�IATRANSPORT RESEARCH LABORATORY

Design of Irish bridges. fords and causeways indeveloping countries

T E Jones and J 0 Parry

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Overseas CentreTransport Research LaboratoryCrowthorne Berkshire United Kingdom

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Design of Irish bridges, fords andcauseways in developing countries

By T E jones and j D Parry

T E Jones PhD CEng FICE Fl[HTFGS

Tom Jones is a chartered civil engineerand obtained his doctorate in highwayengineering from the University ofBirmingham. He has worked in theOverseas Unit of the TRL, for over 25years researching a wide range oftopics in over 30 developing countries.The topics have included catchmenthydrology, storm rainfall prediction,soil physics and more recentlypavement maintenance and manage-ment problems for both paved andunpaved roads. Currently he is Headof the Pavement Management Sectionof the Overseas Unit.

J D Parry CEng MIMechE FIHT

John Parry studied mechanicalengineering at Reading College ofTechnology and became a charteredEngineer in 1972. Most of his career,apart from three years as engineer to arefugee team in North Africa, has beenwith the Transport Research Labora-tory. Initially, he was a member of theteam responsible for the design andevaluation of the mini roundabout andguardrail projects. In 1979 he joinedthe Overseas Unit at TRL, where he'has reported on the design andperformance of modular timberbridges, and published manuals onbridge inspection, maintenancemanagement and bridge design. Morerecently he has been responsible forthe design and supervision of concretepavement trials in several developingcountries.

Vented causeway in Zimbabwe.

River crossings are vital to road networks and can absorb asignificant proportion of the cost of both construction andmaintenance. It is important therefore that these structures areappropriate to the category and volume of the traffic carried.In forestry regions of UK and rural areas in many countries,low level water crossings can provide practical, economical andsimple alternatives to conventional bridges. This papersummarises the relative merits of water crossings for lowvolume roads from the simplest fords to the engineeredbed-level causeways, vented causeways and submersiblebridges.There are two basic types of low level

crossing:T-fords and bed-level causeways

- and vented causeways andsubmersible bridges.

The success or life of these structuresdepends very much on the hydraulicdesign. Fords and bed-level causeways,like conventional bridges, may beconstructed so that they cause littleinterference with the design flood.Vented causeways and submersiblebridges inevitably disrupt river flow andso are liable to sustain damage themselvesor indirectly cause scour damage to theriver bed or banks, Which in turn mayaffect the road approaches to the crossing.

AS fords and bed-level causeways areover-topped by any water flowing in theriver channel, there is no advantage inraising the road surface above the streambed. Vented causeways and submersiblebridges usually present a dry carriageway

for ordinary flows and are overtoppedonly during the design flood.

All four crossing types are suited to lowtraffic flows or where an all-weatherbridge is available on a reasonably shortdetour. They should be designed so thatfor most of the year there will be a flowof water over the carriageway no morethan 150mm deep.

Site selectionThe best location for a low level crossing

is similar to that for a conventionalbridge, with the exception that a widestretch of the river provides easier roadapproaches and slower, shallower water.The stream should be straight, with welldefined banks and a uniform gradient andthe bed material should be strong enoughto support traffic. (The submersiblebridge requires different considerationsand is described briefly under thatsub-heading.)

28JANUARY 1993 HIGHWAYS AND TRANSPORTATION

Design of Irish bridges, fords and causeways in developing

Table 1. Maximum rate of change per cent

Traffic speed Descending Ascending

45 mph (72 kph) 10 1s60 mph (96 kph) 8 12

Road approaches The equal cut and fill construction ofClearly, gentle slopes are required for the approach roads shown in Fig Ia

both traction and safety. The maximum requires less work than the cut andgradient for motor traffic should be 10%, remove spoil of Fig lb but the placing ofand for animals 5%. Change of gradient the spoil in the river channel, shownshould be gradual to prevent the hatched in ~Fig la, may cause scourunderside of vehicles touching the road problems during a flood. Fig 2 shows inand to preserve long sight distances. A plan how steep approaches may besmall change in horizontal alignment of relieved by a diagonal descent of the riverthe road at the crossing helps to draw the bank for roads where speeds are naturallyattention of drivers to a dip that may slow and the horizontal curve on theconceal an obstacle, approach side is clearly visible.

