design report print

8/18/2019 Design Report Print

1/227

i

TABLE OF CONTENTS

1.0 - GEOMETRIC DESIGN OF ROADS ................................................................... 1

1.1 INTRODUCTION ............................................................................................... 2

1.1.1 GENERAL .................................................................................................... 2

1.1.2. PROJECT DESCRIPTION .......................................................................... 3

1.2. DESIGN CONTROL & CRITERIA .................................................................. 5

1.2.1. TOPOGRAPHY ........................................................................................... 5

1.2.2. TRAFFIC ..................................................................................................... 6

1.2.2.1. ROAD USER ........................................................................................ 6

1.2.2.2. TRAFFIC DATA .................................................................................. 6

1.2.2.3. THE VEHICLE ..................................................................................... 7

1.2.3. SPEED ......................................................................................................... 8

1.2.4. SAFETY ...................................................................................................... 91.2.5. ECONOMIC CONSIDERATIONS .......................................................... 10

1.3. WIDTH OF TRAFFIC LANES & SHOULDERS ........................................... 11

1.3.1. GENERAL ................................................................................................. 11

1.3.2. FACTORS AFFECTING THE LANE WIDTH ........................................ 11

1.3.2.1. VEHICULAR TRAFFIC .................................................................... 12

1.3.2.2. GENERAL LANE WIDTHS .............................................................. 12

1.3.2.3. DERIVATIONS OF NUMBER OF LANES ..................................... 12

1.3.3. SHOULDER WIDTHS .............................................................................. 18

1.3.3.1. FACTORS AFFECTING SHOULDER WIDTH ............................... 18

1.3.3.2. GENERAL SHOULDER WIDTH ..................................................... 18

1.4. CROSS SECTION ELEMENTS ...................................................................... 191.4.1. GENERAL ................................................................................................. 19

1.4.2. CARRIGEWAY ........................................................................................ 21

1.4.3. SHOULDER .............................................................................................. 21

1.4.4. MEDIAN.................................................................................................... 21

1.4.5. PLATFORM .............................................................................................. 22

1.4.6. CROSSFALL ............................................................................................. 22

1.4.6.1. GENERAL .......................................................................................... 22

1.4.6.2. PAVEMENT CROSSFALL ............................................................... 22

1.4.6.3. SHOULDER CROSSFALL................................................................ 23

1.4.7. DRAINS ..................................................................................................... 23

1.4.7.1. SIDE DRAINS .................................................................................... 241.4.7.2. INTERCEPTER DRAINS .................................................................. 24

1.4.7.3. SUB-SOIL DRAINS ........................................................................... 24

1.4.7.4. CROSS SHOULDER DRAINS .......................................................... 25

1.4.8. BATTERS .................................................................................................. 25

1.4.8.1. CUT BATTER .................................................................................... 25

1.4.8.2. FILL BATTER.................................................................................... 25

1.4.8.3. SLOPE BENCHES ............................................................................. 26

1.4.8.4. SIGHT BENCHES.............................................................................. 26

1.4.8.5. FILL SLOPE TREATMENT .............................................................. 26

1.4.9. PARKING STRIPS .................................................................................... 27

1.4.9.1. GENERAL .......................................................................................... 27


2/227

ii

1.4.9.2. WIDTH OF PARKING STRIPS ........................................................ 27

1.4.9.3. PARKING BAYS ............................................................................... 27

1.4.9.4. CROSS FALL OF PARKING STRIPS AND BAYS......................... 28

1.4.10. RIGHT OF WAY ..................................................................................... 28

1.5. SIGHT DISTANCE .......................................................................................... 28

1.5.1. GENERAL ................................................................................................. 281.5.2. CONSTANTS USED IN DESIGN FOR SIGHT DISTANCE ................. 28

1.5.3. STOPPING SIGHT DISTANCE (SSD) .................................................... 29

1.5.4. EFFECT OF GRADE IN BREAKING ..................................................... 30

1.5.5. OVERTAKING SIGHT DISTANCE (OSD) ............................................ 31

1.5.6. CONTINUATION SIGHT DISTANCE (CSD) ........................................ 32

1.5.7. SIGHT DISTANCE AT GRADE CRESTS .............................................. 33

1.5.8. HEAD LIGHT SIGHT DISTANCE .......................................................... 33

1.6. HORIZONTAL ALIGNMENT ........................................................................ 33

1.6.1. INTRODUCTION ..................................................................................... 33

1.6.2. MOVEMENT ON A CIRCULAR PATH ................................................. 33

1.6.3. SIDE FRICTION FACTOR ...................................................................... 351.6.4. HORIZONTAL CURVATURE ................................................................ 36

1.6.4.1 GENERAL ........................................................................................... 36

1.6.4.2. GENERAL CONSIDERATIONS IN DESIGNING HORIZONTAL

ALIGNMENT .................................................................................................. 36

1.6.4.3. SIMPLE CIRCULAR CURVE........................................................... 37

1.6.4.4. TRANSITION CURVE ...................................................................... 39

1.6.4.5. COMPOUND CURVE ....................................................................... 39

1.6.4.6. REVERSE CURVE ............................................................................ 40

1.6.4.7. SIMILAR CURVES ........................................................................... 40

1.6.4.8. WIDENING OF CARRIGEWAY ON CURVES .............................. 40

1.7. SUPERELEVATION ....................................................................................... 42

1.7.1. GENERAL ................................................................................................. 42

1.7.2. STANDARDS FOR SUPERELEVATION ............................................... 42

1.7.3. CURVES WITH ADVERSE CROSSFALL ............................................. 42

1.7.4. DEVELOPMENT OF SUPERELEVATION ............................................ 43

1.7.4.1. AXIS OF ROTATION ........................................................................ 43

1.7.4.2. SUPERELEVATION DEVELOPMENT LENGTH .......................... 44

1.7.4.2.1. RELATIVE GRADIENT METHOD .......................................... 44

1.7.4.2.2. RATE OF PAVEMENT ROTATION METHOD ....................... 45

1.7.4.3. POSITIONING OF SUPERELEVATION ......................................... 48

1.7.4.4. SUPERELEVATION DEVELOPMENT ON SHOULDERS............ 511.8. VERTICAL ALIGNMENT .............................................................................. 53

1.8.1. GENERAL ................................................................................................. 53

1.8.2. GENERAL CONSIDERATIONS OF IN DESIGNING THE VERTICAL

ALIGNMENT ...................................................................................................... 53

1.8.3. VERTICAL CURVES ............................................................................... 54

1.8.3.1. CREST VERTICAL CURVES........................................................... 55

1.8.3.1.1. GENERAL ................................................................................... 55

1.8.3.1.2. LENGTH OF CREST CURVES ................................................. 55

1.8.3.2. SAG VERTICAL CURVES ............................................................... 61

1.8.3.2.1. GENERAL ................................................................................... 61

1.8.3.2.2. LENGTH OF SAG CURVES ...................................................... 611.8.4. GRADES.................................................................................................... 64


3/227

iii

1.8.4.1. GENERAL .......................................................................................... 64

1.8.4.2. STANDARDS FOR GRADES ........................................................... 65

1.8.4.3. GRADES STEEPER THAN GENERAL MAXIMUM ..................... 67

1.8.4.4. CRITICAL LENGTH OF GRADES .................................................. 67

1.9. OTHER CONSIDERATIONS.......................................................................... 68

1.9.1. GENERAL ................................................................................................. 681.9.2. CO-ORDINATION OF HORIZONTAL AND VERTICAL ALIGNMENT

FOR SAFETY ...................................................................................................... 68

1.9.3. CO-ORDINATION OF HORIZONTAL AND VERTICAL ALIGNMENT

FOR AESTHETIC REASONS ............................................................................ 69

1.10. REFERENCES ............................................................................................... 70

2.0 - GEOMETRIC DESIGN OF INTERSECTION .................................................. 72

2.1. INTRODUCTION ............................................................................................ 73

2.1.1. GENERAL ................................................................................................. 73

2.1.2. PROJECT DESCRIPTION ........................................................................ 74

2.2. BASIC DATA FOR DESIGN .......................................................................... 75

2.2.1. TRAFFIC DATA ....................................................................................... 752.2.1.1. PEAK HOUR TRAFFIC VOLUMES INCLUDING TURNING

