Good Ground Day (Ii Jiban no Hi)

What Is Good Ground Day?

November 28 is "Good Ground Day," established by Jibannet Research Institute (now Jibannet Co., Ltd.), a company specializing in ground survey analysis, and certified by the Japan Anniversary Association in 2016, based on a wordplay: "11 (ii/good) 2 (ji) 8 (ban)" sounds like "ii jiban" (good ground). The day encourages people to learn about the ground in their own area, since correct knowledge can help reduce damage from earthquakes and floods and lead to safer homes.

What Is "Ground," and Why Does It Matter?

"Ground" refers to the soil and earth layers that a building's foundation rests on. No matter how excellent the building above is, if the ground beneath it is weak, the risk of subsidence, tilting, or collapse during an earthquake increases. Soft ground, reclaimed land, riverside areas, and low-lying land all require particular attention to ground strength.

Methods of Ground Surveying

• Screw Weight Sounding (SWS) test (formerly called the Swedish Weight Sounding test): The most common method for residential ground surveys. A weighted rod is rotated and driven into the ground to measure soil strength
• Boring survey: A more detailed method for large buildings and public facilities, sampling and analyzing deeper soil layers
• Ground improvement work: If the ground is judged to be soft, it can be reinforced with column-shaped soil improvement or small-diameter steel pipe piles

The Good Ground Day Awards and Ranking

Tied to this day, "Good Ground Day" awards have honored companies and groups for advanced work on natural-disaster resilience. In December 2016 the company also released its own "good ground ranking" of Japan's 47 prefectures, with Okinawa, where plateaus of Ryukyu limestone formed from coral reefs are widespread, in first place. For an actual home, an individual ground survey is still essential.

Ground Checks to Make When Buying a Home

• Hazard maps: Check the Ministry of Land, Infrastructure, Transport and Tourism's hazard map portal for liquefaction and flood risks
• Place names: Place names containing characters meaning "swamp," "wetland," "rice field," or "valley" can indicate historically soft ground
• Tilt of nearby buildings: Areas where surrounding buildings appear tilted or cracked warrant caution
• Ground survey report: Always request disclosure of the ground survey results when buying a newly built home