3 PROJECT DESCRIPTION
3.1 General
The Port Louis conurbation, is currently experiencing severe morning and afternoon traffic congestion, estimated to cost the Mauritian economy around Rs 2 billion annually.
The Road Development Authority (RDA), Ministry of Public Infrastructure and Land Transport of the Government of Mauritius is thus implementing a Road Decongestion Programme, entailing the establishment of a network of roads, in order to alleviate the severe traffic congestion being experienced along the Port Louis-Curepipe corridor and around Port Louis itself.
The Government of Mauritius (GOM) has set a high priority for development of road infrastructure to support the achieved and planned pace of economic growth. The main focus is set on the National Road Decongestion Programme to provide an effective approach of decongestion of the road network in Mauritius.
The whole extent of Port Louis to St Jean Road (A1) is heavily congested throughout the day and traffic jam reach an unbearable level during peak hours. Severe traffic congestion occurs along the M1 Motorway from Caudan waterfront through the M2 Motorway towards Quay D and Mer Rouge Roundabouts, especially during peak times. Also, the traffic volume of the inner roads in Port Louis City has been largely exceeded its capacity and acute traffic jams occur during business hours, especially in the vicinity of Place d'Armes.
Therefore, in order to ease the traffic congestion, a re-design of the Pont Fer, Jumbo and Dowlut Roundabouts as part of the Road Decongestion Programme (RDP) has been prioritised.
The proposed project has been selected based on the recommendations of traffic modelling performed by KEC whose parameters are considered to remain valid as well as the following parameters:
Function Cost Constructability Social Impact Environmental Impact Maintenance.
The project description, as given in this chapter, are extracts from the Front End Engineering Design Report (FEED) prepared by KEC.
EIA GSJ at Pont Fer / Jumbo / Dowlut Roundabouts 3-1 May 2017 M191/GSJ/3.1
3.1 Project Description
3.1.1 Project Location
The Grade Separated Junctions (GSJ) involving three roundabouts at Pont Fer, Jumbo and Dowlut display typical characteristics of the downtown road and are located in the heart of the busiest downtown area in Mauritius. The Sodnac Link Road provides a direct connection from the town of Quatre Bornes to the Phoenix Mall, Offices, Industrial Zone in the Pont Fer area and on a broader scale to the M1 motorway in the Southern direction.
The Current Traffic Volume witnesses more than 50,000 vehicles/day. Whereas according to international standards a daily traffic volume of 25,000 vehicles/day is recommended for a 2 lane roundabout. Accordingly, severe traffic congestion is observed at Pont Fer and Jumbo Roundabouts.
3.1.2 Main Line Carriageways
The M1 Motorway at Phoenix Roundabout is a dual carriageway with 6 lanes to northbound and dual carriage way with 4 lanes to southbound.
The B63 road from Jumbo Roundabout, is a dual carriageway with 4 lanes.
As a method for relieving the congestion around Pont Fer roundabout, one lane as an independent road is currently in operation for Airport bound from Port Louis and Port Louis bound from Vacoas.
3.1.3 Roundabouts Concerned
The distances between the Roundabouts are:
(1) 500m between Pont Fer and Dowlut. (2) 350m between Pont Fer and Jumbo.
All 3 roundabouts; Pont Fer, Jumbo and Dowlut, have 2 lanes in operation. The close proximity between the roundabouts increases the traffic congestion from one intersection to the other.
3.1.4 Operation of Pont Fer Roundabout
The Pont Fer Roundabout operates with 2 lanes. However, the single lane as an independent road for Airport bound from Port Louis adds an additional lane to the part of the roundabout. This results in a partial 3 lane operation, which then requires the approaching traffic into the factory area to turn and cut across 3 lanes simultaneously. This raises an alarm in the road safety and seeks an immediate remediation.
3.2 Brief Project Description
The proposed project consists of the following:
Construction of the Pont Fer viaduct which is shown schematically in Figure 3-1 Improvement of the Jumbo viaduct alignment on B63 road which is shown schematically in Figure 3-2
EIA GSJ at Pont Fer / Jumbo / Dowlut Roundabouts 3-2 May 2017 M191/GSJ/3.1
Improvement to the existing 3 legs intersection at Jumbo Roundabout as shown in Figure 3-3 Improvement of the alignment of the ramp at Dowlut Roundabout as shown in Figure 3-4 A new link road from, Swami Baktivedanta Road (Sodnac Link Road) B94 to Phoenix-Beau Songes Road B130 as shown in Figure 3-3.
Figure 3-1: Proposed Pont Fer Viaduct Figure 3-2: Proposed Jumbo Viaduct
Figure 3-3: Proposed 3-leg Intersection at Figure 3-4: Alignment of Ramp at Dowlut Jumbo Roundabout
EIA GSJ at Pont Fer / Jumbo / Dowlut Roundabouts 3-3 May 2017 M191/GSJ/3.1
Overall, the project is as shown schematically Figure 3-5.
