Bridging-The Arabian-Gulf Between Qatar And Bahrain-Conceptual Design and Construction (1).pdf

8/19/2019 Bridging-The Arabian-Gulf Between Qatar And Bahrain-Conceptual Design and Construction (1).pdf

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+ Written comments on this Paper are invited and will be received upto 31 st

December, 2004.

* Advisor to Govt. of Bahrain, Bahrain.

Paper No.507

“BRIDGING” THE ARABIAN–GULF BETWEEN

QATAR AND BAHRAIN-CONCEPTUAL DESIGN &

CONSTRUCTION”+

By

DR.V.K. RAINA*

CONTENTS

Page

1. Introduction ... ... 463

2. Geometrical and Functional Requirements ... ... 4723. Structural Capacity Requirements .. ... 473

4. Materials and Workmanship Requirements ... ... 474

5. Bridge Locations and Type ... ... 475

6. Conceptual Design Philosophy .. ... 476

7. Conceptual Design ... ... 478

8. Construction and Erection Concept ... ... 481

9. Prefabrication Yards ... ... 484

1. INTRODUCTION

1.1. The Challenge

This most mighty of ‘bridges’, a 40 km. long Sea-Link, will connect

Bahrain with Qatar, cutting through the Arabian Gulf waters that are one

of the most highly charged with the attacking chlorides, sulphates and

moluscs–the tripple killers of structure–durability in the Gulf waters.

Above all, its alignment, effecting the location and extent of the

navigation channels, the effect of associated dredging for these channels,

the location and extent of island–embankmants, their effect on the delicate

balancing of water and salt exchange across, the unobstructed migration

of shrimps and minimum disturbance to the fresh water acquifers mid-sea,


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in order to have an almost zero impact on environment, are only some of

the terrible challenges to be met in designing and constructing this trulyunique civil engineering feat of the century!

Hence evolvement and fixing of the various technical requirements

and parameters of design for this classical structure are a class of their

own that call for unstinted clinical attention of the best minds there are

in bridge structure location, design and construction in such trying

circumstances.

1.2. The Project

For a ‘large’ project of this nature, a decision on the number and

scope of contracts constituting it may be critical to the eventual success

of the project. Having multiple contracts will give the Employer more

control than under a single contract for the entire project, and it may be

more economic by maximising competitive pricing. On the other hand,

multiple contracts will require more interface managemeent from the Client’s

organisation.

The contract “packaging” could perhaps be divided into the following

contracts:

· Contract No 1: Coast-to-Coast Fixed Link (39.94 km. long)

·

Contract No 2: Qatar Land Works· Contract No 3: Bahrain Land Works

The ‘offshore’ part of the Works could perhiaps be done on “Design-

and-Build” basis (for reasons of ‘clearer risk allocation’ and ‘speedier

construction – since Design-Build allowes an overlap between Design

and Construction activities’), while the “on-shore” works - where the

‘risk allocation’ is simpler - need not necessarily be of a “Design-and-

Build” nature and therefore could be done as item rate constructioncontracts. However, no final decision has been taken as yet.

The characteristcs and scope of each contract will determine the

party responsible for the design of the Works. A brief description of the

scope might then be as follows:

Contract No. 1, The Causeway __ Coast-to-Coast (i.e. offshore) part

of the Work:

The characteristics and overall scope of Contract No.1 may be


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“BRIDGING” THE ARABIAN-GULF BETWEEN QATAR AND BAHRAIN CONCEPTUALDESIGN & CONSTRUCTION

465

summarised as follows:

· A total causeway length of 39,940 m from coast to coast;

· A dual carriageway road on the causeway with two lanes in

each direction;

· Six embankments in the causeway alignment with individual

lengths between 500 m and 6,245 m, and with a total embankment

length of approximately 17,990 m;

·

Five bridges with individual lengths between 1,320 m and10,070 m, including high-level navigation spans in two of the

five bridges, and with a total bridge length of approximately

21,950 m;

· Miscellaneous works including ancillary buildings, utilities

and services from coast to coast;

· Dredging from two navigation channels;

· Two extended embankments for the purposes of rest areas,

and

· Two fill depots for surplus dredged material.

Contracts No. 2 & 3, the Land Works (i.e. on-shore) parts of the

work:

The Land Works under Contracts No. 2 and 3 will comprise all

works on land in each country from the “Coast-to-Coast Fixed Link”interface up to and including tie-ins with existing infrastructures. These

Contracts could include the following:

· Directional interchanges in Qatar and Bahrain

· Road Works

· Border and Tolling facilities

· An Operation and Maintenance complex in Qatar

· Miscellaneous works, including ancillary buildings, fencing,parking areas, utilities and services, and

· Landscaping

1.3. Materials & Quantities:

The construction of the Causeway will require import of considerable

quantities of materials. Most of the materials will be imported directly to

the work sites through temporary harbours established near the landfalls

of the Causeway. Some materials will require land transport or combined


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DR. RAINA ON466

land/sea transport. The bulk parts of the materials will be imported from

other countries as available resources both in Qatar and Bahrain arelimited.

