CE413 Highway Eng II[1]

8/12/2019 CE413 Highway Eng II[1]

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KYAMBOGO UNIVERSITY

CE 413HighwayEngineering IILecture NotesF.E. Okello

8/23/2010

The notes deal with the design of flexible pavements based on the TRL Standard together

with aspects of drainage design based on the Ministry of Works and Transport Guidelines


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Kyambogo University | P. O. Box 1, Kampala-UgandaCE413 Highway Engineering II, FEO- 2010. E-mail: [email protected]. Mobile No.: (256) 712 806514, (256) 701 806514

i

iPreamble

Preamble

Global developments in highway engineering are evolving at an incredible speed. New

materials, planning and design concepts together with radically distinctive construction

practices are emerging as road users demand high quality roads. Following the recent

restructuring of the Ministry of Works and Transport, Uganda is due to modernise its road

transport system with subsequent construction of high speed highway facilities. Highway

Engineering IIis hereby presented as part of the comprehensive courses taught in Kyambogo

University in partial fulfilment of the award of the Bachelor of Engineering in Civil and

Building Engineering of Kyambogo University. This course is delivered in the second

semester of fourth year and carries a credit unit of four (4) with a total of 60 contact hours. It

introduces and examines the major principles and practices encountered during the Flexible

Pavement Design Processof highway systems. The main Objective of the course is to impart

and equip the student with the values, knowledge and skills necessary to design a flexible

pavement. By the end of the course the candidate should be able to: Survey possible routes,

assess traffic flow, measure subgrade strength, select pavement materials and select the

appropriate pavement structures required for flexible pavement construction. The authors hope

is that this revised edition of the notes will be simpler to read, revise, comprehend and apply to

the daily challenges that are faced by the student and the practicing Highway Engineer.

May the Almighty God richly bless you!

F.E. Okello


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ii

iiTable of Contents

Table of Contents

Preamble ....................................................................................................................................... i

Table of Contents ......................................................................................................................... ii

List of Tables .............................................................................................................................. iv

List of Design Charts .................................................................................................................. vi

Acronyms ................................................................................................................................... vii

Chapter One: ................................................................................................................................ 1

Introduction to Pavement Design................................................................................................. 1

General ......................................................................................................................................... 1

1.1 Types of Pavements .......................................................................................................... 2

1.2 Elements of a Flexible Pavement ...................................................................................... 3

1.3 Overview of the Design Process ....................................................................................... 5

1.4 Design Approaches ........................................................................................................... 7

1.5 Design Standards .............................................................................................................. 7

1.6 Questions........................................................................................................................... 8

1.7 Bibliography ..................................................................................................................... 8

Chapter Two: ............................................................................................................................. 10

Traffic Assessment..................................................................................................................... 10

2.1 General ............................................................................................................................ 10

2.2 Estimation of Traffic Flows (F) ...................................................................................... 10

2.3 Determination of Cumulative Standard Axles (T) .......................................................... 18

2.4 Example 2.1: Traffic Assessment ................................................................................... 19

2.5 Questions......................................................................................................................... 212.6 Bibliography ................................................................................................................... 23

Chapter Three: ........................................................................................................................... 24

Subgrade Strength Assessment .................................................................................................. 244.1 General ............................................................................................................................ 24

4.2 Climatic Regime ............................................................................................................. 24

4.3 Testing Subgrade Soils ................................................................................................... 25

4.4

Example 3.1: DCP Test................................................................................................... 374.5 Defining Uniform Sections ............................................................................................. 39

4.6 Design of Earth Works .................................................................................................... 39

4.7 Questions......................................................................................................................... 45

4.8 Bibliography ................................................................................................................... 48

Chapter Four: ............................................................................................................................. 49

Selection of Pavement Materials ............................................................................................... 49

5.1 General ............................................................................................................................ 49

5.2 Unbound Pavement Materials ......................................................................................... 49

5.3 Bitumen Bound Pavement Materials .............................................................................. 57

5.4 Bituminous Surfacings .................................................................................................... 63

5.5 Bituminous Roadbases .................................................................................................... 70

5.6

Surface Dressing ............................................................................................................. 73

5.7 Example 4.1: Surface Dressing ....................................................................................... 86

5.8 Questions......................................................................................................................... 89

5.9 Bibliography ................................................................................................................... 92

Chapter Five: .............................................................................................................................. 93

The Structure Catalogue ............................................................................................................ 93

5.1 Basis for the Structure Catalogue ................................................................................... 93

5.2 How to use the Structure Catalogue................................................................................ 93

5.3 Key to Structural Catalogue ............................................................................................ 95

5.4 Pavement Design Charts ................................................................................................. 96


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iii

iiiTable of Contents

5.5 Questions....................................................................................................................... 104

5.6 Bibliography ................................................................................................................. 104

Chapter Six: ............................................................................................................................. 105

Highway Drainage ................................................................................................................... 105

6.1 General .......................................................................................................................... 105

6.2 Main functions of Drainage .......................................................................................... 105

6.3 Highway Drainage Terminologies ................................................................................ 105

6.4

Surface Drainage ........................................................................................................... 1066.5 Sub-Surface Drainage ................................................................................................... 111

6.6 Cross Drainage .............................................................................................................. 112

6.7 Culverts ......................................................................................................................... 113

6.8 Questions....................................................................................................................... 119

6.9 Bibliograhy ................................................................................................................... 120Chapter Seven: ......................................................................................................................... 122

Conclusion ............................................................................................................................... 122

7.1 Road Deterioration ........................................................................................................ 1227.2 Economic Considerations ............................................................................................. 122

7.3 Effects of Climate ......................................................................................................... 123

5.7 Variability in Material Properties and Road Performance ............................................ 123


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iv

ivList of Tables

List of Tables

Table 2.1: Selection of Design Life ........................................................................................... 17

Table 2.2: Axle Factor Multipliers............................................................................................. 19

Table 2.3: Traffic Classes .......................................................................................................... 19

Table 2.4: Two-Way Traffic Volume at the Project Road (in vehicles/day) ............................. 19

Table 2.5: Axle Weights (in tonnes) .......................................................................................... 19

Table 2.6: Axle Loads (in tons) ................................................................................................. 21

Table 2.7: Friday 24-hr count summary .................................................................................... 22

Table 2.8: Sunday 24-hr count summary ................................................................................... 22

Table 2.9: 16-hr counts on the remaining days .......................................................................... 22

Table 3.1: Water Table Correction Factors for Soil Type PI ..................................................... 26

Table 3.2: Subgrade Strength Classes ........................................................................................ 28

Table 3.3: Number of blows required in the compaction test .................................................... 29

Table 3.4: Typical CBR Values ................................................................................................. 29

Table 3.5: CBR Preparation Sheet ............................................................................................. 31

Table 3.6: CBR Penetration Sheet ............................................................................................. 32

Table 3.7: DCP Field Results..................................................................................................... 37

Table 4.1: Properties of Unbound Materials .............................................................................. 49

Table 4.2: Grading Limits for crushed stone base materials (GB1,A; GB1,B) ......................... 51

Table 4.3: Mechanical strength requirements for the aggregate fraction of crushed stone

roadbases (GB1,A; GB1,B) as defined by the Ten Percent Fines Test ..................................... 51

Table 4.4: Typical Coarse aggregate gradings for Dry-bound (GB2,A) and Water-bound

Macadam (GB2,B) ..................................................................................................................... 53Table 4.5: Recommended Particle size distribution for mechanically stable natural gravels and

weathered rocks for use as roadbases (GB3) ............................................................................. 53

Table 4.6: Recommended Plasticity characteristics for granular Sub-bases (GS) ..................... 56Table 4.7: Typical Particle Size distribution for sub-bases (GS) which meet strength

requirements ............................................................................................................................... 56

