CHAPTER2

2 LITERATURE REVIEW

2.1 Selection of pipe material

Poly Vinyl Chloride (PVC), Steel and Ductile Iron (DI) are the commonly

used material for water pipe lines. However. PVC is not used in pipelines

across waterways and highways unless it is encased for protection from sun

light and external impact and shock loads.

Steel pipes are liable to the actions of acids and alkalis in water and slight

trace of these will produce rust and encrustations. Therefore. pipes must be

protected against it inside and out. and also couplings and bolts. Steel pipes

need frequent painting. Thinner walls of steel pipes and greater susceptibility

to corrosion are likely to cause high maintenance costs and shortened life. In

Sri Lanka 800mm, 900mm. lOOOmm and 1200mm diameters of steel pipes are

used in exposed rising mains and bridge crossings.

The largest practical advantage of Ductile Iron pipe compared with steel pipe

is that Ductile Iron pipe is much easier to install properly. Handling.

assembling, and adapting to field conditions all are other aspects in vvhich

Ductile Iron pipe offers distinct benefits. Therefore, at present DI pipes are

used for most of the waterway and highway crossings.

This study focuses on Ductile Iron pipes. -·-

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2.2 Selection of the Type of the pipe joint

Ductile Iron pipe is furnished with several different types of joints: push-on,

mechanical, restrained, flanged, ball and socket, and grooved and shouldered

joints. It is centrifugally cast in 4m, 5.5m, 6m and 9m nominal laying lengths.

Nominal diameters range from 80mm to 1600mm with a variety of pressure

and special thickness classes.

Typically, bridge crossings involve push-on joints, mechanical joints.

restrained joints or combinations thereof.

Push-on joints are excellent for bridges with properly designed and

constructed supports. Ample deflections in these joints are possible when

properly braced support structures are provided to carry the weight of the pipe

and its contents and resist any forces acting against the pipe supports.

Normally, expansion and contraction of the pipe due to temperature changes

can be adequately provided for with such joints due to its deeper socket depth.

if more adjustment is needed, expansion couplings should be considered.

(DIPRA 2007)

Figure 2-1 Push-on Joint

Mechanical joints are often used for fittings but are not generally used for

straight runs of pipe. Since standard push-on and mechanical joints are not

·'restrained,'' due consideration should be given to proper design and

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construction of supports or anchorages to resist thrust forces, dead loads,

impact and shock load and thermal changes.

~-~1 J CJI 1----------

~-~ l Figure 2-2 Mechanical Joint

The restrained joint complements the push-on and mechanical types by

maintaining flexibility and also by providing both ease of assembly and a

locking feature to resist pull-out. Numerous types are available employing

modifications of the push-on and mechanical joint designs. This depends on

the pipe manufacturer regarding the use of standard push-on joints with

gripping type gasket products on bridges. In a pressurized system, some

flexible restrained joints are subject to significant joint extension. Therefore.

when utilizing restrained joints, proper design and construction techniques

normally should include provisions for extending each joint so as to engage its

restraints. Cumulative joint extension due to thrust pressure over a long

crossing could result in over-deflection, excessive movements. and excessive

beam loadings to fitting connections at the end of a crossing.

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~~

Figure 2-3 Restrained Joint

According to the present practice, the frequently used types of pipe joints for

crossings are the push-on joint and mechanical joint and these two types are

selected for the further study.

2.3 Methods of crossings

There are two methods of Surface water crossings and highway crossings.

They are:

a) Above ground crossings,

b) Under ground crossings.

Whether the crossing is over or under ground, they present special problems.

• Above ground crossings - In above ground crossing, pipe shall he

adequately supported and anchored, protected from damage or temperature

effects and accessible for repair or replacement.

• Under ground crossings - In under ground crossings, if it is under water,

a minimum cover of 600mm shall be provided over the pipe. When

crossing water courses which are greater than 4.5m in width, the following

shall he provided:

a. the pipe shall be of special construction, having f1exible, restrained

or welded watertight joints,

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b. valves shall be provided at both ends of water crossings so that the

section can be isolated for testing or repair; the valves shall he easily

accessible, and not subject to flooding,

c. Permanent taps or other provisions to allow insertion of a small

meter to determine leakage and obtain water samples shall he made

on each side of the valve closest to the supply source.

But considering the climatic and the river hed variations in Sri Lankan

conditions, and the common practice in Sri Lanka this study will focus only on

above ground water crossings without disturbing the flow.

