AttJ7_Feeder_Raods_design_and_typical_drawings.pdf

This Design Manual has been prepared by the Ministry of Public Works (MPW) under the bilateral agreement for Institutional Capacity Building between Government of the Republic of Liberia represented by MPW and the Swedish Government represented by SIDA.

The Manual gives guidance and recommendations to the Engineers from Government and Private Firms (consultants and contractors) who are responsible for the design of rural feeder roads in Liberia. It complements the Government of Liberia efforts to provide guidelines for construction by setting uniform standards to be used in development of infrastructure to meets the needs-of the society.

The Manual will serve as nationally recognized document, the application of which is deemed to serve as a standard reference and ready source of good practice for the design of rural feeder roads, and will assist in a cost effective, safe and an environmentally optimized development of the country's rural feeder road network.

The benefits to be gained in applying this Manual are the harmonization of professional practice and the ensuring of appropriate levels of safety and economy with due consideration of the objective conditions and need of the country.

It is a technical document, which, by its very nature, requires periodic updating from time to time arising from the dynamic technological developments and changes. Future technological innovations or proposals will be welcome from implementing partners cooperating with Ministry of Public works in development of the road network.

The Ministry of Public Works wishes to acknowledge the efforts of Hifab International AB for the Technical Assistance provided in the preparation and compilation of the document, and the Department of Feeder Roads for their significant contributions towards the preparation work. The stakeholders in particular from ILO Project, German Agro-Action, USAID Liberia, GIZ Road Maintenance and Capacity Building Project, African Development Bank Funded Labour-based Unit, Engineering Society of Liberia, Liberia Swedish~eeder Roads Project, MPW i1ndCounty Resident Engineers are gratefully acknowledged for their input and comments from the preparation workshop.

Ministry of Public Works Monrovia

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The Feeder Road Design Manual sets forth the policy and standards to be adopted for the design of rural feeder roads in Liberia. It may also find application in some r1Jral roads whose functions and characteristics-may be similar to feeder roads. The manual is divided into two .parts and the works designed in accordance with this manual are also specified in another manual for specifications. The parts and sister manuals are:

1. Part I : Feeder Road Design Manual

2. Part II: Standard Structure Design Manual

3. Technical Specifications For Feeder Road Works

The purpose of this design manual is to give guidance and recommendations to the engineers responsible for the design of rural feeder roads in Liberia.

The contents of the manual are partly guidelines and recommendations to be considered, and partly standards which as a general rule should be adhered to. In some instances special conditions may demand modifications to these standards, in which case special consideration should be given in consultation with the Engineer-In-Chief.

The design engineer should realize that the adoption of the guidelines and standards in this manual does not automatically ensure safe, economic, structural sound and aesthetic road and structures design.

Road design to the aforementioned properties can only be achieved through a careful consideration and balancing of criteria or elements, controls and conditions that prevail. This and many other manuals used in road or engineering design often set limiting conditions of standards, whose uncritical use, more often than not, may produce an inadequate design ..

It is recommended that every design engineer familiarizes herself jhimself with the basic ideas, principles and derivatives in this manual and any other associated manuals or policies within the land

·····--·----as5O€iated~-tJ:H~·d~t----

Chapters 1:Introduction Chapter 2: The Road System Chapter 3: Design Controls and Criteria Chapter 4: Geometric Design Elements

Chapter 5: Pavement Chapter 6: Drainage Chapter 7: Road Furniture

Chapter 1 discusses aspects of purpose, arrangements, and units of measure and definitions which may be used for the interpretation of the rest of other contents.

Chapter 2 which discusses the road system in Liberia is largely helps to understand the classification and other policies relating to the road classes.

Chapter 3 contains discussions on various factors that influence road design, which the design engineer needs to use to base her/his design decisions on.

The Chapters from 4 - 7 specifically address the issues of design principles, methods, recommendations and in some cases limiting standards for the various physical elements of a road. The major caregeries in which the elements are set form the Chapter title.

Part II: Standard Structure Design Manual

The contents of this part include typical structures presented by the following:

a. Typical Structure Drawings

b. Format or typical Standard specifications

c. Typical Standard details

The standard units of measurement to be used are based on the International system (SI) units.

However, the units applicable to road design also include some units which are not strictly part of SI.

Multiples and sub-multiples of SI units are formed either by the use of indices or prefixes. Definitions of applicable prefixes are given in table 1.1 below. The basic units and the derived 'and supplementary units which willl)ormally be required for road design are listed in the table 1.2 below.