The context of the proposed project is shown in Drawing M191/GSJ/EV/LA/02.
Figure 3-5: Overall Proposed Improvement.
Through the proposed Road Decongestion Programme the RDA expects to meet, amongst others, the following objectives:
To provide efficient and effective transport services in support of development, business, trade, tourism and production sectors of the economy To reduce vehicle operating costs, travel time costs and road maintenance costs To reduce traffic congestion on the existing motorways by: providing increased road capacity and bypass alternatives to Port Louis; responding to traffic growth by future expansion of road capacity; and implementation of suitable and effective traffic congestion management procedures To reduce traffic accidents and enhance road safety.
3.3 Proposed Works and Road Design
The key components of the proposed works include the following:
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Improvement of entrance and exit for industrial area on Pont Fer Roundabout Improvement of Pont Fer viaduct alignment on M1 Motorway Improvement of the Jumbo viaduct alignment on B63 road Change of the existing Jumbo Roundabout to 3 legs intersection Improvement of alignment of the ramp at Dowlut Roundabout Diversion and protection of all affected utility services, cables, etc. Construction and installation of all required ancillary buildings, offices and road furniture Construction of additional lane at Pont Fer and extension of culvert at River Mesnil will entail the following: . Temporary diversion of existing river flow to proceed with construction works . Realignment of existing river to original alignment.
The road design involves the following main fields, amongst others:
Geometric design Pavement design Bridge Geotechnical engineering Traffic analysis.
The following sections 3.4 to 3.7 have been extracted from the FEED Report produced by KEC in association with Cheil Engineering Co. Ltd. and Kyongdong Engineering .Co Ltd.
3.4 Geometric Design
As a basic element of the road design, the road design standard should be based on various factors: function of roads, traffic and site conditions, geological and geotechnical conditions, social environmental and economic factors.
According to the design standard stipulated in the Contract Agreement between RDA and KEC (signed on 25 Nov., 2016), all design fields will basically follow local standards and AASHTO standards. The Consultant will comply with the followings as reference standards in the order of priority listed below, unless otherwise specified.
Local standards AASHTO Standard, 2011, AASHTO, USA(Design Standard for the Project) Design Manual for Road and Bridges Volume 6, 2015, British Standard, UK Road Design Standard, 2012, Ministry of Land, Infrastructure and Transport, Republic of Korea
EIA GSJ at Pont Fer / Jumbo / Dowlut Roundabouts 3-5 May 2017 M191/GSJ/3.1
South African Pavement Engineering Manual Second Edition, 2014, - The South African National Road Agency Southern African Transport and Communications Commission,1998, - The division of Roads and Transport Technology.
3.4.1 Road Classifications
According to the Road Traffic (Speed) Regulations 2011 of Mauritius, the roads in Mauritius are classified into four categories: Motorway, A roads, B roads, and Un-rated roads.
Regarding the road classification for the project, the RDA informed that all RDP projects would normally consist of 'A Class' road. Based on this information, the project road sections to be newly constructed are classified as 'A road' while current classifications remain the same for the existing roads subject to upgrade. The road classifications for each section of the Grade Separated Junction at Pont Fer-Jumbo-Dowlut (GSJ) are as follows:
Table 3-1 below shows road classification according to the AASHTO Manual.
Table 3-1 - Road Classification on Project road
Local AASHTO Project Road Classification Standard,2011
M1 Motorway Motorway Freeway
A10 Road A road Arterial Grade Separated B63 Road B road Arterial Junction Sodnac Link B road Arterial Road(B94)
3.4.2 Design Speeds
The Road Traffic (Speed) Regulations 2011 provided by the RDA defines the maximum speed ranges by road classification and vehicle type as follows:
Motorway : 80 ~ 110 km/h A road : 60 ~ 80 km/h B road : 40 ~ 60 km/h.
Considering operating speed limit on the A1 road, the design speed of the A1-M1 Link Road is decided as 60km/h. The adopted design speed of the existing roads to be improved in the Grade Separated Junctions at Pont Fer / Jumbo / Dowlut Roundabouts ranges from 60 km/h to 80 km/h as shown in Table 3-2. In the selection of design speeds for existing road upgrades, the traffic and site conditions as well as function of each road were taken into consideration.
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Table 3-2 - Adopted Design Speeds of the Grade Separated Junctions Design Speed Component (km/h) Remark M1 Motorway (Viaduct) 80 M1 Motorway (Frontage 40 Road) Existing Road A10 Road (Viaduct) 40 Upgrades A10 Connector Road 30 B63 Road (Viaduct) 60
Sodnac Link Road (B94) 60 Alternative Route
Note 1: For the design speed of a connector road, 50km/h is adopted for direct ramps while 40km/h is adopted for loops in the FEED report.
