Ductile Iron Water Mains

GENERAL INFORMATION

Ductile Iron water pipe consists of the mains and branches that serve residential, commercial, and industrial structures. Watermains are pressure pipe systems so they are more adaptable to existing underground or above ground obstructions (via fittings) than gravity pipe systems.

Water mains are typically placed at least 6” below the lowest recorded frost depth which in colder northern climates can range anywhere from 3’ to 7’ or more. Larger trunk mains however may be deeper ranging anywhere from 7-14’ feet deep or more. Also when requiring clearance underneath existing utilities or obstructions, depths beyond frost minimums may be necessary as well.

The depth of water service laterals commonly ranges from +/- 2-7’ as well, depending on frost depths expected.

DIMENSIONAL INFORMATION

PROPERTIES

Available sizes are 4”, 6”, 8”, 10”, 12”, 14”, 16”, 18”, 20”, 24”, 30”, 36”, 42”, 48”, 54”, 60”, 64”. Pipe lengths come in 20’ standard but 14’ lengths may be available as well.

Pipe deflection is of negligible concern with ductile iron, unlike plastic watermain pipe, because of it’s high strength. Ductile Iron Pipe commonly has a cement lined pipe interior.

HANDLING

When the delivery truck arrives, all pipes should be inspected and inventoried. Suppliers often will not remedy issues that aren’t identified upon delivery as they have no control over what happens on site after the delivery driver leaves. A forklift, front end loader with forks, or backhoe with forks is commonly used to unload ‘bundles’, ‘bunks’, or ‘pallets’ of ductile iron pipe. A spreader bar with straps spaced +/- 8’ may also be used with a crane or excavator to lift the pipe bundle off the truck.

If bundles are to be stored prior to ‘stringing’ individual pipe, they will need wood blocking supports underneath, spaced +/- 8’. During cold weather, PVC becomes more brittle so extra care must be used during handling.

Trench width is often determined by the installer, via trench box sizes, pipe depth, pipe size etc, however minimum trench widths for C900 pipe areoften set forth by many manufacturers.

‘Full Profile’ trench boxes are commonly used for trench wall shoring and should be set on shelves in sidewalls of the trench above the springline of the pipe. This ‘sub-ditch- condition helps ensure peak compaction around the pipe.

For larger sections needing replaced, the piece is commonly removed/cut, and a new piece is replaced with a ‘solid sleeve’ type fitting on one or both ends.

JOINTS

Ductile iron Pipe typically is shipped with molded synthetic rubber gaskets which are placed within the bell’s gasket socket which allow for a water tight push together jointing system. Spigot ends come from the factory with bevels on the spigot end made to allow for easier entry into a bell and to mate with the bells inner taper.

When installing ductile iron pipe, the bell interior, gasket, and spigot should be cleaned to remove any foreign dirt or debris from permitting proper joint seal, or entering the pipe and compromising the anti-bacterial environment needed to pass testing. The gasket is placed within the bell. The spigot end is lubricated, aligned with the bell, and inserted until it contacts the gasket uniformly. A spade with a block buffer, lever pullers, or ‘eagle claw’ devices may be used to push the pipe ‘home’ to the reference mark on the spigot. A pipe has been pushed home when the previously laid bell end lines up flush with the newly laid spigot end. Many installers elect to used a backhoe bucket to push the pipe home, which is typically more acceptable to ductile iron pipe than plastic, but still rarely recommended by manufacturers.

More joint deflection is allowed with ductile iron pipe than plastic pipe, typically +/- 5 degrees depending on manufacturer. Ductile iron pipe however can not be bent or radiused like plastic pipe.

Eagle claw pushing joint 'home'

FITTINGS

Ductile Iron ‘Mechanical Joint’ fittings are used for ductile pipe and are usually available in sizes up to 48” and in working pressures of 150, 250, and 350psi. Many different types of fittings are available for ductile iron pipe:

45 Degree Bend

90 Degree Bend

Tee

Reducer

Plug

Cross

Plain end cap and tapped cap

They are most commonly used in conjunction with ‘Wedge-Action Restraint’ devices at the joint, also referred to as megalugs.

For example, a 6” megalug is composed of a ‘gland’ (ductile iron ring with premachined bolt holes), torque-limiting bolts/nuts (anchorage to pipe), and ‘T-Bolts/nuts (imparting wedge action, anchors to flanged end of fitting). The number of bolts needed with megalugs depends on the size.

Megalugs are commonly coated with a thermoset epoxy material and/or electrostatically applied polyester powder for corrosion/impact/UV resistance. It is important to note that the wedge action restraints used for ductile iron pipe and ductile iron fittings are specific (commonly black in color) and cannot be confused with C900 wedge action restraint products (commonly red in color).

