In the shape of the graphite in the metal, which changes the material's behaviour completely.

In grey cast iron the graphite forms flakes. Those flakes act as internal cracks, so the material is strong in compression but brittle - it fails suddenly, without warning, and old cast iron mains fracture when the ground moves. In ductile iron a treatment during casting makes the graphite form spheres instead. Spheres do not concentrate stress the way flakes do, so the material yields and deforms before it breaks.

For a buried main that difference is everything. Ductile iron tolerates ground settlement, traffic loading, thermal movement and impact during installation, absorbing them by deforming slightly rather than cracking.

It also allows thinner walls for the same duty, so modern ductile pipe is lighter than the cast iron it replaced while being far tougher.

Most commonly with a push-fit socket and spigot joint sealed by an elastomeric gasket.

One end of each pipe is formed as a socket containing a moulded rubber gasket; the other is a plain spigot. The spigot is cleaned, lubricated and pushed into the socket, and the gasket compresses to seal. Line pressure acts on the gasket to improve the seal rather than to open it.

The joint is fast, needs no heat, welding or curing, and can be made in a wet trench. It also permits a few degrees of angular deflection at each joint, so a pipeline can follow a gentle curve or accommodate settlement without fittings.

Flanged joints are used above ground and in chambers where the pipe must be bolted to valves and pumps and later dismantled. Restrained joints are used where thrust has to be carried by the pipeline itself.

Wherever the pipeline changes direction or terminates, and thrust cannot be taken by a concrete anchor block.

Pressurised water pushes outward at every bend, tee, taper and closed end. A standard push-fit joint resists pressure but not axial pull, so without restraint the fitting is driven out of the joint. Traditionally that thrust is resisted by casting a concrete thrust block against undisturbed ground.

Thrust blocks are not always possible - in congested ground full of other services, in poor soil that will not take the bearing, on steep gradients, or where excavation is limited. Restrained joint systems lock the spigot into the socket mechanically, so the pipeline carries the thrust in tension through several pipe lengths either side.

The restrained length required is calculated from the pipe size, pressure, bend angle and soil properties. It is a design calculation, not a rule of thumb.

Cement mortar lining internally as standard, and an external system matched to how aggressive the soil is.

The cement mortar lining protects the bore from corrosion and keeps the pipe hydraulically smooth, and it maintains water quality. It is the default for potable water; special linings are used for wastewater and for aggressive fluids.

Externally the choice depends on the ground. A metallic zinc or zinc-aluminium spray with a finishing layer suits most soils and works sacrificially. Where the ground is corrosive - high chloride, low resistivity, contaminated or made ground - a polymeric or bonded coating is specified, often with a loose polyethylene sleeve over the pipe.

This decision deserves a soil survey rather than a default, because it is the main determinant of how long the main lasts and it cannot be revisited once the pipe is buried.

From the maximum pressure the pipeline will see including surge, not from the normal operating pressure.

Ductile iron pipe is supplied in pressure classes denoting the allowable operating pressure. Selecting on the steady working pressure alone ignores surge - the pressure spike produced when a valve closes quickly, a pump trips or a pump starts against a closed valve. Surge pressures can substantially exceed normal operating pressure and are what actually break pipelines.

A surge analysis is normal practice on anything other than a short simple main, and it may lead either to a higher pressure class or to surge protection such as slow-closing valves or a surge vessel.

Also check the burial condition. The class governs internal pressure, but external loading from cover depth and traffic is a separate check, and a shallow main under a road may be governed by that rather than by pressure.