Ductile iron access cover FAQs

Ductile iron is a unique form of graphite-rich cast iron which has been treated before the casting process to produce a solid and durable material with high impact resistance.

It is also known as ductile cast iron, nodular cast iron, spheroidal graphite iron, spheroidal graphite cast iron and SG iron. It has a much superior strength-to-weight ratio than traditional grey cast iron.

This combination of properties allows us to design and produce our innovative features and make significant weight savings that could not be achieved with less sophisticated materials.

A15 to C250 ductile iron manhole covers have a single integral, ‘dry-labyrinth’ (tongue and groove) seal, whereas D400 to F900 have no integral seals unless otherwise stated in their datasheets. Some steel manhole covers have a single integral seal, where others have no seal (at the same load category). The datasheets provide details of the existence of seals.

Standard manhole cover seals are designed to resist water/odour egress/ingress at normal atmospheric pressure only. For higher pressure requirements (up to 0.5Bar), a flood & odour control plate should be used.

The standard finished coating found on our iron covers is a barrier coat intended to provide some aesthetic appeal and a level of corrosion protection commensurate with the functional requirements of transit and short-term external storage. Under normal UK non-marine exposure conditions, it is not necessary to provide additional protection to ductile iron castings as normal (and expected) light surface oxidation (rust) prevents further corrosion. A normal light level of surface rust is known to not have a detrimental effect on the service life of ductile cast iron.

To produce a casting, a pattern must be used which is an expensive piece of equipment. For this reason, generally, only standard chamber sizes are accommodated by castings unless quantities allow economic manufacture of covers to non-standard chamber sizes. Custom-sized covers are often accommodated by the manufacture of a bespoke steel fabrication.

Most of Wrekin’s products offer screw-fastener locking as a factory-fitted option, whilst some products incorporate dedicated lock assemblies as standard. Where locking is available or as a standard fitment, this is illustrated on the respective product datasheet. In some circumstances, it may also be possible to factory lock other products on request.

The water evacuation rates for Wrekin gully gratings vary depending on the design of the gully. Generally, for conventional designs, the larger the grating the greater the capacity for water evacuation. CD526 categories are available for all Wrekin gratings within the product data.

The purpose of link fasteners between adjoining cover elements is to provide a loose coupling that reduces the possibility of dropping a joined pair of covers down a chamber during cover lifting. At the same time, they must allow the covers to perform independently under traffic loads so must remain loosely fastened in all circumstances.

Note: Link fasteners are not actually applied to help traffic security but will make some contribution to it in the event of unrestrained cover movement.

Not all of them are reversible. Usually they are reversible where provisions are made in the frame to allow it. This is usually restricted to loose-fit, flat seated designs like Highway double-triangular gully gratings. Reversibility is not possible where there are no provisions in the design. It is usually restricted to interference-fit, wedge seated designs which are manufactured to a specific orientation and component pairing.

We can provide datasheets and safety information which provide the fundamental dimensions and product details. On request, additional illustrative drawings and documents may be provided at Wrekin’s discretion, including further data for BiM purposes.

How access covers are measured:

Access cover dimensions are always referred to by their ‘clear opening size’ and sometimes also by their frame depth (see diagram).

Clear Opening:

The clear opening is the inside frame dimensions and these should normally be marked on the cover. If these are missing or have become illegible over time then it will be necessary to remove the cover and physically measure these dimensions. The clear opening size of the access cover frame must always be equal to or greater than the size of the chamber it is covering to ensure the frame is adequately supported.

Base Opening:

These are the most important dimensions when establishing the correct size of frame intended to fit on a given chamber or gully-top opening. i.e. The central opening at the base of a frame must never be smaller than the opening at the chamber top so that the frame flange is always completely supported by the chamber top, usually via bedding materials. This harmonisation of component dimensions is vital in achieving the most effective traffic-load transfer possible to an underlying chamber structure.

Clear Area:

The Clear Area dimensions are the two gross measurements that are taken perpendicular to each other and a product’s permanent frame walls or supports and coincide with the narrowest points at that location. In view of this, it is common for one of these dimensions to be the same as the Clear Opening.

A frame product’s Clear Area often disregards the presence of cover seating or locking-feature encroachment in the corners of the frame. i.e. Such features are generally ignored as their presence is generally not considered detrimental to unrestricted chamber access unless the equipment in a given chamber requires the full use of a frame product’s Clear Area.

Minimum frame depths for NRSWA road category Types 0 to 4 are listed below. Manhole covers in shared driveways that could be subject to occasional loads from heavy vehicles (e.g. refuse vehicles) should see the requirements for Type 4 roads.

Although the frame depths are prescribed in the NRSWA, the location of the (immovable) chamber top relative to the finished road surface, may sometimes dictate the depth of the frame that can be accommodated.

It is possible to conduct a component exchange, but it is generally not recommended. The reasons for this include, but are not limited to:

• Iron assemblies have their components specifically paired during manufacture making interchange with other components, uncertain.
• Some cover-to-frame seating designs (e.g. interference fit) are tolerance sensitive and therefore intolerant of component interchange.

Further, if ironwork assemblies requiring cover exchange are in-situ and have undergone a period of ‘bedding-in’, the exchanging of worn components for new ones may create unforeseen stability and noisy operation issues.

If a cover/grating exchange is the only practical or preferred remedy available, we recommend that Wrekin personnel are in attendance to provide any necessary adjustments in order to ensure stable assembly operation.

BS EN 124 requires load classes of D400 and above to demonstrate quietness in use during trafficking. In the UK, this is commonly achieved by the use of a 3-point cover/grating suspension design.

This principle is based on the fact that cover support at the three outermost points provides stable behaviour no matter where traffic loads are applied on the cover/grating’s surface. Indeed, it’s a lesson learned from [splayed] three-legged stools which were commonly used on farms but which remained stable irrespective of the uneven floor profile.

This is because all three legs form the extremes of a triangle, the simplest geometric shape that can provide stability under load. i.e., no matter how you place a triangular three-legged object on a given surface, all three points (legs) will always touch the ground, ensuring that the object remains steady.

In access covers and gratings, typically, 2 triangular covers are used to form a square, therefore still meeting the ‘quietness in use’ operation requirement whilst accommodating a more-useful rectilinear chamber opening. In this instance, where the corners of each triangle make contact with the frame (the seats), stable 3-point seating is maintained.

Additionally, as the load transfer to the triangular corner locations is entirely deliberate and predictable, it means that frame component ultimately receiving the traffic loads can then be locally optimised (around the triangular cover’s corner positions) to more sympathetically dissipate traffic loads into the surrounding and underlying foundation structure.