When installing the spigot end into a fitting, a clean/square cut is required (no bevel necessary) with lubricant applied to gasket and spigot. The gasket is pushed into the fittings’ gasket socket recess and the bolts are installed hand tight only.

Any deflection at joint if needed should be made at this time. Then the T-Bolts are installed and tightened to 75-90 lbs of torque (maintaining equidistance from gland to flange). Then the torque limiting bolts are tightened until the nuts shear off indicating full torque.

WATER SERVICES

Diagram of a common water service connection system.

Saddle - for Service tap location. A good rule of thumb for ductile iron pipe services is that 5/8, ¾, and 1” taps can be made as ‘direct taps’, anything larger will often require a tapping ‘saddle’. Anything larger than 2” however should utilize a tapping sleeve.

Corporation Stop - Threads into saddle/water main at tap location

Copper or Polyethylene Tube most often ranging from 3/4" to 3".

'Curb Stop' Valve - Often used by local water authority to turn water service on and off near the curb

Curb Stop 'Box' - Housing to protect the curb stop valve and provide a pathway for the valve 'key'. A rod is often installed in the box allowing the key access just below grade.

Curb Stop 'Box' - Housing to protect the curb stop valve and provide a pathway for the valve 'key'. A rod is often installed in the box allowing the key access just below grade.

Exterior Water Service Meter Pit - For newer services with meter not located inside home, a water meter box or 'pit' is set somewhere in yard, often near property line. For such conditions, the curb stop valve is often placed inside this meter pit box, instead of it's own box. The water service meter pit is often used as the customer/municipality 'responsibility' borderline.

Compression Fitting with a 'flared connection'. Most often recommended for exterior below ground water service applications.

Compression fitting with a 'Ferrule' connection. (Ferrule ring, fitting, and nut). Alternate to 'flared connection' for exterior below ground water service applications.

Thrust blocking is almost always required when installing ductile iron pipe and fittings. It is most commonly done with cast-in-place concrete or in some cases precast concrete blocks. Either option must rest against virgin soil. (To left is a cast-in-place concrete thrust block being measured for appropriate size by inspector)

Precast Concrete Thrust Block - (To right is a precast thrust block resting on virgin soil, often accepted as an alternate to cast-in-place thrust blocks when installed as sized appropriately)

Also additional joint restraint is often required by engineers within a certain calculated distance of changes in direction (fittings) which with ductile iron pipe is often accomplished with a gripping/locking gasket mechanism. This will resist joint pullout at the change in flow direction (tees, bends, etc), especially during pressure surges. Gripping/locking gaskets, used in lieu of the standard gasket, grip the 'homed' pipe with metal teeth which dig into the iron pipe. They are also commonly used in directional drilling and auger/casing bores as well. Gripping/locking gaskets are typically available in sizes from 4-30” diameter.

An alternate to a gripping/locking gasket might be a bell restraint assembly (often used for larger ductile iron pipe when locking gaskets are not practical or otherwise not allowed by the project engineer.

Restraint directly at the mechanical fitting joint

Restraint is also often required at the nearby spigot/bell joint within a certain distance of the 'thrust vulnerable' location. This thrust calculation/joint requirement is calculated by the engineer using factors such as pipe size, water mains intended working pressure, and type of thrust vulnerability (i.e. bend degree, vertical bends vs. horizontal bends, tees, end-of-line caps, etc)

CUTTING

Cutting ductile pipe can be done with a power saw with a steel blade. Cutting is easiest and safest when done outside of the trench, prior to lowering it into place. Assuring the cut is ‘square’ is essential for proper joint connections. This is done by marking the cut around the entire circumference, while referencing proper length from the end of the piece of pipe. After cutting, the plain end of the pipe will need to be ‘beveled’ to fit into the taper which exists inside the bell. This bevel can be done with a portable sander, or abrasive disc. The bevel should match the look of the factory spigots. Also a new insertion or ‘home’ line should be drawn on the cut end to assure the cut pipe piece is ‘home’ in the gasket/bell. This ‘bell depth’ can vary depending on manufacturer or fitting type.

Note cut pieces which are joined with mechanical joint or flanged fittings will not require beveling, only cut square and deburred.

STRUCTURE CONNECTIONS

When an in-line valve and manhole are required on a project, the manhole is commonly precast with ‘doghouse’ openings at the bottom to provide a little vertical ‘give’ considering invert elevation doesn’t need to be dead on for pressure pipe systems. Also, it is quite common for watermain valve manholes to be bottomless and meant to be rested on a compacted gravel or concrete base (though some engineers specify full bottoms).

There are 2 common connection types of ductile iron pipe to valve manholes: mortared and booted.

A mortar connection refers to simply adding filler brick/block to the annular space, and mortaring it all closed, limiting infiltration and exfiltration.