3.4.3 Intersections
(a) Profiles
In selecting locations of intersections, it is generally not desirable to locate intersections in excess of 3 percent grade. Where conditions make such designs too expensive, grades should not exceed about 6 percent, with a corresponding adjustment in specific geometric design elements.
In the case of locating roundabouts on grades, grades steeper than 4 percent are not recommended in the roundabouts (An Informational Guide by FHWA).
(b) Intersection Sight Distances
The general considerations are as follows:
Sight distance is provided at intersections to allow drivers to perceive the presence of potentially conflicting vehicles. This should occur in sufficient time for a motorist to stop or adjust their speed, as appropriate, to avoid colliding in the intersection
The intersection will be designed according to stopping sight distance and intersection sight distance by each design speed, considering sight triangles as skewed intersections.
(c) Intersection Controls
Intersections with traffic signal control (Jumbo Roundabout) Jumbo Roundabout is considered to change from roundabout to three-leg signalized intersection in consideration of the Metro Express plan. In the design of intersection, the sufficient sight distance for the right-turning will be given if the site condition is allowed.
EIA GSJ at Pont Fer / Jumbo / Dowlut Roundabouts 3-7 May 2017 M191/GSJ/3.1
3.4.4 Design Surveys
Specific surveys carried out included:
A topographical survey to determine the elevation along the road alignment and to facilitate the production of accurate topographical maps Soil investigations An investigation of the natural and man-made drainage in order to determine the direction of flow An investigation of services and utilities in the area Traffic surveys including traffic volume, OD (Origin & Destination) and Speed and Delay studies An economic analysis with vehicle operating costs A road furniture study.
For the Front- End Engineering Design (FEED) stage, the topographical survey covers the following:
a) Ground Control Survey- Trigonometric networks connecting to the national survey control points, polygon networks, Benchmarks setting at appropriate intervals. b) Levelling along centreline at 25m intervals for longitudinal profiles. c) Cross-section levelling at 200m intervals. Cross sections are to cover at least 50m on either side of proposed centreline or up to the land acquisition limit whichever is greater. d) The survey shall cover the location from the centreline of physical features like buildings, monuments, posts, pipe, existing roads and railways lines, stream/river crossings, cross drainage structures, land use etc. that are likely to affect the road construction works. e) Topographic Mapping:- drawing scale; 1:1000 for general route alignment, and others as necessary, Contour line interval; 1.0m for scale 1:1000 maps, and 0.5m for scale 1:500 maps.
For the Detailed Design stage, the topographical surveys covered the following:
a) Cross-section levelling at 25m intervals. Cross sections are to cover at least 50m either side of proposed centre line or up to the land acquisition limit whichever is greater; b) Underground surveys and consultations with relative institutions regarding relocation of existing utilities (both of underground and ground) and data acquisition.
3.5 Typical Cross Sections
The elements of the cross section of a road are, mainly, the carriageway, the shoulder, and the median strip. The width of the lanes, which consists of the carriageway, changes depending on the design speeds (road classifications) and significantly
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influences the traffic capacity. It would be rational to determine the road width based on the design speed and the volume of the traffic. The condition of the existing roads and the location conditions should also be taken into account.
The decisions on the cross section design elements were based on the existing roads and rights-of-way, the existing developments as well as the AASHTO Design Standards - A Policy on Geometric Design of Highways and Streets, 2011.
3.5.1 Review of Cross Section Elements
The following cross section elements were reviewed:
Width of lane Width of shoulder Median Strip Sidewalks.
3.5.2 Lane Widths - Pont Fer / Jumbo / Dowlut Roundabouts
According to the AASHTO Design Standards, lane widths of 2.7 to 3.6m are generally used, with 3.6m lane predominant on most high-speed, high-volume highways. Lane widths of 3.6m are used on freeways and rural and urban arterials roadways.
In urban areas where pedestrian crossings, rights-of-way, or existing developments become stringent controls on lane widths, the use of 3.3m lanes may be appropriate as per AASHTO recommendations. Lanes 3.0 m wide are acceptable on low-speed facilities, and lanes 2.7 m wide may be appropriate on low-volume roads in rural areas.
Based on the site survey results, the lane widths were measured ranging from 3.3m to 3.65m wide on the M1, A10 and B63 roads which need to be adjusted in consideration of narrow road condition for road widening.
Therefore, it is considered appropriate to set the lane widths as 3.5m for this project, considering the site constraints such as narrow existing roads due to road widening, and the reduction of encroachment of the river and the buildings.
Figure 3-6: Satellite Image of Pont Fer - Jumbo - Dowlut Roundabouts
EIA GSJ at Pont Fer / Jumbo / Dowlut Roundabouts 3-9 May 2017 M191/GSJ/3.1
3.5.3 Shoulder Widths
According to AASHTO Design Standards, a minimum shoulder width of 0.6m is considered for low-volume highways and 1.8m to 2.4m shoulder width is preferable. A minimum shoulder width of 3.0m wide and preferably 3.6m wide are recommended for high-speed highways.