MOVEMENTS ................................................................................................ 75

2.2.1.2. VEHICLE CHARACTERISTICS ...................................................... 80

2.2.1.3. PEDESTRIAN TRAFFIC ................................................................... 81

2.2.1.4. ACCIDENT DATA ............................................................................ 81

2.2.1.5. PARKING DATA ............................................................................... 81

2.2.1.6. PUBLIC TRANSPORT REQUIREMENTS ...................................... 81

2.2.2. SITE TOPOGRAPHY ............................................................................... 82

2.2.3. PHYSICAL FEATURES AND ITEMS OF CULTURAL, HISTORICAL

AND RELIGIOUS SIGNIFICANCE .................................................................. 82

2.2.4. OVERHEAD AND UNDERGROUND UTILITY SERVICES ............... 82

2.2.5. RIGHT OF WAY PROBLEMS AND LIMITATIONS ............................ 83

2.3. ANALYSIS OF DATA..................................................................................... 83

2.3.1. ANALYSIS OF DATA FOR THE SELECTION OF DESIGN VEHICLE

.............................................................................................................................. 83

2.3.2. ANALYSIS OF DATA FOR SELECTION OF TYPE OF

INTERSECTION ................................................................................................. 84

2.3.2.1. TYPE OF INTERSECTION BASED ON TREATMENT ................. 84

2.3.2.2. FACTORS AFFECTING SELECTION OF TYPE OF

INTERSECTION ............................................................................................. 86

2.3.3. BEST INTERSECTION TREATMENT FOR THE PROBLEM IN HAND.............................................................................................................................. 86

2.4. ROUNDABOUTS ............................................................................................ 87

2.4.1. TYPES OF ROUNDABOUTS AND SELECTION OF THE TYPE FOR

THE DESIGN ...................................................................................................... 87

2.4.1.1. MINI ROUNDABOUTS .................................................................... 87

2.4.1.2. SMALL ROUNDABOUTS ................................................................ 87

2.4.1.3. LARGE ROUNDABOUTS ................................................................ 87

2.4.1.4. DOUBLE ROUNDABOUTS ............................................................. 88

2.4.1.5. CONVENTIONAL ROUNDABOUTS .............................................. 88

2.4.2. ELEMENTS OF A ROUNDABOUT ........................................................ 88

2.4.2.1. CENTRAL ISLAND .......................................................................... 882.4.2.2. CENTRE ISLAND DIAMETER ........................................................ 89


4/227

iv

2.4.2.3. INSCRIBED CIRCLE DIAMETER................................................... 90

2.4.2.4. CIRCULATING CARRIAGEWAY .................................................. 90

2.4.2.5. CRCULATING CARRIAGEWAY WIDTH ..................................... 90

2.4.2.6 GIVE WAY LINE ............................................................................... 90

2.4.2.7. ENTRY CURVE ................................................................................. 91

2.4.2.8. EXIT CURVE ..................................................................................... 912.4.2.9. APPROACH WIDTH ......................................................................... 91

2.4.2.10. ENTRY / EXIT WIDTH ................................................................... 91

2.4.2.11. DEPARTURE WIDTH ..................................................................... 92

2.4.2.12. CORNER KERB RADIUS (KERB RETURN) ............................... 92

2.4.2.13. SPLITTER ISLAND ......................................................................... 92

2.4.3. GEOMETRIC DESIGN STANDARDS ................................................... 93

2.5. CAPACITY ANALYSIS AND DELAY CALCULATIONS .......................... 93

2.5.1. METHODS OF CAPACITY ANALYSIS ................................................ 93

2.5.1.1. GAP ACCEPTANCE THEORY ........................................................ 93

2.5.1.2. ANALYSIS PACKAGE ..................................................................... 94

2.5.2. THE METHOD SELECTED FOR THE CAPACITY ANALYSIS ......... 942.6. PRELIMINARY DESIGN AND SELECTION OF A LAYOUT .................. 123

2.7. GEOMETRIC DESIGN CALCULATIONS .................................................. 130

2.7.1. CIRCULATING TRAFFIC SPEED ........................................................ 130

2.7.2. THROUGH TRAFFIC SPEED ............................................................... 130

2.7.3. CIRCULATING CARRIGEWAY WIDTH ............................................ 132

2.7.4. ENTRY AND EXIT CURVES ................................................................ 134

2.7.5. SIGHT DISTANCE ................................................................................. 135

2.7.6. VERTICAL CURVES ............................................................................. 135

2.7.7. SPLITTER ISLAND ................................................................................ 135

2.8. REFERENCES ............................................................................................... 137

3.0 - DESIGN OF SURFACE WATER DRAINAGE SYSTEM FOR

INTERSECTION ....................................................................................................... 139

3.1. INTRODUCTION .......................................................................................... 140

3.1.1. GENERAL ............................................................................................... 140

3.1.2. PROJECT DESCRIPTION ...................................................................... 140

3.2. STORM WATER RUNOFF ESTIMATION ................................................. 141

3.2.1. INTENSITY OF RAINFALL .................................................................. 141

3.2.2. SELECTION OF DESIGN STORM RECURRENCE INTERVAL ....... 143

3.2.3. TIME OF CONCENTRATION ............................................................... 143

3.2.4. PEAK RUN-OFF ..................................................................................... 144

3.2.4.1. DIFFERENT METHODS FOR ESTIMATING OF PEAK RUN-OFF........................................................................................................................ 145

3.2.4.2. RATIONAL METHOD .................................................................... 146

3.2.4.2.1. RATIONAL FORMULA........................................................... 146

3.2.4.2.2. ADVANTAGES AND DISADVANTAGES OF THE

RATIONAL FORMULA........................................................................... 147

3.2.4.2.3. COEFFICIENT OF RUN-OFF .................................................. 147

3.2.4.3. APPLICATION IN THE PROJECT ................................................ 148

3.2.5. DEPTH OF FLOW AND LENGTH OF FLOW PATH .......................... 150

3.2.5.1. DEPTH OF FLOW ........................................................................... 150

3.2.5.2. LENGTH OF FLOW PATH ............................................................. 150

3.3. FLOW IN HIGHWAY GUTTERS ................................................................ 1503.3.1. INTRODUCTION ................................................................................... 150


5/227

v

3.3.2. FORMULA FOR FLOW IN TRIANGULAR GUTTER ........................ 151

3.3.3. PONDED WIDTH ................................................................................... 153

3.3.4. CALCULATIONS FOR FLOW IN GUTTERS...................................... 153

3.4. INLETS ........................................................................................................... 154

3.4.1. INTRODUCTION ................................................................................... 154

3.4.2. TYPES OF INLETS ................................................................................ 1543.4.2.1. GRATED INLETS............................................................................ 154

3.4.2.2. KERB OPENING INLETS............................................................... 155

3.4.2.3. COMBINATION OF GRATED AND KERB OPENING INLETS 156

3.4.3. SELECTION OF TYPE OF INLET FOR THE DESIGN ....................... 156

3.4.4. CAPACITY OF INLETS ......................................................................... 156

3.4.4.1. GENERAL ........................................................................................ 156

3.4.4.1.1. CAPACITY OF KERB OPENING INLET ON A

CONTINUOUS GRADE ........................................................................... 156

3.4.4.1.2. CAPACITY OF KERB OPENING INLET ON SAG ............... 157

3.4.4.2. SPECIMEN CALCULATIONS ....................................................... 157

3.4.5. GENERAL RULES FOR SPACING OF INLETS ................................. 1573.4.6. INLET COMPUTATIONS ...................................................................... 158

3.4.6.1. DESCRIPTION OF TABLE FOR INLET COMPUTATIONS AND

SPECIMEN CALCULATIONS .................................................................... 158

3.5. FLOW IN RECTANGULAR DRAINS ......................................................... 163

3.5.1. INTRODUCTION ................................................................................... 163

3.5.2. DESIGN CRITERIA ............................................................................... 163

3.5.3. CAPACITY OF RECTANGULAR DRAINS ......................................... 164

3.5.3.1. GENERAL ........................................................................................ 164

3.5.3.2. CALCULATIONS AND PREPARATION OF GRAPHS FOR

DESIGN ......................................................................................................... 164

3.5.4. STORM DRAIN COMPUTATIONS ...................................................... 165

3.6. REFERENCES ............................................................................................... 170

4.0 – BILL OF QUANTITIES ................................................................................... 172

4.1. SUMMARY OF THE BILL OF QUANTITIES ............................................ 173

4.2. BILL OF QUANTITIES ................................................................................. 175

4.3. TAKING OFF SHEETS ................................................................................. 180

4.4. RATE ANALYSIS FOR B.O.Q. ITEMS ....................................................... 208

LIST OF TABLES

Table 1.2.1 – Classification of Terrain .......................................................................... 5

Table 1.2.2 – Traffic data for Palugama-Boralanda-Haputale road ............................... 7

Table 1.2.3 – Relationship of the Design Speed related with the Road Classification,

Terrain and the Design Volume. .................................................................................... 9