A booted connection, uses a pre-sized flexible rubber boot inserted into the structures cored hole. Rubber boots are often coupled with a stainless steel band that is tightened around the pipe penetration. The entire assembly creates a virtually watertight connection, typically required of all sanitary sewer lines.

BACKFILL

Trench backfill material for ductile iron will depend on whether the pipe resides underneath or within the ‘zone-of-influence’ of pavement (often 1:1 slope from edge) or a structure foundation. If the pipe is within this condition, crushed stone with a gradation from 2-1/2” down to fines is commonly used. Engineers and pipe manufacturers typically consider 95% or greater proctor density backfill compaction to be a requirement to resist settlement and potential pipe deflection below.

If outside of the ‘zone-of-influence’ native soils from the trench excavation can typically be used given they are of reasonable condition.

Backfill can be done with plate compactors, rollers, or hydraulic hammer attachments (for excavators and backhoes). When hydraulic hammers are used for ductile iron pipe, manufacturers often recommend they are not used within 3’ of the top of pipe.

Albeit not as much as plastic watermain pipe, ‘Long Term Deflection’ is a concern for many engineers which involves trench backfill material settling over a long period of time based on backfill material type, depth, and compaction effort. Premium backfill compacted above 90% Proctor Density will typically be sufficient to resist long term deflection on cover heights up to 10’ or more. Sandy/Gravely and Silty/Clayey soils as backfill however will start to become a Long Term Deflection concern at depths of 5’ or more, if not compacted above 80% proctor density.

BEDDING

Bedding is used with ductile iron pipe for several reasons: to provide for consistent support under the pipe bending lengthwise, to increase the loading strength of the pipe, to spread loading pressure away from the joint and out more evenly among the length of each piece, to provide a platform for lining up and leveling the pipe, and to protect the pipe from potential damage as a result of backfill.

The most commonly utilized bedding material types for ductile iron are clean mason/concrete sand or course sand. ¼” – 1-1/2” crushed stone and gravel may also be used. ‘Bedding’ is commonly 4-6” thick under the pipe, and will often require hand grading for proper pipe grade/slope and bell cradling. After placement the pipe will be embedded along the sides and often 4-12” above the top of the pipe. This second ‘lift’ of bedding is often referred to as the ‘haunch’ on either side of the pipe and is critical to ensuring pipe performance and deflection resistance. With ductile iron pipe, a tube of polyethylene fabric is wrapped around the pipe, helping isolate the iron from reacting with caustic/corrossive soils over time.

TESTING

The 2 required testing processes for ductile iron water main pipe are pressure testing and bacteria testing. For a detailed description of watermain testing, see ‘Pressure Testing Water Mains’ and ‘Water Main Disinfection and Bacteria Testing’ articles.

It’s important to note that during pressure tests, compressed, entrapped air can cause difficulty in pumping to required pressure. Also, a pressurized water main may leak compressed air at the joints when it is actually watertight. For this reason, air venting and blowoffs should be placed at high points in the main if at all possible. Also, Engineers will design water mains with periodic automatic relief valves, slow closing/opening valves, etc. Testing pressure approximately 25% above intended operating pressure is typically sufficient, unless higher pressures are called for by engineer.

PROS

Ductile Iron pipe is stronger/stiffer than C900 pipe and it has higher working pressures as well. The highest Pressure Class available in C909 is 305psi while the lowest pressure class of Ductile Iron pipe available is 350psi.

It is typically readily available in sizes up to 12”, and sometimes even in the larger diameter sizes depending on region and demand.

Ductile iron pipe is considered by many engineers to have a longer service lift than c900 pipe.

CONS

Ductile Iron pipe is often less economical than C900 pipe of the same size. It is relatively heavy in comparison to C900 pipe making it not quite as easy to work with, cut, and handle.

Ductile Iron pipe can be affected by corrosion or corrosive soils, unlike C900 pipe, though it will likely take a considerable amount of time, especially when polyethylene encasement is used.

PRICE

While regional factors and demand can influence cost, generally speaking Class 350 Ductile Iron pipe is +/- 30-50% more costly than C900 pipe.

CAUTION

Beware of chipping, breaking, or faulty joints when handling. Overlooking such defects could become costly situations to fix if failures exist during testing.

Use caution to properly bed the pipe, backfill the haunches, and backfill the trenches as excessive deflection can cause joint failure. Investigation, leak chasing, and repairs can be frustrating, contentious, and costly.

MANUFACTURERS

Top manufacturers in the U.S. are:

American Cast Iron – Birmingham, AL

Griffin Pipe – Council Bluffs, IA

McWane Ductile – Coshocton, OH

Tyler Pipe – Tyler, TX

US Pipe and Foundry – Birmingham, AL