Table 4.8: Coarse Aggregate for Bituminous mixes .................................................................. 61

Table 4.9: Fine Aggregate for Bituminous Mixes ..................................................................... 62Table 4.10: Asphaltic Concrete Surfacings ............................................................................... 66

Table 4.11: Suggested Marshall Test Criteria ............................................................................ 66

Table 4.12: Voids in Mineral Aggregate (VMA) ...................................................................... 67

Table 4.13: Bitumen Macadam Surfacings ................................................................................ 68

Table 4.14: Suggested Marshall Criteria for Close Graded Bitumen Macadams or DBMs ...... 68

Table 4.15: Hot Rolled Asphalt (HRA) Surfacings ................................................................... 69

Table 4.16: Bituminous Macadam Roadbase ............................................................................ 71

Table 4.17: Rolled Asphalt Roadbase ........................................................................................ 71

Table 4.18: Job-mix Tolerances ................................................................................................. 73

Table 4.19: Manufacturing and rolling temperature (in degrees centigrade) ............................ 73

Table 4.20: Category of Road Surface Hardness ....................................................................... 78

Table 4.21: Traffic Categories for Surface Dressing ................................................................. 78

Table 4.22: Recommended maximum chipping size (mm) ....................................................... 78

Table 4.23: Condition Constants for determining the rate of application of Binder ................. 83

Table 4.24: Typical Bitumen Spray Rate Adjustment Factors .................................................. 85

Table 5.1: Summary of Material Requirements for the Design Charts ..................................... 94

Table 6.1: Run off coefficient for the rational method ............................................................ 108

Table 6.2: Run off coefficient for the rational method ............................................................ 110

Table 6.3: Maximum Permissible velocity in open channels .................................................. 110

Table 6.4: Mannings n Values ................................................................................................ 111

Table 6.5: Entrance loss Coefficient (Outlet Control, Full or Partially full) ........................... 118


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v

vList of Tables

List of FiguresFigure 1.1: Definition of Pavement layers ................................................................................... 3

Figure 1.2: The Pavement Design Process .................................................................................. 6

Figure 2.1: Form for Manual Classified Counts ........................................................................ 12Figure 2.2: Axle Load Survey Form A for recording vehicle survey data ................................ 15

Figure 2.3: Axle Load Survey Form B for recording vehicle wheel loads ................................ 16

Figure 3.1: CBR Test Machine .................................................................................................. 30Figure 3.2: Dynamic Cone Penetrometer in use ........................................................................ 34

Figure 3.3: The TRL Dynamic Cone Penetrometer ................................................................... 35

Figure 3.4: DCP-CBR relationships .......................................................................................... 36

Figure 3.5: The DCP Test Result ............................................................................................... 36

Figure 3.6: Defining Uniform Sections ..................................................................................... 39

Figure 3.7: Dry density moisture content relationships for a gravel-sand-clay ...................... 43

Figure 4.1: Type of Surface Dressing ........................................................................................ 76

Figure 4.2: Surface temperature/choice of binder for surface dressings ................................... 81

Figure 4.3: Determination of average Least Dimension ............................................................ 82

Figure 4.4: Surface Dressing Design Chart ............................................................................... 84

Figure 6.1: Road Drainage features ......................................................................................... 106

Figure 6.2: Nomograph for the Calculation of Headwater Depth with Inlet Control .............. 116

Figure 6.3: Headwater Losses for Concrete Pipe Culverts Flowing Full ................................ 117

Figure 6.4: Headwater Losses for Concrete Pipe Culverts Flowing Full ................................ 118


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vi

viList of Design Charts

List of Design Charts

Chart 1: Granular Roadbase / Surface Dressing ........................................................................ 96

Chart 2: Composite Roadbase (Unbound & cemented) / Surface Dressing .............................. 97

Chart 3: Granular Roadbase / Semi-Structural Surface ............................................................. 98

Chart 4: Composite Roadbase / Semi-Structural Surface .......................................................... 99

Chart 5: Granular Roadbase / Structural Surface ..................................................................... 100

Chart 6: Composite Roadbase / Structural Surface .................................................................. 101

Chart 7: Bituminous Roadbase / Semi-Structural Surface ....................................................... 102

Chart 8: Cemented Roadbase / Surface Dressing .................................................................... 103


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vii

viiAcronyms

Acronyms

AADT Annual Average Daily Traffic

AASHTO American Association of State Highways and Transportation Officials

ADT Number of average daily traffic

ALD Average Least Dimension

CBR California Bearing Ratio

ESA Equivalent Standard Axle

GB3 Granular Base-material type 3

GIS Graphical Information Systems

HW Allowable Headwater depth

KUTIP Kampala Urban Transportation plan

LL Liquid Limit

LS Linear Shrinkage

M.S.A Millions of equivalent standard axle

MC Moisture Content

MDD Maximum Dry Density

OMC Optimum Moisture Content

ORN Overseas Road Note

PI Plasticity Index

PL Plastic Limit

TRRL Transport Road Research Laboratory

TW Tailwater depth


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1

1Chapter One:

Chapter One:

Introduction to Pavement Design

General

A road is a path established over land for the passage of vehicles, people, and animals.

Roads provide dependable pathways for moving people and goods from one place to

another. They range in quality from dirt paths to concrete-paved multilane highways.

Roads are used by various forms of transportation, such as trucks, automobiles, buses,

motorcycles, and bicycles. Roads allow trucks to move goods from points of production,

such as fields and factories, directly to markets and shopping centres. Private individuals

rely on roads for safe and efficient automobile, motorcycle, and bicycle travel. Fire

departments, medical services, and other government agencies depend on an organized

system of roads to provide emergency services to the public in times of need [Urbanik,

2007].

There are many different types of roads, from multilane freeways and expressways to two-

way country roads. One important quality of a road is known as control of access. This

term describes how vehicles are allowed to enter and exit a road. By controlling access to

a road, the road can support more traffic at higher speeds. Roads can be classified into

three broad categories: highways, urban or city streets, and rural roads. Each type of road

controls access to different degrees. Each type also differs in location, the amount of

traffic it can safely support, and the speed at which traffic can safely travel.

To support heavy vehicles moving at high speeds, a modern road is made up of severallayers. Each layer helps the layers above it support the weight and pressure of moving

traffic. Roads that carry more traffic at higher speeds, like highways, are built to stronger

standards than roads that carry less traffic, such as rural collector roads. The number of

layers in a road often depends on the intended use of the road, but generally roads have

three distinct layers. From bottom to top, the layers are the subgrade (or roadbed), the

roadbase (or base course), and the Surfacing (or wearing course). These layers form part

of what is known as the pavement structure of a road [Urbanik, 2007].

Road traffic is carried by the pavement, which in engineering terms is a horizontal

structure consisting of superimposed layers of selected and processed material supported

by in-situ natural material.

The task before the pavement designer normally entails the development of the most

economical combination of layers that will guarantee adequate dispersion of the incident

wheel stresses so that each layer in the pavement does not become overstressed during the

design life of the highway. The major variables in design of a highway pavement are: the

type of vehicles in the traffic stream, the volume of traffic predicted to use the highway

over its design life, the strength of the underlying subgrade, the material contained within

each layer of the pavement and the thickness of each layer in the pavement [Rogers,

2003].


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2

2Types of Pavements

In order for the pavement to support the design year traffic without exceeding its support

capacity existing records must be examined, possible routes surveyed, traffic flow

assessed, subsurface explorations conducted and appropriate materials selected. Based on

this information, a pavement structure can then be selected from a structure catalogue, the

maximum slopes for embankments and cuttings established, the degree of compaction to

be achieved during construction determined, and drainage needs specified before the

construction process is undertaken.