Table 1.3.1 – Minimum width of sealed pavements of undivided roads ..................... 13

Table 1.3.2 – Design Level of Service ......................................................................... 14

Table 1.3.3 – Adjustment factors for directional distribution (f d) .............................. 16

Table 1.3.4 - Adjustment factors for the combined effect of narrow lanes and

restricted shoulder width (f w) ...................................................................................... 16

Table 1.3.5 - Average Passenger Car Equivalent for Trucks and Buses on Two Lane

Highways over Different Terrain Segments ................................................................ 16Table 1.4.1 - Recommended Cross-falls on Straight for Different Surface Types ...... 23


6/227

vi

Table 1.4.2 - Recommended Shoulder Cross-falls on Straight .................................... 23

Table 1.5.1 – Longitudinal Friction Factors ................................................................ 30

Table 1.5.2 - Design Values for Stopping, Overtaking & Intermediate Sight Distances

on Bituminous and Concrete Pavements ..................................................................... 32

Table 1.6.1 – Maximum Superelevaion Values ........................................................... 35

Table 1.6.2 - Maximum Design Values of Co-efficient of Side Friction .................... 36Table 1.6.3 – Minimum Radii for Different Superelevation ....................................... 38

Table 1.6.4 – Radii selected, with corresponding Superelevation ............................... 38

Table 1.6.5 – Design values for pavement widening on open highway curves ........... 41

Table 1.7.1 - Minimum Radii with Adverse Cross Fall ............................................... 43

Table 1.7.2 – Maximum Relative Gradients ................................................................ 45

Table 1.7.3 - superelevation values and corresponding length of superelevation

development for various radii and design speed .......................................................... 47

Table 1.8.1 - Minimum Crest Vertical Curve Length Based on Sight Distance Criteria

...................................................................................................................................... 56

Table 1.8.2 – Minimum Vertical Curve Length Based on Appearance Criterion ....... 58

Table 1.8.3 – Minimum Vertical Curve Length Based on Comfort Criterion ............. 60Table 1.8.4 - Minimum Sag Vertical Curve Length Based on Head Light Sight

Distance Criterion ........................................................................................................ 62

Table 1.8.5 – Maximum Gradients .............................................................................. 66

Table 1.8.6 – Critical length of Grades ........................................................................ 68

Table 2.2.1 (a) - Summary of Peak Hour Turning Moments in Morning Peak (0915 –

1015 Hrs) at Gabada Junction (in Veh / h) .................................................................. 77

Table 2.2.1 (b) - Summary of Peak Hour Turning Moments in Mid day Peak (1100 –


Table 2.2.1 (c) - Summary of Peak Hour Turning Moments in Evening Peak (1700 –


Table 2.2.2 - Equivalent Passenger Car Units ............................................................. 80

Table 2.3.1 - Composition of Vehicles at Hambantota Intersection during Peak Hours

of the Day ..................................................................................................................... 84

Table 2.5.1(a) - Summary of Peak Hour Turning Moments in Morning Peak (0915 –

1015 Hrs) at Gabada Junction (in PCU / h) ................................................................. 96

Table 2.5.1(b) - Summary of Peak Hour Turning Moments in Mid day Peak (1100 –


Table 2.5.1(c) - Summary of Peak Hour Turning Moments in Evening Peak (1700 –


Table 2.5.3 – No. of entry lanes and approach lanes in Hambantota intersection ..... 103

Table 2.5.4 – Widths of entry lanes and approach lanes in Hambantota intersection103Table 2.5.5 - Dominant- Stream Follow-up Headways (tfd) (Initials Values) in seconds

.................................................................................................................................... 105

Table 2.5.6 - Adjustment Times for the Dominant Stream Follow-up Headway ...... 106

Table 2.5.7 - Sub-dominant Steam Follow-up headway tfs ....................................... 106

Table 2.5.8 - Ratio of the Critical Acceptance Gap to the Follow-up Headway(tad / tfd )

.................................................................................................................................... 107

Table 2.5.9 - Average headway between bunched vehicles in the circulating traffic (τ)and the number of effective lanes in the circulating carriageway. ............................ 108

Table 2.5.10 - Proportions of Bunched Vehicles, θ ................................................... 109Table 2.5.11 (a) - Geometric Delay for Stopped Vehicles (Seconds per Vehicles) .. 116

Table 2.5.11 (b) - Geometric Delay for Vehicles, Which Do Not Stop (Seconds perVehicles) .................................................................................................................... 117


7/227

vii

Table 2.5.12 - Geometric Delay Values for Gonnoruwa Leg of Hambantota

Roundabout ................................................................................................................ 118

Table 2.5.13 - Geometric Delay and Total Average Delay Values for Movements of

Gonnoruwa Leg of Hambantota Roundabout. ........................................................... 119

Table 2.5.14 - Data Required for Capacity Calculations ........................................... 120

Table 2.5.15 - Capacity Calculations & Queuing Delay Calculations ...................... 121Table 2.5.16 (a) - Geometric Delay Calculations & Total Delay Calculations –

Morning Peak ............................................................................................................. 122

Table 2.5.16 (b) - Geometric Delay Calculations & Total Delay Calculations – Mid-

Day Peak .................................................................................................................... 122

Table 2.5.16 (c) - Geometric Delay Calculations & Total Delay Calculations –

Evening Peak ............................................................................................................. 123

Table 2.7.1 - Entry and Exit Curve Radius of Hambantota Roundabout .................. 134

Table 2.7.2 - The splitter island areas for Habmantota Roundabout ......................... 136

Table 3.4.1 – Inlet Computations ............................................................................... 160

Table 3.5.1 - Q/S½

values for varying depth ............................................................. 165

Table 3.5.2 – Storm Drain Computations .................................................................. 169

LIST OF FIGURES

Figure 1.1.1 – Location Map of Palugama-Boralanda-Haputale road. .......................... 4

Figure 1.4.1 – Typical cross section of a road ............................................................. 20

Figure 1.7.2 - Edge profile for a Circular curve .......................................................... 49

Figure 1.7.3 - Edge profile for Reverse curve ............................................................. 50

Figure 1.7.4 - Edge profile for Compound curve......................................................... 51

Figure 1.7.5 – Development of superelevation in sealed shoulder .............................. 52Figure 1.7.6 – Development of superelevation in unsealed shoulder .......................... 52

Figure 1.8.1 – Basic Elements of a typical vertical curve ........................................... 54

Figure 1.8.2 – Crest vertical curves ............................................................................. 55

Figure 1.8.3 – Sag vertical curves ................................................................................ 61

Figure 2.2.1 – Traffic Data at the intersection from 7.00 to 19.00 hrs ........................ 76

Figure 2.4.1 – Geometric Elements of a Roundabout .................................................. 89

Figure 2.5.1(a) – Traffic turning volumes in Morning Peak hour (9.15-10.15) .......... 99

Figure 2.5.1(b) – Traffic turning volumes in Mid day Peak hour (11.00-12.00) ....... 100

Figure 2.5.1(c) – Traffic turning volumes in Evening Peak hour (17.00-18.00) ....... 101

Figure 2.5.2 – Required No. of Circulating and Entry lanes ................................... 102

Figure 2.5.3 –Proportion of vehicles stopped on a multi lane entry roundabout ....... 115

Figure 2.5.4 – Definitions of the terms used in Table 2.5.11(a) and 2.5.11(b) .......... 118

Figure 2.5.5 – Alternative 1 ....................................................................................... 125





Figure 2.7.1 – Derivation of turning roadway width on curves at intersection ......... 132

Figure 3.2.1 – Rainfall Intensity – Duration Curves .................................................. 142

Figure 3.2.2 – Coefficient of Run-off ........................................................................ 149

Figure 3.3.1 – Flow in Triangular Gutter ................................................................... 151


8/227

viii


9/227

1

1.0 - GEOMETRIC DESIGN OF ROADS

REPORT

ON

GEOMETRIC DESIGN

OF

PALUGAMA-BORALANDA-HAPUTALE ROAD

(B 353) FROM 0+000 TO 27+200(SECTION FROM 2+800 TO 5+450)


10/227

2

1.1 INTRODUCTION

1.1.1 GENERAL

Geometric design is an important aspect of Highway Design dealing with the visible

dimensions of a roadway. To produce a well balanced design, all geometric elements

such as

Horizontal alignment

Vertical alignment

Sight distances

Cross-section components

Lateral and vertical clearance

Intersection treatment

Control of access

should economically feasible as far as possible, while providing safe continuous

operation at a speed likely under the general conditions for the highway. Mainly this

is achieved through the use of design speed as the overall control. In the design of

highway curves, it is necessary to establish the proper relation between design speed

and curvature and their joint relations with super elevation and side friction.