Above ground water crossings can be classified in to two groups:

• when the pipe line can be run directly across the bridge and straight in

to the soil- this can often simplify many aspects of the design,

• Crossing by raising the level (by using multiple offsetting bends in the

approach areas of the water way/highway) of the pipe line at the

abutments to avoid the disturbance to the flow of the waterway. - In

the event that bends must be used, external anchorages to be provided

to control the thrust and the thermal movement at these down bend

locations. It may be advantageous to use smaller angle change as

feasible to accommodate the needed elevation offset.

The method of crossing selected for this study is that where the pipe line runs

directly across the bridge and straight in to the soil as it is not feasible to

carryout a detailed study on both types within a limited duration of the study.

2.4 Types of supports for water pipelines

Typical installation of Ductile Iron pipes above ground on supporting system

involves a basic ''pipe-on-support" approach.

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It is of the utmost importance that sufficiently sturdy and properly located pipe

lines support structure be provided to prevent lateral and vertical movement of

the pipe or joints and to also prevent any damaging due to axial bending of the

supports in response to axial pipeline movements.

There are several types of supports frequently used in crossings of watervvays

and highways in all parts of the world. Spac~ trusses with angle iron sections

and plane structure with Universal beams and parallel tlanged channels are

used for detailed design.

2.5 Supporting pipes above ground

Pipes frequently have to span over waterways and highways. For short spans

rigid jointed pipes may have sufficient strength to support themselves plus the

fluid. For larger spans. it may be necessary to support the pipe on trusses or

concrete bridges or hang the pipe from an existing traffic bridge.

(Stephenson,1989)

Stephenson ( 1989) describes that, if a truss bridge is to be constructed for the

pipe, the pipe could act as the tension member at the bottom. or the

compression member at the top.

Stephenson ( 1989) proposed an attractive form of bridge as the pipe supported

from suspension cables. He further explained the details of this supporting

arrangement that two cables are preferable to one. as they may be spaced apart

and the hangers attached at an angle to the vertical plane through the pipe. I Ic

says that, this arrangement helps to support the pipe against wind forces and

reduces wind vibrations and pipe could also be designed to act as an arch but

again lateral support would be necessary- either two pipes could be designed

to act together with cross bracings, or cable stays could be erected.

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.. Figure 2-4 Pipeline suspension bridges in USA

Stephenson ( 1989) further described that, in all cases except if the p1 pe

supported on an independent bridge, the pipe should be rigid jointed

preferably welded steel. He said that this means there will have to be some

form of expansion joint along the pipe to prevent thermal stresses developing.

But this type of pipe support is not being used in Sri Lanka.

For spans up to 50m, trusses are generally less economic than plate girders

because of higher fabrication cost. They arc therefore adopted only where the

available construction depth is not sufficient. (Buick D. and Graham W.O.

2003)

2.6 Pipes supported on road bridges

There are several pipe cradle support methods for supporting pipes on road

bridges described by Ductile Iron Pipe Research Association (DIPRA-2007).

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In this publication about cradle support methods, it recommends to have a

support for each and every pipe. It is important that this support should be

sufficiently sturdy and properly located to prevent any detrimental axial

bending of the supports in response to axial pipeline movements. When a

flexibly joined Ductile Iron pipeline is pressurized, some thrust forces

develop- even at slightly deflected joints. If not adequately stabilized, these

forces can cause the joints to deflect to th_eir maximum, creating a ·'snaking"

of the pipeline and possibly even separation of unrestrained joints.

It is recommended in DIPRA (1995) that the saddle angle of the support is

between 90 and 120 degrees. Little or no benefit is gained by increasing 120

degrees but by decreasing the angle less than 90 degrees, stresses tend to

increase rapidly.

a. Types of pipe cradle supports are:

(i) Inside a concrete box utility corridor - normally provided by

the ceiling or the floor. Lateral and axial movement is restricted

by bracing the support structure against the side of the box

corridor. Long rod lengths are not advisable due to the

possibility of twisting or buckling and the lack of proper

support resulting from critical movements ..

(ii) Underneath the deck between girders - Though this has not

been practiced in Sri Lanka, due to the maintenance difficulties

with the available facilities and financial restraints in obtaining

those facilities. In addition to above. another reason is that, the

different organizations are responsible in road bridges and

water supply. But this method is practiced in Japan. This

information is received from Mr. S. Muraoka - structural

Engineer, JICA study team of Outer Circular Higlnvay project.

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This is further explained in the section "crossing types use m

other countries".