Before adopting any material for use within the pavement structure, the engineering

properties of the local rock and soil are established, particularly with respect to strength,

stiffness, durability, susceptibility to moisture, and propensity to shrink and swell over

time. The relevant properties are determined by either field tests, empirical estimates

based on soil type, or by laboratory measurements. The material is tested in its weakest

expected condition, usually at its highest moisture content. Probable performance under

traffic is then determined, soils unsuitable for the final pavement are identified for removal

and suitable replacement materials are reserved for use on the pavement [TRL, 1993].

1.1

Types of PavementsPavements are called either flexible or rigid depending on their relative flexural stiffness.

Two main types of pavements are usedbituminous, or flexible, pavement and concrete,

or rigid, pavement. Generally, bituminous pavements are cheaper and easier to construct,

but they require more maintenance. Concrete pavements, however, last for a very long

time with minimal upkeep but are much more expensive and time-consuming to build

[Urbanik, 2007].

a)Flexible PavementsThese pavements are rather flexible in their structural action under loading. They are

surfaced with bituminous or asphalt materials. Flexible pavements consist of several

layers of materials and rely on the combination of layers to transmit load to the subgrade.

As a result of this action, flexible pavements distribute load over a small area of subgrade.

b)Rigid PavementsRigid pavements are made of Portland Cement Concrete (PCC). The concrete slab ranges

in thickness from 6 to 14 inches (or 152.4 - 355.6mm). These types of pavements are

called rigid because they are substantially stiffer than flexible pavements due to the high

stiffness of PCC. As a result of this stiffness, rigid pavements tend to distribute load over a

relatively wide area of subgrade. The concrete slab that comprises a rigid pavement

supplies most of its structural capacity.

In deciding whether to use flexible or rigid pavements, engineers take into account the

following factors:

Traffic disruptions due to maintenance;

Riding characteristics;

Ease and cost of repair;

Effect of climatic conditions.

Lifetime costs.


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3

3Elements of a Flexible Pavement

1.2 Elements of a Flexible PavementA flexible pavement is built up of layers namely; surfacing course, roadbase, sub-base,

capping layer and subgrade [Kadiyali, 2006].

Wearing Course

Subgrade

Base Course or Binder Course

Roadbase

Sub-base

Surfacing

Figure 1.1: Definition of Pavement layers

Source: TRL (1993)

a) SubgradeThis is the top surface of a roadbed on which the pavement structure and shoulders

including kerbs are constructed. Generally the top soil portion up to 500mm of the

embankment or cut section is referred to as the subgrade [Bindra, 1999].

It may be undisturbed local material or may be soil excavated elsewhere and placed as

fill. The loads on the pavement are ultimately received by the subgrade layer; it istherefore, essential that the layer should not be over-stressed. The top part of the layer

requires preparation to receive the layers above either by stabilizing it adequately (which

reduces the required pavement thickness) or designing and constructing a sufficiently

thick pavement to suit subgrade strength [TRL, 1993].

The subgrade strength depends on:

The type of material;

Moisture content;

Dry density;

Internal structure of the soil particles;

Type and mode of stress applied.

The major factors that influence pavement thickness are; design wheel load, strength of

subgrade (and other pavement materials), climatic and environmental factors [Singh,

2001].

b) Capping Layer (Selected or Improved Subgrade)A capping layer may consist of better quality subgrade material brought in from

somewhere else or from existing subgrade material improved by mechanical or chemical

stabilisation. It is usually justified where weak soils are encountered [TRL, 1993].


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4

4Elements of a Flexible Pavement

c) Sub-baseThis is the secondary load-spreading layer underlying the roadbase. It will normally

consist of a material of lower quality than that used in the roadbase (but of higher quality

than the subgrade) such as unprocessed natural gravel, gravel-sand, or gravel-sand-clay.

It may or may not be present as a separate layer since its presence is justified by the

insufficiency of the subgrade or to enhance reliability of the pavement performance

[TRL, 1993].

The functions of this layer are to:

Distribute stresses to the subgrade - the sub base material must therefore be

stronger than the subgrade material;

Act as a drainage layer in case the subgrade is poor - A good drainage layer

should be able to drain very fast if water is logged, but also must be able to retain

some moisture in times of extreme drought;

Prevent capillary attraction effect;

Serve as a separating layer preventing contamination of the roadbase by the

subgrade material;

Protect the subgrade from damage by construction traffic especially under wet

conditions.

The sub-base is omitted when the subgrade is a hard intact rock or if it is granular and

has a CBR greater than 30% and without a high water table [TRL, 1993].

d) RoadbaseThe roadbase is the main load-spreading layer of the pavement. It is structurally the most

important layer of a flexible pavement. It distributes the applied wheel load to the

subgrade in such a way that the bearing capacity of the subgrade soil is not exceeded.

This layer requires higher quality material often obtained by stabilizing sub-base

materials. It will normally consist of crushed stone or gravel, or of gravely soils,

decomposed rock, sands and sand-clays stabilised with cement, lime or bitumen [TRL,

1993].

e) SurfacingThe surfacing forms the topmost solid layer of the pavement usually designed to be

smooth and to withstand erosion from traffic and weather [Urbanik, 2007].

It usually consists of a bituminous surface dressing or a layer of premixed bituminous

material. It is comparatively thin, but resists abrasion and the impacts caused by wheel

loads and the effects of weather condition [Bindra, 1999].

The functions of this layer are to:

Provide a safe and comfortable riding surface to traffic;

Take up wear and tear stresses caused by traffic; Provide a water tight surface against infiltration of water;

Provide a hard surface which can withstand tyre pressure.

Where premixed materials are laid in two layers, these are known as the wearing course

and the base course (or binder course) as shown in Figure 1.1 [TRL, 1993].


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5

5Overview of the Design Process

1.3 Overview of the Design ProcessORN31 (TRL, 1993) sums up the overall process of pavement design as constituting the

following major activities:

1. Surveying possible routes - which are part of the feasibility study process;

2. Assessing traffic;

3. Measuring subgrade strength;

4. Selecting pavement materials;

5.

Selecting the type of pavement structure with subsequent design of the drainagesystem. This is then followed by the actual implementation of the project.

However, the three major steps to be followed in designing a new road pavement are:

Traffic Assessment; which involves estimation of the amount of traffic and the

cumulative number of equivalent standard axles that will use the road over the

selected design life;

Subgrade strength assessment; which entails assessing the strength of the

subgrade soil over which the road is to be built;

Material selection;which involves the selection of the most economical

combination of pavement materials and layer thicknesses that will provide

satisfactory service over the design life of the pavement. It is usually necessary toassume that an appropriate level of maintenance is also carried out throughout the

design life of the road [TRL, 1993].

The following chapters will consider each of these steps in turn and put special emphasis

on five aspects of design that are of major significance in designing roads in most tropical

countries:

The influence of tropical climates on moisture conditions in road subgrades;

The severe conditions imposed on exposed bituminous surfacing materials by

tropical climates and the implications of this for the design of such surfacing;

The interrelationship between design and maintenance. If an appropriate level of

maintenance cannot be assumed, it is not possible to produce designs that will

carry the anticipated traffic loading without high costs to vehicle operators throughincreased road deterioration;

The high axle loads and tyre pressures which are common in most countries;

The influence of tropical climates on the nature of the soils and rocks used in road

building.

The overall process of designing a road is illustrated in Figure 1.2. Some of the

information necessary to carry out the tasks may be available from other sources e.g. from

a feasibility study or from Ministry records, but all existing data will need to be checked

carefully to ensure that it is both up-to-date and accurate. Likely problem areas are

highlighted in the relevant chapters of these notes.


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6

6Overview of the Design Process

Figure 1.2: The Pavement Design Process

Source: TRL (1993)


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7

7Design Approaches

1.4 Design ApproachesThere are two basic approaches to design namely empirical and semi-empirical methods

[Arora, 2000].

