Don’t forget street ironworks

There are over six million manholes and inspection chambers on sewer systems in the UK and the water industry spends over £40M annually on repair and maintenance of the ironwork that covers them. It is estimated that over 70,000 manhole covers are replaced each year, and although the unit price of a cover assembly is relatively inexpensive (in the range of £90 to £150); the total cost including traffic management can be in excess of £1000 per site.

Where frequent replacement is required the whole-life cost at a single site can be significant. As well as those costs to the water industry, defective covers can cause a variety of problems for wider society, ranging from noise nuisance to causing serious traffic accidents.

A recent study undertaken by WRc highlighted that up to 85% of manholes on heavily trafficked routes had failed in some manner. It was also revealed that there are a small, but significant, number of locations where the service life of the cover installation is measured in months rather than years.

As statutory undertakers, water companies have a duty under the New Roads & Street Works Act to ensure street ironwork is adequately maintained. This requires companies to have means of identifying those covers that require maintenance.

In practice, water companies have generally replaced manhole covers and frames following notification by highway authorities of a defect, taking a reactive approach to these ‘forgotten assets’. This predominantly reactive approach makes it difficult to establish a clear view on the overall condition of these assets and thereby effectively plan expenditure. Has the rate of deterioration increased due to the type of traffic on today’s roads? Is the current rate of replacement sustainable?

This reactive approach is also ineffective in reducing the overall number of failures, so is the industry just storing up problems for later generations? A groundbreaking collaborative Portfolio research project completed by WRc this year classified the common failure mechanisms for street ironworks and identified the factors that contributed to their failure. There are three basic mechanisms by which manhole covers and frames fail.

A common type of failure is the cracking of the road surface or pavement adjacent to the cover. This is caused by differences between the stiffness of the manhole chamber and the road surface, which leads to high tensile stresses in the pavement material under traffic loading.

Cover assemblies may also sink under traffic loading if there is inadequate support for the ironwork, for example, mortar or shaft brickwork. And finally the ironwork itself can fail, either by becoming polished, by fracturing or by the cover sinking within the frame due to seating wear. Onsite inspections of covers and frames during the course of the project highlighted some alarming statistics. More than 50% of those covers inspected in the carriageway suffered from cover seating wear, over 40% suffered from failure of the surrounding road surface and nearly 40% exhibited some degree of polishing.

WRc have used the technical knowledge obtained during this study to develop a proposed proactive maintenance approach for managing street iron works. The proactive approach is based on the risk of cover failure, taking likelihood and consequence of failure into account. This risk based approach will

enable water companies to target inspection programmes and plan replacement schemes more effectively. The adoption of a more proactive approach to the management of these assets will concentrate inspections where the risks of failure are the highest.

Water companies will therefore have a greater understanding of the overall condition of these assets and be able to predict future changes in condition. All of this information is important to enable an economic case to be made to the regulator for investing in street ironwork, which is vital if these assets are to continue to function as part of a busy traffic network.

WRc has recently embarked on a second phase of the Portfolio project, working in collaboration with a number of water companies, manhole assembly suppliers and manufacturers. The previous work had significantly moved forward the understanding of the issues surrounding street ironwork failures, highlighting the importance of these assets, which had previously been largely overlooked.

The overarching aim of this new project is to increase awareness of these issues and ultimately promote products, materials and installation methods of a more appropriate specification. This will be beneficial to the water industry as is will reduce the cost of repeated replacement of manhole covers.

The water industry needs to give greater consideration to the reasons why street ironworks fail and the consequences of these failures. By having a good understanding of these issues, the water industry can influence the development of solutions to address them. This understanding can only be gained through a proactive approach to the management of street ironworks. It is unrealistic to believe that a largely reactive approach is sustainable.