Prefix Symbol Factor By Which the Unit Is Multiple Mega M 106

Kilo K 103

Hecto H 102

Deca da

Oeci d 10'1

Centi C 10'2

Milli mm 10'3

Micro 11 10'6

Item Unit Symbol Recommended Multiples

1000 Meter (m) = 1 Kilometer (Km); 1 Meter (m) = Length Meter m 1000 Millimeter (mm)

1000Kg = 1 Mega-gram (Mg) or 1 tonne (ton); 1Kg = Mass Kilogram Kg 1000grams (g)

Time Second s 60s = 1 Minute (min); 3600s = 1 hour (h);

Area Square Meter m2 10,000m2 = 1 Hectare (ha) Volume (Solids) Cubic meter m3 1 m3 = 10,000 cm3 Volume (liquids} Litre I 1000 litres (I) = 1 m3; 1 litre (I) = 1000mililitre (ml)

Kilogram per cubic Density meter Kg/m3 1000 Kg/m3 = 1 ton/m3; 1kg/m3 = 1000g!cm3

1000N = 1 Kilonewton (KN); iN = 1 Kgm/s2; 1 kgf = Force Newton N 9.81N

Pressure and Newton per Stress square meter N/m2 1 KN/m2 = 1000N/m2; 1 KN/m2 = 0.001N/mm2 Velocity Meter per second m/s 1 Km/h = 1/3.6 m/s Angle Degree or grade Temperature Degree celcius °c

Basic conversion table is provide to allow conversion of linear or other measures found in this manual from Sl to Imperial System: This conversion can be found in table 1.3 below.

Item Measure and Unit Conversion Factor and Unit Length 1 m 3.2808 Feet (ft)

39.3701 Inches (In)m - -. -

1 m - 1.0936 Vard (V)

1 Km 0.6214 Miles ..

Mass 1 Kg 2.2050 Pounds.

Area 1 m2 10.7639 Square ft.

1 m2 1.1960 Square Vard 1 ha 2.4711 Acres

Volume (Solids) 1 m3 35.3147 ft3

1 m3 1.3080 y3 Volume (liquids) 1 I --- 0.2200 gallons (UK)

1 I 0.2642 gallons (USA) Density 1 Kg/m3 0.0624 pounds/ft3 Force 1 N 0.2248 pounds - force Velocity 1 m/s 3.2808 ft/s

1 km/h 0.6214 Miles/h

Terms to be used to describe the road parts and works must be clearly understood by all members from both from the design, employer and the contractors technical team. The proper use of clearly defined and well-understood terminology will avoid misunderstandings and ensure clarity of purpose.

Feeder Road: Lowest level of road in the network hierarchy with the function of linking traffic to and from rural areas, either directly to adjacent urban centers, or to the collector road network.

Paved Roads: Paved roads are those which have a-homogenous and strengthened carriageway surface, which consists of bitumen, concrete or other pavement materials such as stones and bricks.

Construction: The process by which a road is built according to established design standards and work methods.

Rehabilitation: Activities which restore a road's geometric characteristics to the original recommended design standards.

Upgrading: Is the process by which the standard of an existing road is improved to allow safe use by a greater volume of traffic than originally designed for.

Maintenance: The work required to keep the road, its structures and property within the road margins as near as possible to their as-constructed or rehabilitated condition.

Cross-Section: Vertical section showing the elevation of the existing ground, ground data and recommended works, usually at right angles to the centerline.

Horizontal Alignment: Arrangement of a road on the plan view showing a series of straight lines connected by curves.

Vertical Alignment: Longitudinal section of a road referring to the surface level of the completed road along the carriageway centerline.

Road Reserve: Strip of land legally awarded to the Road Authority in which the road is or will be situated and where no other work -or construction may take place without permission from the Road Authority.

The width of the road reserved is measured at right angles to the centerline of the road and varies according to the classification of the road.

Shoulders: Paved or unpaved width of the road between the edge of the carriageway and the shoulder breakpoint. The shoulder provides side support for the pavement or gravel surface and allows vehicles to stop or pass in an emergency.

Shoulder Breakpoint: The line along which the extended flat planes of the surface of the shoulder and the ditch inside slope or inside slope of the embankment /pavement intersect.

Camber: The lateral slope(s) of the cross-section of the carriageway and shoulder, constructed to drain the rainwater from the carriageway to the side drains.

Crossfall: The difference in level measured traverse across the surface of the carriageway expressed as a percentage (%).

Road Surface: The top layer of the pavement on a paved road. It consists of wearing course and sometimes a a base ceurse or binder course. On the gravel road, it is the gravel surface.

Subgrade Surface: Constructed upper layer of the natural or imported soil (free from unsuitable material) which supports the pavement layer or gravel surface.,0-

Fee'der Road Design ,Manual

Embankment: Constructed fill material below the pavement or gravel surface raising the road above the surrounding natural ground level.

Drain Back Slope (Drain Outside Slope): The outer slope of the side drain with an appropriate angle to prevent soil from sliding into the ditch.

Superelevation: Inward tilt or transverse indination given to the cross-section of a carriageway throughout the length of a horizontal curve to reduce the effects of centrifugal forces on a moving vehicle. Superelevation is expressed as a percentage.