Suggested maximum rates of change of Even where the road is a single track, itapproach gradient are given in Table I is usual to make the crossing and the(Binghamn, 1979). approaches of two-lane width to allow

Fig 2. Plans of low level crossings and approaches.

traffic to pass a broken down vehicle orone which fails to mount the gradient.

FordsFords are unpaved and only suitable for

the lowest of traffic flows. These are thesimplest form of river crossing where thestream is wide, shallow and slow, theapproaches gentle, and the surface firm.Improvements to the approaches arechiefly concerned in lessening thegradient. The running surface can bestrengthened and made more even withstones which are brought in and buriedjust below the surface. Alternatively, ifstones are carried in the flow, these maybe trapped by barriers made of boulders,gabions or piles.

BouldersLarge stones placed across the river bed

at the downstream side of the crossing arereputed to filter the flow of water andretain gravel and sand, which eventuallyform a more level and even surface forvehicles. However, if the stones are toolarge or form too high a wall, scour willresult. If they are not heavy enough, theywill be washed away at the first flood. Fig3a shows a typical cross section.

GabionsA more expensive but durable

improvement may be made by replacingthe boulders with gabions to trap rivergravel or retain imported material, asshown in Fig 3b.

The standard gabion is a rectangularbasket made of hexagonal steel wiremesh. It is strengthened by edges ofheavier wire and by mesh diaphragmswhich divide it into lim long compart-ments. It is usually supplied as a flat packand assembled on site and is normallyfilled in-situ with quarried stone orrounded shingle of sufficient size that thestones cannot pass through the mesh. The

Fig 3. Types of ford.

HIGHWAYS AND TRANSPOR~nTION JANUARY 199329

countries

C. -- F. odi-b.

Fig I. Vertical alignment of road approaches.

(al 5,,aight crossing

(b) Road f0110-5 lime bank, S

b) C., ..d ------

Design of Irish bridges, fords and causeways in developing countries

. bion structure is more stable anddurable if the stones are packed carefullyby hand, almost as if building a wall.Internal tie wires at about 0.5m intervalshelp to retain the shape. At a ford,gabions are securely wired together inposition to form a continuous revetment.

The gabion baskets (typically 2 or 3mlong x Im wide and 0.5 or Im high) arewired together and dropped into aprepared trench. The central gabion isfirst filled and, using that as an anchor,the line of gabions is pulled taut andstraightened by a chain attached to a truckor winch. This tension is maintainedwhile the remaining baskets are filled.When filling is complete, finaladjustments are made to the top courseof rock and the baskets are closed.

It is important that gabions protrudeonly 150-300mm above the natural bedlevel of the river, depending on the nature*the bed material, otherwise they willetas a weir and cause heavy scourdownstream of the crossing.

As an alternative to conventionalgabions, tube baskets can be made froma roll of fencing mesh filled with stone orshingle. During filling, the edges areraised and then bent over at the top toform a tube and tied; finally a wire ropeis attached as shown in Fig 3c andsecurely anchored at each end. As withgabions, tube baskets need to be installedin a previously excavated trenchapproximately half the depth of thebasket, ie 0.2 to 0.3m. After installation,sand and gravel transported by the streamis trapped behind the baskets andprov'ides a firmer fairly level surfacesuitable for vehicles.

Piles

Where gabions are unobtainable, timber~ie, driven into the river bed are suitablemacut-off wall, as shown in Fig 3d.WIube effective, timber piles need to be

about 2m long and placed at no more than0.6m centres. If the river is fast flowing,a continuous line of piles may be needed.As with the gabions and wire baskets, thetop of the piles should be no more than0.3m above bed level.

Provision of a curtain cut-off wall madeof gabions or piles may be necessary onthe upstream as well as the downstreamside if the road bed is erodible. Note thatall four types of ford may require rip rapscour protection on the downstream side,as shown in the Figures.