(iii) On the exterior side of the bridge where the structural material

of the bridge exterior and its design configuration \vill

generally dictate the arrangement for installing the Ductile Iron

Pipe.

b. Pipes Supported on rollers- Pipe supported on rollers normally has

only two point contact locations at each roller support. Due to

smaller contact areas, this point loading results in much higher

localized stress concentrations that are dependent on pipe size, pipe

wall thickness, distance between rollers, location of rollers along the

pipe length, radius of rollers, loading etc. These stress analysis is

difficult and the results arc rendered uncertain by doubtful boundary

conditions,

The types a (ii) and b of above are not practiced in Sri Lanka.

Figure 2-5 Support on exterior side of the bridge ,>''

/,

/

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2. 7 Pipes not supported on road bridges I pipe bridges

2. 7.1 Spigot & Socket pipes

2. 7.1.1 Normal spans (Saint Gobain manual-2001)- Support at every p1pe

In Saint Gobain Manual (200 1) for water pipe line products it is

recommended that above ground installations of spigot and socket pipes be

provided with one support per pipe, the support being positioned behind the

socket of each pipe.

This results in a normal maximum distance between supports of 5.5m for

pipes ON 80 to ON800 and 8.0m for pipes ON 900 to ON 1600 shown as

dimension A in Figure 2-6. The lengths of 5.5m and 8.0m are their

manufactured lengths.

The above manual further says that, the pipes should be fixed to the supports

with steel straps, so that the axial movement due to the expansion or

contraction resulting from temperature f1uctuation is taken up at individual

joints in the pipeline. In addition, joints should be assembled with the spigot

end withdrawn 5 to 1 Omm from the bottom of the socket to accommodate

these thermal movements.

Pipes supported in this way arc capable of free def1ection and axial

movement at the joints which accommodate small movements of the pipe

supports.

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... ;\ .,.., A = 5.5m Pipe length

Figure 2-6 Supporting Pipes above ground (Socket & Spigot)

Figure 2-7 Each pipe supported (Sammanthurai Sri Lanka)

2. 7.1.2 Unsupported large spans (Saint Gobain manua/-2001)

(Supporting Socket & Spigot Pipes above ground, where support cannot be

provided at every pipe.)

In Saint Gobain Manual (200 1) for water pipe line products says that, where

a support cannot be provided at every pipe, e.g. at stream crossings etc ..

unsupported spans as short as possible up to 11m (where manufacturing

length of pipe is 5.5m) for pipes DN80 to DN 800, 16m (where

manufacturing length of pipe is 8.0m) for pipes DN 900 to DN 1600 can be

installed by positioning supports relative to joints as indicated in Figure 2-8.

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When unsupported span exceeds 16m a special pipe support such as Pipe

Bridge using truss or any other method should be provided.

The above manual recommends that the length of dimension B should not

exceed one quarter of total span length.

It further states that, the cut pipes, fittings, valves etc., which are adjacent to - -

the span, must be positioned outside the joints marked X and the length

between the joints X-X must be equal to 3 full length pipes, i.e. 16.5m for

pipes DN 80 to DN 800 and 24m for DN 900 to DN 1600. The inner saddles

carry a double load and in some cases require having increased seating

angles which are specified in the manual.

To prevent excessive stresses in the pipe, the joints at each end of the centre

suspended pipe should not be deflected.

Considering the maintenance point of view, it is better to have some sort of a

structure to allow for maintenance work, rather than support the pipe.

B < % Unsupported Span

Unsupported Span (m) X-X (m) Diameter (mm)

Up to 11 16.5 80- 800

Up to 16 24 900- 1600

Figure 2-8 Un-Supported Large Spans

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''When designing the construction of a new bridge, there arc many

considerations to be taken into place, but ultimately the span of the area to

be bridged is important to ensure the right materials are chosen for the

construction.''- RETDsteel

2. 7.2 Flanged pipes (Saint Gobain maJJua/-2001)

"'*

The following figures show typical installations where spans greater than

the nominal 1Om or 15m can be obtained

Figure 2-9 Flanged pipes DN80 to DN250 using 5m long pipes

Figure 2-10 Flanged pipes DN300 to DN800 using Sm long pipes

Figure 2-11 Flanged pipes DN900 to DNl600 using 6m long pipes

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•

2.8 Crossing types use in other countries

In most of the countries pipelines across waterways and highways arc

amalgamated with the road bridges. Sometime they were installed underneath

the interior of the deck in-between girders or support on exterior side of the

bridge.

The sketches obtained from a Japanese Consultant Mr. S. Muraoka. Structural

Engineer JICA study team of the outer circular highway project are shown in

Fig. 2-12 and Fig 2-13.

Figure 2-12 Support on exterior side of the bridge

Figure 2-13 Support under the deck in between bridge beams

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Figure 2-14 Pipe bridge across the Umlaas River- permitting safe pedestrian access across the river for the first time

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