A well designed highway has to be consistent with the economy. There are some

other factors influence the geometric design are the availability of funds and the

adjacent land use. If design is carried out improperly it may result in early

obsolescence of the new highway with considerable economic losses.


11/227

3

1.1.2. PROJECT DESCRIPTION

Palugama-Boralanda-Haputale road, which is approximately 27 km in length “B

353” route, connects the two major suburb towns, Keppetipola and Haputale in Uva

Province. The road stretch bears an Annual Daily Traffic of 1399 vehicles. Currently

it functions as two lane two way road with an average carriageway width of 5.0 m

and it runs on a rolling terrain of average gradient being 8%.

Carriageway is not divided into lanes and no built up drains are provided. Bends are

not met with geometric standards. The travelers those who are going visit the

Adisham Bungalow which is a historic place popular among the locals and foreignershave to go along the this particular road. If the mobility had been improved in this

road it will be helpful for the tourism industry. These factors had been inspired to

improve this road.

The other factors lead to improve the above road is the poor vertical alignment,

narrow carriageway width, poor sight distance and humpy road surface.

This report describes the general principles of the geometric design and its

application to the detailed design of Palugama-Boralanda-Haputale road from

chainage 2+800 to 5+450.


12/227

4

Figure 1.1.1 – Location Map of Palugama-Boralanda-Haputale road.


13/227

5

1.2. DESIGN CONTROL & CRITERIA

The design criteria determines roadway geometric. The principal factors influencing

the choice of design standard for a particular road section are as follows.I. Topography.

II. Traffic.

III. Speed.

IV. Safety.

V. Economic consideration

VI. Financial level.

1.2.1. TOPOGRAPHY

Terrain type is one of the main factors which govern the vehicular speed. Terrain also

has a significant effect on the costs of achieving high geometric standards. Therefore

terrain can be considered as one of the main element used in selecting the design

criteria. In geometric design, roads are divided into sections with uniform terrain

characteristics, i.e. Flat, Rolling, Mountainous. Table 1.2.1 gives the classification ofterrain based on its representative slope.

Table 1.2.1 – Classification of Terrain

Terrain Type Code Corridor Slope %

Flat

Rolling

Mountainous

F

R

M

10

10-25

>25

The topography of the section from 2+800 to 3+500 and from 4+750 to 5+450 of

Palugama-Boralanda-Haputale road can be considered as flat. The remaining section

is a Rolling terrain.


14/227

6

1.2.2. TRAFFIC

Traffic composes of the following groups.

1.

The road user2. Traffic data

3. The vehicle

1.2.2.1. ROAD USER

As the road user (Drivers and Pedestrians) is the main part of the road system. Human

behavior and limitations should be considered in all road designs.

1.2.2.2. TRAFFIC DATA

The geometric design of roads is mainly based on the traffic data. Traffic data is

classified as follows.

I. Volume of Traffic.

A road should be designed so that it will accommodate, or can be readily

changed to accommodate, the number of vehicles which is estimated will be

using at the end of its service life with a desired level of service. The traffic

volume is usually the Annual Average Daily Traffic (AADT) and number of

traffic lanes required is a function of traffic volume. The number of vehicles

using a road in a given time determines the number of traffic lanes required.

II.

Composition of Traffic.

Road traffic is a mixture of passenger cars and commercial vehicles. Compared

to passenger cars, commercial vehicles are slow in speed occupy a greater length

and width of road, and on ascending grades, their speeds fall to lower figures

quicker. Therefore such vehicles on a road reduce its capacity and a road

capable of carrying a set number of passenger cars will carry fewer mixed

vehicles. The composition of traffic is a factor in the determination of width of

traffic lanes and to some extent, the maximum grade on a road.


15/227

7

Traffic data given by the Traffic & Planning Division of Road Development Authority

in Palugama-Boralanda-Haputale road is tabulated in Table 1.2.2.

Table 1.2.2 – Traffic data for Palugama-Boralanda-Haputale road

Vehicle Composition ADT = 1399

Percentage %

Motor Cycle

Three wheel

CarVan

Medium Bus

Large Bus

Light Goods Vehicles

Medium Goods Vehicles

Large Lorries

Farm Vehicles

28.99

31.39

8.155.66

0.69

7.89

5.49

6.17

2.49

3.09

1.2.2.3. THE VEHICLE

The vehicle data which affect the design are axle load, axle spacing, wheel base,

overall length, width, height and minimum turning radius.

The design vehicle is a motor vehicle whose weight dimensions and operating

characteristics are used to establish design controls. For this purpose vehicles are

classified into following groups.

I. P design vehicle - Passenger Car class

II. SU design vehicle - Single unit trucks and buses

III. WB-40 design vehicle - Semi-trailor intermediate

IV.

WB-50 design vehicle - Semi-trailor combination


16/227

8

1.2.3. SPEED

Speed maintained by a driver for a given road section depends upon following

conditions.

I. Type of terrain.

II. Weather.

III. Volume of traffic.

IV. Speed limitations.

In a road section having uniform topographical character all curves should be of a

uniform speed value. At the places where there is a change of topography and if it is

necessary to change the speed from one section to another it is desirable to change the

speed in steps of 10km/h contributing to safe driving condition.

Design speed applies to individual geometric elements and is the speed that is used to

co-ordinate sight distance, curve radius, super elevation and friction demand for

elements of the road so that the driver negotiating each element at its design speeds.

The design speed for this particular road is 40km/h. But the topographic constrains

limit the design speed to 30km/h in some sections.

Table 1.2.3 gives the design speed values corresponding to the road class, terrain type

and design volume.


17/227

9

Table 1.2.3 – Relationship of the Design Speed related with the Road Classification,

Terrain and the Design Volume.

Type of

RoadRoad Class Terrain

Design

Volume

PCU/day

Design Speed

Km/h

Rural Urban

R0 A F72,000-

108,00080 70

R1 AF

40,000-72,00080 70

R 70 60

R2 A,B

F

25,000-40,000

80 70

R 70 70

M 60 60

R3 A,B

F

18,000-25,000

70 60

R 60 60

M 50 50

R4 C,D

F

300-18,000

60 50

R 50 50

M 40 40

R5 D,E

F


18/227

10

traffic control devices. To provide safety of pedestrians, sufficient width of sidewalks,

proper signing and pedestrian crossing to be accommodated.

The existing trace is having poor geometric features such as inadequate sight

distances at the bends and inadequate carriageway and shoulder width. As these sub

standard will tend to promotes accidents, in designing new trace adequate amounts

were provided for above.

1.2.5. ECONOMIC CONSIDERATIONS

Before taking the final decision on the horizontal alignment on a particular road, it isnecessary to compare the economic aspect of various alternative of horizontal design.

Above mentioned economic aspect should be made on,

The cost required for,

I. Construction.

II. Acquisition.

III. Vehicle operation.

A properly designed highway afford benefit to the community as it improve

transportation facilities, reduction of travel time, reducing accidents, dropping of

vehicle maintenance cost and enhancing vehicle lifetime.

In the design of Palugama-Boralanda-Haputale Road, attempts were made to

establish more favorable geometric features by following the existing centre line of

the road. Cost components of the project, when providing the improved facilities will

be the cost of land acquisition, cost of construction and cost of maintenance. Due

consideration was paid in establishing limitations for geometric elements such as

maximum grade, minimum curve radius, sight distance requirement etc. to keep the

total cost to minimum. Efforts were taken to reduce the number of buildings affected

due to the improvements.


19/227

11

1.3. WIDTH OF TRAFFIC LANES & SHOULDERS

1.3.1. GENERAL

Width of traffic lane influence the safety, comfort of driving and Level Of Service of

that particular road. With the increase of lane width vehicles could be driven freely in

a broaden width. Hence it provides safety and comfort of driving. The width of

carriageway is determined in terms of the number of traffic lanes and width of a

traffic lane. Number of lanes to be provided depends on the present and future

anticipated traffic volume. Shoulder is the portion traveled adjoining to the outer edge

of the traffic lane. Wider shoulder will be provided emergency stops for the vehicles.

1.3.2. FACTORS AFFECTING THE LANE WIDTH

When calculating the lane widths following factors are take into account.

I. Traffic –

The volume and composition of traffic are the major factors which

determine the width of traffic lanes. Average Annual Daily Traffic

(AADT) of the particular road and peak hour traffic volume are required.

II. Vehicle Dimension –

Commercial vehicles in traffic stream also influence the lane width.

Normal steering deviations as well as the tracking errors and pavement

imperfections reduce the clearance between passing vehicles.

III. Speed environment –

Drivers have less control over the lateral position of a vehicle at high

speed. Therefore at higher Design Speeds, high width of traffic lane is

required.