Table 1 provides a summary of main construction material quantities.

The table summarises permanent materials as per theoretical bulit-in

volumes. In addition are quantities for temporary works which for some

of the materials (e.g. stone and sand fill) can reach approximately 10 per

cent of the volumes for the permanent materials.

For the permanent materials an add-on for waste and tolerances

should be made to reach the actual required quantity of the respective

materials. This could add a volume of as much as 20-25 per cent for some

of the material categories (e.g. blinding layers and sand fill).

TABLE-1. MAIN QUANTITIES AND POTENTIAL SOURCES OF MATERIALS

Material Unit Quantity Potential Source

Stone fill m3 ~ 5,630,000 UAE, Iran, Oman

Sand fill, offshore 1) m3 ~10,400,000 Locally dredged material

Sand fill, onshore 2) m3 ~ 850,000 Local sources

Geotextile m2 ~ 691,000 Europe, Far East, USA

Structural concrete m3 ~ 650 ,000 Coarse aggregates from UAE

Fine aggregates either locally

dredge’d sea sand or desertsand from Saudi Arabia

Cement from Qatar, Europe,

Far East or USA

Reinforcement & tonnes ~ 116,000 Europe, Far East, USA, Qatar

prestressing

Cable stay tendons tonnes ~ 840 Europe, Far East, USA

Structural steel tonnes ~ 2,200 Europe, Far East, USA

Road base m3 ~ 274,000 UAE, Oman

Asphalt m2 ~ 1,418,000 Coarse aggregates from UAE,

Oman Bitumen from sources

in the Gulf Area

Crash barriers & railings m ~ 201,000 Europe, Far East, USA

Buildings m2 ~ 17,000 Miscellaneous sources

Landscaping m2 ~ 600,000 Local sources

1) Sand fill in embankments, rest areas and protection islands2) Sand fill/imported fill in embankments for interchanges & link roads in Qatar and

Bahrain.


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1.4. Studies, Surveys, Site-lnvestigations, Conceptual Design,

Time and Cost

The works carried out so far were accomplished in two phases.

1.4.1. During Phase 1 the following tasks were undertaken:

· Studies, Surveys and Site-investigations in a 15 x 40 km study

corridor,

· alternative alignments and recommending in favour of one

alignment,

· a Sketch Design for the Causeway.

1.4.2. During Phase 2 the following tasks were accomplished:

· detailed studies, surveys and site investingations for the

selected alignment,· developing a conceptual design for the Causeway

1.4.3. A very brief summary of Phase 2 accomplishments is as

follows:

Planning Study: A further review of the existing and planned land

use and drawing up Conceptual Local Area Plans for the areas adjecent

to the Causeway landing points.

Traffic Study: Updating and detailing of the Phase 1 Traffic Study

for the selected alignment. The traffic forecast made in Phase 1 was

confirmed with Causeway average daily traffic of 3,900 vehicles in 2005,

5,000 in 2010 and eventually reaching 12,000 in 2050.

Topographic Survey: Detailed survey of a 300 m wide corridor

around the onshore part of the alignment to create a digital terrain model.Also, the selected alignment has been staked out on site in both countries.

Utility Survey: The information collected in Phase 1 on existing

and planned services close to the landing points has been updated and

further detailed.

Bathymetric and Geophysical Surveys: Close grid Bathymetric and

Geophysical Surveys have been conducted in the selected alignment.


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Geotechnical Investigations and Evaluation: A total of 14 onshore

boreholes, 8 trial pits and 110 offshore boreholes have been made inPhase 2 along with laboratory testing. These have been used as basis

for a geotechnical evaluation to establish feasible foundation methods

for bridges and embankments.

Marine Studies: The Phase 1 marine surveys and measurements

have been supplemented during Phase 2. The numerical hydraulic modelling

has been refined and optimised. The modelling concludes that the ‘Zero-

solution’ on water exchange can be achieved by dredging of two channels,that also serve for navigational purposes.

Environmental Impact Assessment: An EIA study has been

conducted, reported and presented to the Qatar Supreme Council for

Environmental & Natural Reserves and to Environmental Affairs in Bahrain.

Conceptual Design, Time & Cost Study: Conceptual Design has

been made for the following components of the project:

Alignment and Road: Drawings of horizontal alignment and vertical

profile for the selected road including the interchanges onshore Qatar

and Bahrain.

Bridges: Toal bridge length of 22 km, with two main navigation

span bridges made as cable stayed, each with a main span of 225 m. Allother bridges are low level viaduct bridges using a concept of span-long

(50 m) pre-cast units made onshore and placed by heavy marine lifting

equipment.

