DESIGN FEATURES OF THE ADDIS ABABA RING ROAD PROJECT

Consultant: - Parkman Limited.


1.         Introduction

The Addis Ababa Ring road was initiated as part of the city's commitment towards implementing the city master plan and enhancing peripheral development.

With this objective and the principle of efficient utilization of resources and time and also with the need to create construction ease, the Ring Road was divided into three major phases. These phases connect all the five main gates in and out of Addis with all other Regions (Jimma, Debre Zeit, Asmara, Gojjam and Ambo)

When the ring Road is completed, it is expected that heavy vehicles entering the city of Addis Ababa from the five main radial routes will be diverted into the ring road either to bypass Addis Ababa completely or to transfer to another radial route to suite their eventual destination and in doing so avoid the city center. There will also be transfer of traffic to the Ring Road from the traffic that currently uses a network of roads within the city.

2.         Traffic

The task of traffic study is concerned with the development of future traffic and axle-load forecasts in order to provide suitable data to assist with the design work. For this propose, a diversion curve analysis has been utilized to assess the potential diversion of existing radial traffic to the Ring Road, if it were open today. Growth factors have then been applied to this diverted traffic to produce estimates of forecast traffic flows. A forecast year of 2020 has been adopted, 20 years after the proposed completion of Ring Road in the year 2000. In addition, consideration has also been given to the impact of major development proposals adjacent to the Ring Road line. Such development is likely to contribute significantly to the levels of traffic on the Road.

Traffic surveys have been carried out to provide up-to-date data of the flow of vehicles on the five main radial routes that the proposed Ring Road will intercept. In addition, surveys have also been undertaken on those sections of existing road that lie on the line of the proposed Ring road and those that will form part of the Ring Road. Sample turning movement count surveys have been undertaken at two key existing junctions, and a sample origin destination survey (O/D Survey) of’ heavy goods vehicles was carried out at the same time and location as the axle-load survey. Table 1 presents the result of design years traffic forecasts

For the maximum flow of’ 30500 24 hr ADT, AASHTO Highway capacity manual recommends 4-lane divided urban arterial road with some at-grade junctions. In addition to the 4 lane divided carriageway, a 6.6 m frontage road will be provided on each side. The maximum flow of the frontage roads is expected to he of the order of 23500 24 hour ADT, or about 12000 ADT on each frontage road.

 

Table I Design Year (2020) Traffic Forecasts

 

Ring Road Section

2020 tow-way 24 Hr ADT flow

 

Main Route

Frontage Roads

Total Flow

Ambo Rd to Jimma Rd

14,700

11,300

26,000

Jimma Rd to Debre Zeit Rd

22,000

 

22,000

Debre Zeit Rd to Bole Rd

24,000

 

24,000

Bole Rd to Asmara Rd

30,500

23,500

54,000

 

3.         Road Cross Sections and Other Features

The basic design philosophy for the Ring road Project was to ensure ease of movement for through traffic. To achieve this, there was need to segregate through traffic from local traffic. This has been achieved by providing a dual two-lane carriageway for the main roads and two separate two-lane frontage roads for local traffic.

The proposed typical cross section consists of the following elements and is shown schematically in Figure. 1

Sidewalk                                 3.0m

Frontage Road                       6.6m

Separator                                1.7m

Mainline                                 7.2m

Median                                   3.0m

Mainline                                 7.2m

Separator                                l.7m

Frontage Road                       6.6m

Sidewalk                                 3.0m

Fig. 1. Schematic diagram of road cross - section geometry

3.1       Junction Design

Five major junctions were originally identified along the ring road at the five radial routes from Addis Ababa, namely:

Ambo Road

Jimma Road

Debre Zeit Road

Bole Road

Asmara Road

 

In order to cater for the projected traffic defined in the Traffic Study and deal with the predicted turning movements at these junctions at grade roundabouts were envisaged originally. Where the turning movement within the tight junction became excessive, consideration was given to grade separation of the junction. Therefore grade-separated flyovers will be provided at Bole and at Old Airport to take the 4 lanes divided carriageway of the main through route over the ground level. Grade - separation is also provided at the junction between the Ring Road and Deberzeit Road, at Kality. Because of’ the topography at this location and the proposed vertical profile of the alignment, the Ring Road will pass under the roundabout junction of the Debreziet Road.

The location of these five major junctions resulted in the road being divided into segments of 4 to 11 km lengths. As a means of limiting these excessive journey times, intermediate junctions in the form of roundabout are proposed at specific locations. These junctions will provide additional turning facilities and where appropriate, give access to development areas already indicated in Addis Ababa’s Master Plan.