20/227

12

IV. Combination of speed and traffic volume –

When both the speed and traffic volume are high, narrow lane width

should be avoided.

1.3.2.1. VEHICULAR TRAFFIC

The importance of vehicular traffic in the event of determination of traffic lane width

was explained under Section 3.2.

1.3.2.2. GENERAL LANE WIDTHS

The width of carriageway is determined in terms of the number of traffic lanes and

width of a traffic lane. A traffic lane is defined as the width used for single line of

traffic operation. The desirable lane width is taken as 3.7m. The absolute minimum

lane width shall be 3.1m.

1.3.2.3. DERIVATIONS OF NUMBER OF LANES

a) By tables in NASSRA

Table 1.3.1 gives the recommended minimum width of sealed carriageway of

undivided roads.


21/227

13

Table 1.3.1 – Minimum width of sealed pavements of undivided roads

Design

Speed

Km/h

Minimum carriageway width(m) for design traffic

volumes(AADT)

1-140 141-300 301-1100 1101-2200 Over 2200

40 3.7 5.6 6.2 - -

50 3.7 5.6 6.2 6.8 7.4

60 3.7 6.2 6.8 6.8 7.4

70 3.7 6.2 6.8 6.8 7.4

80 3.7 6.2 6.8 7.4 7.4

90 3.7 6.2 6.8 7.4 7.4

100 3.7 6.8 6.8 7.4 7.4

110 3.7 6.8 7.4 7.4 7.4

120 3.7 6.8 7.4 7.4 7.4

As per Table 1.3.1 minimum carriageway width required, corresponding to design

speed of 30-50km/h and traffic volume (AADT) of 1399 is 6.8m. However taking in to

account the acquisition problems and construction cost a 6.4m wide carriageway is

provided.

b) Capacity analysis – level of service concept

The concept of Level Of Service (LOS) is the different operational qualities of the

traffic flow. Level Of Service is a qualitative measure of speed and travel time,

freedom to maneuvers, traffic interruption, driving comfort, convenience, safety and

operating cost.

Six Level Of Service are categorized from A to F and it cover entire range of traffic

condition which may occur.

Level Of Service A – Free flow with high speed and low volumes. Drivers can hold

their desired speeds without delays.


22/227

14

Level Of Service B – Reasonably free flow. Stable flow and drivers have reasonable

freedom to select their speed.

Level Of Service C – Stable flow. Most drivers are restricted in their freedom to select

their own speed, change lanes, overtake etc... But operating speeds are still

reasonable.

Level Of Service D – Approaches unstable flow with nearly all drivers restricted.

Comfort and convenience are low but may be tolerated for short periods. Fluctuations

in conditions cause substantial drops in speed. As this service volume corresponds to

what is referred as tolerable capacity, this level of service should be used to determine

the upper limit of traffic demand which should be tolerated.

Level Of Service E – Unstable flow and there may be momentary stoppages. This

LOS is obtained with traffic volumes with near or at capacity.

Level Of Service F – Forced or breakdown flow operating at low speed caused by the

demand exceeding capacity. There is stop-start operation with large queues and

delays. In the extreme both speed and volume can drop to zero.

New roads are normally designed for LOS C or even D. The design level of service

may be as in the Table 1.3.2.

Table 1.3.2 – Design Level of Service

Road ClassDesign

speed(km/h)

Design Level Of

Service

Volume/Capacity

ratio

A

B

C,D,E

70 or more

60 or more

50 or more

C

D

E

0.6-0.8

0.8-0.9

>0.9


23/227

15

The capacity is the maximum hourly rate at which vehicles can reasonably be

expected to traverse a point or uniform section of a lane at LOS E.

The service flow rate is the maximum hourly rate of a lane or roadway under

prevailing roadway, traffic and control conditions while maintaining a designated

level of service. To calculate the service flow rate, equation 1.3.1 and 1.3.2 is applied.

HV wd i

i f f f cvSF ****2800= ……. (1.3.1)

( ) ( )[ ]1111 −+−+= B BT T HV E P E P f ……. (1.3.2)

Where:

SFi = Total service flow rate in both directions for prevailing roadway and

Traffic conditions, for level of service i, in vph

(v/c)i = Ratio of flow rate to ideal capacity for level of service i, obtained from

Table 1.3.2

f d = Adjustment factor for directional distribution of traffic, obtained from

Table 1.3.3. In the absence of directional distribution factor, it is better to use

60/40 as the directional distribution factor

f w = Adjustment factor for narrow lanes and restricted shoulder width, obtained

from Table 1.3.4

f HV = Adjustment factor for the presence of heavy vehicles in the traffic stream

PT = Proportion of trucks in the traffic stream, expressed as a decimal

PB = Proportion of buses in the traffic stream, expressed as a decimal

ET, EB = Passenger car equivalent for trucks and buses obtained from Table 1.3.5


24/227

16

Table 1.3.3 – Adjustment factors for directional distribution (f d)

Table 1.3.4 - Adjustment factors for the combined effect of narrow lanes and

restricted shoulder width (f w)

Usable

Shoulder

Width

(m)

3.7 m Lanes 3.4 m Lanes 3.0 m Lanes 2.7 m Lanes

LOS

A - D

LOS

E

LOS

A - D

LOS

E

LOS

A - D

LOS

E

LOS

A - D

LOS

E

>=1.8 1.00 1.00 0.93 0.94 0.84 0.87 0.70 0.76

1.2 0.92 0.97 0.85 0.92 0.77 0.85 0.65 0.74

0.6 0.81 0.93 0.75 0.88 0.68 0.81 0.57 0.70

0 0.70 0.88 0.65 0.82 0.58 0.75 0.49 0.66

Table 1.3.5 - Average Passenger Car Equivalent for Trucks and Buses on Two Lane

Highways over Different Terrain Segments

Vehicle Type Level Of Service Type of Terrain

Level Rolling Mountainous

Trucks, ET

A 2.0 4.0 7.0

B and C 2.2 5.0 10.0

D and E 2.0 5.0 12.0

Buses, EB

A 1.8 3.0 5.7

B and C 2.0 3.4 6.0

D and E 1.6 2.9 6.5

Directional Distribution 100/0 90/10 80/20 70/30 60/40 50/50

Adjustment Factor, f d0.71 0.75 0.83 0.89 0.94 1.00


25/227

17

Specimen Calculation for Palugama-Boralanda-Haputale road;

Average Daily Traffic = 1399(In 2009)

Present peak Hour Traffic = 10% of ADT

= 140

f d =0.94(From Table 1.3.3- assuming 60/40

distribution)

f w = 0.77 (from Table 1.3.4)

(v/c) I =0.8 (from Table 1.3.2)

PT =17.24% (5.49%+6.17%+2.49%+3.09%)

P B =8.58% (0.69%+7.89%)

E T =5.0

E B =2.9

From Equation 1.3.2,

F HV = 1 /{ 1+[17.24 x (5.0 –1)/100 + 8.58 x (2.9 –1)/100]} = 0.54

From Equation 1.3.1

SF i = 2800 x 0.8 x 0.94 x 0.77 x 0.54 = 875

Traffic growth rate is 5% and assuming the road will reach to its capacity after n

years.

875 = 140 x (1.05)n

n =37.5

The road will maintain the LOS D for 37 years from 2009.


26/227

18

1.3.3. SHOULDER WIDTHS

1.3.3.1. FACTORS AFFECTING SHOULDER WIDTH

The width of shoulder should be chosen in relationship to the traffic and topography

after due consideration of the following

a) Provision of space for -

• Maneuvering to escape potential accidents.

• Emergency and rest stops.

• Emergency operations

•

Guide posts or guard fence

b) Reduction of driving strain by the provision of extra width.

c) Separation of drains from pavement with resultant decrease in seepage effects.

d) Structural support for the pavements.

e) Appearance and traffic capacity of the road.

1.3.3.2. GENERAL SHOULDER WIDTH

The minimum shoulder width for two-way rural roads unless the volumes are below

150 vpd should be 1.0 m. Shoulders less than 1.0 m width result in vehicles on the

adjacent lane travelling closer to the road centre line. A width of 1.5 m to 2.0 m

ensures capacity of the adjacent lane is unaffected by obstructions outside the

shoulder. These widths allow a vehicle to stop, or a maintenance vehicle to operate

with only partial obstruction of traffic lanes.

A lane width of 2.5 m is needed to allow a passenger vehicle to stop clear of the

traffic lane. A width of 3.0 m allows a passenger vehicle to stop clear of the traffic

lanes and provides an additional clearance to passing traffic. It also allows a

commercial vehicle to stop clear of the traffic lanes.