Embankment and Dredging: About 18 km of the Causeway will be

made as embankment using dredged fill as core material with stone bunds

and armour slope protection at the sides. Extended embankments at two

locations provide rest and turn-around facilities for the Causeway users.Surplus dredged materials will be placed in two fill depots located close

to the dredged shipping channels.

Tolling and Border Facilities: Tolling facilities will be located

onshore in the two countries. Border facilities and Causeway Operation

& Maintenance complex will be located onshore in Qatar.

Utilities and Services: Utilities and Services are required for theCauseway itself in addition to space provision for a possible future

national electrical link.


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Construction Time: A review of the expected construction period

has concluded with 4¾ years as a realistic, albiet tight constructionprogramme.

Construction Cost: Cost estimates have been prepared and conclude

with a total estimate for the tender sum of QR 6,200 million (USD 1,700

million) for the coast-to-coast Causeway including services and onshore

facilities and interchanges, but excluding wastage, VAT and various

other items but including 7 per cent escalation of price; rates and costs

based on January, 2002 values and best guesses of Contractor’s DirectCosts. However, the actual overall cost today is likely to exceed the

above figure, which will soon be known when the job is put to tender.

1.5. Special Features of Qatar-Bahrain Causeway (QBC)

The most important special features of the Conceptual Design for

the Qatar-Bahrain Causeway are:

Environmental Impact

· A zero-solution for exchange of water and salt from Bay of

Salwa has been obtained for the conceptual design solution.

· Optional culverts can be made in long embankments to improve

exchange of water-mainly to benefit shrimp spawning and

nursery.· All reclamation including the surplus material from dredging

of navigation channels are placed behind bunds to keep spillage

low.

Alignment & Layout

· Large radii horizontal curves assure a view to the oncoming

parts of the Causeway, variatioion in the view and avoidanceof sun glare over longer distances.

· A free view over the sea area and other Causeway elements

is assured all along the causeway.

· The longitudinal profile places the bridge girders high enough

to avoid wave loads and severe salt water spray.

· Provisions are made in the longitudinal profile for free vertical

clearances for the main bridges of 35 m (Qatar) and 27 m

(Bahrain) in addition to two secondary navigation routes withvertical bridge clearances of 15 m.


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Bridges

· Rational prestressed concete box girder design over entire

length and cable stayed altenative over two large navigation

portions.

· Same generic shape is applied for all bridge piers in order to

obtain a coherent design and the appearance as a same family

of bridge components.

· Large diameter bored pile design for pier foundations assures

a great flexibility in the contractor’s design and work schedule.(Precast tubular concrete piles placed in pre-drilled vertical

holes).

· The overall structural systems are designed for efficient

construction methods.

· The components are well suited for mass production onshore,

which will help to obtain a good quality and durability for the

elements.

· Landmark qualities of the main bridges are aimed at.

· Design is for a 100 years service life of the Causeway.

Embankments & Dredging (Marine Works)

· Conventional revetment design.

· use of materials available from the Gulf region is emphasized.

· Revetment protection designed for wave action.

· Limited inconvenience from overtopping waves for travellers.

· Limited likelihood of soft soil and need for soil replacement.

· Low spillage dredging methods for the works.

Tolling & Border facilities

· Tolling and border facilities located onshore.

·

Tolling, immigration, car insurance check and customs check in Qatar.

· Tolling and arrrival check in Bahrain.

· Access to rest areas for Tourists and visitors.

Installations & Services

· Provision of power, water and sanitary systems at border

stations and rest areas.· Road lighting sysyem all along the Causeway.

· Illumination of landmark main bridge structures.


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· Illumination and marking of navigation spans of bridges.

·

Traffic Monitoring and Surveillance System (TMSS) to securesafe and efficient use of the Causeway facilities.

· Electroinic Payment Systems for collection of toll and transfer

of information to banking systems and administrative systems

at the Causeway Authority.

· SCADA – Supervisory Control and Data Acquisition System

for collection of all measurement data and information on the

status of Causeway systems.

· Communication systems for internal and external communicationby the Causeway Authority and communication by travellers.

· Povisions for 3 future HV-Interconnection Cables between

Qatar and Bahrain.

· Provisions for Telecommunication Cables between Qatar and

Bahrain.

Time, Quantities and Cost

· Design & Build contract assumed at conceptual design stage

for coast-to-coast part of Causeway.

· 4¾ years construction period is estimated for the project (very

tight).

· A tender price estimate (mentioned above) is provided for the

Causeway including the land works.

Aesthetics

· A smooth crossing experience for the travellers.

· Efficient handling at check points by a linear layout of the

facilities.

· A pronounced visual variation is obtained by the curved

alignment, variations in longitudinal profile due to navigation

bridges, rest areas and landmark structures like main bridges

and canopy structures at the land facilities and rest areas.