TYPICAL CROSS - SECTION FOR GRAVELED ROAD

Feeder Roa~ Design Manual I

I I

I

LABEL CODING AND TERMS

1- ROAD RESERVE (RIGHT OF WAY)

2 - FORMATION WIDTH

3 - ROADWAY

4 - CARRIAGEWAY

5 - SHOULDER

6 - SHOULDER BREAK POINT

7 - CAMBER

8 - CROSSFALL

9-CROWN

10 - PAVEMENT (GRAVEL ROAD)

11 - PAVEMENT (SURFACED ROAD)

12 - ROAD BASE

13 - SUB-BASE

14 -IMPPROVED SUBGRADE SURFACE

15 - ORIGINAL GROUND LEVEL

16 - EMBANKMENT

17 - EMBANKMENT SLOPE

18 - CUT

19 - CUT SLOPE

20 - DRAIN INVERT

21· DRAIN INSIDE SLOPE

22 - DRAIN BACK SLOPE (OUTSIDE SLOPE)

23 - ROAD CENTRELINE

I

-I

I i

TYPICAL CROSSI- SECTION FOR PAVED ROAD

I I

LABEL CODING AND TERMS

1- ROAD RESERVE

2 - FORMATION WIDTH

3· ROADWAY

4 - CARRIAGEWAY

5· SHOULDER

6· SHOULDER BREAK POINT

7 - CAMBER

8 - CROSSFALL

9- CROWN

10 - PAVEMENT (GRAVEL ROAD)

11 • PAVEMENT (SURFACED ROAD)

12 • RqAD BASE

13 • SJB-BASE

14 - SUBGRADE SURFACE

15 - ORIGINAL GROUND LEVEL

16 - EMBANKMENT

17 - EMBANKMENT SLOPE

18 - CUT

19 - CUT SLOPE

20 - DRAIN INVERT

21- DRAIN INSIDE SLOPE

22 - DRAIN BACK SLOPE (OUTSIDE SLOPE)

23 - ROAD CENTRELINE

Sloe Of'V,JNLEFT UNEXCAVATEO TO OIVEfH WA1'ER INTO MITRE DRAIN

Side Drain: Long flat-bottomed excavation running along the road side, designed to collect and drain surface runoff water from the carriageway and adjoining land, away from the roadway to a suitable point of disposal.

Mitre Drain: Mitre drain leads the water out of the side drains and safely disperses it onto adjoining land.

Catch Water Drain: Catch water drain is a ditch constructed on the uphill side designed to intercept or collect and drain away surface runoff water flowing towards the road from the uphill side, and lead it to a suitable point of disposal.

Scour Check: Scour check is a small structure placed across the-'drairrun-sreep--gradients-cmttis-designed to slow down the flow of water to prevent erosion of drain invert and slopes.

Headwalls: Retaining wall at the entry or exit of the culvert to retain and protect the embankment or retained soil/gravel.

Wingwalls: Retaining wall at the side of the culvert or large structures to retain and protect the embankment or retained soil.

Cutoff Wall: A vertical wall under the headwall to prevent water seeping under the structure and undermining it.

Invert: The lowest point on the cross section through the culvert opening. This usually varies through the length of the culvert.

Culvert: The culvert is a structure constructed under the road and is designed to allow water from the drains and/or natural water course to safely cross-under the roadway.

Drift (Ford): Drift is a low level structure constructed to allow water from the drains and/or natural water course to safely cross over the road at bed level.

Vented Ford (Vented Drift or Causeway): Vented ford/drift is a medium level structure designed to allow the normal flow of water in a natural water course to pass safely through openings below the roadway and to be overtopped during, periods of heavy rainfall.

Bridge: A structure providing a Figures1.7: Single Span Bridge means of crossing safely above water, railway or other obstruction whether natural or artificial.'-'"""'" ••,,_----·-

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Roads have two basic service functions as to provide traffic mobility between centers and areas; and to provide access to land and properties adjoining the roads.

-Roads that function to provide mobility, for through and long-distance traffic, high and uniform-speeds and uninterrupted traffic flows are desirable. Roads whose function is to provide land access, high speeds are unnecessary and undesirable for safety reason. The function of a particular road in the national, regional and local road network has a significant impact on the design and careful consideration be given in the early stages of the design. The design process should include following steps:

1. Classify the road in accordance with its major function.

2. Determination of the level of access control compatible with the function of the road.

3. Selection of geometric design standards compatible with function and level of access control.

Design features that can convey the level of functional classification to the driver include carriageway width, continuity of alignment, spacing of functions, standards of alignment and grades, traffic controls and road.reserve widths.

The Road Functional Classification establishes a hierarchy of roads according to the importance of each road in the road network, the socio-economic function they serve and intend to serve. The classification enables the agency that is responsible for the development and maintenance of the roads to plan and programme road maintenance and upgrading works. Design standards and level of maintenance_are directed by the road functional class, together with other indicators such as existing and predicted traffic on the road.

1. Primary Roads: Linking the major urban centers, County centers and main border posts.

2. Secondary Roads: Providing the connections between adjacent counties and linking main local centers to the primary network.