27/227

19

1.4. CROSS SECTION ELEMENTS

1.4.1. GENERAL

The cross section of a road consists of the following items. (Refer Figure 1.4.1)

I. Carriageway.

II. Shoulder.

III. Median.

IV. Platforms.

V.

Cross fall.

VI. Drains.

VII. Batters.

VIII. Parking Strips.

IX. Right Of Way.


28/227

20

Figure 1.4.1 – Typical cross section of a road


29/227

21

1.4.2. CARRIGEWAY

The carriageway is the surface on which the vehicles are expected to run.

Carriageways are classified as single-lane, two-lane or multi-lane. A traffic lane is

defined as the width used for single line of traffic operation. The desirable lane width

is taken as 3.7m. The absolute minimum lane width shall be 3.1m. For multi-lane

roads carriageway is divided into two parts by introducing a median. Then vehicles

are moving in opposite direction at each part. Such carriageway is called a duel-

carrigeway.

The width of the carriageway selected for Palugama-Boralanda-Haputale Road is 6.4

m (3.2m x 2).

1.4.3. SHOULDER

Shoulder is the portion traveled adjoining to the outer edge of the carriageway that

accommodates stopped vehicles, emergency use and lateral support of sub base, base

and surface course. In the absence of foot walk, shoulders could be used for

pedestrians also. It may be of compacted earth or gravel. In some cases shoulders may

be totally or partly sealed. The desirable width of shoulder is 3.0m and minimum

shoulder width may be 2.4m and absolute minimum width may be 1.8m.

The shoulder width selected for this particular road is 1.2 m.

1.4.4. MEDIAN

Median is the land of dividing the carriageway in order to avoid the collision of

opposing vehicles. Normally center median accommodate for 4-lane or multi-lane

roads. To clearly identify the centre median from the carriageway, median may be

raised, flush or provide small humps at regular intervals. The main advantages of

providing the centre median are to separate opposing traffics, provide refugee area for


30/227

22

crossing pedestrians, provide a stopping area in case of emergency situation, allow

space for U-turning vehicles, provide open green space and minimize opposing head

light glare at night.

No centre median is provided for Palugama-Boralanda-Haputale road since it was

designed as two way two lane road.

1.4.5. PLATFORM

Carriageway and shoulder on either side when taking together is called as the

platform.

A platform width of 10.6 m is selected for Palugama-Boralanda-Haputale road.

1.4.6. CROSSFALL

1.4.6.1. GENERAL

The carriageway crossfall is the slope of the carriageway measured at right angles to

the horizontal alignment and shoulder crossfall could be similarly defined. The main

purpose of introducing crossfall is to ensure sufficient drain the road surface.

Generally shoulder crossfall is steeper than the carriageway crossfall and should be

varied with the surface material.

1.4.6.2. PAVEMENT CROSSFALL

The carriageway is cambered to form an inverted “U” which is rounded at its highest

point known as crown. The recommended crossfall values for various surface

materials are tabulated in Table 1.4.1.


31/227

23

Table 1.4.1 - Recommended Cross-falls on Straight for Different Surface Types

Type of surface on carriageway Recommended Crossfall

Portland Cement Concrete

Asphalt Concrete

Surface Seals

Unsealed Gravel

2.0%

2.5%

3.0%

4.0%

Since Asphalt Pavement surface is applied for Palugama-Boralanda-Haputale road,

2.5 % slope is selected as normal cross-fall.

1.4.6.3. SHOULDER CROSSFALL

The shoulder cross-fall is generally steeper than the carriageway cross-fall and the

cross-falls should be varied with the surface material.

The recommended crossfall values for the shoulder are tabulated Table 1.4.2.

Table 1.4.2 - Recommended Shoulder Cross-falls on Straight

For Palugama-Boralanda-Haputale road, 4.0 % slope is selected as the soft shoulder

cross-fall.

1.4.7. DRAINS

Drains are provided for collecting and conveying of water from the road surface. The

selected cross section of drain should have the sufficient capacity to dispose the rain

water collected from the road platform.

Type of Shoulder Recommended Crossfall

Bitumen or other all weather surface

Gravel

3-4%

4-5%


32/227

24

1.4.7.1. SIDE DRAINS

These are located on outside of shoulder in either side. The surface water should be

drained away as quickly as possible before seepaging water from top of pavement

layers to subgrade. To overcome this issue side drain of adequate section shall be

provided. The cross section may be either trapezoidal if that is earth drain or

rectangular if that is concrete drain.

For Palugama-Boralanda-Haputale road 0.9m wide side drains are provided on

either side of the road where necessary.

1.4.7.2. INTERCEPTER DRAINS

Intercepter drains are also known as catch or cut-off drains. These are located at the

top of cut slopes behind the rounded batter. Their purpose is to interrupt the seepage

water within the upper soil layer and flow of surface water and prevent the scour of

the cut slope face. Intercepter drains are occupied as open channels.

No intercepter drains are provided for this particular section on Palugama-

Boralanda-Haputale road since embankment cut height is always less than 6m.

1.4.7.3. SUB-SOIL DRAINS

Sub-soil drains deals with drainage of underground water. The main purpose of

providing sub-soil drains are:• Intercept groundwater.

• To drain the subgrade and pavement during and after construction.

• Drain wet areas below the surface or outside the carriageway.

• Stabilize or lower the water table.

As the finished level of Palugama-Boralanda-Haputale road lies considerably above

the water table, the necessity of providing of sub-soil drains is not arisen.


33/227

25

1.4.7.4. CROSS SHOULDER DRAINS

Cross shoulder drains (mitre or boxing drains) are designed to drain the pavement

through the shoulder, usually via a coarse permeable filter media. This type of drains

is generally occupied to places where water is collected during the construction stage.

Therefore it is not included in the design.

Cross Shoulder Drains were not provided in the design of Palugama-Boralanda-

Haputale road.

1.4.8. BATTERS

1.4.8.1. CUT BATTER

Cut batter slope will depend on the depth of cut and soil properties. For general

conditions a cut slope of 4:1 could be adopted. It is preferable to limit the depth of cut

to 6m when adopting 4:1 cut slope. For special condition of soil properties and site

conditions it is better to investigate the batter stability.

A cut slope of 4:1 is adopted, in the design of Palugama-Boralanda-Haputale road

where necessary.

1.4.8.2. FILL BATTER

Fill batter slope will depend on the soil properties. For general conditions a fill slope

of 1:1.5 could usually be adopted.

A fill slope of 1:1.5 is adopted for the design of Palugama-Boralanda-Haputale road

where required.


34/227

26

1.4.8.3. SLOPE BENCHES

When material in a cut slope is sufficiently unstable to warrant flatter than average

slopes, then providing of benches may be more suitable in the case of deep cuts. The

necessity of benches in unstable material, their width and vertical spacing should be

determined only after an adequate materials investigation. When cuts exceed 12m

consideration shall be given to benching of batters to facilitate maintenance and

possibly improve aesthetics. Benches should be sloped to form a valley along the

centre so that rain water can be drained off toward the ends of bench and discharged

onto natural ground where possible. Minimum width of bench should be 3m with a

maximum crossfall of 0.10m/m. The slope should be suitably protected by grassing.

In the design of Palugama-Boralanda-Haputale road, slope benches were not

provided as no deep cuttings were involved.

1.4.8.4. SIGHT BENCHES

When benches are provided mainly for purpose of improving the sight distances, they

are called as sight benches.

In the design of Palugama-Boralanda-Haputale road, the minimum sight distance

was provided without adopting sight benches.

1.4.8.5. FILL SLOPE TREATMENT

The run-off from the pavement surface and the shoulders ultimately flows over the

slopes of the embankment. The extent of erosion of fill slope from surface run-off

depends on various factors such as soil type, slope of the embankment, and rainfall

intensity. The measure adopted overcome the erosion is called fill slope treatment.

Various types of treatments are available such as

a. Turfing

b. Pitching with rip-rap


35/227

27

c. Raised curbs, gutters and flumes

d. Geotextiles

e. Bituminious treatment

Turfing is used as fill slope treatment in designing of Palugama-Boralanda-Haputale

road.

1.4.9. PARKING STRIPS

1.4.9.1. GENERAL

Every vehicle owner wishes to park the vehicle as close as possible to his destination

so as to minimize his walking. Also spaces are required to vehicles to load or unload

goods. This results a great demand for parking spaces in the urban area where bare

land around the road is limited.

1.4.9.2. WIDTH OF PARKING STRIPS

The most commonly used angle parking arrangement provides for 450 parking on a

strip 4.9 m wide, measured at right angle to the kerb. For parallel parking

arrangement, a 2.4 m strip is the minimum acceptable width and this is the usual

width marked on the pavement.