In order to accommodate the dual two-lane mainline and the two - lane frontage roads on either side, it is proposed to merge the section into a dual three - lane carriageway prior to any roundabout junction. These dual three - lane will then be carried through the roundabout before diverging back to original dual two-lane cross-section.

 

3.2       Pedestrian Facilities

Roughly 70 percent of all inhabitants of Addis Ababa travel by foot and the provision for pedestrian movement is an important aspect of the Ring Road Design.

Pedestrian facilities will be provided at each junction by way of delineated footpath crossings. In addition footbridges will be provided at numerous locations along the length of the road.

 

3.3       Side Roads

In the design of the Ring road, all side road access is restricted to the two - lane frontage roads. To allow side roads direct access to and from the main line, will multiply the potential points of conflict between all forms of transport i.e. whether by vehicle, pedestrian or animal. The roads, operating in one direction only, would merge and diverge from the mainline ‘at grade’ roundabout junction in accordance with AASHTO’s guidelines on lane balance.

The two-lane frontage road will be reduced to a single lane and merged with the two lane mainline to form a three lane approach to the junction. As the road leaves the junction the process is reversed with the three-lane road diverging into two lanes and one single lane. Once the separator is formed the single lane will be then widened to form the two lane and the frontage lane, this procedure will also be used where the road alignment approaches any bridge structure, which will be designed to carry a dual three -lane road.

4.         Materials and Pavement

4.1       Existing Ground Condition

Analysis of test pit investigations indicates that black cotton soil is the predominant soil type. The black cotton soil generally exhibits very high swell values and is therefore classified as unsuitable material as fill.

4.2            Pavement Design

 

4.2.1    Axle Load

An axle- load survey was conducted, and based on the traffic forecast result, the equivalent standard Axle was calculated on a year basis. The Table 3 indicates the cumulative ESAs for one direction and for different design life periods for three road sections.

In Highway capacity terms the Ring Road has been designed as a dual two-lane road, Principal Arterial Road (AASHTO definition). A lane distribution factor should be taken into account which allows for the distribution of the heavy vehicle traffic into the slow (inner) and the fast (outer) lanes.

For a principal arterial road AASHTO recommends as a guide, a distribution between 80% to 100% of heavy vehicles using the slow lane. Therefore a median of 90% of the traffic was taken for design of the inner lane (slow lane).

The pavement construction of the frontage road will be significantly different from the mainline due to the fact that the majority of heavy vehicles will be using the Ring Road and not the frontage road. For the frontage roads, a figure of 1.5 million EA is considered to be sufficient for a 15 years design-life.

4.2.2    Design

Using the AASHTO nomograph for each of the road sections, various pavement options were derived for 10,15 and 20 years design periods. Individual options were considered by reducing the thickness of the asphalt surfacing layers and increasing the thickness of the underlying layers.

An exercise in costing was then carried out for each option and it was found that the overall cost per square meter of the entire pavement continued to drop upto the absolute minimum thickness of asphalt concrete recommended by AASHTO, 10 cm (i.e. for greater than 7 million ESA). It is therefore proposed that the minimum thickness of 10 cm of asphalt surfacing be applied.

The Structural Number and hence the layer thickness of the section between Jimma Road and Debre Zeit to Bole Road and the section between Jimma Road and Ambo are so similar that it is recommended the same design be used for both sections.

The structural number and the layer thickness for the Bole to Asmara Road section are significantly different from the remainder of project and thus different layer thickness are recommended for this section. Table 4 indicates the pavement designs for the main line and frontage road.

In Table 4, the sub-base is prescribed to have a minimum CBR value of 30 percent and the capping layer 10 percent.

The following construction methods are recommended:­

For the Main Line;

        Surfacing to be 6 cm of binder course and 4 cm of wearing course placed in two layers

        Crushed stone base course to be placed in two equal layers

        Crushed or ‘as -dug’ sub base to be placed in two equal layers.

Table 4 proposed pavement layer thickness (in cm) based on 20-years design life

 —        Surfacing to be 8 cm of wearing course to be placed in one layer

 —        Crushed stone base course to be placed in one layer

        Crushed or ‘as - dug’ sub-base to be placed in one layer

In order to take care of the problem which may arise from the presence of black cotton soil; it is recommended that this soil be removed up to a depth of 1.5 m and be replaced with selected material fill up to the bottom of the capping layer. The required thickness of this till material varies from 20 cm to 50 cm.