· Landmark bridges are focus points along the Causeway.

· A unique, strong and simple visual entity.

· Transparent and light structures are at the border facilities

and at the rest areas.


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2. GEOMETRICAL AND FUNCTIONAL REQUIREMENTS

2.1. The geometrical requirements to roadway layout and width are

to accommodate an emergency lane of 3.0 m and two travel lanes of

3.65 m each, with a shoulder of 1.2 m, thereby giving two 11.5 m wide

roadways kerb to kerb.

2.2. The soffit of girders shall be at a certain height above water,

the three criteria being:

Requirement 1

Dictated by two main navigation channels at stations 34335 and

57175, both with a width of 160 m and vertical clearances of 35 m and

27 m respectively, corresponding to heights of 35.3 m and 27.3 m above

QBC2001. (The 0.3 m portions allow for the long term sea – level rise

due to feared Global Warming Effect.)

Requirement 2

Dictated by two secondary navigation channels at stations 21210

and 54045, both with a width of 45 m, and vertical clearance of 15 m

corresponding to a height of 15.3 m above QBC2001. (The 0.3 m portions

allow for the long term sea-level rise due to feared Global Warning

Effect.)

Requirement 3

Dictated by the requirement that the deck-soffit should be at a

certain height above the sea to avoid direct hit of waves and to have only

a limited amount of salt spray on the structure. The required height is

7.2 m above QBC2001 for bridge structures from station 20500 (BR1,

begin-station) to station 36000, and 5.8 m above QBC2001 for bridgestructures from station 36000 to station 58647.5 (BR5, end-station).

2.3. Various ‘Requirements’ for the Bridges have already been

reported in the other related Papers dealing with “Conceptual Design

Basis”1 and “Conceptual Study & Strategy for Durability”2 for the QATAR

– Bahrain CAUSEWAY (QBC), to which reference may be made.

2.4. A longitudinal and a plan profile have been developed for theselected alignment according to the above requirements.


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2.5. Accordingly following three Bridge – types have been

developed:

· Viaduct Bridge – Low Level type

· Viaduct Bridge – Elevated type

· Main Bridge – Cable Stayed type

The Viaduct Bridge – Low Level portions cover majority of the

bridge structure with a length of 18,450 m out of the total bridge length

of 21,950 m. The Viaduct Bridge – Elevated portions cover a length of 2,672 m while the two Main Bridges each have a length of 414 m (main

span and anchor span).

The location of the bridge structures is defined in Fig. 1 and

Table 1.

Fig. 1. Naming and numbering of bridge structures and embankments

The requirement for two navigational channels each with a navigation

width of 160 m has led to the introduction of a cable-stayed bridge type

that has landmark qualities and will be the central attraction for the

Causeway.

3. STRUCTURAL CAPACITY REQUIREMENTS

The structural capacity has been verified according to the AASHTO

Load and Resistance Factor Design specifications. The LRFD specifications

have been followed in general with the exceptance where it is found not

to represent the local conditions for the Causeway.


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The modifications to the LRFD have been introduced in the following

areas:

· Wind pressure:

The available extreme wind data at the Al Arish site during the

years 1985-1995 has been studied. A design gust wind velocity VG at

10 m above design water level of 36.2 m/s has been found to represent

the conditions at the site. This value has been used in the conceptual

design as the VG value when defining the wind pressure according toAASHTO LRFD, section 3.8. This represents a reduction from the default

design gust wind velocity value of 44.4 m/s. The corresponding design

mean wind velocity averaged over 10 minutes and measured in 10 m

height is V10

=24 m/s.

· Site-specific ship collision forces have been considered as per

Conceptual Design Basis document mentioned above, and

not according to AASHTO Load and Resistance Factor Design.

· Durability aspects against the harsh environmental exposure

dictate modifications and additions to concrete specifications

and workmanship.

4. MATERIALS AND WORKMANSHIP REQUIREMENTS

The Causeway is located in a severe marine environment with extreme

exposure to salt from water, water spray and salt-laden dust in combination

with high temperatures. The exposure calls for the highest attention to

durability during design, construction and operation and maintenance of

the Causeway. In combination with state-of-the art material and

workmanship specifications the structural detailing requires all external

surfaces to have sloping surfaces to secure good run-off of water with

minimum collection of salt-laiden dust and water. Corners are rounded or

chamfered to avoid acute corners likely to deteriorate and spall. With

regard to workmanship following have been suggested.

· Use of slip-forming should be prohibited and in-situ concrete

work in the sea severely limited.

· Compulsory use of permeable form liner shall be considered in

selected areas (to give clean solid surfaces).· The curing requirements shall be strict and comprehensive.


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· Steam curing shall not be allowed (to cut out possibility of

differential temperature cracks from probable human error).· Strict restictions shall be applied to maximum and differential

temperatures during casting and curing of concrete.