1.4.9.3. PARKING BAYS

In urban areas, parking bays should be provided for commercial vehicles to load and

unload merchandise and customer’s vehicles. Location of parking bay is important as

it influence the success in attracting motorists.

In the design of this particular section of Palugama-Boralanda-Haputale road,

parking bays were not provided, as the trace was located in un-urbanized area.


36/227

28

1.4.9.4. CROSS FALL OF PARKING STRIPS AND BAYS

Generally 3% -4% cross fall is adopted for parking strips in both angle and parallel

parking.

1.4.10. RIGHT OF WAY

Right of Way is the clear width required to accommodate all the facilities of the road.

In the design of Palugama-Boralanda-Haputale road, 12 m wide right of way is

provided.

1.5. SIGHT DISTANCE

1.5.1. GENERAL

Sight distance in road design is the distance at which a driver of a vehicle can see an

object of specified height on the road ahead of him assuming satisfactory conditions

prevail to ensure visibility. A driver ability to see ahead is of the most importance in

the safe and efficient operation of a vehicle on a road. For safety on road the designer

should provide sight distance of sufficient length that drivers can control the vehicles

to avoid striking an unexpected object on traveled way.

1.5.2. CONSTANTS USED IN DESIGN FOR SIGHT DISTANCE

In calculating various sight distances, following values of reaction time and heights of

objects have been adopted.

a) Total reaction time

This is the time between the instant the hazard comes into view and the

instant the vehicle begins to slow after breaks have been applied. (Taken

as 2.5sec.)


37/227

29

b) Driver eye height

Passenger car = 1.05m

Commercial Vehicle = 1.80m

c) Object cut off height above road surface

Approaching vehicle = 1.15m

Stationary Object = 0.20m

Vehicle tail height/Stop light = 0.60m

Height of head light = 0.75m

Upward divergence angle = 1.00deg

Vertical clearance = 5.20m

1.5.3. STOPPING SIGHT DISTANCE (SSD)

Stopping sight distance is the minimum distance required by an average driver of a

vehicle traveling at a given speed to react and stop before reaching an object in its

path. Stopping sight distance is measured from the driver’s eyes, which are assumed

to be 1.05m above the pavement, to an object 0.20m high on the road.

Stopping sight distance has two compartments; the distance traveled during total

reaction time and the distance traveled during breaking time.

………….. (1.5.1)

SSD =Stopping Sight Distance (m)

t R =Total Reaction Time (sec)

V =Speed of vehicle (km/h)

µ =Coefficient of Longitudinal Friction between vehicle tyres and road

pavement assumed constant throughout the breaking period.

µ 2546.3

2V V t

SSD R +=


38/227

30

The value of Coefficient of Longitudinal Friction ( µ) varies with speed, tyre pressure,

tyre condition, type of pavement and whether the surface is dry or wet. The

Coefficient of Longitudinal Friction ( µ) values assumed are tabulated in Table 1.5.1.

Table 1.5.1 – Longitudinal Friction Factors

Design Speed (kmph) Friction Factor ( µ)

30

40

50

60

70

80

90

100

0.40

0.38

0.35

0.33

0.31

0.30

0.30

0.29

Specimen Calculation for Palugama-Boralanda-Haputale road,

V = 40 km/h

f = 0.38 (Refer Table 1.5.1)

t R = 2.5s

From Equation 1.5.1,

SSD = 2.5 x 40/3.6 + 402 / (254 x 0.38)

= 44.35 m

By rounding, SSD = 45 m

1.5.4. EFFECT OF GRADE IN BREAKING

The distance a vehicle travels while being breaked to a halt is longer on downhill

grades and shorter uphill. Therefore above breaking distance formula adjusted to take

into account the effect of grade is;


39/227

31

………….. (1.5.1)

G = Longitudinal grade as a percentage (+ uphill, - downhill)

The calculated stopping sight distances for each speed are given in Table 1.5.2.

1.5.5. OVERTAKING SIGHT DISTANCE (OSD)

The distance needs to see ahead to safely overtake the front vehicle moving at

constant speed is called the Overtaking Sight Distance (OSD). Vehicles in two-lane,

two-way highways need to overtake slower moving vehicles. If passing is to be

accomplished safely, the passing driver should be able to see a sufficient distance

ahead, clear of traffic, to complete the passing manoeuvre without cutting off the

passed vehicle before meeting an opposing vehicle that appears during the

manoeuvre. The actual safe overtaking sight distance depends on many variables, but

the following simplifying assumptions could be made.

• The overtaken vehicle travels uniformly at one step lower than the

design speed.

• The overtaking vehicle trails the overtaken vehicle as it enters the

overtaking section.

• Overtaking manoeuvre is accomplished by accelerating in the early part

of the manoeuvre up to reaching design speed and completing the

manoeuvre at the same speed.

• Only one vehicle at a time is overtaken.

With these assumptions, calculated sight distances required for overtaking at various

speeds are also tabulated in Table 1.5.2.

( )GV V t

SSD R

01.02546.3

2

±+=

µ


40/227

32

As the road is located in mountainous terrain and due to land acquisition problems

overtaking sight distance was not strictly followed.

1.5.6. CONTINUATION SIGHT DISTANCE (CSD)

Continuation sight distance is equal to twice the stopping sight distance needed for

two drivers traveling at design speed approaching each other head on to stop before

colliding. It provides reasonable opportunity to overtake assuming that a driver has a

choice to either proceed or withdraw when he is nearly opposite the overtaken

vehicle.

If continuation sight distance could be provided, it will automatically fulfill the

stopping sight distance values for speeds greater than the specified design speed.

The design Continuation sight distance values are tabulated in Table 1.5.2

Table 1.5.2 - Design Values for Stopping, Overtaking & Intermediate Sight Distances

on Bituminous and Concrete Pavements

Design speed(kmph)Stopping Sight

Distance(SSD) - m

Continuation Sight

Distance(CSD) - m

Overtaking Sight

Distance(OSD) -

m

30

40

50

60

70

80

100

30.0

45.0

65.0

85.0

115.0

140.0

205.0

60.0

90.0

130.0

170.0

230.0

280.0

410.0

160

220

280

350

430

520

690

Height of Eye (m)

Height of Object (m)

1.05

0.20

1.05

1.15

1.05

1.15


41/227

33

1.5.7. SIGHT DISTANCE AT GRADE CRESTS

Stopping, Overtaking and Intermediate Sight Distances required can be achieved by

designing appropriate vertical curve radius and curve lengths for different speeds.This will be discussed in the Section 1.8.

1.5.8. HEAD LIGHT SIGHT DISTANCE

This illuminated distance is important traveling at night, because the sight distance at

night is governed by the distance illuminated by the head light beam. This effect is

discussed in Section 1.8.

1.6. HORIZONTAL ALIGNMENT

1.6.1. INTRODUCTION

The important consideration in determining the horizontal alignment of a road is the

provision of safe and continuation operation for substantial lengths of roadway. The

horizontal alignment of a road is usually a series of straights and circular curves

connected by transition curves.

When introducing horizontal curves, following aspects such as the minimum radii,

transition lengths, pavement widening and superelevation required has to pay special

attention.

1.6.2. MOVEMENT ON A CIRCULAR PATH

When a vehicle moves in a circular path it undergoes a centripetal force that act

toward the centre of curvature. In road design it is assumed that the above force is

balanced by the side friction developed between tyre & pavement and by the

superelevation.


42/227

34

From the first principals the following equation could be derived for normal values of

superelevation;

RV f e

127

2=+

………….. (1.6.1)

Where;

e = Superelevation.

f = Coefficient of side friction between vehicle tyres & road pavement.

V = Design speed (kmph).

R = Curve radius (m).

Superelevation is a percentage that the pavement to be superelevated. The

superelevation to be adopted is chosen primarily for safety, other factors being

comfort and appearance.

By rearranging the above formula, a minimum curve radius can be determined using

the maximum values for superelevation (emax) and maximum side friction factors

(f max) for a given design speed(V).

( )maxmax

2

min127 f e

V R

+=

………… (1.6.2)

Therefore maximum values for superelevation and side friction factors have to be

determined. The maximum superelevation used on highways is controlled by many

factors. Those are climate condition, terrain type, type of area and frequency of slow

moving vehicles whose operation might be affected by high superelevation rates.

Consideration of these factors jointly leads to the conclusion that no single maximum

superelevation rate is universally applicable that a range of value should be used.

Therefore the range of values suit Sri Lanka for Maximum superelevation (emax) was

tabulated in Table 1.6.1.