Empirical methods include:

Group index method;

CBR method (or thickness design method)

Semi - empirical methods include:

AASHTO method;

Tri-axial test;

Nottingham method;

California Resistance Value Test;

McLeod method; and

Banister method.

In Uganda, the AASHTO and Thickness design methods are most commonly applied.

These methods will be looked at in more detail during the assessment of subgrade

strength. The Group index method is limited as it considers only the particle distributionof the soil and its atterberg limits.

1.5 Design Standardsa) General

Design of flexible pavements in Uganda has been based on a number of design standards

that include the TRL, Overseas Road Note 31 (1993), Uganda Road Design Manual

(2005), the Kenya Road Design Manual and the American Association of State Highways

and Transportation Officials (AASHTO) interim guides for design of pavement structures

1972-1986. The latest version of the AASHTO design guide was printed in 1993. The

above design guides have been adopted to suit most materials and climatic conditions

found in developing countries. The AASHTO design equation in the design guide 1972-

1986 was also modified through research done by the World Bank to suit conditions in

developing countries.

b) Uganda Road Design ManualThe Uganda Road Design Manual 2005 has incorporated the pavement design guide

prepared for SATCC countries. The SATCC design guide was developed for Southern

Africa Transport and Communication Commission for use in Tanzania, Zambia,

Zimbabwe Mozambique, Malawi, Swaziland, Lesotho, Angola, and Botswana [Thagesen,

1996]. The method follows the AASHTO design concept as set forth in AASHTO interim

guides for design of pavement structures 1972-1986 published by the American

Association of State Highways and Transport Officials. The pavement strength requiredfor a given combination of subgrade bearing capacity, traffic load, service level and

climate is expressed by means of the subgrade structural number. Layer coefficients,

according to the position in the structure, are given to determine the structural number of

the pavement. For each type of pavement, the thickness of the base and sub base layers are

determined so that the required structural number is satisfied [Uganda Road Design

Manual, 1994].


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8

8Questions

c) Kenya Road Design ManualThe materials and pavement design in the Kenya Road Design Manual sets forth the

standards for structural design of new bitumen surfaced roads in Kenya. The Kenya Road

Design Manual includes design of gravel wearing course on unpaved roads.

d) TRL Overseas Road Note (ORN) 31The British Transportation and Road Research Laboratory (TRRL) published the first

version of Road Note 31 in 1962 and subsequently revised it in 1976 and 1977. The Road

Note 31 has in 1993 undergone a comprehensive revision by the transport research

laboratory (TRL) and now includes the structural catalogue where a layer thickness can be

selected for a whole range of common pavement combinations. The guidelines are based

on an empirical method taking into account the organisations vast experience in

understanding the behaviour of road building materials and their interactions in composite

pavements.

e) Conclusive RemarkIt is important for engineers, especially those in developing countries like Uganda, to

exercise judgement in the use of a given design standard to ensure that they come up with

an economical solution for a pavement design. Use of local materials has to always be

taken into consideration. Sometimes, more than one design standard is used for the

purposes of comparing one pavement design with another so that the comparison guides

the engineer in selecting the most economical option.

1.6 Questionsa) In less than 300 words, write an executive summary about the pavement design process.

b) Discuss the elements that make up a flexible pavement structure and their significance.

c) Discuss the three major activities that constitute the pavement design process.

d)

Mr. Juan Chvez Fernndez is a Spanish Highway Consultant who wishes to open up anew Highway Engineering firm in Kampala. He is faced with a problem of deciding

which highway design standards he should use in order to come up with the most

economical pavement designs. Using the knowledge you have acquired from your recent

training in highway engineering, advice Mr. Fernndez.

1.7 Bibliography1. Arora, K. R., 2000, Soil Mechanics and Foundation Engineering, 5

thEdition.

2. Bindra, S.P, 1999, A Course in Highway Engineering, 4th

Edition, Dhanpat Rai

Publishers, New Delhi.

3.

Gupta, B.L., 1995, Roads, railways Bridges and Tunnels engineering, 4th

edition,Standard publishers Distributors, Nai sarak, Delhi.

4. Kadiyali, L.R., 2006. Principles and Practices of Highway Engineering (including

Expressways and Airport Engineering), 4th

Edition. Khanna Publishers, New Delhi.

5. Ministry of Works, and Transport, 1994. Road Design Manual, Republic of Uganda,

Kampala.

6. OFlaherty C.A., 2002.Highways: The Location, Design, Construction and Maintenance

of Pavements.4th

Edition, Oxford, Butterworth Heinemann.

7. Rogers, M., 2003,Highway Engineering, Oxford, Blackwell Publishing Ltd.


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9

8. Ruhweza, D., 2005, Highway Engineering I. Lecture notes, Department of Civil

Engineering, Kyambogo University.

9. Singh, G., 2001,Highway Engineering, 3rd

edition, Standard publishers and Distributors,

Delhi.

10.Transport Research Laboratory, 1993,A Guide to Design of Bitumen Surfaced Roads in

Tropical and Sub Tropical Countries, Overseas Road Note 31, Crowthorne, England.

11.Urbanik, T., 2007, Road", Microsoft Student 2008 [DVD]. Redmond, WA: Microsoft

Corporation.


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10

10Chapter Two:

Chapter Two:

Traffic Assessment

2.1 GeneralOn completion of the route location process (not discussed in these notes), the designer is

then expected to estimate the amount of traffic and the cumulative number of equivalent

standard axles that will use the road over the selected design life. In this step, other sub-

activities include: measurement of traffic volume by class; measurement of axle loads;

choosing the design life and the calculation of the total traffic in esa or msa. The final step

at this stage is the assignment of a traffic class to the amount of traffic ascertained with

guidance from the structure catalogue in chapter five.

It should be noted that information on traffic flow of vehicles past a given point in a

specified time period provides a key input to decisions on the planning, design and

operation of transport systems. This data is used in highway planning and helps in thedesign of road pavements, establishment of control measures, carrying out of cost benefit

analyses and studying accident patterns in relation to traffic volume. Accuracy of traffic

flow data is therefore extremely critical as any inaccurate data is useless for any design

purposes [OFlaherty, 2002].

2.2 Estimation of Traffic Flows (F)a) Introduction

Generally, heavier loads require thicker pavements provided other design factors remain

constant. The structural design of a pavement largely depends on the traffic (or design

wheel load) projected to use that pavement. In design of a pavement, knowledge of themaximum wheel load is more important than gross weight of vehicles [Gupta, 1999].

During design, emphasis is placed on commercial and heavy goods vehicles whose axle

weight is greater than 1,500 kg. It is these classes of vehicle that are most damaging to the

pavement making their volumes a critical parameter in design [TRL, 1993]. When

designing a new road, the total flow of commercial vehicles in one direction per day at the

roads opening are normally required in order to determine the cumulative design traffic

over the design life. For purposes of pavement design, vehicles weighing less than 1500

kg may be ignored. If the traffic flow figures available are for two way flow, the

directional split is assumed to be in ratio 1:2 (in favour of the heavily trafficked lane)

unless traffic studies show otherwise [Kadiyali, 2006].

The distribution of commercial vehicle traffic can be expected to vary at particular points

along the road e.g. where lanes leave or join a carriageway, or at traffic signals or at

roundabouts. Nonetheless in the design of new roads the traffic distribution considered is

that away from junctions. All lanes are designed to carry the heaviest traffic load assessed

from the most trafficked lane.