Considering the road passes through an area which is heavily populated with obstacles and in order to minimize any contacts with obstacles on right side of the roads, a minimum width of shoulder ranging from 0.6m to 1.0m was applied required for the minimum width of gutter. The outer shoulder widths ranging from 1.0m to 2.5m are to be used for M, A, and B roads in consideration of the restrictive site conditions.
3.5.4 Median Strips
Median width is defined as the width between edges of the travelled way for the roadways in opposing directions of travel, including the width of the right shoulders.
Most median widths are in the range from 1.2m to 24.0m according to AASHTO Design Standard but median width can be limited in consideration of economic factors and the features of the project site.
The median width of ranging from 2.0m to 3.0m will be reflected in consideration of a restrictive site condition such as viaduct for the Grade Separated Junction at three roundabouts. The median width of 3.0m will be applied for the A1-M1 Link Road, which is the minimum width of a median strip for a road to install lighting poles and sign boards.
3.5.5 Sidewalks
Sidewalk widths in residential areas vary from 1.2m to 2.4m. Sidewalks less than 1.5m in width require the addition of a passing section every 60m for accessibility.
3.5.6 Applied Cross Sections
KEC- CHEIL- Kyongdong set the cross section elements of the project based on the review and the comparison of each design elements defined in the three design standards as well as road measurements through the site surveys: AASHTO-2011 (U.S.), DMRB Vol.6- 2015 (UK), and RDS-2012 (Korea) standards refer.
Grade Separated Junctions comprise the M1, A10, B63 and B94 roads and their cross section elements and typical cross sections are as follows:
(a) Typical Cross Section for M1 Motorway – as shown in Figure 3-7.
(b) The typical cross section for M1 Motorway (STA.0+000~STA.0+135) as shown in Figure 3-8.
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Table 3-3 - Typical Cross Section (M1) – Duel Carriageway with 3 to 4 Lanes in each Direction
Northbound Southbound Paved Outer Paved Outer Lane Median Lane Shoulder Shoulder
1.0 m 43.5=14.0 m 3.5m 3@3. 5=10.5 1.0 m Total Width : 30.7m(VAR.)
Figure 3-7: Typical Cross Section (M1 Motorway)
(a) Typical Cross Section for M1 Motorway and M1 Frontage Road
The typical cross section for the M1 Motorway and the M1 Frontage Road is as follows:
Table 3-4 - Typical Cross Section (M1) - Duel Carriageway with 2 Lanes in each Direction
Northbound Southbound Paved Outer Paved Outer Lane Median Lane Shoulder Shoulder 1.0 m [email protected]=7.0 m 2.0 m 2@3. 5=7.0 m 1.0 m Total Width : 18.0 m
Table 3-5 - Typical Cross Section (M1 Frontage Road)
Northbound Southbound Paved Paved Paved Paved Lane Lane Outer Inner Inner Outer 1.0 m [email protected]=10.5 m 1.0 m 1.0 m 2@3. 5=7.0 1.0 m Total Width : 21.5 m
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Figure 3-8: Typical Cross Section (M1 Motorway and M1 Frontage Roads)
(b) Typical Cross Section for the M1 Motorway (STA.0+801~STA.1+355)
Table 3-6 - Typical Cross Section (M1) - Duel Carriageway with 3 Lanes in each Direction
Northboun Southboun Paved Outer d d Paved Outer Lane Median Lane Shoulder Shoulder 2.5 m [email protected]=10.5 m 2.0 m 3@3. 5=10.5 m 1.5 m Total Width : 27.0 m
Figure 3-9: Typical Cross Section (M1 Motorway)
EIA GSJ at Pont Fer / Jumbo / Dowlut Roundabouts 3-12 May 2017 M191/GSJ/3.1
(c) Typical Cross Section for Dowlut Ramp-1, 2 of A10 Road and A10 Connector Road
Table 3-7 - Typical Cross Section (A10) - One-way Ramp with 1 Lane
One Direction Paved Outer Shoulder Lane Paved Inner Shoulder 1.5~5.8 m 3.5 m 1.5 m Total Width : 6.5 m~10.8m
Table 3-8 - Typical Cross Section (A10 Connector Road)
One Direction Sidewalk Paved Outer Shoulder Lane Paved Inner Shoulder 1.5 m 1.0 m 3.5 m 1.0 m Total Width : 7.0 m
Typical cross section for Dowlut Ramp-1 of A10 and A10 connector road is as follows:
Figure 3-10: Typical Cross Section (Dowlut Ramp-1of A10 and A10 Connector Road)
Typical cross section for Dowlut Ramp-2 of A10 is as follows:
Figure 3-11: Typical Cross Section (Dowlut Ramp-2 of A10 Road)
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(d) Typical Cross Section for Sayed Hossen B63 Road and B63 Frontage Road
Figure 3-12: Typical Cross Section for Sayed Hossen B63 Road and B63 Frontage Road
Table 3-9 - Typical Cross Section (B63) - One-way with 2 Lanes
One Direction Paved Outer Shoulder Lane Paved Inner Shoulder 1.0 m 2@3. 5=7.0 m 1.0 m Total Width : 9.0