Bed level causewaysWhere the type of traffic or the distance

to an alternative crossing justifies theexpense, a pavement may be laid on theriver bed. A paved ford is also called abed-level causeway, drift, paved dip or

,,ish bridge. Three common designs arewn in Fig 4. To protect the pavement

W mscour damage, curtain walls oraprons are usually required on both theupstream and the downstream side and

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Fig 4. Bed-level causeways.

lnsh bridge in central Kenya

JANUARY 1993 HIGHWAYS AND TRANSPORTATION

Design of Irish bridges, fords and causeways in developing countries

Vented causeway in Zimbabwe

these must continue up the approaches tothe height of the design flood.

It is recommended that if aprons are notinstalled curtain walls should be lm deepon the upstream side and at least 2m deepon the downstream side unless a rockstrata is reached before that depth. If thebed is inerodible, the causeway need notbe provided with curtain walls, but thebed on both the upstream anddow~nstream sides of the crossing shouldbe trimmed flat to reduce turbulence.

Fig 4a (Gillett 1979) shows a sectionthrough a basic bed-level causewaysuitable for maximum water flows below2m/sec and for light traffic. The crossingshown in Fig 4b (Metschies 1978),requires good concrete technology andmay sustain damage to the apron that isdifficult to repair. Fig 4c shows a designemploying a practical combination ofconcrete pavement with flexibleprotection that is more suited to routinerepair than the rigid concrete curtain wall.

Fig S. Vented causeway.

Generally, a 1:2:4 concrete, by volume,is used and slabs are jointed using crackinducers every 5m. The concrete shouldbe laid on non-erodible material.

Vented causewaysVented causeways are designed to pass

what may be called an ordinary flood withvery little water overtopping thecarriageway, but may still be inundatedand unusable for a few days each year.

These structures present a considerableobstacle to the free flow of both normalflow and the design flood, so they mustbe built massive enough to withstandwater pressure and debris impact. Theymust also be provided with adequatescour protection where the bed iserodible, and marker posts.

Fig 5 shows a typical section andelevation, Fig 6 gives dimensions forconcrete cover and reinforcement (Parry,1992).The vents are usually concrete or

corrugated steel pipes from 0.6 to 1 .Omdiameter, set in a block of concrete ormasonry. Where prefabricated pipes arenot available, vaulted masonry tunnelshave proved successful. Concrete ormasonry retaining walls and aprons arerequired to channel the flow and preventscour at both entrance and exit. For thisreason too, the vents should bedistributed all along the structure so thatflow parallel to the roadway is avoided.

In order to prevent blockage of thestream by debris or silting, carefulattention is required to setting the pipeslevel with the stream bed and at the samegradient. No part of the vents should benarrower than the entrances andwedge-shaped deflector ramps may berequired on the upstream side to; guidelarge floating debris above the vents.Alternatively, a grill of posts installedupstream of the causeway will collect treebranches before they reach the structure.

The capacity of the vents is sufficient topass all ordinary floods without damageand with no more than 150mm of waterover-topping the structure. The

HIGHWAYS AND TRANSPORTATION JANUARY 199331

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S.e Fig. 7.6 f., dim.,nioe, .nd --------- nf~c~en

(b) El.e.io

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Design of Irish bridges, fords and causeways in developing countries

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Fig 6. Vented causeway dimensions and reinforcements.

maximum capacity of the vents is reachedwhen water on the upstream side of theembankment stands at its highest safelevel. Under this condition, the outlet willnormally be submerged.

The operating head h is the differencebetween the upstream and downstreamlevels, and consists of the following threeelements:

h =he + hf + h,

where

h, is the loss at the entrance

hf is the friction loss and

h. is the loss at the outletU,'.

c

The entrance and outlet losses areexpressed in terms of theVelocity head:

h, ke V2

2g

h,= k. V2

2g

where V is the average velocity in thepipe, k, for bevelled entrances to pipesand box culverts can be taken as 0. 15 butfor corrugated metal pipes projectingfrom the fill the ke value can be as high

as 0.9 (Farraday and Charlton, 1983). Useof headwalls forming a square entrancecan reduce this to 0.5.

Ko is 1.0 for all types of pipe.The friction loss

hf = f LV2

D2g

where

L = pipe length (in)

V = flow velocity (mn/sec)

D = the pipe internal diameter (in)

g = acceleration due to gravity(MlSec 2)

f =0.016 for concrete pipes and0.075 for corrugated metal pipes.