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11Estimation of Traffic Flows (F)

b) Baseline Traffic Flows (Fo)In order to determine the total traffic over the design life of the road, the first step is to

estimate baseline traffic flows. The estimate should be the (Annual) Average Daily Traffic

(AADT) currently using the route, classified into the vehicle categories of cars, light

goods vehicles, trucks (heavy goods vehicles) and buses. The AADT is defined as the total

annual traffic summed for bothdirections and divided by 365. It is usually obtained by

recording actual traffic flows over a shorter period from which the AADT is then

estimated. For long projects, large differences in traffic along the road may make itnecessary to estimate the flow at several locations. It should be noted that for structural

design purposes the traffic loading in onedirection is required and for this reason care is

always required when interpreting AADT figures. Traffic counts carried out over a short

period as a basis for estimating the traffic flow can produce estimates which are subject to

large errors because traffic flows can have large daily, weekly, monthly and seasonal

variations. The daily variability in traffic flow depends on the volume of traffic. It

increases as traffic levels fall, with high variability on roads carrying less than 1000

vehicles per day.

In order to reduce error, it is recommended that traffic counts to establish ADT at a

specific site conform to the following practice:

i) The counts are for seven consecutive days.ii) The counts on some of the days are for a full 24 hours, with preferably at least one

24-hour count on a weekday and one during a weekend. On the other days 16-hour

counts should be sufficient. These should be grossed up to 24-hour values in the

same proportion as the 16-hour/24 hour split on those days when full 24-hour counts

have been undertaken.

iii)Counts are avoided at times when travel activity is abnormal for short periods due tothe payment of wages and salaries, public holidays, etc. If abnormal traffic flows

persist for extended periods, for example during harvest times, additional counts

need to be made to ensure this traffic is properly included.

iv) If possible, the seven-day counts should be repeated several times throughout theyear [TRL, 1993].

The following steps are generally taken when carrying out a traffic survey;

i) Traffic count data sheets are made indicating the classification of vehicles, i.e. cars,pick-ups, minibuses, buses, trucks and trailers (See figure 2.1 for a sample of a

classified count data sheet);

ii) Traffic count stations along the road are then identified;iii)Enumerators who are trained to carry out the traffic survey are positioned at the

identified station.


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Figure 2.1: Form for Manual Classified CountsSource: TRL (2004)

c) Projected Traffic (Fp)Forecasting traffic growth is a difficult exercise and may involve uncertainty in growth

predictions. Some factors considered include economic growth, vehicle growth, and land


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use development. These factors are considered together with traffic modelling. To reduce

this uncertainty, sensitivity and risk analyses are involved in the process [TRL, 1993].

Even with a developed economy and stable economic conditions, traffic forecasting is an

uncertain process. In a developing economy the problem becomes more difficult because

such economies are often very sensitive to the world prices of just one or two

commodities.

In order to forecast traffic growth it is necessary to separate traffic into the following

three categories:

a) Normal traffic;Traffic which would pass along the existing road or track even if nonew pavement were provided.

b) Diverted traffic;Traffic that changes from another route (or mode of transport) tothe project road because of the improved pavement, but still travels between the

same origin and destination.

c) Generated traffic;Additional traffic which occurs in response to the provision orimprovement of the road.

For existing roads, the greatest traffic contribution is from the normal traffic. We shall

therefore only examine the methods used to forecast normal traffic (The student isencouraged to find out how the other two types of traffic are forecast).

The commonest method of forecasting normal traffic is to extrapolate time series data on

traffic levels and assume that growth will either remain constant in absolute terms i.e. a

fixed number of vehicles per year (a linear extrapolation), or constant in relative terms

i.e. a fixed percentage increase [TRL, 1993].

A constant growth rate formula shown below is normally used to project the traffic to the

design year.

1

.2.1

Where,

Fp = Cumulative number of commercial vehicles after n years

Fo = Present number of vehicles after the traffic survey;

r = Growth rate of commercial vehicles;

n = Number of years of projection.

d) Axle Loading (W)i) Axle Equivalency

The damage that vehicles do to a road depends very strongly on the axle loads of the

vehicles. For pavement design purposes the damaging power of axles is related to a

'standard' axle of 8.16 tonnes using equivalence factors which have been derived from

empirical studies. 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 over this period in terms of the cumulative number

of equivalent standard axles (esa). Axle load surveys must be carried out to determine

the axle load distribution of a sample of the heavy vehicles using the road. Data collected


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from these surveys are used to calculate the mean number of equivalent standard axles

for a typical vehicle in each class. The axle loading for each category of commercial

vehicle is the sum of the front and rear axles. For commercial vehicles with more than

one rear axle, the total equivalent standard axle for the vehicle will be the sum of the

front and each of the rear equivalent standard axles. These values are then used in

conjunction with traffic forecasts to determine the predicted cumulative equivalent

standard axles that the road will carry over its design life. The wear factor can be

calculated from the following equation;

, 8.16 . . . . 2.2

ii) Axle Load SurveysIf no recent axle load data is available, it is recommended that axle load surveys of heavy

vehicles are undertaken whenever a major road project is being designed. Ideally, several

surveys at periods which will reflect seasonal changes in the magnitude of axle loads are

recommended. It is also recommended that axle load surveys are carried out by weighing

a sample of vehicles at the roadside. The sample should be chosen such that a maximum

of about 60 vehicles per hour are weighed. The weighing site should be level and, if

possible, constructed in such a way that vehicles are pulled clear of the road when being

weighed. The portable weighbridge should be mounted in a small pit with its surface

levelwith the surrounding area. This ensures that all of the wheels of the vehicle being

weighed are level and eliminates the errors which can be introduced by even a small

twist or tilt of the vehicle.


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Figure 2.2: Axle Load Survey Form A for recording vehicle survey data

Source: TRL (2004)


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Figure 2.3: Axle Load Survey Form B for recording vehicle wheel loads

Source: TRL (2004)


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e) Growth Factor (G)Growth is assumed to be compound over the design period. The Portland Cement

Association developed a formula that applies traffic at the middle of the design period as

the design traffic as shown below;

1 . .2.3 Where;

G = the growth factor;

r = the growth rate; and

n = the design period

The Asphalt Institute and the AASHTO Design Guide recommend the use of traffic

over the entire design period to determine the total growth factor as follows;

1 1. . 2.4

Where; G, r and n are as previously defined.

f) Design Life (Y)The design period is the time during which the road will accommodate traffic at a

satisfactory level of service without requiring capital intervention (or further funding) in

the form of rehabilitation or strengthening. For most road projects an economic analysis

period of between 10 and 20 years from the date of opening is appropriate, but for major

projects this period should be tested as part of the appraisal process discussed in

Overseas Road Note 5, TRL (1988). Below is a table used to guide the pavement

designer in choosing the appropriate design life as recommended by the Ministry of

Works and Transport, Pavement Design Manual (2005).

Table 2.1: Selection of Design Life

Source: Uganda Road Design Manual (2005)

A pavement design life of 15 years also reduces the problem of forecasting uncertain

traffic trends for long periods into the future. It should be noted that design life does not

mean that at the end of the period the pavement will be completely worn out and in need

of reconstruction; it means that towards the end of the period the pavement will need tobe strengthened so that it can continue to carry traffic satisfactorily for a further period.

Low High

Low 10 - 15 yrs 15 yrs

High 10 - 20 yrs 15 - 20 yrs

Importance/Level of ServiceDesign Data

Relibility


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18Determination of Cumulative Standard Axles (T)

2.3 Determination of Cumulative Standard Axles (T)A successful outcome of the pavement design process in any given instance is dependent

upon the accuracy with which the total number of standard axle loads, and their

cumulative wear or damage effects, can be predicted for the design lane(s) over the period

of the selected design life. Below is a summary of the steps involved in carrying out a full

traffic assessment:

a)

Estimate the present one-way commercial vehicle flow or the traffic flow, F, at theopening of a new road. For each class of vehicles select the initial design period Y;

b) Determine the appropriate average wear factor, W, to be used with each vehicle class;

c) Determine the growth factor, G, for each category of vehicle;

d) Calculate the cumulative design traffic in each vehicle class using the equation shown

below:

.2.5 Where;

365 10

. 2.6

And;

i = vehicle class

Note:

In case of a two-way single carriage-way pavement, the total design traffic, T, is the

summation of the cumulative design traffic in each category in a given direction.

