While walking along the main streets and alleys, you may not notice the manhole covers that are lest remarkable under your feet. The fact is every manhole cover bears a different pattern and code. Manhole covers are an indispensable part of a city’s public facilities, serving as doors for drainage workers to open for inspection and clearance of manholes and drains when necessary. In celebration of the 30th anniversary of its establishment next year, the Drainage Services Department (DSD) earlier held a first ever Manhole Cover Design Competition. This time, I have invited “a manhole cover expert” to talk about the types and design features of manhole covers, and explore with you the meanings behind these codes on them.

The birth of manhole covers

The first manhole cover had close linkage with the historical development of Hong Kong. In the early days since the founding of the city, a combined drainage and sewerage system was adopted in the City of Victoria (nowadays the Western, Central & Sheung Wan, Admiralty, Wan Chai, Causeway Bay, etc.) to allow the rainwater flushing away the sewage. Furthermore, channels and gullies were not covered, which could easily lead to the breeding and spread of bacteria. In 1894, the outbreak of plague in Hong Kong prompted the Government to speed up the reform of the sewerage system, including the separating of the original combined sewer system into two independent ones and installation of manhole covers. From then on, sewers have been turned to underground. Subsequently, cast iron, as a stronger material, replaced stone and clayware for constructing more durable pipes and manhole covers.

How to identify different manhole covers?

Currently, there are about 160 000 manhole covers in Hong Kong. The former Chief Technical Officer (now the DSD’s Hotline Superintendent), Mr LAI Chiu-leung, Sammy, who has been working in the DSD for more than 20 years, says that notwithstanding the non-eye catching look of the manhole covers, the thoughtfulness put into the casting process is worth our time to ponder and appreciate.

He says that most of the manhole covers were round in the early days, and they were later redesigned into square shape, being formed by two triangles putting together. With this design, it is easier for drainage workers to use a crowbar to open the manhole cover, and the combination of two triangular pieces can also facilitate operation. Regarding the grid patterns, or raised studs, on manhole covers, they not only enhance skid resistance, but also serve the primary purpose of allowing drainage workers to identify the two kinds of pipes underneath – sewers and drains – sewers for sewage and drains for stormwater. The round and radical pattern indicates a manhole cover for stormwater drains while the small squares pattern indicates a manhole cover for sewers, so-called ‘round for stormwater, square for sewer’ (see the picture).

Have you noticed two small sealing plugs on some manhole covers? As it turns out, a manhole cover generally has two keyholes, which are shaped like eyes (see the picture), allowing workers to pull up the cover with special tools. The sealing plugs help prevent a lady’s high-heel shoes from getting stuck in the keyholes or the breeding of mosquitoes arising from accumulation of water.

Hidden codes on manhole covers?

Besides different patterns, codes made up of letters and numerals are often seen on manhole covers. What do these codes represent? According to Mr Sammy LAI, “HK” is a short form of Hong Kong, representing government property. “DC” refers to “Drainage Contract”. In addition, a manhole cover is normally cast with its manufacturing year and a contract number. For instance, “DC/97/05” refers to contract no. 5 of the DSD in 1997. The manhole cover also bears the English name of the manufacturer.

Moreover, manhole covers would use different English alphabets to illustrate their designed loading capacities, total three levels. The most common one is “H”, referring to “heavy duty”, which means the cover can bear heavy vehicles driving on it; “M” refers to “medium duty”, usually found on pavements; while “L” represents “light duty”, usually found in a planter.

Manhole cover design competition

Year 2019 is the 30th anniversary of the establishment of the DSD. A number of activities will be organised for the public to know more about the department’s work and its achievements over the years. Among others, the first manhole cover design competition in Hong Kong closed for entries just yesterday. More than 1 400 design submissions have been received. The results of the competition will be announced on 14 January 2019. Members of the public are welcome to cast their votes online for the most popular design. The winning design may be adopted for use as manhole covers in Hong Kong.

I believe that you now have a basic understanding of the manhole covers in Hong Kong. Though small in size, a manhole cover requires a lot of attention for its manufacture and design. Next time, when you walk on the streets, amidst surfing your phone with a bowed head, you may stop to take a good look at the manhole covers on the ground. By doing so, you will probably discover more interesting things in the city.