Groundwater level（underground water level）Means that the groundwater surface is relative toDatum planeThe elevation of.It is usually calculated in absolute elevation.Elevation scale of water table“Phreatic level”；Elevation scale of confined water surface“Artesian water level”。[1]According to the drilling observation time, it can be divided intoInitial water level、Stable water level, water level in wet season, water level in dry season, water level before freezing, etc.
Chinese name
groundwater level
Foreign name
underground water level
human factor
Overutilization of human beings
Natural factors
The elevation of the terrain caused by the earthquake
The surface of groundwater.There are two types of water table and confined water surface.The diving surface is the free surface of diving;The confined water surface refers to the stable water surface after the confined water is exposed, that is, after the well penetrates the roof of the confined aquifer, the water level in the well reaches the stable water surface.[1]
Groundwater flow system
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The process and state of groundwater flowing from recharge area to discharge area.Groundwater with unified spatiotemporal evolution process.The groundwater flow system does not deliberately distinguish between aquifers and impermeable layers, but is formed by the aquifer geological body with boundary enclosure and unified hydraulic connection.It is a part of the hydrological cycle system and consists of three parts: input, output and hydrogeological entities.[1]
Reason for decline
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The main reasons for groundwater level decline are:
1. Human factors.For example, the excessive exploitation and utilization of human beings.
2. Natural factors.For example, the earthquake caused the elevation of the terrain and the subsidence of underground rivers;Diversion of rivers;Climate change, etc.
Reason for rise
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There are natural and human reasons for the rise of groundwater level.
The rainfall increases in a short time, the flow direction of underground rivers in karst areas changesSeismicity anomalyThese may lead to the rise of groundwater level, which is a natural cause.
There are two kinds of artificial factors disturbing the groundwater level, one is reasonable, the other is unreasonable.
Reasonable, such as JinanBaotu SpringDunhuang, GansuCrescent SpringIt is necessary to replenish water regularly to raise the groundwater level to maintain the tourist landscape.
What is unreasonable is agricultureFlood irrigation, too much irrigation water and untimely drainage will lead to the rise of groundwater level, or evensecondary salinization Disaster.[2]
Manual lowering
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Lowering groundwater level is a very important technical measure in foundation pit construction technology.
The purpose of lowering the groundwater level is:
1. In order to drain the soil in the pit and improve the earthwork construction conditions;
2. Consolidating the soil mass at the bottom of foundation pit is conducive to improving the safety of supporting structure.
According to the construction and test results, the foundation pit with good dewatering effectCohesionC andinternal friction angleφ value can be increased by about 25%~30%.[3]
Foundation pit dewatering principle
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one
In cohesive soil foundation, when the excavation depth of foundation pit is less than 3m, gravity drainage can be used; when the excavation depth exceeds 3m, well point dewatering should be used.
two
In sandy soil foundation, if the excavation depth of foundation pit exceeds 2.5m, well point dewatering should be adopted.
three
When the dewatering depth is more than 6m, multi-stage light well points or jet well points should be used for dewatering, and deep well points can also be used for dewatering, or the comprehensive dewatering method of adding vacuum pumps in deep well points can be used.
four
For the foundation pit with sloping excavation or without waterproof curtain enclosure, the dewatering well point should be set outside the foundation pit. For the foundation pit with waterproof curtain enclosure, the dewatering well point should be set inside the foundation pit.The precipitation depth shall not be greater than the setting depth of the waterproof curtain.
five
The dewatering depth in the foundation pit should be 0.5~1.0m below the bottom of the foundation pit, and should be set in the soil layer with good water permeability.
six
The well point dewatering shall ensure the construction quality of sand filter layer to ensure the pumping effect, and the water shall be always clean.
seven
For dewatering outside the pit, in order to reduce the impact of well point dewatering on the surrounding environment, recharge well points or recharge sand wells and trenches can be set between the dewatering pipe and the protected objects.[3]