In case of a dual carriageway road the proportion of vehicles in the most heavily trafficked

lane is normally obtained and applied to the total accumulation to derive the design traffic.

g) Channelization Factor (Ch)In certain cases, the equation for the cumulative design traffic includes a channelization

factor, thus Ti= 365 F.W.G.Y.Ch (10-6

) msa. In the urban area it is relevant to consider

the effects of vehicle channelization which may be caused by various factors. In 1983 the

County Surveyors Society report entitled Vehicle Damage Factors Present, Past and

Future Values indicated that where the normal tendency for transverse wander is

constrained by, for example , traffic islands then the damaging effect can be at least

twice that normally expected. In narrow urban streets where one street parking is

permitted there is a tendency for buses and Heavy Goods Vehicles (HGVs) to use the

same wheel tracks when passing in both directions [Ruhweza, 2005].

The effect of bus stop areas is another location where an increased axle load factor may

be relevant. The same applies to traffic signal junctions and roundabouts. In the case of

long severe gradients where there are significant HGV flows, there is an indication that

the normal damaging effect calculations may not be adequate. The following overall

multipliers are suggested:


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19Example 2.1: Traffic Assessment

Table 2.2: Axle Factor Multipliers

Effect Multiplier

Traffic island 2.0

Parking 2.0

Traffic Signals 1.5

Roundabouts 1.5

Severe Bends 1.5

Steep Hill 2.0 Source: County Surveyors Society (1983)

N.B: It is usually advised that the total multiplier used should not exceed 3.0. In certain

locations and circumstances, it may be appropriate to consider the multipliers to be

cumulative. Table 2.3 below shows the various traffic classes and their corresponding

equivalent standard axles, in msa.

Table 2.3: Traffic Classes

Traffic Classes Ranges (msa)

T1 < 0.3

T2 0.3 - 0.7

T3 0.7 - 1.5

T4 1.5 - 3.0T5 3.0 - 6.0

T6 6.0 - 10

T7 10 - 17

T8 17 - 30

Source: TRL (1993)

2.4 Example 2.1: Traffic AssessmentThe Kampala Gayaza road is in a state of failure and is due for reconstruction. Tables

2.4 and 2.5 below show the results of a traffic survey at different stations on the above

road. The survey was carried out on 1st January, 2008 and construction is to begin in

December, 2009. The road is expected to be opened to traffic on 1stJanuary, 2010.

Table 2.4: Two-Way Traffic Volume at the Project Road (in vehicles/day)

Table 2.5: Axle Weights (in tonnes)

Description Mini Buses Buses Pick-Ups Cars 2-AxleTrucks 3-AxleTrucksGross Weight (t) 3.00 15.00 3.00 12.00 20.00

Front Axle Load (t) 1.00 3.00 1.00 4.00 4.00

Rear Axle Load 1 (t) 2.00 6.00 2.00 8.00 8.00

Rear Axle Load 2 (t) - 6.00 - - 8.00

Considering the design traffic loading at station D and assuming a 15 year design

period, design the pavement using the TRL approach. (Assume that the traffic grows at

the rates indicated for each vehicle class in Table 2.4 above).

Growth Rate

A B C D r%

Minibuses 4,013 2,271 4,647 4,507 6.0%

Buses 220 82 5 3 4.5%

Pick-Ups 850 348 1,621 1,845 6.0%

2-Axle Trucks 182 255 447 507 5.0%

3-Axle Trucks 23 11 73 44 4.0%

Total 5,288 2,967 6,793 6,906

Vehicle Class Station


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20Example 2.1: Traffic Assessment

Solution

1.0 Design Information(a)Traffic growth rate, r = 6%

(b)Design life, Y = 15 yrs

(c)Construction Period, n = 2 yrs (2010 2008)

2.0 Determination of cumulative design traffic, T

Where; 365 10 2.1 Unidirectional traffic Flow, F

Assuming a 1:2 directional traffic split, then;

23 of the trafic volume for each vehicle classe.g. for minibuses at station D;

234508 3005 veh/day 3005 10.06 3376 veh/day

2.2 Wear factor, WFrom equation 2.2

,

8.16 .

e.g. for minibuses at station D;

1.008.16. 2.008.16

. 0.0001 0.0018 0.0019

2.3 Growth Factor, GAccording to the Portland Cement Association (equation 2.3); the growth factor

for minibuses at station D is given by;

1 0.06.

1.5481


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21Questions

2.4 Table of results

From table 2.3 a cumulative design traffic of 3.335 msa corresponds to a traffic

class of T5i.e. 3.0 < T (in msa) < 6.0.

2.5 Questionsa) On 1

st December 2006, Mr. Fernndez was awarded a contract to upgrade Kansanga

Lukuli road, which is in an alarming state of failure, to a bituminous surfaced road. He

now wishes to carry out a baseline traffic survey together with an axle load survey to

ascertain the amount of traffic currently using the road and the amount of axle loading onthe project road respectively. Describe in detail how you would expect him to carry out the

above surveys in preparation for pavement design.

b) Upon completion of the surveys in part (b) above, Mr. Fernndez summarised his findings

in Table 2.6 to Table 2.9 below. The tables show the results of the traffic and axle load

surveys at station A on the project road. The survey was carried out in December, 2007

and construction is to begin in January, 2009. The road is expected to be opened to traffic

on 31stDecember, 2012 with traffic projected to grow at a rate of 4%. Ascertain the design

traffic loading for station A assuming that the design data reliability is low yet the

expected level of service is high. Assign a traffic class to this section and comment on

your results.

Table 2.6: Axle Loads (in tons)

Gross Front Rear Rear

Weight Axle Axle 1 Axle 2

(tons) (tons) (tons) (tons)

Vans, PickUps & 4WDs 3.00 1.00 2.00

Minibuses and Matatus 3.00 1.00 2.00

Coasters 3.00 1.00 2.00

Buses 9.00 3.00 6.00

Dynas and Tractors 12.00 4.00 8.00

2-Axle Trucks 12.00 4.00 8.00

3-Axle Trucks 20.00 4.00 8.00 8.00

Axle Load Weights (in tons)

Vehicle Class

Front Axle Rear 1 Rear 2 Rate, r Fo Fp W G Y Ti

(esa) (esa) (esa) % (Veh/d) (Veh/d) (esa) (yrs) (msa)

Minibuses 0.0001 0.0018 0.0000 6.0% 3,005 3,376 0.0019 1.5481 15 0.053

Buses 0.0111 0.2507 0.2507 4.5% 2 2 0.5124 1.3911 15 0.008

Pick-Ups 0.0001 0.0018 0.0000 6.0% 1,230 1,382 0.0019 1.5481 15 0.022

2-Axle Trucks 0.0404 0.9147 0.0000 5.0% 338 373 0.9552 1.4418 15 2.8133-Axle Trucks 0.0404 0.9147 0.9147 4.0% 29 32 1.8699 1.3420 15 0.440

3.335Cumulative Design Traffic, T (in msa)

Wear Factor, W F

Vehicle Class


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22Questions

Table 2.7: Friday 24-hr count summaryLight Goods Vehcles Heavy Commercial Goods Vehicles Two Wheeled

