Ground improvement - Jibankairyo

When constructing a structure on the ground, or when excavating the ground or using soil as a construction material, the natural ground often lacks stability. In such cases, artificial measures are taken to improve the properties of the ground or soil, which is called ground improvement. Improvement of relatively shallow foundation ground such as roadbeds and runway subgrades is sometimes called soil stabilization.

Problems in ensuring the stability of structures and the ground can be summarized as problems related to the destruction and settlement of the ground, and water infiltration and drainage, and the main purpose of ground improvement is to improve the strength, compression and permeability of the soil properties. Methods include (1) a replacement method in which poor soil is replaced with good soil, (2) a method in which the soil is compacted to artificially increase density, (3) a method in which the moisture between the soil particles is removed to promote consolidation and increase density, (4) a method in which a consolidating material is mixed and/or injected between the soil particles to consolidate the soil, and a combination of these methods.

In recent years, there has been a remarkable trend for structures to become larger and heavier, and moreover, due to the circumstances of the construction site, it is often necessary to build them on soft ground such as reclaimed land or alluvial lowlands. Furthermore, the construction area has expanded not only to land but also to marine areas, and improvement of the seabed ground is often required. In addition, rapid construction is required by society, and it has become difficult to proceed with construction work as the ground settles naturally, as in the past. Against this background, soft ground improvement technology has developed rapidly, and currently a wide variety of construction methods are in practical use, and are used appropriately depending on the purpose and conditions of the construction work.

[Akira Kono, Hitoshi Shimizu, and Takashi Shigitani]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

構造物を地盤上に構築する場合、あるいは地盤を掘削したり、土を建設材料として用いるような場合に、自然のままの地盤では安定性が不足することがしばしばある。このようなとき、人工的な手を加えて地盤または土の性質を改良することを地盤改良という。道路、滑走路の路床、路盤など比較的浅い基礎地盤を対象とした改良は土質安定処理との呼び名で区別されることもある。

　構造物および地盤の安定性を確保するうえで問題となるのは、地盤の破壊・沈下および水の浸透、排水にかかわる問題に集約され、土の性質のうち強度特性、圧縮特性および透水性の改良が地盤改良の主目的とされる。方法としては、(1)不良土を良質土に置き換える置換(ちかん)工法、(2)土を締め固めて人工的に密度を増加させる方法、(3)土粒子間の水分を除去し圧密を促進させることにより密度を増加させる方法、(4)固結材を攪拌(かくはん)混合したり土粒子間に注入して固結させる方法、およびこれらの併用工法がある。

　近年、構造物の大型化、大重量化の傾向が著しく、しかも建設用地の事情から埋立地や沖積低地などの軟弱な地盤に立地せざるをえないことが多くなり、さらに陸上だけでなく海域にまで施工領域が拡大され、海底地盤の改良が必要とされることも多くなってきた。また、急速施工が社会的に要請され、従来のように自然に地盤が落ち着くのにあわせて工事を進行させるような方法をとることが困難になってきた。このような背景のもとに軟弱地盤改良技術は急速に発展し、現在、多種多様な工法が実用に供され、工事の目的と条件に応じ適宜使い分けられている。

［河野　彰・清水　仁・鴫谷　孝］

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