3. Feeder Roads: These constitute the tertiary and other access roads. As tertiary roads, they provide additional connections within the county and districts, accessing and linking to secondary and primary roads. Some of the feeder roads provide access to one or more villages or settlements.

The Feeder and Secondary roads generally serve similar functions and accommodate shorter trips mainly feeding the primary road or the major centers directly. Some feeder roads are even more local in trip lengths providing access between villages or their local collection center. The level of service for these roads may be intermediate or low and therefore require intermediate or low design speeds.

In order to preserve major roads as high standard, efficient and safe traffic facilities it is necessary to exercise access control, whereby the right of providing access to the surrounding is controlled by the authority responsible for roads.

In case of rural feedeF--Feads access is largely unrestricted, meaning that preference is given to local traffic, with the road serving the adjoining areas through direct access connection. For these roads the location and layout of the accesses should be subject to approval by the authority in order to ensure adequate standards of visibility or aspects like surfacing and drainage.

Right-of-way also known as road reserves, are provided in order to accommodate the ultimate planned roadway or to meet the future needs of expansion during the life of the road and includes all cross sectional elements and areas to enhance the safety, operation and appearance of the roads. The width of right-of-way should depend on class ofthe road, the cross section elements of the road, topography and other physical controls together with economic considerations. Right-of-way is a matter of National Ordinance and any applicable measures are to be determined by the Ministry of Public Works.

Class for Road Right of Way Reduced Right- of- Way Primary Roads 50m [150'] 30m [100'] Secondary Roads 30m [100'] 20m [50'] Feeder Roads 15m [50'] 15m [50']

Red~ced widths may be adopted when these are found necessary for economic, financial or environmental reasons in order to preserve valuable land, resources or existing development or when proviSion of the desirable width would incur unreasonably high costs because of physical constraints .

.~ 12

The factors that influence or control road design decisions like location, configuration, standards and their combination may include the following:

1. Physical features - Topography and Land Use

2. Environmental Considerations

3. Road Safety Considerations

4. Road Service and Access Level

5. Traffic Considerations

6. Economic and Financial Constraints

7. Soil and Materials

8. Technology Options

In terms of road design, the topography traversed in Liberia is described as flat terrain, rolling terrain and hilly to mountainous terrain. The Country is dominated by the rolling hills most of which fall under the rolling terrain. The various terrain types are often defined by both subjective description and by the average ground slope. •

Ser. Terrain Percent cross-slope of Classification the land

1 Flat (Level) 0% to 5% Generally level or slightly rolling terrain with largely terrain unrestricted horizontal and vertical alignment.

Minimum alignment standards will rarely be necessary. Roads will,~for the most part, follow the

--_ ... ____ 0 _________ ._..._ .. ground contours and amounts of cut and fill will be.

. very small.

2 Rolling terrain Over 5% to 12.5% Rolling terrain with low hills introducing moderate levels of rise and fall with some restrictions on vertical alignment. Whilst low standard roads will be able to follow the ground contours with small amounts of cut and fill, the higher standards will

<-' require more substantial amounts 3 Hill and Over 12.5% Rugged, hilly and mountainous with substantial

Mountainous restrictions in both horizontal and vertical alignment.

terrain Higher standard roads will generally require large amounts of cut and fill.

Millistry of P~blic Works I Feeder Road Design Manual

The cost of road improvement, construction and maintenance will be greater as the terrain becomes steeper. Higher standards will become less justifiable or achievable in such situations than for roads in either flat or rolling terrain. ~peed limitation will normally become an option to check in order to reduce accidents while providing minimum standards in conditions where they are unjustifiable. Design speed and road geometry will therefore vary with terrain.

Man made features like agricultural, industry, commercial, residential and recreational developments are important controls for road alignment. Care should be taken to avoid avoiding unnecessary destruction, demolition or severance of valuable properties. Information regarding land use and other physical features should be obtained from physical planners or the local authorities' in-order to coordinate the project with other planned land use.

Safety is one of the main objectives in geometric design. It is important that the design engineer considers safety features to be built into the road from the very start of the design. Safety considerations in roads have the objectives to provide design features to:

a. Provision of physical separation between motor vehicles in opposing directions and also with other road users (especially pedestrians and cyclists);

b. Provision of a balanced design, Le. compatibility between the various design elements;

c. Avoidance of surprise elements for the drivers, for example abrupt changes in sti\ndard, insufficient visibility or poor phasing of horizontal and vertical alignment;

d. Avoidance of situations where drivers must make more than one decision at the time;

e. Proper location and design of intersections;

f. Proper design, application and location of traffic signs, road markings and other traffic control devices;

g. Provision of design elements compatible with traffic volumes and type of traffic;

h. Provision of proper drainage of the road surface;

L Upgrading bridge safety features such as bridge rails, approach rails, connections, and terminals; Reducing the potential hazard of existing roadside features such as trees~ .