· Strict restrictions shall apply to ensure careful handling during

transport and erection of elements.

· Strict QA procedures shall apply.

5. BRIDGE LOCATIONS AND TYPE

The proposed Causeway bridge structures are numbered according

to the figure below. The exact locations of the three bridge types are

identified with regard to ‘station begin’ and ‘station end’.

TABLE-2. BRIDGE NUMBER, TYPE, LENGTH AND STATIONING

Bridge Bridge type Length Station (m)

No: (m) Begin End

BR1 Viaduct type, Low Level 10070 20500,0 30570,0

BR2 Viaduct type, & Main type 2620 33012,5 35632,5

Viaduct type, low level 342 33012,5 33354,5

Viaduct type, elevated 686 33354,5 34040,5

Main

(cable stayed portion) 414 34040,5 34454,5



BR3 Viaduct type, low level 1320 41420,0 42740.0

BR4 Viaduct type, low level 5220 48985,0 54205,0

BR5 Viaduct type, & Main type 2720 55927,5 58647,5



Main

(cable stayed protion) 414 57055,5 57469,5




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6. CONCEPTUAL DESIGN PHILOSOPHY

6.1. The objective of the conceptual design has been to identify,

visualise, verify and document a feasible rational design and construction

method that fulfils overall functional and engineering requirements taking

into account prevailing conditions and restraints.

6.2. However, if the conceptual design were to be developed into

a tender design, it may not necessarily turn out to be the lowest cost

solution among alternative tender design submittals. An alternativetender desing has the benefit of being suited to a specific contractor’s

work methods and available equipment and on the marked situation with

regard to material cost. The presented conceptual design uses a

prestressed concrete deck girder but the possibiliy of a steel bridge

solution or a composite steel/concrete solution being cost competitive

exists and cannot be ruled out at this stage although prestressed concecte

has more advantages. The preparation of tender documents will therefore

consider carefully the requirements that would be applicable to thesealternative solutions.

The durability and maintenance requirements are more stringent for

the structural steel as also for the composition of the wearing course for

the ‘steel only’ solution. The hot climate and a relatively flexible steel

deck plate requires careful considerations into a suitable wearing course.

More importantly it should be noted that the steel box decks willrequire to be permanently equipped with perenially power – driven de-

humidifiers to control humidity perpertually to control corrosion. This

factor, together with other Durability considderations, itself is likely to

tilt the balance in favour of prestressed concrete, as happened in the case

of King Fahad Causeway.

6.3. Some key factors have been identified in achieving the rational

design in ‘concrete’ that is considered to yield a cost efficient and

durable design. These key factors are:

· Relatively, ‘short’ span lengths:

-economical best solution for relatively shallow foundations

for the given substrata conditions.

·

Prefabrication in a precast yard on–shore:-‘maximum work on-shore/and minimum work at sea’ is the

economical best solution.


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· Prestressed concrete:

-material preferred in achieving specified design requirementsand ‘durability’ with low maintenance cost.

· Repetition in prefabrication:

- results in a rational and cost effective solution.

· Large prefabricated elements:

-shortens construction time and reduces cost but limit weights

to 1000 T (because of limitation on available draught).

· Minimum draught required for the barges to carry heavy P/c

elements:

-the average 4 to 5 m deep stretches along the alignment will

allow just about enough draught for these barges to cary

about 1000 T P/c elements. This sets the weight limit if these

stretches do not have to be dredged generally. (Bridge to be

replaced by Embankment in shallower stretches.)

6.4. The draught for a barge loaded by a 1000 t carrying crane is

around 2.0 m to 2.4 m and allowance for waves and clearance under keel

is required for a weather-independent and safe construction progress.

TABLE-3. WATER DEPTH AT BRIDGE LOCATIONS

Bridge Min. water Max. water Depth Average water depth (m) depth (m)

BR1 2.0 7.4 5.6

BR2 1.3 4.5 3.0

BR3 1.4 6.2 5.0

BR4 2.8 6.9 5.7

BR5 3.8 6.1 5.3

6.5. The listed water depths at various bridge locations given in

Table 3 confirms the generally shallow water along the Causeway location.

The very shallow water at BR2 over a fairly long section is considered to be

insufficient for the construction without dredging. Dredging has to done for

water depth less than 4 m. Lesser water depths for the remaining bridges are

very localised and do not represent a significant problem for the construction.

The average water depth is more than 4 m, except for BR2.


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6.6. The selection of a limiting lifting capacity of 1000 t allows the

construction to be carried out with existing standard equipment that can beobtained in the construction market at reasonable commercial rates. (KFC

could use slightly heavier lifting capacity for its water depth.).

