Completely destroyed by mountainside landslide

In Daiwa Town, Mihara City, Hiroshima Prefecture, which was hit by a series of disasters due to a landslide on July 5 caused by torrential rains, a solar power plant for business use connected to a low-voltage transmission line was nearly completely destroyed by earth and sand that flowed into the site (Fig.6).

Fig. 6: The damaged site in Daiwa Town, Mihara City, Hiroshima Prefecture (picture: Nikkei BP)

Daiwa Town is in the northern part of Mihara City and is on a gently-sloped plateau, where a number of buildings, roads and fields are located between small hills. Low-voltage solar power plants for business use with an output of about 50kW, as well as mega (large-scale) solar power plants, are scattered among the fields along the roads and at the foot of the mountains.

When we visited the site in late July after the disaster, traces of the landslide could be seen on mountainsides along the roads, and earth and sand containing trees covered roads in some areas. According to a survey by Mihara City, roads in the city were damaged at 1,114 spots by the torrential rains in western Japan, of which 349 locations are in Daiwa Town.

Houses and offices hit by the earth and sand were damaged. Throughout the city, the number of damaged houses totaled 399 while 187 office buildings were also hit. The solar power plant was one of the damaged buildings. The panels were installed on a narrow site that extends from north to south between an agricultural road and a forest.

The panels were arranged in a latitudinal direction in 4 levels and 6 rows (24 panels in total) in one array. The arrays were installed on mounting bases, which were fixed on pile foundations consisting of two piles in the north-south direction and three piles in east-west direction. Around 10 arrays were laid out from north to south. It seems that the earth, sand and fallen trees hit the middle of the arranged arrays and washed away the panels together with the mounting systems (Fig.7).

Fig. 7: The panels were washed away together with the mounting systems by earth and soil that flowed down from the mountain. (picture: Nikkei BP)

Two arrays, one at the southern end and the other at the northern end, barely kept its shape, but they are tilted and twisted, and the plant was nearly completely destroyed. Some of the pile foundations remain in place. It is thought that the mounting systems were disconnected from the pile foundations, while some of the pile foundations were pulled out due to the weight of earth, sand and fallen trees, and they were destroyed after being pushed toward the road (Fig. 8).

Fig. 8: Some of the pile foundations remain in place (picture: Nikkei BP)

In Daiwa Town, there is a solar power plant that was not damaged at all although earth and sand from a landslide reached the area several meters away from the plant (Fig. 9). Solar power plants are easily damaged when earth and sand flow into the sites. The importance of careful investigation of the risks of damage caused by earth and soil in constructing power plants in front of mountains was re-acknowledged.

Fig. 9: The power plant that escaped damage from landslide by a narrow margin (picture: Nikkei BP)

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