Duration Saloon Vans, Pick- Minibuses Coasters Buses Dynas & Truck Truck Motor Bicycles Carts

of count Cars Ups & 4WD & Matatus Tractors 2-Axle 3-Axle Cycles

07:00-08:00 a.m. 136 125 100 3 0 6 1 0 68 19 0

08:00-09:00 111 144 70 0 0 2 4 0 109 14 0

09:00-10:00 50 57 34 0 1 1 3 0 0 12 0

10:00-11:00 69 65 24 1 0 6 0 0 63 10 1

11:00-12:00 p.m. 40 47 30 1 0 4 9 0 60 5 0

12:00-01:00 41 50 32 5 0 0 4 0 67 11 0

01:00-02:00 25 44 23 1 0 3 3 0 53 5 002:00-03:00 58 79 44 2 0 0 4 0 38 3 1

03:00-04:00 98 120 52 3 0 3 0 0 63 5 0

04:00-05:00 50 49 27 5 0 7 2 0 61 5 0

05:00-06:00 34 55 40 3 0 4 1 0 62 8 0

06:00-07:00 41 43 38 1 0 2 0 0 60 9 0

07:00-08:00 52 41 27 0 0 5 2 0 84 8 0

08:00-09:00 46 26 34 0 0 2 0 0 124 3 0

09:00-10:00 38 24 35 0 0 2 1 0 102 6 010:00-11:00 55 32 55 0 0 1 0 0 84 2 0

11:00-12:00 a.m. 27 12 25 0 0 0 1 0 65 2 0

12:00-01:00 19 3 17 0 0 0 1 0 74 1 0

01:00-02:00 9 6 3 0 0 0 0 0 50 0 0

02:00-03:00 7 4 0 0 0 0 0 0 26 0 0

03:00-04:00 7 5 0 0 0 0 0 0 29 0 0

04:00-05:00 4 3 1 0 0 0 0 0 41 0 0

05:00-06:00 11 4 6 0 0 0 0 0 43 2 0

06:00-07:00 18 3 26 0 0 2 0 0 36 8 0

Table 2.8: Sunday 24-hr count summaryLight Goods Vehcles Heavy Commercial Goods Vehicles Two Wheeled

Duration Saloon Vans, Pick- Minibuses Coasters Buses Dynas & Truck Truck Motor Bicycles Carts

of count Cars Ups & 4WD & Matatus Tractors 2-Axle 3-Axle Cycles

07:00-08:00 a.m. 9 19 38 0 0 6 1 0 59 11 1

08:00-09:00 30 25 23 0 0 1 0 0 28 15 0

09:00-10:00 43 58 32 0 0 4 0 0 88 10 0

10:00-11:00 38 50 32 0 0 4 0 0 111 4 0

11:00-12:00 p.m 48 57 21 1 0 2 0 0 72 5 2

12:00-01:00 45 58 23 1 0 4 0 0 58 12 1

01:00-02:00 48 62 32 0 0 4 0 0 66 6 0

02:00-03:00 50 74 33 1 0 1 0 0 56 9 0

03:00-04:00 34 41 27 0 0 4 0 0 44 2 0

04:00-05:00 32 48 28 1 0 7 0 0 56 9 0

05:00-06:00 50 42 34 0 0 0 0 0 50 4 0

06:00-07:00 60 36 37 0 0 0 3 0 43 0 1

07:00-08:00 53 52 49 1 0 3 1 0 84 8 0

08:00-09:00 41 36 51 0 0 5 1 0 77 8 0

09:00-10:00 38 16 45 0 0 3 1 o 44 1 110:00-11:00 33 10 34 0 0 1 0 0 46 0 0

11:00-12:00 a.m. 11 12 30 0 0 0 0 0 56 3 0

12:00-01:00 18 4 21 0 0 0 0 0 49 1 0

01:00-02:00 3 3 9 0 0 0 0 0 39 0 0

02:00-03:00 3 0 2 0 0 0 0 0 30 0 0

03:00-04:00 1 1 1 0 0 0 0 0 27 0 0

04:00-05:00 1 1 1 0 0 0 0 0 28 0 0

05:00-06:00 4 3 12 0 0 0 1 0 30 2 0

06:00-07:00 28 18 38 1 0 2 0 0 40 11 0

Table 2.9: 16-hr counts on the remaining daysDay Saloon Vans, Pick- Minibuses Coasters Buses Dynas & Truck Truck Motor Bicycles CartsCars Ups & 4WD & Matatus Tractors 2-Axle 3-Axle Cycles

WED 921 836 553 17 0 89 18 1 1080 142 10

THU 1044 1236 950 60 0 91 51 2 1482 240 8

SAT 646 576 606 39 0 71 8 0 1183 119 2

MON 804 3290 785 16 2 54 41 5 1281 130 9

TUE 974 1075 671 16 2 52 59 4 1259 152 1


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23Bibliography

2.6 Bibliography1. Kadiyali, L.R., 2006. Principles and Practices of Highway Engineering (including

Expressways and Airport Engineering), 4th

Edition. Khanna Publishers, New Delhi.

2. Ministry of Works, and Transport, 2005.Road Design ManualVol.III, Pavement Design

Manual, Republic of Uganda, Kampala.

3. OFlaherty C.A., 2002. Highways: The Location, Design, Construction and

Maintenance of Pavements.4th

Edition, Oxford, Butterworth Heinemann.4. Ruhweza, D., 2005, Highway Engineering I. Lecture notes, Department of Civil

Engineering, Kyambogo University.

5. Transport Research Laboratory, 1993,A Guide to Design of Bitumen Surfaced Roads in

Tropical and Sub Tropical Countries, Overseas Road Note 31, Crowthorne, England.

6. Transport Research Laboratory, 2004,A Guide to Axle Load Surveys and Traffic Countsfor Determining Traffic Loading on Pavements, Overseas Road Note 40, Crowthorne,

England.


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24Chapter Three:

Chapter Three:

Subgrade Strength Assessment

4.1 GeneralThis third stage of the pavement design process can be carried out concurrently with the

traffic assessment immediately following the route location process. The sub-activities

involved in assessing the subgrade strength of the soil are: Assignment of climatic a

regime, testing of soils, definition of uniform sections, and designing of earth works [TRL,

1993].

Properties of the subgrade soil are important in designing the depth of the pavement.

Weak subgrade material requires higher thickness to protect it from traffic loads.

Pavement deformation mainly depends on the subgrade properties and drainage. During

design and construction, proper drainage has to be maintained in order to control

pavement deformation.

Climatic factors are important here because rainfall affects the moisture of the subgrade

and pavement layers. The daily and seasonal variations of rainfall are important in the

design and performance of the pavement. Where the water table is close to the formation

level of the roads, adjustments in the design of the pavement layer thicknesses are

necessary. Embankment heights and the depth of water table below the embankment have

an effect on the performance of an embankment and must be examined [Kadiyali, 2000,

Arora, 2000].

The strength of road subgrades is commonly assessed in terms of the California Bearing

Ratio (CBR) and this is dependent on the type of soil, its density, and its moisture content.

For designing the thickness of a road pavement, the strength of the subgrade should betaken as that of the soil at a moisture content equal to the wettest moisture condition likely

to occur in the subgrade after the road is opened to traffic. In the tropics, subgrade

moisture conditions under impermeableroad pavements can be classified into three main

categories. Some of the key tests in the design of the subgrade include the Compaction

test, the Dynamic Cone Penetrometer test and the California Bearing Ratio (CBR) test.

4.2 Climatic Regimea) Category (1)

Category (1) subgrades are those in which the water table is sufficiently close to the

ground surface to control the subgrade moisture content. The type of subgrade soilgoverns the depth below the road surface at which a water table becomes the dominant

influence on the subgrade moisture content. For example, in non-plastic soils the water

table will dominate the subgrade moisture content when it rises to within 1 m of the road

surface, in sandy clays (PI40 per cent) the water table will

dominate when it rises to within 7m of the road surface. In addition to areas where the

water table is maintained by rainfall, this category includes coastal strips and flood plains

where the water table is maintained by the sea, by a lake or by a river.