6.7. Longer spans, requiring heavier lifts, would increase the required

draught and hence the water depth. This would require additional dredging

in localised areas for all bridges and for deeper dredging for the entire length

of BR2. After due consideration, this idea of heavier lifts was given up finally.

However, the issue can be re-visited during detailed design for the possibilityof heavier lifts.

6.8. The 50 m optimum span lengh for the Causeway has been

considered at a conceptual level but the cost difference between 50 m span

and longer spans up to 65 m is within a few percent. This can be considered

in the detailed design.

6.9. Infact even the span articulation of ‘50 m spans with 8 m cantileverarms’ and ‘34 m drop spans’, as in KFC, could also be looked into in detailed

design stage, yielding a determinate structural system too.

7. CONCEPTUAL DESIGN

7.1. Foundation

7.1.1. For viaduct bridges: It is ‘pile’ foundation for all viaductbridges, consisting of precast prestressed concrete hollow 4.5 m diameter

piles, installed into predrilled holes. The conceptual design of the piles

has established that piles are to be installed with pile tip elevation at a

depth of approximaterly 10 m into competent rock. The average pile

length has been found to be just short of 17 m with a variation from 10

to 27 m. The magnitude of the soft sand, silt and clay layers varies along

the alignment according to the geological profile of sub strata.

An alternative open foundation for the viaduct bridges is feasible

but is not favoured for the following reasons:

· The pile foundation is more flexible in obtaining the required

load bearing capacity as the depth of the boring/pile can be

increased to adjust for corrections to the assumed bearing

capacity during construction.· The pile foundation working is less prone to adverse weather


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conditions in comparison, with the benefit of an almost

uninterrupted construction cycle. Interruptions are likely tobe more problematic for the open foundation where adverse

weather conditions will cause longer delays during installation

of numerous such foundations in the open sea.

The difference in cost was found to be marginal with a small cost

advantage to the pile alternative, even ignoring the possible weather-

related delay problems with open foundation work.

The (deeper) pile foundation could present a problem if fresh-water

aquifers were to be found at their locations. The bore hole information

obtained during Phases 1 and 2 has not identified the existence of aquifers

but it is considered too early to rule out the existence of aquifers all

together. Fresh water aquifers are known to exist below the sea bed close

to the coastline of Bahrain and therefore the Bahrain Water Resource

Directorate was contacted. Their initial conclusion is that the proposed

deeper pile foundation along the selected alignment will not be a problemto aquifers. This conclusion should be verified by the selected contractor

once his pile design has been completed. There are no records of fresh

water aquifers close to the Qatar coast.

The open foundation could be an alternative solution. It is

recommended to keep it as an alternative in the preparation of tender

documents to allow contractors such option.

The foundation for the low level sections of the Viaduct Bridges is

a single pile per pier (i.e. per carriageway). The single pile solution has

been found to possess sufficient structural capacity up to a roadway

elevation of approximately 19 m above QBC2001. The soil-structure

interaction has not been fully studied and the maximum elevation of the

roadway with a single pile could be further raised when taking the soil-

structure interaction into considerat ion. The information available inPhase 2 has not been sufficient to fully determine further relief from the

soil-structure interaction. This should be the focus of the tender Document

and the detailed design when the soil data collection has been carried out

rigorously.

In the elevated portions of Vaiduct–Bridges at a roadway height

above approximately 19 m above QBC2001, a single pile is inadequate. A

two-pile support is introduced under each pier and foundations underadjacent piers at the same station are tied togerther with a cross beam

connector.


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7.1.2. For main bridges (Cable Stayed) : The pylon is founded on

a huge open founation (40x40 m). The bottom of this foundation of theQatar side Main Bridge (at station 34222.5) is determined to be at -17.0

m. This foundation level at the Bahrain side Main Bridge (at station

57287.5) is determined to be at – 16.0 m. Both elevations secure the open

foundation in competent material in the Pleistocene deposits. This open

foundation is a cast in-situ concrete footing suggested to be cast in

dry within a cofferdam enclosure. The footing is below the MSL, leaving

the pylon shaft elegantly protruding uninterrupted through the water

line.

7.2. Substructure

7.2.1. In the low level portions of the viaduct bridges: The pier

emerges from the pile from an elevation of approximately 0.75 m below

sea-bed, extending to approximately 0.5 m below soffit of deck girders.

the pier shafts have been shaped with a curved surface for aesthetic

reasons, and are precast prestressed concrete hollow shafts. The piershafts have been sized to support Bearings, provide required load carrying

capacity and have, to some degree, been shaped to minimise the blocking

of the water flow. Up to a roadway elevation of 16 m, the piers have a

depth of 2.50 m in the longitudinal direction of the bridge. For highter

elevations (taller piers) the depth is increased to 3.0 m.

Scour protection shall be provided around the pier shafts unless

rocky sea floor provides sufficient protection against scour.