/bushes which obstruct,

2. Reduce the severity of accidents:

a. Road side lighting poles, or other utility poles to be placed far enough;

b. Roadside slopes should not be too steep, (where resources can allow side slopes of 1:4 or flatter);

c. Safety barriers should be provided to protect vehicles from hitting dangerous obstacles that cannot be removed or made breakaway and also to protect vehicles from falling of the road down embankments.

a. Provide shoulders where the traffic is relatively higher;

b. separate foot tracks - in some circumstances it may also be used by cyclists.

c. For roads through trading centre and towns the options are to provide improved shoulders and where the road is paved, pave the shoulders; walkway / bicycle way; and where the road is paved, raise the kerbed footway

The location and design of the road should aim at maximizing the positive effects and minimizing the negative effects. Environmental impact assessments are therefore required for every road design, the scope of which depends on the size and complexity of the road project.

• Effects on areas of with plant and animal life worth of protecting e.g. forest land and wild life sanctuaries.

• Effects on areas with historic and cultural remains or landscapes.

• Effect on land of high agricultural value or potential

• Effect on settlement of population

• Potential sedimentation or progressive soil erosion

• Whether there will be very high rates of use of scarce materials

• Whether it will causes illegal logging

• Changes in the way of life of the local population

• Potential of health hazards like water ponding or swamp formation

• Effect on natural water bodies like swamps, or surface and ground waters e.g. pollution, drying up

• Effect on manmade works e.g. drainage systems, properties etc.

• Increase in noise (from traffic) and air pollution (especially with gravel roads)

• Potential effects of vibrations during construction and operation

• Potential conflicts regarding land use and ownership of land.

• Illegal invasion of squatters on communal land

• Effect on the be-aut¥ or aesthetic values of the surrounding e.g. severance of areas, reduction in visual intrusion5-

Feeder roads are often located from settled centers as they must be accessed. therefore traffic related ----------nuisance (Ii~e dust and accident) need other types of solution like speed reduction controls near settled centers.

Therefore the design engineer needs incorporate environmental mitigation measures of the road project under design in his designs, specifications, execution methods and project budget estimates. The design engineer shall ensure that an environmental checklist and management plan is prepared and submitted for approval.

The costs of road infrastructure, its operation and maintenance to a large extent depend on its design.

The economic outcome of the design will be decided both in the route selection and in the geometric design of the chosen route .

Ministry of Public Works I Feeder Road Desig'1 Manu'!l

Although it may be desirable to construct or reconstruct all improvements to high or full design standards, the reality is that there are not nearly enough funds available for all sectors needed by the society at anyone time. Therefore, in design the engineer must apply some economic analysis or judgment for each road improvement and come up with the best balance between the desired design standards, the safety and mobility of the public, and the available finances. A cost-benefit and objective analysis including as many effects as possible should be made for all studied alternatives.

Roads that may not have high level traffic will have low benefit accruing from traffic operation. For such roads the most important economic considerations is the construction and maintenance cost over the design life of the infrastructure. In order to make improvement of these roads justifiable, costs must be saved on the construction and maintenance process. This can be possible by designing these projects with appropriate standards, using appropriate local resource based construction and maintenance technology, and targeting opportunities that reduce the maintenance cost while improving the design life. Limiting the geometric standard to what is needed for the level of traffic is often considered appropriate.

The design of road, or any part thereof, should be based upon factual information on traffic volumes which the road will have to accommodate. Traffic directly affects the decision on improvement of geometric features of road such as widths, horizontal and vertical alignments. Traffic is also responsible for the deterioration of road pavements among other factors.

Traffic data for road design include volumes for days of the year and times of the day as well as the distribution of vehicles by types and by weights. The data also include information on trends from which the designer may estimate the traffic to be expected in the future.

For low volume roads the design control is Average Annual Daily Traffic (AADT) in the "design year". For routes with large seasonal variations the design control is Average Daily Traffic (ADT) during the peak months of the "design year". The "design year" is the last year of the design life of the road or any other facility.

Design volume is the volume of traffic estimated or expected to use a certam.faGilit.y..4tiAAg.the-design.-----year, which may be 8-10years (unpaved gravel roads) or 10 - 20 years in the future.

(a). Average Annual Daily Traffic (AADT) :The total traffic volume for the year divided by 365. For two-lane rural road it includes traffic in both directions.

(b). Average Daily Traffic (ADT): The total traffic volume during the given time period (in whole days), greater than one day and less than one year, divided by the number of days in that time period. For two-lane rural road iyfncludes traffic in both directions. Knowledge of ADT is important determining annual usage in justifying proposed expenditures and for design of structural elements of a road. Its direct use in geometric design of road is not appropriate because it does not indicate the variation in the traffic occurring during the various months of the year, days of the week, and hours of the day.

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The prediction of future traffic growth for rural roads constructed from earth or gravel is difficult. The design life and maintenance cycle for such roads is relatively short. As a result, the design of earth and gravel roads should be based on the existing AADT.