Table 3-10 - Typical Cross Section (B63 Frontage Road) - Duel Carriageway with 2 Lanes in Each Direction
Northbound Southbound
Paved Paved Outer Shoulder Sidewal Lane Median Lane Outer k Shoulder 1.5 m 1.0m [email protected]=7. 3.0 m 2@3. 1.0 m 0 m 5=7.0 m
Total Width: 20.5m
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(e) Typical Cross Section for Sodnac Link Road Table 3-11 - Typical Cross Section - Sodnac Link Road
Northbound Southbound
Paved Outer Paved Outer Lane Median Lane Shoulder Shoulder
1.5 m 3.5 m - 3.5 1.5 m
Total Width : 10.0 m
Figure 3-13: Typical Cross Section - Sodnac Link Road
(f) Typical Cross Section for B130 Road
Table 3-12 - Typical Cross Section (B130) - Two-Way with 2 Lanes in Each Direction
Northbound Southbound Paved Outer Paved Outer Sidewalk Lane Median Lane Sidewalk Shoulder Shoulder 1.5 m 1.0m [email protected]=6.5 m 1.7 m [email protected]=6.5 m 1.0 m 1.5m Total Width : 19.7 m
Figure 3-14: Typical Cross Section (B130 Road)
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3.6 Pavement Design
3.6.1 Design Standards
The design manuals, standards and guidance documents that have been used in the development of the pavement design are as follows:
a) AASHTO guide for the design of pavement structures (DOPS) 1986, 1993 and supplementary guide to the design of pavement structures 1998 b) Guidelines for 1993 AASHTO Pavement Design Virginia Department of Transportation 2003 c) Highways Capacity Manual (2000) National Research Council d) Recalibration of the Asphalt Layer Coefficient, NCAT Final Report 09-03, 2009 e) Material Properties of the 2003 NCAT Test Track Structural Study, NCAT Report 06-01, 2006.
The primary function of a pavement is to provide a suitable running surface and remain serviceable for the anticipated traffic expected over the design life. The overall traffic loading is therefore a key element of the pavement design process.
The road and interchange pavement design is based on traffic forecasts for 20 years. Traffic growth from 2020 to 2039 as the basic years were used for the pavement design calculation.
3.6.2 Design Life
The pavement structure is designed for a life of 20 years after the opening of the road which is currently planned for 2019. The performance period is the length of time the initial pavement should last before it requires major rehabilitation and therefore has deteriorated from its initial serviceability level to its terminal serviceability level. No major rehabilitation is planned prior to 2039. Therefore, the performance period is 20 years.
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3.6.3 Grade Separated Junction Pavement Types
In this section, the four main types of pavement structures are described in the following tables :
1) Type A: Port Louis to Pont Fer Roundabout Section (Type A)
Table 3-13 - Pavement Layers for Port Louis to Pont Fer Roundabout Section
Thickness Total SN Categories Description (mm) (mm) provided
Layer 1 HMA Surface Course 50
Layer 2 HMA Intermediate Course 60
Layer 3 Asphaltic Concrete Base Course 100
Asphalt sub-total thickness 210 3.244 Crushed Subbase Course Layer 4 220 (with CBR 80%)
Total Pavement Thickness 430 4.457
2) Type B: Pont Fer to Jumbo Roundabout Section
Table 3-14 - Pavement Layers for Pont Fer to Jumbo Roundabout (Type B)
Thickness Total SN Categories Description (mm) (mm) provided
Layer 1 HMA Surface Course 50
Layer 2 HMA Intermediate Course 60
Layer 3 Asphaltic Concrete Base Course 100
Asphalt sub-total thickness 210 3.244
Crushed Subbase Course Layer 4 200 (with CBR 80%)
Total Pavement Thickness 410 4.346
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3) Type C: Pont Fer to Dowlut Roundabout Section
Table 3-15 - Pont Fer to Dowlut Roundabout (Type C)
Thickness Total SN Categories Description (mm) (mm) provide Layer 1 HMA Surface Course 50
Layer 2 HMA Intermediate Course 60
Layer 3 Asphaltic Concrete Base Course 100 Asphalt sub-total thickness 210 3.244
Crushed Subbase Course (with 190 Layer 4 CBR 80%)
Total Pavement Thickness 400 4.291
4) Type D: Sodnac Link Road Section
Table 3-16 - Pavement Layers for Type D
Thickness Total SN Categories Description (mm) (mm) provided Layer 1 HMA Surface Course 50
Layer 2 HMA Intermediate Course 60 Layer 3 Asphaltic Concrete Base Course 100
Asphalt sub-total thickness 210 3.244
Crushed Subbase Course (with 150 Layer 4 CBR 80%)
Total Pavement Thickness 360 4.071
3.7 Bridges
Three road bridges are planned over three roundabouts at the Grade Separated Junctions.