 

5.         Structures

5.1       General Philosophy

The considerations underlying the design of all the structures of the Addis Ababa Ring Road can be summarized under the following headings:

          Maintenance

          Aesthetics

          Economics of construction

          Appropriate Technology

 

5.2 Bulbula River Bridge

5.2.1    General Description

The structure is a four span continuous reinforced concrete deck supported on intermediate reinforced concrete columns and concrete cross heads with masonry abutments.  The deck is haunched and monolithic over the two main piers and is of beam and slab construction (known as girder and slab) beyond the haunches. The columns are supported upon spread footings founded on rock.

5.2.2    Philosophy

The initial aim is to locate the principal intermediate supports in such a position that they and their supporting bases are not affected by the highest flood levels and hence are not vulnerable to scour. Due to the 25 degree skewed crossing, the north bound and south bound carriageways are separated and staggered by approximately 5.0 m. The main spans are thus fixed at about 42m distance. Utilizing proportionate end spans on the south side a new abutment is to he located clear of the steep and unsafe ground that is apparent near the existing abutment.

The northern end spans have been adjusted to utilize the existing abutment, although there will be need for raising its height in order to accommodate the new level of the roadway.

 

5.2.3 Materials

The flexibility of reinforced concrete for the purpose of accommodating varying shapes is fully utilized on the Bulbula River Bridge. Here, the bridge is located on a vertical curve throughout its length and horizontally it is partly on a transition curve and partly on a straight axis. The realization of this complex alignment will be easily attained through use of concrete to be cast in situ.

5.2.4 Technical

The choice of a haunched construction helps concentrate the strength and the self-weight of the deck at the piers. This solution increases the hogging moment at the piers and reduces the sagging moment at mid span. This also eases construction, enabling the contractor to utilize the pier and the pier base for supporting false work. The construction of the central half of the spans can be undertaken either through erecting the supporting false work from ground level or spanning temporary beams between the false work that is supporting the haunch. The temporary beams in turn are to support the false work for the central half of the spans. The general arrangement for the bridge is shown in Figure 2.

Figure 2. Bulbula River General Arrangement.

 

5.3       Bole and Old Airport Viaducts

5.3.1    General Description

Both viaducts are multi span continuous reinforced concrete slab structures, supported on discrete intermediate reinforced concrete supports and on masonry abutments at the two ends. The intermediate supports will be constructed on pile foundations owing to the presence of highly expansive clays overlaying igneous rock.

5.3.2 Philosophy

The designs of both viaducts are dictated by the geometry of the roadway which resulted in lengthy structures. It is designed in simple but aesthetically pleasing form that allows repetition and is relatively simple in its construction. At both locations, there is significant taxi-traffic using the two areas as terminus. The areas beneath the viaducts will eventually also serve this purpose. And more, the viaduct at Bole shall provide a major aesthetic feature for visitors arriving at Bole International Airport.

5.3.3 Materials

As for the Bulbula Bridge both viaducts are to be constructed from reinforced concrete. Use of reinforced concrete will ease construction of the super elevation as well as the variations designed in horizontal and vertical alignments within the stretch of the viaducts.

At the approach sections of these viaducts, it is proposed to use masonry retaining walls and abutments, which shall provide an attractive contrast to the expanse of the concrete deck. The intermediate columns will be featured in order to enhance their appearance. The features will be carried along the parapets to the deck all the way to the concrete capping of the masonry walls.

5.3.4 Technical

In the choice of spans and determination of the number of discrete supports, the design attempts to balance the need for maximizing the space beneath the structures and that of minimizing the dimensions of structural components.

The general arrangement of the proposed Bole Viaduct is shown in Figure 3.

 

Figure 3. Bole Viaduct General Arrangement.

5.4       Quarry and Abo Over bridges

5.4.1    General Description

It is proposed that both bridges shall be of two span continuous reinforced concrete slab structures to be supported on single intermediate supports. The abutments are to be of masonry and the intermediate support is to be constructed from reinforced concrete.

5.4.2    Philosophy

At both over bridges, approach ramps are found necessary in order to achieve the required carriageway clearances. At Quarry, the Ring Road is partly in cut and this helped to reduce the length of the ramps. Here, the deck depth and ramps had to be minimized due to the close proximity of adjacent properties. As a result, a slab-deck type of structure was the choice for these over bridges.

6.         Drainage

Piped drainage system has been designed for throughout the Project. The pipes are to be installed underneath the separator lane between the Ring Road and the frontage roads or right under the sidewalks. Button entry gratings with gully pots and manholes are incorporated in the design of the drainage systems.

The article appears in the publication of the EACE (Ethiopian Association of Civil Engineers) who owns the copyright. All due acknowledgements and copyright belong to EACE (POBox 20930, Code 1000, Addis Ababa)

EACE Bulletin Vol 1, No 1, 1998.

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