Jiban (English spelling) soil

Japanese: 地盤 - じばん（英語表記）soil

There is no clear definition, but it is commonly used to refer to the surface layer of the earth's crust that supports structures from a construction engineering standpoint. Geologically, the ground is classified into Paleozoic, Mesozoic, Tertiary, Pleistocene, and Alluvial layers according to the age at which it was formed.

[Akira Kono, Hitoshi Shimizu, and Takashi Shigitani]

Ground Composition

The ground is composed of soil and rocks. In general terms, soil is soft and weakly bonded, while rock is hard and made up of consolidated mineral particles. Bedrock is broadly divided into hard rock layers of igneous rock, metamorphic rock, and sedimentary rock from the Tertiary period or earlier, and soft rock layers with a relatively low degree of consolidation. Bedrock generally poses few problems as a foundation for structures, and the general rule is to build the foundations of large structures such as dams, piers of long bridges, and reactor pressure vessels on bedrock. In engineering terms, the issues are landslides and faults, the stability of cut slopes, and the difficulty of tunnel excavation, rather than bearing capacity. Soil layers are composed of most of the less consolidated diluvial layers and weathered soil from alluvial layers and rock layers, and are usually found at shallow depths of up to several tens of meters below the ground surface. This is the ground on which the majority of general civil engineering structures and buildings are built. This type of ground poses many engineering problems, as it will sink, change, or break down to some extent when excavated or the weight of a structure is added, and will cause ground subsidence when groundwater is pumped up. In terms of geotechnical engineering, it is broadly classified into clayey soil, sandy soil, and gravel soil, depending on the soil grain size composition.

[Akira Kono, Hitoshi Shimizu, and Takashi Shigitani]

Alluvial soil

Alluvial deposits are stratum formed after 10,000 years ago mainly by hydrological deposition. They are the accumulation of sediment and rotting soil from rivers and ocean currents, and constitute the majority of plains. Since these alluvial deposits are the result of recent formation, they generally have no history of bearing loads other than their own weight, so they are weak in strength and highly compressible. They are generally not suitable as support layers for large structures, and foundations are usually supported by embedding them in hard foundations such as diluvial deposits or Tertiary layers below the alluvial deposits. In particular, soft clayey soils can cause uneven settlement of structures, damage to foundation piles due to negative friction, and widespread ground subsidence, so special consideration is required in the design and construction of structures. In addition, loose sandy soils can cause liquefaction during earthquakes, significantly reducing the bearing capacity and causing damage to buildings. Among alluvial deposits, this type is often found in sand layers that accumulate on the surface of beach bars and deltas facing inland bays downstream of large rivers.

[Akira Kono, Hitoshi Shimizu, and Takashi Shigitani]

Alluvial ground

Diluvium is a geological layer dating back more than 2 million years ago, and is distributed under terraces, plateaus, hills, and alluvium. It includes marine deposits, fluvial deposits, and volcanic deposits, and is composed of gravel, silt, clay, volcanic ash soil, etc. Diluvium is older than alluvial soil, and is generally more consolidated than alluvial soil. In other words, the sand layer is tight, the clay layer is consolidated, and in many cases it has been subjected to a preload, so the amount of consolidation settlement due to the load is small. Diluvium is generally better as a foundation than alluvium. Volcanic ash soil has considerable strength in natural ground that has not been weathered or destroyed, but its strength decreases significantly when it is kneaded, making it undesirable as an earthwork material.

[Akira Kono, Hitoshi Shimizu, and Takashi Shigitani]

Ground Survey

Ground surveys clarify the geological composition, the engineering properties of each layer (shear characteristics, deformation characteristics, etc.), the properties of groundwater, etc., to obtain the information necessary for the design and construction of structures. Survey methods include on-site observations, geophysical surveys, boring surveys, standard penetration tests, plate load tests, and various laboratory tests using sampled specimens.

[Akira Kono, Hitoshi Shimizu, and Takashi Shigitani]

[Reference items] | Ground improvement | Ground subsidence | Uneven settlement

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

Japanese:

明確な定義はないが、慣用的にはおもに建設工学的な立場から構造物を支持する地殻表層部の地層をいう。地質学的には、その地盤のできた年代別に古生層、中生層、第三紀層、洪積層、沖積層などに分類される。

［河野　彰・清水　仁・鴫谷　孝］

地盤の構成

地盤は土または岩石から構成される。一般の概念からいえば、軟らかく粒子の結合の弱いものを土といい、硬く鉱物粒子が固結されているものを岩石という。岩盤は、火成岩、変成岩および地質年代で第三紀層以前に属する堆積(たいせき)岩の硬い岩層よりなる地盤と、固結度がやや低い軟岩層よりなる地盤とに大別される。構造物基礎地盤としては一般に問題が少なく、ダム、長大橋の橋脚、原子炉圧力容器といった大型構造物の基礎は岩盤につけるのを原則としている。工学的には、地耐力よりも地すべりや断層などや、切取り斜面の安定やトンネル掘削の難易などが問題となる。土層は、洪積層の固結度の低いほとんどの部分と、沖積層および岩層の風化土から構成され、通常地表面下数十メートルまでの深さの浅い部分に存在する。一般の土木建造物、建築物の大半が立地している地盤である。掘削したり構造物の重量が加わると多少とも沈下、変化あるいは破壊を生じ、地下水を汲(く)み上げると地盤沈下を生じるなど工学上問題が多い地盤である。土質工学的には、土の粒度組成により粘性土地盤、砂質土地盤、および砂礫(されき)地盤に大別される。

［河野　彰・清水　仁・鴫谷　孝］

沖積地盤

沖積層は1万年前以降に主として水成堆積によって形成された地層で、河川または海流により土砂が堆積したり、植物性の腐朽土が集積したもので、平野のほとんど大部分を構成している。これらの沖積地盤は新しい時代の生成によるものであるため、一般に自重による圧力以外の荷重を受けた経歴がないので強度が小さく、圧縮性も大きい。通常、大型構造物の支持層としては不適で、沖積層の下の洪積層や第三紀層などの堅硬な基盤に基礎を根入れし支持させるのが普通である。とくに軟弱な粘性土地盤では、構造物の不同沈下、ネガティブフリクションnegative friction（負の摩擦）による基礎杭(くい)の障害、広範囲な地盤沈下などを生じることがあり、構造物の設計・施工上特段の配慮が必要である。また、緩い砂地盤では地震時に液状化現象を生じ、支持力が著しく低下して建造物に被害を与えることがある。沖積層のうち、海浜砂州や大河川の下流内湾に臨む三角州などの表層部に堆積した砂層にこの種のものが多い。

［河野　彰・清水　仁・鴫谷　孝］

洪積地盤

洪積層は地質学的には200万年前以降の地層で、段丘、台地、丘陵地および沖積層下に分布している。海成層、河成層および火山性堆積土層があり、砂礫、シルト、粘土、火山灰土などよりなる。洪積地盤は沖積地盤よりその生成時期が古いため、一般に沖積土に比し固結度が高い。すなわち砂層は締まっており、粘土層は固結し、かつ多くの場合先行荷重を受けており、載荷重による圧密沈下量は小さい。洪積層は沖積層に比べて基礎地盤としては一般に良好である。なお、火山灰土は風化や破壊を受けていない自然地盤ではかなりの強度を有するが、こね返すと強度が著しく低下し土工材料としては好ましくない性質をもっている。

［河野　彰・清水　仁・鴫谷　孝］