43/227

35

Table 1.6.1 – Maximum Superelevaion Values

Terrain typeemax

Open Build-up

Flat

Rolling

Mountainous

6%

8%

10%

6%

6%

6%

Also it is necessary to specify a minimum value for superelevation due to a minimum

crossfall is defined for drainage considerations. Therefore it is recommended to take

minimum superelevation equal to normal crossfall although for larger radii, a small

superelevation is sufficient for stability.

1.6.3. SIDE FRICTION FACTOR

The side friction factor represents the vehicle’s need of side friction. The upper limit

of the side friction factor is the point at which the tyre would begin to skid. This is

known as the point of impending skid. Because the highway curves are designed to

avoid skidding condition with a margin of safety, side friction factor value used in

design should be substantially less than the coefficient of friction at impending skid.

By considering safety and comfort the maximum values of side friction factor adopted

for the design of horizontal curves for paved and unpaved roads are tabulated in Table

1.6.2.


44/227

36

Table 1.6.2 - Maximum Design Values of Co-efficient of Side Friction

Design speed(kmph)Maximum design values of coefficient of side friction

Bituminous Roads Gravel Roads

30

40

50

60

70

80

90

100

0.210

0.190

0.170

0.160

0.150

0.140

0.130

0.128

0.140

0.130

0.120

0.110

0.100

0.090

-

-

1.6.4. HORIZONTAL CURVATURE

1.6.4.1 GENERAL

Horizontal curvature is a shape to provide for places where change in direction occurs

of the designed centre line. Depending on the changes of the direction, one of the

following curve types could be introduced at the design stage.

I. Simple circular curve.

II. Transition curve.

III. Compound curve (Unidirectional curve).

IV.

Reverse curve.

1.6.4.2. GENERAL CONSIDERATIONS IN DESIGNING HORIZONTAL

ALIGNMENT

The following general controls for horizontal alignment should be kept in view in a

sound design practice.


45/227

37

I. Alignment should be as directional as possible but should be consistent with the

topography and with preserving developed properties.

II. The number of short curves should be kept to a minimum.

III. For a given design speed, use of minimum radius for that speed should be

avoided wherever possible.

IV. Sharp curves should not be introduced at the ends of long tangents.

V. Sudden changes from areas of flat curvature to areas of sharp curvature should

be avoided.

VI. At places where sharp curvature must be introduced it should be approached by

successively sharper curves from the generally flat curvature.

VII. The minimum length of horizontal curve on main highways should be about

three times the design speed.

VIII. On high long fills, tangents or flat curves should be used as far as possible.

IX. Compound curves with large difference in curvature should be avoided. The

radius of the flatter circular curve should not be more than 50% greater than the

radius of the sharper circular curve.

X. Introducing short tangents between two curves in the same direction should be

avoided except where topography or right-of-way restrictions make their use

necessary.

1.6.4.3. SIMPLE CIRCULAR CURVE

These are plain circular curves and simplest to use. In designing the curves followings

will be governed.

I. Minimum radius of curve.

II. Minimum length of curve.

By substituting values of maximum superelevation (emax) and maximum side friction

factors (f max) minimum curve radius could be calculated for a given design speed.

These values are shown in the Table 1.6.3.


46/227

38

Table 1.6.3 – Minimum Radii for Different Superelevation

Design

Speed(kmph)

Superelevation

2.5% 3.0% 4.0% 5.0% 6.0% 7.0% 8.0% 9.0% 10.0%

30 35 30 30 30 30 30 25 25 25

40 60 60 55 55 55 50 50 45 45

50 105 100 95 90 90 85 80 80 75

60 155 150 145 135 130 125 120 115 110

70 225 215 205 195 185 180 170 165 155

80 310 300 280 270 255 240 230 220 210

90 415 400 380 355 340 320 305 290 280

100 515 500 470 445 420 400 380 365 350

In this design the radius selected for two curves and their corresponding

superelevation values are as in the Table 1.6.4

Table 1.6.4 – Radii selected, with corresponding Superelevation

Chainage Radius of the

Curve (m)Superelevation

3+272.04 – 3+327.28 70 2.5%

3+737.00 – 3+791.99 160 2.5%

To accommodate the super-elevation development length (Ls) at two edges of the

curve, curve length should be sufficient enough. The length of 32 Ls is required to

accommodate superelevation development at two edges and full superelevation is

accommodated over length equal to Ls.

Thus minimum total length of the curve should be equal to 35 Ls.

Curve lengths are provided to fulfill above requirement in Palugama-Boralanda-

Haputale road.


47/227

39

1.6.4.4. TRANSITION CURVE

Transition curves are introduced between tangents and circular curves, between two

tangents, between two similar curves or between two reverse curves. Clothoid(spiral

curve) is used as the transition curve.

The advantages of adopting the transition curve in horizontal alignment are the

following.

I. To provide a natural, easy-to-follow path for drivers, such that the lateral

force increases and decreases gradually as a vehicle enters and leaves a

circular curve.

II. To provide a length over which the super elevation development can be

applied.

III. To provide flexibility in accomplishing the widening of sharp curves.

IV. To improve the appearance of the road by avoiding sharp discontinuities in

alignment at the circular curves.

Transition curves are used when the design speed is higher than 70km/h. Since

maximum design speed used in Sri Lanka is less or equal to 70 km/h use of transition

curve has been limited.

In designing Palugama-Boralanda-Haputale road, transition curve is not been used.

1.6.4.5. COMPOUND CURVE

This type comprises of successive curves of different radii in the same direction, with

common or closely spaced tangent points. The radius of the larger curve should not be

higher than 50% of the radius of the smaller curve. Generally compound curve should

be avoided and a single curve shall be adopted if topography and ground control is not

making an obstacle.

In designing of Palugama-Boralanda-Haputale road, compound curves are used.


48/227

40

1.6.4.6. REVERSE CURVE

These curves consists with successive curve having common or closely spaced

tangent points but in opposite direction. Reverse curve having common tangent point

make difficulty in achieving change of superelevation rates. Therefore short tangent

could be introduced between two curves to overcome the above problem.

In designing of Palugama-Boralanda-Haputale road, reverse curves are used.

1.6.4.7. SIMILAR CURVES

This consists of two curves in the same direction formed by a short tangent. Attempt

should be made to avoid this type of alignment by adopting a Simple Curve or a

Compound Curve. When Similar Curves are unavoidable, tangent lengths in the range

of 0.6 V to 3 V should be avoided. (Where V is the Design Speed in km/h for the

particular trace)

In designing of Palugama-Boralanda-Haputale road, similar curves were not used.

1.6.4.8. WIDENING OF CARRIGEWAY ON CURVES

Carriageways are widened on some curves because a vehicle passing through on a

curve occupies a greater width than it goes on a straight and to avoid difficulty in

steering their vehicles in the centre of the lane. The amount of widening required

depends on;

I. Radius of the curve.

II. Vehicle length and width.

III. Width of lane on straight.

IV. Lateral clearance between two vehicles.


49/227

41

In general practice minimum amount of widening is limited to 0.6m. The amount of

widening for a SU vehicle and for two lane highway is given in the table 1.6.5.

Table 1.6.5 – Design values for pavement widening on open highway curves

Radius of

curve (m)

6.2 meters

Design speed (km/h)

30 40 50 60 70

300 0.5 0.6 0.6 0.7 0.7

250 0.5 0.6 0.7 0.7 0.8

200 0.6 0.7 0.8 0.8 0.9

150 0.8 0.9 1.0 1.1 1.1

140 0.8 0.9 1.0 1.1 -

130 0.9 0.9 1.0 1.1 -

120 0.9 1.0 1.1 1.2 -

110 0.9 1.0 1.1 1.2 -

100 1.0 1.1 1.2 - -

90 1.1 1.2 1.3 - -

80 1.1 1.2 1.3 - -

70 1.2 1.3 - - -

60 1.3 1.5 - - -

50 1.6 1.7 - - -

40 1.8 - - - -

30 2.0 - - - -


50/227

42

1.7. SUPERELEVATION

1.7.1. GENERAL

When vehicle travels along a curve it experiences a force called centrifugal force.

Centrifugal force increases when the velocity of the vehicle increases and the radius

of the curvature decreases. To balance the effects of the centrifugal force curves

should be superelevated. The amount of superelevation will depend on the radius of

the curve, design speed and pavement surface characteristics.

1.7.2. STANDARDS FOR SUPERELEVATION

The superelevation to be adopted is selected for safety, comfort and appearance. Use

of maximum superelevation should be confined to mountainous terrain or places

where there are physical obstructions which confine the radius of the particular curve

with respect to the adjacent curves.

The maximum superelevation varies from 10% in m

design report print

Documents