上総掘り(かずさぼり)とは?

千葉県上総地域で生まれた伝統的深井戸掘り工法

河岸段丘が発達した千葉県君津市の小櫃川・小糸川流域では、飲料水や農業用水の確保が困難でした。江戸時代までの井戸掘りと言えば、人力で竪穴を掘る「掘り井戸」、やがて地中に鉄棒を突き入れる鉄棒式(突抜工法、掘抜工法)が普及しますが、鉄棒式で掘削できるのは深さおよそ20間(約35m)までで、充分な飲料水や農業用水を確保できません。そこで小櫃地区では大村安之助が、小糸地区では池田久吉と池田徳蔵らが中心となり、技術の改良に努めました。

 

明治15年、大村と池田久吉は鉄棒の代わりに樫の木を使う樫棒式を考案。用具の軽量化と労力の軽減が実現し、50~60間(90~110m)の掘削が5~6人で行えるようになりました。

その後、大村と池田徳蔵は竹ヒゴと掘り鉄管、スイコを組み合わせた掘削技術を考案し、さらに200~300間(360~540m)の掘削が可能となっていきました。

明治19年には小糸地区で、石井峯次郎がハネギを考案し、沢田金次郎がシュモクとヒゴグルマを考案したことにより、明治28年頃には現在の上総掘り技術が完成されたと言われています。

 

その後、上総掘りは全国に広まり、天然ガス・温泉・石油の採掘などに活用されましたが、昭和40年代には灌漑用井戸の需要減・水道の普及・機械掘りの普及などによって人件費のかかる上総掘りは行われなくなっていきました。

しかし近年、水不足に苦しむ途上国などで国際貢献に役立つ技術として、重機や電力を使わずに少人数で深く掘れる点などが世界的に注目を集めています。また東日本大震災後は、災害用として自宅や公共施設内に井戸を掘削する事例が増えています。

What is "KAZUSA-BORI"?

Kazusa-bori is a method of digging a deep well which was devised in the city of Kimitsu, Chiba prefecture. This area was called ‘Kazusa’ hundreds of years ago. ‘Bori’ is a variation of ‘hori’, which means digging in Japanese.

 

Since river terraces have been structured well along Obitsu river and Koito-river in Kimitsu city, it was difficult to have water for drinking and agricultural use in those areas. Until the early 19th century, they had been using ‘Hori-Ido’ method, where they dig pits with bare hands, and ‘Tetsubo-shiki’ method, where they dig the ground with an iron stick. However, we cannot reach more than 35m deep with those methods and thus cannot gain enough water. Then directed by Yasunosuke Omura in Obitsu area, Kyukichi Ikeda and Tokuzo Ikeda in Koito area, people tried to improve the technique.

 

In 1883, Omura and Kyukichi Ikeda devised ‘Kashibo-shiki’ method where they use oak wood in place of iron sticks. With lighter devices and less human power, they started to dig wells 90~110m deep. Later Omura and Tokuzo Ikeda devised a technique in which they use materials called Take-higo*¹, Suiko*² and an iron pipe at the same time. Then they reached 360~540m deep.

In 1887, Minejiro Ishii devised the Hanegi*³while Kinjiro Sawada devised the Shumoku*⁴ and Higoguruma*⁵ in Koito area, Kimitsu. It is said that around 1896, the current type of Kazusa-bori technique came out.

After that, Kazusa-bori spread across the nation as it was used for drilling oil, natural gas and hot springs. But around 1970, Kazusa-bori had become unpopular because of its big labor costs, widespread of sewage and mechanical drilling skills, and lower demand of irrigation wells.

Nevertheless, these days it attracts considerable attention as a promising technique which enable us to drill deep into the ground without any machineries. This is useful especially in developing countries that suffer from water shortage. After the Great East Japan Earthquake, there are many cases where we installed wells in homes or in public spaces for emergency use.

 

*¹Take-higo

a string made of bamboo which is used to hang an iron pipe inside an underground hole.

*²Suiko

a pipe made of tin. It is used for removing mud at the bottom of the hole after digging.

*³Hanegi

a pair of poles sustaining Hori-Tekkan or Suiko from above with their resilience.

*⁴Shumoku

a handle vertically driven into Take-higo. It conveys our power to the Hori-Tekkan underground.

*⁵Higoguruma

a large wheel which rolls part of Take-higo which is not in use. It stands out with the structure like a waterwheel.