3.7.1 Bridge Locations and Surrounding Conditions
The locations of bridges at the grade separated junctions are as shown in Figure 3-14 and the list of bridges is given in Table 3-17.
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Figure 3-15: Location of Bridges at Grade Separated Junctions
Table 3-17 - Details of Bridges at Grade Separated Junctions
Bridge Bridge Composition of No. of name Place type span length(m) Length(m) Lanes Remark Pont Fer Overpass the Pont Fer BR1 Roundabout PSC I Girder 7@45=315 315 4 Roundabout Overpass the Jumbo BR2 Jumbo PSC I Girder 4@45=180 180 2 crossway Dowlut BR3 Ramp RC Rahmen 2@14=28 28 1 Overpass M1 Motorway Total Sum 523 -
3.7.2 Investigation and Analysis of Site Conditions for Bridge Selection
The project area consists of three roundabouts at Pont Fer, Jumbo and Dowlut, in close proximity from one another. They are surrounded by industrial, commercial and residential areas, and their capacity has reached the saturation with heavy traffic flows experienced especially during peak hours.
The Metro Express is planned to pass within a few meters of the Jumbo Roundabout. The River Sèche and River Mesnil flow under the existing road at Pont Fer and Dowlut Roundabouts. Utility lines such as communications, conduit tubes, water supply pipes and sewage pipes are currently buried underground along the project roads.
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Pont Fer Roundabout Existing Culvert
Figure 3-16: Pictures of Pont Fer Roundabout
Jumbo Roundabout Access Road To Shopping Mall
Figure 3-17: Pictures of Jumbo Roundabout
Dowlut Roundabout Existing Culvert
Figure 3-18: Pictures of Dowlut Roundabout
3.7.3 Geotechnical Survey
A total of five (5) boreholes investigations for the FEED phase were carried out at Grade Separated Junctions. According to the results of the investigations, the composition of strata is in order of silty clay/residual basalt, highly/completely weathered basalt and
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moderately/slightly weathered basalt. The bearing layer such as moderately/slightly weathered basalt for the bridge foundation was encountered on the range of 19.0m to 21.0m in depth below the ground level. Considering the depth of bearing layer by boring investigations, a type of pile foundation is adopted for the bridge at Grade Separated Junctions.
3.7.4 Construction Materials and Equipment Investigation
The Consultant has looked for locally available construction materials and equipment that can be used for the construction. As Mauritius is an island, the procurement of the required construction materials and equipment becomes an important factor to consider compared to other countries.
Firstly, the main materials for the construction of this project are concrete, reinforcing bars, steel and prestressing steel. Steel bridges, which are light in weight and have high tensile strength, are commonly used in urban areas because of the application of curved alignments and having a superior and longer span.
Had there been steelworks and a steel structure manufacturing plant in Mauritius, then a diverse type of steel bridge could have been adopted. Unfortunately, supply of steelwork is scarce in Mauritius and imported steel from another country will add extra transportation costs and increase construction period. Sand and aggregate, which are the raw materials used to make concrete, are readily available and concrete is widely manufactured in Mauritius. However, cement, reinforcing bars and prestressing steel cables have to be imported.
As for construction equipment, scaffolding & stage, cranes, form traveler and climbing form are expected in place. 200T Cranes are available locally, however, more than 300T cranes must be imported. Climbing forms or slip forms, which are required for construction of an extradosed bridge would be also expected to be imported. The quality of shoring available in Mauritius is relatively poor.
3.7.5 Design Standard for Bridge
The design standards of bridges are based on AASHTO-LRFD Bridge Design Specifications.
3.7.6 Design Clearance
The vertical clearance of overpasses and underpasses is decided based on the road categories and discussion with the local authorities.
Table 3-18 - Vertical Clearance for Roads Category Vertical Clearance(m)
For M motorway, A road, B Road 5.00 m For Pedestrian 2.50 m
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3.7.7 Pont Fer Viaduct: BR1
a) Site Conditions
The Pont Fer Roundabout is a three-leg interchange, which connects the M1 motorway and B63 road, and the bridge planned at Pont Fer Roundabout will cross over the M1 motorway. The Pont Fer Roundabout has a complicated structure with a number of access roads connecting the nearby industrial area as well as other inner roads, and currently these suffer from heavy traffic flows throughout the day.
The main obstacles around Pont Fer Roundabout include the existing culvert and five frontage roads including access roads. Also the Metro Express is planned to pass on the left hand side of the project alignment.
