On Jan. 26, a sinkhole formed under the sidewalk running in front of the Mitsukoshi Sakae department store in Naka Ward, Nagoya.

The hole, measuring 3 meters across and 2 meters deep, formed when an aging sewer pipe running beneath the sidewalk caved in and the soil went into the pipe.

The sewer system on that street was built around 1954, when the Oriental Nakamura department store, the predecessor of the Mitsukoshi outlet, opened.

Then, on Feb. 13, another sinkhole formed underneath the street running in front of the Matsuzakaya Nagoya department store, only 200 meters from Mitsukoshi. The city said this mini cave-in was likewise caused by a decaying sewer line, this one laid in 1972.

“The total number of cases is decreasing, but cave-ins still occur somewhere in the city once a day,” said an official in the Nagoya City Waterworks and Sewerage Bureau.

More than 300 sinkholes appear beneath Nagoya streets every year as sewer pipes, many of them installed more than 50 years ago, grow old and develop problems.

In response, the municipal government is stepping up efforts to repair damage inside the pipes to prolong their lifespan and prevent the streets above from collapsing.

According to the city, there were 399 street cave-ins above deteriorating pipes in fiscal 2009 and 337 in fiscal 2010.

The problem is worse in summer, when heavier rains loosen the ground. Many cave-ins also occur in Nakamura, Naka and Nakagawa wards, where the sewer pipes are oldest.

Roads cave in when joints connecting sewer pipes deteriorate and break, or when hydrogen sulfide gas from waste water eats holes in the concrete pipes.

“There are so many decrepit sewage pipes that have passed the acceptable period of use,” the official from the waterworks bureau said.

In fact, this year marks the 100th anniversary since Nagoya first introduced a sewage system in 1912.

The city has 7,656 km of sewer lines. That’s equivalent to 10 round trips between Nagoya and Tokyo by shinkansen.

In Nagoya, 60 percent of the sewer pipes are made of reinforced concrete and 32 percent are basically made of clay. Because of Nagoya’s proximity to the homes of the famed Tokoname and Seto ceramics, the city uses clay pipes at a higher rate than other municipalities.

Laying sewer lines in Nagoya was intense during the rapid economic growth after World War II, hitting a peak in fiscal 1978 when 270 km were installed. According to the city, 1,100 km of pipes will pass their estimated life of 50 years in the next decade.

The basic way to replace old pipes is to dig up the streets, but not only is this method disruptive to the surrounding area, it’s expensive as well.

Thus, Nagoya has decided to focus more on refurbishing than replacing old pipes.

Currently, the city uses small cameras or endoscopes to check the inside of sewer pipes that have been in use 35 years. When damage is found, repairs are made right away. In some cases, a vinyl chloride material is applied to the inner part of the pipe, a practice almost equivalent to replacing it.

The dangers of street collapses triggered by aging pipes was first recognized in 1980 after a massive cave-in accident occurred on Route 22 in Nishi Ward. Then 1,400 cave-ins were observed in 1981, or an average of four across the city every day.

Nagoya isn’t the only city facing this problem.

A recent study by the Land, Infrastructure, Transport and Tourism Ministry identified about 5,300 road cave-ins nationwide caused by broken old sewer pipes in 2010. The figure excludes Iwate, Miyagi and Fukushima prefectures.

The study also found that sewer pipes used for more than 30 years are highly vulnerable to breaking apart.

In fiscal 2010, Nagoya spent ¥5.2 billion repairing the sewage system, and the cost is expected to increase in the coming years.

“We will try to take a proactive approach to prevent collapse of roads rather than fixing them after it happens,” a city official said.

This section, appearing Saturdays, features topics and issues from the Chubu region covered by the Chunichi Shimbun. The original article was published March 3.