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25Testing Subgrade Soils

b) Category (2)Category (2) subgrades are those with deep water tables and where rainfall is sufficient to

produce significant changes in moisture conditions under the road. These conditions occur

when rainfall exceeds evapotranspiration for at least two months of the year. The rainfall

in such areas is usually greater than 250 mm per year and is often seasonal.

c) Category (3)Category (3) subgrades are those in areas with no permanent water table near the groundsurface and where the climate is dry throughout most of the year with an annual rainfall of

250 mm or less.

Direct assessment of the likely strength or CBR of the subgrade soil is often difficult to

make but its value can be inferred from an estimate of the density and equilibrium (or

ultimate) moisture content of the subgrade together with knowledge of the relationship

between strength, density and moisture content for the soil in question. This relationship

must be determined in the Laboratory.

The density of the subgrade soil can be controlled within limits by compaction at suitable

moisture content at the time of construction. The moisture content of the subgrade soil is

governed by the local climate and the depth of the water table below the road surface. Inmost circumstances, the first task is therefore to estimate the equilibrium moisture content.

A method of direct assessment of the subgrade strength, where this is possible, will be

discussed later together with less precise methods of estimation which can be used if

facilities for carrying out the full procedure are not available.

d) Equilibrium Moisture ContentAn impervious road surface isolates soil from rainfall, evaporation and plant transpiration.

After the construction of an impervious pavement, the moisture content within the soil

tends to settle to a set of more or less steady values. For each depth there is a particular set

of values referred to as the equilibrium moisture content. It has a value between the wetter

and drier values of moisture content in an unprotected subgrade during the wetter and drier

months respectively. For economic reasons, a roadway should be designed to suit the

subgrade when it has reached equilibrium moisture content conditions. For small works

the equilibrium moisture content can be taken as being equal to the moisture content

occurring in the natural soil at a depth of 1m, provided that this soil is the same as the soil

that will be the formation level [Ruhweza, 2005].

4.3 Testing Subgrade SoilsBefore the subgrade strength can be determined the probable equilibrium moisture content

of the pavement during its design life must be determined.

a) Equilibrium Moisture ContentCategory (1);The easiest method of estimating the design subgrade moisture content is to

measure the moisture content in subgrades below existing pavements in similar situations

at the time of the year when the water table is at its highest level. These pavements should

be greater than 3m wide and more than two years old and samples should preferably be

taken from under the carriageway about 0.5m from the edge. Allowance can be made for

different soil types by virtue of the fact that the ratio of subgrade moisture content to

plastic limit is the same for different subgrade soils when the water table and climatic

conditions are similar. If there is no suitable road in the vicinity, the moisture content in

the subgrade under an impermeable pavement can be estimated from knowledge of the


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depth of the water table and the relationship between suction and moisture content for the

subgrade soil [TRL, 1993]. The test apparatus required for determining this relationship is

straightforward and the method is described below:

The subgrade moisture content under an impermeable road pavement can increase after

construction where a water table exists close to the ground surface. This ultimate moisture

content can be predicted from the measured relationship between soil suction and moisture

content for the particular soil and knowledge of the depth of water table. Measuring thecomplete relationship between suction and moisture content is time consuming and a

simpler, single measurement procedure can be used. A small sample of soil, compacted to

field density and moisture content, is placed within suitable laboratory equipment that can

apply a pressure equivalent to the 'effective depth' of the water table (e.g. a pressure plate

extractor). The 'effective depth' of the water table for design purposes comprises the actual

depth from the subgrade to the water table plus an apparent depression of the water table

due to the pressure of the overlying pavement. This apparent depression varies with soil

type and approximate correction factors used in calculating effective depth of water table

are given in table 3.1 below:

Table 3.1: Water Table Correction Factors for Soil Type PI

PI Correction Factor, SF0 0.00

10 0.30

15 0.55

20 0.80

25 1.10

30 1.40

35 1.60

>35 2.00 Source: TRL, 1993

To calculate the effective depth Dwhich is used to determine the applied suction in the

pressure plate extractor, the following equation is used:

. 2.7Where;

WT = Depth of water table below subgrade (at its highest expected seasonal

level),

SF = Correction factor from table 3.1,

t = Pavement thickness, with consistent units for WT, t, D

When equilibrium is attained in the pressure plate extractor, the sample is removed and its

moisture content measured. This moisture content is the value at which the CBR for

design should be estimated following standard soil tests to be discussed later in this

chapter.

Category (2);When the water table is not near the ground surface, the subgrade moisture

condition under an impermeable pavement will depend on the balance between the water

entering the subgrade through the shoulders and at the edges of the pavement during wet

weather and the moisture leaving the ground by evapotranspiration during dry periods.

Where the average annual rainfall is greater than 250 mm a year, the moisture condition

for design purposes can be taken as the optimum moisture content given by the British

Standard (Light) Compaction Test, 2.5 kg rammer method. When deciding on the depth of


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the water table in Category (1) or Category (2) subgrades, the possibility of the existence

of local perched water tables should be borne in mind and the effects of seasonal flooding

(where this occurs) should not be overlooked.

Category (3); In regions where the climate is dry throughout most of the year (annual

rainfall 250 mm or less), the moisture content of the subgrade under an impermeable

pavement will be low. For design purposes a value of 80 per cent of the optimum moisture

content obtained in the British Standard (Light) Compaction Test, 2.5 kg rammer method,should be used.

The methods of estimating the subgrade moisture content for design outlined above are

based on the assumption that the road pavement is virtually impermeable. Dense bitumen-

bound materials, stabilised soils with only very fine cracks, and crushed stone or gravel

with more than 15 per cent of material finer than the 75 micron sieve are themselves

impermeable (permeability less than 10-7 metres per second) and therefore subgrades

under road pavements incorporating these materials are unlikely to be influenced by water

infiltrating directly from above. However, if water, shed from the road surface or from

elsewhere, is able to penetrate to the subgrade for any reason, the subgrade may become

much wetter. In such cases the strength of subgrades with moisture conditions in Category

(1) and Category (2) should be assessed on the basis of saturated CBR samples asdescribed in the following section. Subgrades with moisture conditions in Category (3) are

unlikely to wet up significantly and the subgrade moisture content for design in such

situations can be taken as the optimum moisture content given by the British Standard

(Light) Compaction Test, 2.5 kg rammer method.

b) Determination of Subgrade StrengthHaving estimated the subgrade moisture content for design, it is then possible to determine

the appropriate design CBR value at the specified density. As a first step, it is necessary to

determine the compaction properties of the subgrade soil by carrying out standard

laboratory compaction tests. Samples of the subgrade soil at the design subgrade moisture

content can then be compacted in CBR moulds to the specified density and tested to

determine the CBR values. With cohesionless sands, the rammer method tends to

overestimate the optimum moisture content and underestimate the dry density achieved by

normal field equipment. The vibrating hammer method is more appropriate for these

materials. If samples of cohesive soils are compacted at moisture contents equal to or

greater than the optimum moisture content, they should be left sealed for 24 hours before

being tested so that excess pore water pressures induced during compaction are dissipated.

If saturated subgrade conditions are anticipated, the compacted samples for the CBR test

should be saturated by immersion in water for four days before being tested. In all other

cases when CBR is determined by direct measurement, the CBR samples should not be

immersed since this results in over design. In areas where existing roads have been built

on the same subgrade, direct measurements of the subgrade strengths can be made using adynamic cone penetrometer.

Except for direct measurements of CBR under existing pavements, in situ CBR

measurements of subgrade soils are not recommended because of

Documents

CE413 Highway Eng II[1]