Submersible bridgesWhere the traffic is dense enough to

justify a dry crossing of a substantialordinary flood and the design flood ismuch greater, a submersible bridge is analternative to a vented causeway.Submersible bridges are able to pass alarger flow than the vents of a causewayof the same height but are moresusceptible to damage by the designflood. The overturning moment at thepier foundations becomes very largeunless the piers are kept short, and thehorizontal and vertical forces on the deckrequire solid restraint.

Because of these difficulties,submersible bridges are notrecommended for any foundation otherthan rock, and even then a ventedcauseway or conventional bridge may bea more durable alternative.

Construction is usually of reinforcedconcrete with continuous reinforcementbetween the sub-structures and thedeck.

Fig 7. Repairs to gabion causeway.

3 2 J N A Y 1 9 I H A S A D T A S O T T O

R 120C 1 50--

Concrete sxrength 30 N/m2

(28 day cr~be)Dimentions (mm)

D A a c

600 250 175 150750 250 175 150

900 275 200 175

1000 300 225 200

Gabion wall before scour damage

After damage

After repair

=Cip CO =_

JANUARY 1993 HIGHWAYS AND TRANSPORTATION

Design of Irish bridges, fords and causeways in developing countries

Signs and markersAll water crossings should be well

signed in advance. Low level crossingspresent more of a hazard to traffic thanconventional bridges because of thechange in vertical alignment as well as thepossibility of encountering water on thecarriageway.

Depth gauges should indicate the depthof water at the lowest point of thecrossing. Simple black and whitemarkings are best with an indication ofthe units used. Posts should be about300mm diameter or square, placed withineasy vision of the approach but well awayfrom possible impact damage by vehicles.

Guide posts should be set each side ofthe carriageway between 2 and 4m apart,according to the probability of catchingfloating debris. There are two opinionsabout their height:a) They should be high enough to be

visible during the highest expectedfloods.

b) They should be visible only when thewater is shallow enough for vehiclesto cross.

The posts may be of durable timber,metal or concrete, according to thematerials used for the carriageway surfaceand curtain walls; eg concrete posts onconcrete bases and timber posts set intogabions etc.

An additional guide for vehicles may be

provided by building a ridge down thecentre of a concrete causeway, as shownin Fig 4b. This also offers restraint againstsideways drifting in strong currents butis an additional restriction to flow.Maintenance

Submersible crossings of all typesrequire more frequent maintenance thanmost conventional bridges. Therefore thestructural design should allow for easyrepair of anticipated damage, which isusually caused by scour. On very erodiblebeds it is often more successful to buildgabion curtain walls and use renomattress aprons rather than a rigidconcrete structure, and to accept thatsome rebuilding will be required eachyear. Fig 7 from BCEOM (1975)illustrates the principle.

SummaryProperly designed submersible

crossings can be an economical solutionto river crossings with low levels of traffic.They are only viable where normal dailyflow over the structure is less than 1 50mmndeep and where flooding occurs for nomore than about two weeks per year.Hydraulic design is of primaryimportance because most damage to thestructures results directly from scour. Forthis reason it is also recommended thatthe design includes provision for ease ofmaintenance.

Acknowledgements W.The work contained in this paper forms

part of the research programme of theOverseas Unit (Unit Head: j S Yerrell) ofthe Transport Research Laboratory.

References1 Binghamn j (1979). Low Water Cross-

ings, Compendium 4, Transporta-tion Research Board. WashingtonDC.

2. BCEOM (1975). Manual for smallworks in Africa. Paris: Ministry of

*Cooperation.3. Farraday R V and F G CharIton

(1983). Hydraulic factors in bridgedesign. Hydraulics Research Station,Wallingford, UK.

4. Gillett D (1979). Low Cost RiverCrossings, Ministry of Works Report.Kenya.

5 . Metschies G (1978). Manual on Road~1Construction. Economic and SocialCommission for Asia and the Pacific.ESCAP.

6. Parry j D (1992). A design manualfor small bridges. Overseas Road Note9. Crowthorne: Transport and RoadResearch Laboratory.

7. Roberts P A (1986). The Irish Bridge,Low Cost River Crossing. Dept ofCivil Engineering Report, Universityof Southampton . 1

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