For design purposes, the composition of traffic meaning the composition of each of the different types of vehicles and also percentage of truck traffic during peak seasons or hours should be known. Besides being heavier, trucks generally are slower and occupy more roadway space and consequently impose a greater traffic effect on roads than passenger vehicles do. Trucks-normally have 4ton or greater gross vehicle mass rating of the manufacturer and dual tires on rear axle.

Speed is a design control and criteria most important to the traveler in selecting alternate routes or transportation modes. The attractiveness of a public transportation system and a new road are each weighed by the traveler in terms of time, convenience and money saved, all of which directly relate to speed.

Vehicle classification and evaluation of equivalent axle loads is an essential aspect and can be broken down as follows:

1. Non motorized traffic, such as animal drawn carts

2. Cars, including passenger cars, vans, minibuses (up to 24-passenger seats), taxis, pick-ups, 4WD vehicles etc

3. Buses, including medium and large buses above 24 passenger seats

4. Light trucks, including small and medium sized trucks with 2 axles, agricultural tractors etc.

5. Medium trucks, including larger trucks with 3 or 4 axles, agricultural tractors with trailers etc.

6. Heavy trucks, including trucks with more than 4 axles and articulated trucks

The deteriol'ation of paved roads caused by traffic results from both the magnitude of the individual wheel loads and the number of times these loads are applied. It is necessary to consider both number of vehicles and wheel loads (axle loads) of these vehicles. Equivalency factors are used to convert traffic volumes into cumulative standard axle loads. As a result, traffic surveys must distinguish between the different types of vehicle.

The mechanism of deterioration of gravel roads differs from that of paved roads and is directly related to the number of vehicles using the road rather than the number of equivalent standard axles. The traffic volume is therefore used in the design of unpaved roads., /

Where there is need to use capacity (volume) values in the design conversion into passenger car units or other characteristic standard vehicle is done to take into account the influence of capacity of different vehicle mixes on different gradients (see Table 3.2 for conversion factors)

Vehicle Type Level Terrain Rolling Terrain Mountainous Terrain

Passenger cars 1.0 pcu 1.0 pcu 1.5 pcu

Light goods vehicle 1.0 pcu 1.5 pcu 3.0 pcu Medium goods vehicle 2.5 pcu 5.0 pcu 10.0 pcu

Heavy goods vehicle 3.5 pcu 8.0 pcu- 20.opcu

Buses 2.0 pcu 4.01lCO 6.0 pcu Motor cycles, Scooters 1.0 pcu 1.0 pcu 1.5 pcu

Pedal cycles 1.0 pcu 0.5 pcu NA

Notes:

1. pcu = passenger car unit

2. Light good vehicle = vehicles of < 1500 kg (3307.5 pounds) un-laden (minibus, land cruiser etc)

3. Medium goods vehicle = maximum gross weight of 8500kg (18742.5pounds)

4. Heavy goods vehicle = vehicle with gross weight greater than 8500kg (18742.5pounds)

5. Buses = all passenger vehicle larger than minibus

The estimate of the initial traffic volume should be the MDT currently using the route (or the MDT expected to use it during the first year the road is put to service). For major projects this can be carried out as follows:

a. 7 consecutive days counts

b. Counts should be for 12 hour daytime (usually 7.00 am to 7.00pm)

c. Counts should be take for all seasons of the year but should avoid holidays, ceremonies or severe weather.

d. Vehicles should be categorized in the groups

e. Counts should be done for both directions

Where there is no information on a factor to convert 12hour to 24 hour traffic, an hourly factor of 1.33 shall be applied. ADT = 12 hour count x Conversion factor (or 1.33).

Where it is difficult to conduct 12hour count, a moving observer method can be used for at least 1 hour count period. Hourly traffic (Th) = (x + y - z) / t, [where x = vehicles moving in the opposite direction of the observer; y = vehicles moving in the same direction and overtaking the observer; z= vehicles in the same direction and being overtaken by the observer; t = period in hours for the count]. ADT = 16 x Th•

Pedestrian facilities should be provided where significant numbers of children are walking to and from school, or where other pedestrian traffic warrants, such as in commercial areas.

. ' . . 18

It may be necessary to provide a widened lane or shoulder, appropriate drainage grates, or other bicycle facilities that can permit bicycle use where such bicycle traffic is considered large in volume.

This period is the expected duration for which the road is capable of carrying traffic, given good maintenance (routine and periodic). The TRLOverseas road note 31 proposes pavement design life of 15 years in order to reduce the problem of uncertainties in forecasting traffic over long periods. Traffic, and indeed many other factors, cannot be reliably predicted for a longer period, a design period longer than 10-15 years is unrealistic for unpaved roads but recommended for low cost sealed or paved roads. In Liberia, the effect of severe environment for roads, affects the life of pavement especially for unpaved roads. With proper routine and periodic maintenance interventions the life of gravel roads can reach 8-lOyears.

The concept of appropriate standards for labour based works should be accepted. It is a foregone conclusion that the best technologies for feeder road works lies somewhere between labour intensive and equipment intensive methods. There is usually little change in design procedure as both functional and condition for feeder roads aims to address construction cost savings just as LBAT does. Accordingly the standards for feeder road design suits the criteria to use LBAT.