7.2.2. In the elevated portions of the viaduct bridges: The 3.0 m

deep pier section depth with the same shape as for the low level section

is used for the elevated sections.

7.2.3. In the main bridges (Cable stayed): Each Main Bridge has

a single central pylon. The shape of the pylon shaft is in family withthe pier shapes for the Viaduct Bridges. The pylon is a hollow cast in-

situ concrete pylon with variable wall thickness from 400 mm at the top

of the pylon to 800 mm at the base. The pylon is tapered with decreasting

cross sectional dimensions from its base at footing towards its top. The

top of the pylon can be accessed through a ladder and entry door at the

deck level with an access system inside the pylon. Access to the top

of the pylon is required not only during construction but also for inspection

and maintenance of the cable anchorages and pylon top light.


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Submerged ship collision fender islands have tentatively been

assumed to be provided at the pylon, at the anchor pier and at the pierclosest to the naviagation channel. Ships not on course will hit shallow

ground at the island edge and the island will absorb the kinetic energy

of the ship’s blow.

Each pier shaft at either end of the Main Bridge will be of the

elevated Viaduct Bridge pier category but will be tied-down to its pile–

cap by 4 Nos: 19 multistrands (through the box decks).

7.3. Superstructure

7.3.1. For viaduct bridges: The Viaduct Bridges have span lengths

of 50 m with a superstructure consisting of two precast prestressed

concrete box girders. The conceptual design of the superstructure for

the Viaduct Bridges consists of two independent identical psc box girders

for both low as well as for elevated portions.

Structural depth of box girder is 2.8 m.

7.3.2. For main bridges (Cable stayed): The standard Viaduct

Bridge deck precast box girder is to be used also for the Main Bridges.

The outside dimentsions of the box will be retained with modifications to

the web thickness and with some additional details to allow for a different

static system, with stay cable ‘support’ at every 7.5 m in the main span

and at every 5.0 m in the back span. A system of in-plan and crossgirder bracings by steel trusses between the boxes is proposed to carry

the horizontal and vertical load component of the stay forces in the main

span; while an in-plan thick cast in-situ concrete slab between the box-

flanges and steel cross-girders, carry such components of the stay forces

in the back span. Box girders are fabricated and installed in 50 m lengths

as for the Viaduct Bridges. The impression will therefore be one of a

soothing single continuous girder all along BR2 and BR5 where the Main

Bridges are located.

Plates 1 to 11 may be referred for explanation.

8. ‘CONSTRUCTION’ AND ‘ERECTION’ CONCEPT

8.1. Introduction

The conceptual study of the optimal construction method resultedin the selection of large prefabricated prestressed concrete elements as

the preferred construction method. This applies to both super and


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substructure elements. The same erection equipment is assumed to be

used for all components throughtout the construction period.

8.2. For Superstructure

8.2.1. In viaduct bridges: Typical box elements are fabricated in

span-long lengths. Box sections are constructed as being balanced

cantilevers on piers once erected with internal cantilever post-tensioning

cables installed and stessed in the precast yard. After erection on site,

the external continuity cables are installed and stressed after concreting/ grouting of in-situ ‘stitches’ between successive elements.

Precast prestressed elements are carried by crane or barge to the

location of installation and then lifted into their final positions. The

elements are erected with utmost care as balanced cantilevers with the

pier support at the centre of gravity for each span-long p/c psc double-

cantilever ‘butterfly’ element. Positioning the lifting points is of paramount

importance as also the presence of fair weather and wind. It is considerednecessary to erect the elements on temporary jacks with the capacity to

adjust the final positioning in both vertical and horizontal directions.

Once the girder is located correctly the permanent Bearings are built into

the structure.

The next step in the erection will be to construct the “in-situ stitch”

between the successive erected girders (comprising consprusign the semi

continuous unit between the Expansion Joints) at the Halving Joints.

Before concrete is placed, a positive fixation of the stitch is established

by providing precast concrete pads to maintain the specified gap distance

and at the same time tension temporary post-tensioning bars across the

in-situ stitch. Once the fixation is in place, the in-situ stitch concrete is

cast, using rapid hardening cement. At the prescribed curing age of the

in-stiu stitch, the external continuity tendons can be installed and tensioned.

The temporary post-tensioning bars can then be released and removed.

The erection method does not allow support from a previously

erected girder element as each element in principal is balanced around its

pier-support and would not require external balancing. However, in practice,

the girder elements would not be 100 per cent balanced and during

erection the wind load on the structure would also cause some out-of-

balance forces. These are however to be resisted by the temporary jacks

that are located on either side of the permanent Bearings. Extremecaution is called for this construction method.


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The adjustment facility in the temporary jacks and the independence

of the neighbouring girder element is considered an advantage whenachieving the correct alignment. At the same time it will cause minimal

restraints to the operations during erection with regard to progress of

previously erected girder elements.