The planned bridge is also surrounded by the Ministry of Education and Human Resources and an industrial area.
b) Proposed Bridge Bridge Alignment, Size and Type
The horizontal alignment within the bridge consists of a straight line with 186m from the beginning of the bridge, and a clothoid of 103m. The last 26m of the bridge consists of a curve radius of 560m. This is shown in Figures 3-18 and 3-19.
The proposed bridge is planned to cross over the box culvert at the start of the bridge, and has the total length of approximately 300m with a 6.0m-7.0m height retaining wall. The location of the bridge is surrounded by commercial / industrial areas, which are connected by a number of access roads as well as the frontage road leading to the main road. Owing to the fact that the bridge will be a good distance from the nearest pedestrians, the height of the retaining wall is not expected to intimidate anyone’s viewpoint. However, in case the height of the retaining wall is less than 5m, then the space below and between the side spans of the bridge is reduced to less than 2m. This makes it difficult to utilize the space under the bridge and is less effective in terms of giving a sense of openness.
Following the determination of the total length of the bridge the next step is to determine the span of bridge and the bridge type. This bridge needs at least a 40m span in order to secure the required clearance for the intersection underneath. Considering constructability, economic efficiency, aesthetics, maintenance management as well as securing minimum 40m span length, the Consultant reviewed the three alternatives below.
The Consultant proposes a PSC Ι Girder bridge as Alternative 1. The construction of this type of bridge can be done on site and the completed bridge can be simply erected using a crane. Therefore, it has a huge advantage in constructability as the construction period is shortened and the cost is low. However, the relatively thick depth of the girder may create a poor aesthetics compared to other alternatives.
EIA GSJ at Pont Fer / Jumbo / Dowlut Roundabouts 3-22 May 2017 M191/GSJ/3.1
Figure 3-19: Plan of Pont Fer Viaduct: BR1
Figure 3-20: Profile of Pont Fer Viaduct: BR1
3.7.8 Jumbo Viaduct: BR2
a) Site Conditions
The existing Jumbo intersection is planned to change from a roundabout type to a signalized 3-leg intersection, and the bridge is planned to be only for Port Louis bound traffic with the aim of relieving the congestion on the B63 road. As the Jumbo intersection under the bridge is a plane signalized intersection, it is critical to secure the sight distance for the drivers turning left and right, and sidewalks for the users of nearby Bus Station should be installed.
The Phoenix shopping mall is situated in the vicinity of the start of the bridge, and the Metro Express is planned to pass through the narrow area on the left hand side of B63 Road, resulting in unavoidable conflicts with the project alignment.
b) Proposed Bridge Alignment, Size and Type
The horizontal alignment within the bridge consists of straight line and a curve radius of 500m. The bridge is situated at a gradient of (+)3.850%~ (-)6.150% on a vertical curve. The road lanes consist of the total two lanes (one lane for each direction) and the bridge width is 10m. This is shown in Figures 3-20 and 3-21.
The length of the bridge at the Jumbo intersection is planned in such a way that there is sufficient clearance under the bridge for passing vehicles to secure the sight distance at the start of the bridge and also for providing enough room for a bus
EIA GSJ at Pont Fer / Jumbo / Dowlut Roundabouts 3-23 May 2017 M191/GSJ/3.1
station (H=2.5m) at the end of the bridge. In this case, the retaining wall with height of around 7.0m seems to be a little high. However, as both sides of the bridge are surrounded by two lane roads and inner roads providing enough spatial margin, a sense of intimidation from the high wall is expected to be low. The Consultant has decided that the optimal length of the bridge should be 180m in consideration of economic efficiency and constructability.
A minimum 45m span is required to secure clearance under the bridge even though the piers are planned to be installed on the traffic island in the middle of the interchange. In consideration of economic efficiency, constructability and maintenance, the Consultant reviewed three alternatives.
The Consultant proposes PSC Ι Girder as Alternative 1. The thick depth of girder gives poor aesthetics compared to other alternatives. However, the girder can be manufactured at the time of construction of the substructure and placed using a crane The construction period is therefore shortened and construction becomes easier with the merit of economic efficiency and maintenance.
Figure 3-21: Plan of Jumbo Viaduct: BR2
Figure 3-22: Profile of Jumbo Viaduct: BR2
3.7.9 Dowlut Ramp Bridge: BR3
(a) Site Conditions
The Dowlut Roundabout, which connects the M1 motorway and the A10 road, is planned to change to a grade separated interchange. The planned bridge crossing over M1 motorway
EIA GSJ at Pont Fer / Jumbo / Dowlut Roundabouts 3-24 May 2017 M191/GSJ/3.1
is planned on the proposed ramp road. The alignment of the ramp has a small curve radius of 41m and widening of the road width that is required to secure the sight distance of drivers, is a crucial point for bridge plan.