Feeder roads must be designed with the idea of making use of materials obtained locally to prevent high purchase and transport costs. It is perhaps the local material availability that has the greatest influence on the design of rural feeder road infrastructure. Opportunities to use for instance stones for a masonry works like culvert end structures, drifts, drainage lining, low level bridge abutments etc should be encouraged. The use of timber decks with steel beams on low-level bridge should be encouraged but must be evaluated with the environmental conditions. These materials can usually be re-usable (in case of stones) or are renewable.

Local soils such as lateritic gravel found in abundance in Liberia and sal}..dsfouQd in areas close to the coastal lines provide opportunity for good road building effort and their use can be done in a sustainable manner by improving their quality for alternative sealing. This surfacing capability of gravel and sanu'C"a11-------··----be improved by use of bitumen such as in otta or sand seals which last longer and help to reduce the burden of maintenance, and address problems of mud and dust.

Geometric design is the first stage in the development of a road after planning and appraisal. Geometric design involves a process in which the layout of the road is designed to meet the needs of the road user.

It principally involves the-se!ectien-ofsWtable geometric elements of horiZGntal and vertical alignments and cross section width. The aim of geometric design is to provide a compromise between the operational efficiencYisafety and economic constraints.

Appropriate standards and combinations of the geometric design elements should be carried out with consideration of the controls and criteria explained in chapter 3.

The principles and standards recommended in this chapter is intended for use of feeder roads which provide basic access and carry low volume vehicular traffic of between 0-100 vehicles per day. The traffic is mostly local and comprise of trucks, buses, small cars, motor cycles, bicycles and pedestrians.

The chapter emphasizes on inexpensive appropriate designs that should depend on the functions of the road.

The functions and characteristics of road network determine the principles and approach to design and standards of roads in that network. Conventionally adoption of design speeds for various terrain types are the major determinants for establishing geometric design parameters. However, in rural feeder roads, design speed may be both a determinant and resultant of design standards.

In networks where roads function to provide increased capacity and efficiency or where service levels are high, increased and uniform speeds and uninterrupted traffic is desirable._A design speed approach can be used. The characteristic of traffic is also important. -

In a network where the function is to provide access to land and properties, a 'basic access approach is use~:LSw:h-!oaLis...in.cWde..r..uIaLfeed.eLand..secondary roads with low volume traffic. These roads have greater need for all-weather accessibmty than provide for operational efficiency. Traffic mix is however an important factor, "and in Liberia it includes motorized transport, and cyclists. As a result there is also greater need for safety. For roads with more than 50 vehicles per day, the increased numbers of vehicles justify the use of a higher or separate standard. The designs may be based on speed approach.

In basic access approach, the geometric layout should allow the largest vehicle which normally uses the road to operate. For feeder roads or low trafficked roads, small trucks are usually the largest vehicles.

The average daily traffic volume will normally guide the designer in selecting the geometric standards of the road.

The standards used will also be dependent upon the terrain. The designer will also consider the implication of the selected alignment standards with the construction and future maintenance costs.

With a wide cross section, the cost of construction and future maintenance will be large. On the other hand, if a gently gradient is chosen in hilly areas, the cost of construction may be high but the cost of future maintenance may be reduced for earth and gravel roads.

It is necessary to choose geometric standards appropriate to the prevailing socio-economic conditions and financial constraints.

A simple design criteria involving a "design by eye" and basic survey instruments liKe (levels, ranging roads, profile boards and boning rods) may be used for horizontal and vertical alignment layout within some tolerances.

Feeder Road Design will usually start with the basic access approach by selecting appropriate cross section based on traffic composition. The designer will then determine the likely speed his/her choice of design will attract. Thislikely speed can be used to define the horizontal alignment standards basing in mind road safety concerns. Traffic composition and volume will help in determining other alignment standards

Conventional design aims to provide a constant level of service over specific road or segments where a consistent speed environment needs to be kept. The approach is therefore governed by speed selection.

In principle, most roads are constrained to minimum parameter values over certain sections. Road costs and benefits are optimized by using different speeds.

Care must be exercised to ensure selection of speed and other standards take into consideration the design controls specified in Chapter 3.

The road surface type and environmental factors have influence in deciding geometric standards as they affect the design life of the road.

Geometric standards for road design should be set at leveTs·-a-ppropnate-lOi"tfie"neeas-oT speCifiC-'road and its functions.

The aim is to select design standards that minimize total transport costs (including the construction, maintenance and operation). The standards proposed in this section are derived from experiences from projects implemented in Liberia in the last 5 years and those from the region.

Design speed is used to link the geometric design features of a road to controls like the road function, traffic and terrain in-order to ensure constant level of service on the road by ensuring consistent speed environment. The design speed is used to correlate various features of the design. For rural feeder roads, the overriding factor in design is the determination of appropriate and safe roadway with speed being a resultant driver's intention, vehicle performance, road geometry and presence of other vehicles.