8.2.2. In main bridges (Cable stayed) : Temporary rigid trestle

supports at 50 m intervals are used to allow same erection procedure as

for the Viaduct Bridges, with the entire deck structure erected before

installation of cable stays. Temporary post-tensioning will be used to

control the stresses during transport and erection of the deck girders.

8.3. For Substructure and Foundation

8.3.1. In viaduct bridges : The substructure comprises a single pile

and pier shaft unit constructed by firstly drilling a 4.75 m diameter hole

into the seabed to the required depth. A steel casing supports the

circumference while drilling and while installing the prefabricated pile

unit. The steel casing is afterwards withdrawn while grouting the annularspace between soil and pile. Installation of pile casings, bottom seal

concreting and grouting are carried out from a jack-up platform – working

for two adjacent piers simultaneously. The pier-pile is installed in the

casing by a 1000 t capacity floating crane. The taller piers near the main

bridges require two piles per pier and an interconnection between pile-

caps, and possibly between piers.

Installation is carried out as for the typical viaduct bridges butadditional work must be restricted to within the limit of 1000 t erection

capacity of the floating crane.

To allow for the variation in pier and pile length due to variable

elevation of roadway and pile tip, a large number of different-length

precast pier and pile units has to be considered. A selection of ‘adjustment-

sections’ in intervals of 0.3 m has been considered in the conceptual

design to allow for any late decision of final pile tip elevation. The‘adjustment-sections’ can be added to the bottom of the pile at the last

minute before it is prestressed and transported for installation.

Confirmatory Geotechnical investigations have to be done for each

pile for deciding its exact length.

Despite all this, where the founding level may, at the last minute,

require to be taken slightly deeper still, suitable concrete can plug be castunder – water in the pre-drilled hole and then the already prefabricated

psc pile installed on it.


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The roadway elevation will be fixed in the design and the variable

length of ‘pier’ can be arranged in adjustment of the shaft length in theprecast yard. Some tolerance in the top of pier elevation of +/-100 mm

is anticipated to be accommodated in an in-situ concrete plinth on which

the Bearing will be positioned with a high degree of accuracy.

8.3.2. In main bridges (Cable stayed) : A temporary island,

constructed at the pylon location, will serve as work area for construction

of the pylon and its foundation. The island will subsequently be modified

to serve as submerged ship collision island which shall be containedwithin stone-bunds and protected by armour stone. Landings for

construction materials and equipment are required. The pylon foundation

is constructed in–situ on the work island within a cofferdam enclousure

– which is first excavated and de-watered. The pylon foundation will be

cast in–situ at the bottom of the cofferdamed/ excavated/de-watared area.

The pylons are constructed in a climbing form, assisted by a tower

crane.

The anchor piers are ‘modified taller Viaduct Bridge piers’ but 4

Nos. 19 strand tie-down cables are required to carry the uplift forces

occurring in adverse situations of heavy traffic on the main span.

9. PREFABRICATION YARDS

One or more prefabrication yards will need to be established on-shore at a location where a temporary harbour facility, with 5 m water

depth, can be constructed.

The fabrication yard provides areas for steel fixing, pre-casting of

units, pre-assembly, prestressing, storage, concrete batching facilities,

offices and stores, workshops, etc.

The temporary harbour is to be protected by breakwaters and has

“load-out” facilities for the prefabricated elements and ‘quays’ for

unloading of concrete aggregates, cement, reinforcement, etc., and berthing

and service of marine construction plant.

A single construction yard for fabrication of all p.c. elements is

considered to be the most cost efficient arrangement and locations for

such a yard appear to be technically feasible both on Qatar and Bahrainside.


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Should it be decided ot establish more fabrication yards, each

facility should be dedicated to fabrication of one type of elements e.g.

piers or deck units, to reap the benefit of repetition in production at same

site. A second fabrication yard with a parallel production would be an

advantage because this provides a full back-up.

The prefabrication yard is arranged such that pile units, pier units

and deck units are constructed ‘between the tracks’ of a 1000 t gantry

crane. The crane lifts fabricated units from casting beds to storage and

transport barges in a ‘load-out’ facility arranged at the end of the track.

Prefabrication of reinorcement cages, small concrete elements and

special parts for sub assemblies is carried out outside the main production

line and moved into the line by cranes.

ACKNOWLEDGEMENTS

I am grateful and highly indebted to the authorities in Bahrain formy association with this mighty and one of the most challenging civil

engineering Projects of the century and cannot thank them enough for

the trust reposed. Participation in this most prestigeous of Projects is

really a lifetime’s dream and opportunity which, for me, nothing else

could equal. Grateful thanks to Cowi Consult who assisted in carrying

out the study.


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“B ” A G Q C


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Documents

Bridging-The Arabian-Gulf Between Qatar And Bahrain-Conceptual Design and Construction (1).pdf