What Is the Groundwater Table?

The underground water level refers to the elevation of the groundwater surface relative to the reference surface. It is usually calculated at absolute elevation. The elevation of the diving surface is called the "dive level"; the elevation of the pressure surface is called the "pressure level". ^[1] According to the drilling observation time, it can be divided into initial water level, stable water level, high water level, low water level, and water level before freezing.

Surface of groundwater. There are two types of diving surface and confined water surface. The diving surface is the free water surface for diving; the confined water surface refers to the stable water surface after the condensed water is exposed, that is, after the well penetrates the confined aquifer roof and the middle water level reaches the well surface after stabilization. ^[1]

The process and state of groundwater flowing from the supply area to the drainage area. Groundwater body with a unified spatiotemporal evolution process. The groundwater flow system does not intentionally distinguish between aquifers and impervious layers, and is formed by aquifer geological bodies with boundary boundaries and uniform hydraulic connections. It is a part of the hydrological circulation system and consists of three parts: input, output and hydrogeological entities. ^[1]

The main reasons for the decline in groundwater levels are:

1. Human factors. For example, humans' excessive exploitation and utilization.

2. Natural factors. For example, the earthquake caused the elevation of the terrain, the sinking of underground rivers, etc .; the diversion of rivers; the change of climate, etc.

There are natural and man-made reasons for the groundwater level to rise.

Short-term increase in rainfall,

Reducing groundwater level is a very important technical measure in the construction technology of foundation pit engineering.

The purpose of lowering groundwater levels:

1. To dry out the soil in the pit and improve the earthwork construction conditions;

2. Consolidating the soil at the bottom of the foundation pit is conducive to improving the safety of the supporting structure.

According to the construction and test results, it is shown that the cohesion of the soil and the internal friction angle of the foundation pit with good precipitation effect can be increased by about 25% to 30%. ^[3]

1. In cohesive soil foundation, gravity excavation can be adopted when the excavation depth of the foundation pit is less than 3m, and well excavation should be adopted when the excavation depth exceeds 3m.

2. In sandy soil foundations, the excavation depth of the foundation pit exceeds 2.5m, and well point precipitation should be adopted.

3. When the depth of precipitation exceeds 6m, multi-level light or jetting well precipitation should be used, deep well well precipitation, or a comprehensive precipitation method of adding a vacuum pump to the deep well well.

4. For foundation pits excavated on slopes or without waterproof curtains, precipitation wells should be located outside the foundation pits. For foundation pits with waterproof curtains, precipitation wells should be located in the foundation pit. The depth of precipitation should not be greater than the setting depth of the water curtain.

5. The depth of precipitation in the foundation pit should be 0.5 ~ 1.0m below the bottom of the foundation pit, and it should be set in the soil layer with better water permeability.

6. The well point precipitation should ensure the construction quality of the sand filter layer to ensure the pumping effect and ensure that the water is always clear.

7. Pre-pit precipitation, in order to reduce the impact of well point precipitation on the surrounding environment, a refilling well point or a sand well or sand trench can be set between the precipitation pipe and the protected object. ^[3]