Recommended design speed is provided in table 4.1. It does not preclude the road being designed for higher speeds when conditions can permit.

Sealed Surface Gravel Surface RoadType AADT Flat or Rolling Mountainous Flat or Rolling Mountainous

Plane Plane Secondary >50 80 Kph 60 Kph 40 Kph 70 Kph 50 Kph 30Kph

[ 50Mph] [40 Mph] [25 Mph]] [45 Mph] [30 Mph] [20Mph] Feeder >50 70 Kph 60 Kph 40 Kph 70 Kph 50 Kph 30 Kph

[45 Mph] [40 Mph] [25 Mph] [45 Mph] [30 Mph] [20 Mph] Feeder 20 - 50 60 Kph 50 Kph 40 Kph 60 Kph 50 Kph 30 Kph

[40 Mph] [30 Mph] [25 Mph] [40Mph] [30 Mph] [20 Mph] Feeder <20 60 Kph 50 Kph 30 Kph 50 Kph 40 Kph 30 Kph

[40 Mph] [30 Mph] [20 Mph] [30 Mph] [25 Mph] [20 Mph] Notes: (1) the design speeds in Miles per hour [Mph] and those in Kilometers per hour [Kph] are not necessarily obtained by direct conversion of the other but rather are the corresponding design speeds.

The physical characteristics of vehicles and the proportions of the various sizes using the road system are important way of deciding geom.etric standards like the cross section, curve widening and junctions layout AASHTO Green Book recommended design vehicle nomenclature should be used for rural feeder road design.

Road in Category of Design Vehicle Nomenclature Secondary Road Bus -- Feeder roads Single Unit Truck/ Pickup Minor Roads Pickup Cul-de-sac Pickup

Wheel Front over- Rear over Overall Overall Design Vehicle Base hang hang length width Height

(m) (m) (m) (m) (m) (m)

Passenger car 2.9 0.9 1.2 5.0 1.9

Single unit truck (9.1m) • 6.1 1.2 1.8 9.1 2.5 3.8

Single unit truck (11m) 6.5 1.5 3.0 11 2.5 3.8

Notes: Conversion factor 1m = 3.2808 ft

The cross section standards are those that pertain to aspects that deal with the width of the road as illustrated in Figure 4.1.

Metric System - Road Type Road Carriageway Shoulder Roadway fall Rig!:lt - -ofADT . Cross or Class Surface width width on each width or Camber -way

Type side

(m) (m) (m) (%)... .~-- --_._. - .,.- - . _ .._- -- .. - _.__ ..- Feeder '>50 Sealed 6 0.5 7 3 15m Feeder 20 - 50 Sealed 6 0 6 3 15m

Feeder <20 Sealed 6 0 6 3 15m Feeder >50 Gravel 6 0.25 6.5 5-7 15m Feeder 20 - 50 Gravel 5 0.25 5.5 5-7 15m

Feeder <20 Grq)lel 4.5 0.2 5-7 15m

Conversion to Imperial System

(ft) (ft) (ft) (%) (ft)

Feeder >50 Sealed 19.7 1.64 23 3 SO'

Feeder 20 - 50 Sealed 19.7 0 19.7 3 SO'

Feeder <20 Sealed 19.7 0 19.7 3 SO'

Feeder >50 Gravel 19.7 0.8 21.3 5-7 50' Feeder 20 -50 Gravel 16.4 0.8 21.3 5-7 50' Feeder <20 Gravel 14.8 0.8 16.4 5-7 50' Notes:

- ADT of base year is used

- There are reasons that certain feeder roads may be sealed.

- Cross section standards may be adjusted in certain conditions of terrain or land use.

The road cross section--elements to be considered for geometric design include width of carriageway, width of shoulders, camber or cross fall, side-drains or drainage features, earth profile, and width of right- of- way. Other aspects to consider include passing lanes and bus lay-bys. In urbanized areas the cross section may also include curbs, pedestrian paths and parking lanes.

The right of way or road reserve is the width of land that is secured and preserved for in public interest for road development purposes. It is important to indicate in the designs what the right of way for the road is.

(b) Roadway width

The roadway width constitutes the carriageway and shoulders. It should be noted that there is no separation which can be seen between the carriageway and shoulder of gravel or earth roads as in most

- cases gravel surfacing extends to the edge of the roadway. Sealed roads may often have seals applied to extend only over the width of the carriageway, although it may be extended over the shoulders making it possible to have a distinctively designed shoulder. Shoulder widths recommended in Table 4.3 for gravel roads are intended to provide support for gravel wearing course by keeping the thickness of the gravel on the carriageway edges to remain uniform with that in the middle areas.

Table 4.3 provides guidelines for selection choice of carriageway width!" The width of the carriageway will depend on the level of traffic and the functional class ofthe road. The designer should therefore use assessed traffic data and determine the functional class before deciding the section to use.