distress will be expected or be considered
acceptable. Hence trunk and link roads are expected to offer a higher standard of
rideability than those carrying lower levels of traffic such as collector roads, access
roads etc.. These differences are implicitly considered in the design, although in
broad terms rather than in precise, measurable economic terms.
1.3 Definitions and abbreviations
1.3.1 Pavement
Figures 1.3.1 and 1.3.2 illustrate the terms used in describing the principal pavement
and cross-section components.
1-2
Ministry of Transport and Infrastructure, Republic of Kenya April 2016
AND BRIDGES
Part III: Materials and Pavement Design
for New Roads
April 2016
,ROAD DESIGN MANUAL PART 3 – Materials and Pavement Design for New Roads
1 GENERAL ........................................................................................................ 1-1
1.1 Introduction ................................................................................................ 1-1
1.2 Design principles ...................................................................................... 1-2
1.3 Definitions and abbreviations ................................................................. 1-2
1.3.1 Pavement............................................................................................ 1-2
1.3.2 General materials .............................................................................. 1-4
1.3.3 Bituminous materials ........................................................................ 1-5
1.3.4 Traffic .................................................................................................. 1-6
Figure 1-1: Cross section terminology ..................................................................... 1-3
Figure 1-2: Pavement terminology ........................................................................... 1-3
1-1
Ministry of Transport and Infrastructure, Republic of Kenya April 2016
,ROAD DESIGN MANUAL PART 3 – Materials and Pavement Design for New Roads
1 GENERAL
1.1 Introduction
The Road Design Manual sets forth the policy and standards to be adopted for the
design of roads in Kenya. The manual is divided into six parts:
Part 1 (a) Geometric Design
Part 1 (b) Road Safety Audits
Part 1 (c) Traffic Surveys
Part 2 – Drainage
Part 3 – Materials and Pavement Design for New Roads
Part 4 – Pavement Rehabilitation and Overlay Design
Part 5 (a) Road Signs
Part 5 (b) Road Markings
Part 5 (c) Traffic Signals
Part 6 Bridge and Culvert Design
This manual makes particular reference to the conditions and materials in Kenya.
It adopts an integrated approach in that full details of possible materials for the
pavement layers are given as well as the details as to how these may be combined,
and in what thicknesses to form a satisfactory pavement for a given traffic volume.
The pavement designs are given in the form of catalogues.
This manual gives recommendations for the structural design of ‘flexible’ pavements
in Kenya. The definition of a flexible pavement is simply a pavement that does not
include a layer of high strength concrete. Thus ‘flexible pavements’ include
pavements with unbound granular aggregate layers and pavements with aggregate
layers that are bound together with bitumen. It also includes pavements that may
contain layers of aggregate that are bound together (or stabilised) with hydraulic
binders such as cement and lime, but with relatively low levels of binder.
Pavements which include a layer of high strength Portland cement concrete are
called ‘rigid’ pavements and are designed on different principles. The design of rigid
pavements is also covered in this manual.
Gravel or ‘unpaved’ roads are also a form of flexible construction. Their design is
similar to that of other flexible structures but the gravel itself wears away, depending
on traffic, rainfall and terrain, hence additional material is required to make sure that
the gravel is always thick enough. The design of gravel roads is also dealt in this
manual.
A chapter on the design of low volume sealed roads is also included in the present
manual.
The contents of this Manual are partly guidelines and recommendations to be
considered, and partly standards which as a general rule should be adhered to. In
some instances special conditions may demand modifications to these standards.
Such instances will usually be brought to the attention of the designer by the Chief
Engineer Roads. However, it is also the responsibility of the designer to put forward
1-1
Ministry of Transport and Infrastructure, Republic of Kenya April 2016
, ROAD DESIGN MANUAL PART 3 – Materials and Pavement Design for New Roads
any proposals for modifications to the standards which he considers will result in a
better and more economical design.
The design engineer should also realise that the adoption of the guidelines and
standards given in this Manual does not automatically ensure a serviceable and
economic road design. This can only be achieved through a careful consideration
and balancing of the various controls, criteria and elements involved.
The Road Des1gn Manual forms a Part of the aforementioned set of manuals, some
of which have a bearing on design. These are:
Part 1 (b) Road Safety Audits
Part 1 (c) Traffic Surveys
Part 5 (a) Road Signs
Part 5 (b) Road Markings
Part 5 (c) Traffic Signals
1.2 Design principles
Road pavements are designed to limit the vertical strain imposed at the subgrade
level as a result of traffic that is travelling on the pavement surface thus preventing
significant deformations of the subgrade. The pavement spreads the concentrated
loads of the vehicle wheels over a sufficiently large area at subgrade level. At the
same time, the pavement materials themselves should not deteriorate to any serious
extent within a specified period of time.
However, it is inevitable that road pavements will deteriorate with time and traffic,
therefore, the goal of pavement design is to limit, during the period considered, the
deterioration which affects the riding quality of the road, such as rutting, cracking,
potholes and other such surface distresses, to acceptable levels.
At the end of the design period, a strengthening overlay would normally be required
but other remedial treatments, such as major rehabilitation or reconstruction, may be
needed (please refer to Part 4 - Pavement Rehabilitation and Overlay Design). The
design method aims at producing a pavement which will reach a relatively low level
of deterioration at the end of the design period, assuming that routine and periodic
maintenance are performed during that period.
An ‘acceptable’ riding quality depends on a match between what the users expect
and what the highway authority (and hence the government) is prepared to provide.
For roads with high traffic levels designed with high geometric standards (and higher
vehicle speeds as a consequence), less distress will be expected or be considered
acceptable. Hence trunk and link roads are expected to offer a higher standard of
rideability than those carrying lower levels of traffic such as collector roads, access
roads etc.. These differences are implicitly considered in the design, although in
broad terms rather than in precise, measurable economic terms.
1.3 Definitions and abbreviations
1.3.1 Pavement
Figures 1.3.1 and 1.3.2 illustrate the terms used in describing the principal pavement
and cross-section components.
1-2
Ministry of Transport and Infrastructure, Republic of Kenya April 2016
