Developing Manhole Covers Not Easily Abraded and Easy to InspectNTT Access Network Service Systems Laboratories

The surface patterns on iron manhole covers were thoroughly reviewed to develop the iron covers that are not easily abraded and are easy to inspect.

There are approximately 700 thousand iron manhole covers in this nation; they are inspected every five years to find abrasion and are replaced depending on the state of abrasion in order to avoid skid accidents by vehicles running over the covers. The technology that leads to the reduction of their maintenance and management cost and to the enhancement of the safety in work processes are wanted for the inspection and replacement of a huge number of iron manhole covers.
Under these circumstances, we have developed a new type iron lid (a taper diamond iron lid) that is not easily abraded and is easy to inspect without changing the production cost by thoroughly reviewing the surface patterns on the iron manhole covers. We have realized the reduction of maintenance and management cost and the enhancement of the safety in work processes by reducing the frequency of the replacement of the iron manholes and by saving the labor for the process of the inspection on the road.

• We have optimized the sizes and arrangements of the patterns on the iron covers to enhance the abrasion durability and to considerably suppress the frequency of the inspection and the replacement of the iron covers.
• We have made a structure that makes appear the patterns in the lower layer (hexagons) when the patterns in the upper layer (rectangles) are worn out due to the amount of the abrasion by arranging the two-layer patterns made from different surface patterns on iron manhole covers (rectangles and hexagons) with a certain distance of gaps. As the patterns on the lower layer (hexagons) appear to show the timing of the replacement of the iron lid, it has become possible to judge the timing of the replacement easily through visual inspection. The measurement of abrasion on the road performed by the personnel has become unnecessary, and we have achieved saving the labor for the inspection on the road.

• Replacing the iron lid of the abraded and degraded manhole installed mainly on roadways with a new-type iron lid (a tapered diamond iron lid).

There are two types of incompatible functions related to the performance of iron covers: enhancing the abrasion durability (realizing a structure not easily abraded) and securing the anti-skidding performance (securing the frictional resistance to suppress skidding). To solve the incompatibility between those, we conducted the following experiment.

1. Enhancing the Abrasion Durability
   The weight of vehicles running on iron covers causes frictional force and in addition sand, asphalt powder, and the like increase the frictional resistance to abrade the surfaces of iron covers. In consideration to the above, we examined the sizes of the iron lid patterns as well as the combination of their arrangement and gaps to enable an effective discharge of sand and other particles that will be the causes of abrasion and, after our driving tests that continued for half a year, we determined the final patterns and shapes, which elongated the abrasion durability performance of current iron covers by approximately 2.8 times.
2. Securing Anti-skidding Performance
   Through our tests to measure the coefficient of dynamic friction, we found out that the arrangement of the two-layer patterns of the shapes in which the patterns on the iron cover surface are different (rectangles and hexagons) made it easier to cause skidding when the shapes change into the hexagons to show the timing of replacement than the current iron covers. Thus, we combined a certain ratio of the two-layer patterns and the rectangular patterns in an alternating way and confirmed that we were able to secure the anti-skidding safety performance that was higher than that of current iron covers.

As a result of (1) and (2) above, we solved the incompatibility between enhancing the abrasion durability and securing the anti-skidding performance, realized the iron covers that would not be abraded easily, and made it possible to considerably suppress the frequency of inspections and replacements of the covers.

NTT Access Network Service Systems Laboratories - Civil Engineering Project