According to the different buried conditions, groundwater can be divided into three categories: upper stagnant water, phreatic water and confined water.
The upper stagnant water is a kind of water storage body formed by local water isolation, which makes the infiltration of atmospheric precipitation stay in the shallow rock cracks or sedimentary layers.
Phreatic water is the first groundwater buried in the first stable aquiclude below the surface of the earth. Most of the groundwater that is usually seen is phreatic water.
Springs form when groundwater flows out of the ground.
Confined water (artesian water) is a kind of groundwater which is deeply buried and exists between two aquifers. This kind of ground water often has a larger water pressure, especially when the upper and lower aquifers are inclined, the soil in the aquifers should bear a greater water pressure.
When the well or borehole passes through the upper roof, the strong water pressure will make the water gush out and form the self flowing water. In modern engineering, the pressure of confined water has a great influence on the safety and stability of foundation pit. In deep and ultra deep foundation pit, measures are often needed to reduce the height of the head of confined water.
The meaning of the phreatic layer
The phreatic layer is the first stable water layer below the surface.
The phreatic layer has free water. There is no continuous water resisting layer above the phreatic layer, and it is not confined or only partially confined. Precipitation and surface water are supplied by infiltration of aeration zone.
Meaning of confined water layer
Confined water, filled with groundwater in the aquifer between the two aquifers.
Because of the aquiclude on the top of the confined water, its recharge area is smaller than the distribution area, its dynamic change is not big, and it is not easy to be polluted. It bears hydrostatic pressure.
Under the suitable terrain conditions, when the borehole reaches the aquifer, the water will be ejected out of the surface, forming a flowing water, so it is also called flowing water. People use this self flowing water as water supply source and farmland irrigation.
However, in engineering, confined water has a certain impact on the safety and stability of foundation pit. In the process of excavation of super deep and deep foundation pit in the area with rich underground aquifer and shallow water level, it is often necessary to reduce the pressure of confined water.
Cause of formation of confined water
The burial condition for forming confined water is that there are aquifers above and below, permeable layer in the middle, and the whole permeable layer must be filled with water.
The following rock formation combinations can form confined water:
Clay is covered on the sand layer; shale is covered on the sandstone; shale is covered on the dissolution limestone; dense and impure limestone is covered on the limestone with developed fissures and dissolution gaps; dense karst is covered on the porous karst.
The formation of confined water is closely related to the geological structure. As long as there is a suitable geological structure, no matter it is loose sediment or bedrock, it can form confined water.
The most suitable geological structure for the formation of confined water is syncline structure and monocline structure.
Classification of formation of confined water
There are two types of geological structures that form water bearing pressure: artesian basin and artesian slope.
Syncline and structural basins with one or more confined aquifers are called artesian basins, such as Paris artesian basin in France, Sichuan artesian basin in China, and Australia artesian basin.
At the lower end of the inclined rock stratum, due to structural fault or lithological change, a artesian slope is formed where the supply area is adjacent to the discharge area and the pressure bearing area is on the other side.
Limited by the water resisting roof, the connection between the confined water and the atmosphere and the surface water is weak, the participation in the hydrological cycle is not as active as the diving, and the dynamic is relatively stable.
It is also not easy to be polluted. Most of the confined water comes from infiltration water, which is suitable for drinking.
Confined water aquifer
Groundwater in an aquifer between two aquifers. The typical confined aquifer can be divided into three parts: recharge area, confined area and discharge area.
The aquifer in the recharge area is exposed and has free water surface. In fact, phreatic water is distributed and can accept external recharge. The confined water in the drainage area is discharged through rising spring and overflow to shallow aquifer.
The aquifer in the confined area is located at a small elevation and is full of water under pressure.
When the well or borehole exposes the water resisting roof, the water in the well will flow up to a certain height above the top surface of the aquifer before it stops.
The elevation of the still water surface is the piezometric water level of the confined aquifer. The distance between the piezometric water level and the aquifer top is the pressure head of the confined water. Under certain terrain and geological conditions, the piezometric water level is higher than the ground, and the borehole erupts the artesian water, which becomes the artesian well.
Difference between confined water and phreatic water
The supply of phreatic water is mainly the local atmospheric precipitation and some rivers and lakes. The confined water is supplied by atmospheric precipitation and river lake water through phreatic water.
Due to the influence of gravity, phreatic water has a free surface (i.e. it floats up and down with the amount of phreatic water), which generally flows from high place to low place.
Confined water is limited by the roof of the water resisting layer and bears hydrostatic pressure. There is a confined water surface limited by the roof of the water resisting layer and a confined water level higher than the roof of the water resisting layer (i.e. the connection between the water level of the supply area and the discharge area). Confined water flows from places with high hydrostatic pressure to places with low hydrostatic pressure.
The phreatic water is shallow, influenced by the climate, especially the precipitation, unstable in flow, easy to be polluted and poor in water quality; The confined water is deeply buried, less affected by the climate directly, stable in flow, not easy to be polluted, and good in water quality.
Phreatic water is discharged by evaporation or exposure of the ground as surface water and spring water, while confined water is transformed into phreatic water, mainly in the form of spring water.
The well drilled into the phreatic water is a phreatic well.
Generally, the water level of the diving well should be consistent with the local water level. If excessive pumping, the water level of the diving well will gradually lower than the local water level, forming a groundwater funnel area.
The well drilled through the roof of the aquiclude and into the confined water is called the confined well. Due to the influence of hydrostatic pressure, the water in the confined well can gush up along the borehole to a height equivalent to the local confined water level.
Under favorable terrain conditions, that is, when the ground is lower than the confined water level, the confined water will gush out of the ground and form a artesian well. Although there is upwelling, it is called semi artesian well.
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