In the vicinity of the Dowlut Roundabout, the Mesnil river intersects with the road at two locations resulting in another constraint to bridge design. As part of a graded separated junction type, overpass and underpass proposals can be considered. For urban areas, an underpass is generally preferred as this gives better aesthetics. However, due to the location of the river at the Dowlut Roundabout, an underpass is not feasible.
(b) Proposed Bridge Alignment, Size and Type
The horizontal alignment consists of a curve radius of 41m. The bridge is situated in the gradient of (+)9.990% ~ (-)3.647% at vertical curve. The road lanes consist of the total one lane for one direction and the bridge width is variable with the range of from 11.7 to 15.2m. This is shown in Figures 3-22 and 3-23.
The proposed bridge intersected with M1 motorway, which has the width of 18.0m, and the intersecting angle between the two roads is 35 degrees. The width of the proposed bridge varies between 20.4m and 12.5m. Therefore, minimum bridge length of 25m is required.
At start and end points of the bridge, an 8m height retaining wall is planned. The large height of the retaining wall is unavoidable as the curve radius on the horizontal alignment is already too small for installation of a bridge. Based on this, the Consultant has decided that the optimal bridge length over the M1 motorway to be approximately 30m.
The location of the bridge has a very small curve radius of 41m. The intersecting angle of 35 degrees is too small as an acute angle of bridge, and the super elevation being 8% is too high. Also, the vertical alignment has reached almost the maximum value of 10%. This acts as a big constraint for deciding appropriate bridge type. Considering this constraint, the Consultant reviewed two alternatives.
The Consultant proposes a Reinforced Concrete Rahmen Bridge as Alternative 1. This type of bridge has a restriction to use the space underneath the bridge during the construction period because it needs shoring, formwork, disposition of reinforced bars and placing concrete on the curved bridge.
Fortunately, as it is situated in the centre of the roundabout, a diversion of the M1 motorway is not required. This type of bridge can provide less driving safety than a single span bridge because it is inevitable to install a pier in the median strip of M1 motorway. However, due to the possibility of lowering the height of the bridge, a slim aesthetic look can be achieved and it has high responsiveness to variable bridge width. The construction cost for this type is low and maintenance is simple. The Consultant plans to widen the width of the bridge to improve the acute angle and subsequently to increase the safety of the structure and its constructability.
EIA GSJ at Pont Fer / Jumbo / Dowlut Roundabouts 3-25 May 2017 M191/GSJ/3.1
Figure 3-23: Plan of Dowlut Ramp Bridge: BR3
Figure 3-24: Profile of Dowlut Ramp Bridge: BR3
3.7.10 Geotechnical Engineering
Based on the geotechnical investigation result, a pile foundation is required to support the superstructure loads. Moderately to slightly weathered basalt which were encountered at GL-14.5m~20.0m, has been chosen as the bearing layer for the foundations. In consideration of the local construction equipment, the construction examples in Mauritius, and constructability, a drilled pile is the best type of the foundation for the structure at the site.
In consideration of the bearing capacity and the settlement of the foundation against the load from the superstructure, the drilled pile, which is to be socketed in the layer of moderately to slightly weathered basalt, is planned to have the length of 2.5 times the diameter of the pile. The minimum thickness of the layer from the bottom of the pile is planned to have a length of 2.0 times the diameter of the pile. From these requirements, the bearing layer thickness should be over 4.5 times the diameter of the pile.
Based on the fact that moderately to slightly weathered basalt and completely weathered basalt are altered and considering minimum bearing layer thickness, an 800mm diameter drilled pile is applied for foundations.
3.7.11 Stormwater Drainage
The existing drainage network was studied and assessed so as to design an optimum storm water network management for the proposed project. This will be taken up during the detailed design stage.
The proposed stormwater drainage scheme for discharge of runoff is as shown in the Figure 3-25.
EIA GSJ at Pont Fer / Jumbo / Dowlut Roundabouts 3-26 May 2017 M191/GSJ/3.1
Figure 3-25: Drainage Scheme for GSJ Pont Fer-Jumbo-Dowlut
EIA GSJ at Pont Fer / Jumbo / Dowlut Roundabouts 3-27 May 2017 M191/GSJ/3.1
3.7.12 Construction Methodology for the River Diversion Scheme
The proposed construction methodology to divert River Sèche is as shown in Figures 3-26 to Figure 3-28.
Figure 3-26: Stage 1 Existing Condition of the River Sèche
Figure 3-27: Stage 2 Excavation in Rivers
Figure 3-28: Stage 3 Construction of Sandbag Barriers for River Diversions
EIA GSJ at Pont Fer / Jumbo / Dowlut Roundabouts 3-28 May 2017 M191/GSJ/3.1
Figure 3-29: Stage 4 Construction of Box Culverts
EIA GSJ at Pont Fer / Jumbo / Dowlut Roundabouts 3-29 May 2017 M191/GSJ/3.1