How the quicksand phenomenon occurs

The formation of quicksand is varied, the main reason is that the sand layer formed by the alluvium of the river through geological changes, in the case of water flow, the whole sand layer flows, thus forming a quicksand layer, along the Yangtze River, along some areas of Huai and Dangshan and Xiao County in our province. Quicksand, as the name suggests, is flowing sand, which is mainly when the sand meets water in the ground, and when the pressure of the water changes, the water flows, so that the sand flows with the water. Under normal circumstances, the pressure of groundwater is fixed, but once the water pressure changes, the entire sand layer will change, so it is of great significance to deal with the impact of quicksand on the foundation.

Correctly dealing with the negative impact of the quicksand layer in the foundation construction process can not only effectively control the cost of the project, but also improve the technical level of the construction.

According to the engineering practice and geotechnical analysis in the area where quicksand often occurs, it can be found that the factors that cause quicksand are roughly as follows:

1) The main external factors depend on the size of the hydraulic slope, that is, the higher the groundwater level in the area, the greater the depth of the foundation pit, the greater the hydraulic pressure difference, and the more likely it is to produce quicksand;

2) The clay content in the particle composition of the soil is less than 10%, and the silt content is more than 75%;

3) The inhomogeneity coefficient of soil d 60 d 10 <5 (where d 60 is the limited particle, that is, when the weight percentage of soil particles smaller than a certain particle size is 60%; d 10 is the effective particle size, that is, when the weight percentage of soil particles smaller than a certain particle size is 10%). The value of the unevenness coefficient obtained in the quicksand-prone area is between;

4) The moisture content of the soil is greater than 30%;

5) The porosity of the soil is greater than 43%;

6) In geological structures with sand interlayers in cohesive soils, the thickness of sandy silt or sand layer is greater than 250mm

The causes of quicksand phenomenon include internal causes, trapped burial and external causes

Internal factors: depending on the nature of the soil, when the porosity ratio of the soil is small, the water content is large, the clay content is less, the powder particles are more, the permeability coefficient is small, and the drainage performance is poor, it is easy to produce quicksand. Therefore, the quicksand phenomenon is very easy to occur in fine sand, silt and sub-clay, but whether the quicksand phenomenon occurs also depends on certain external conditions.

External cause: It is the amount of hydrodynamic pressure (seepage force) generated by the seepage of groundwater in the soil.

The main ways to prevent and control quicksand are: reducing or balancing the hydrodynamic pressure; Try to make the hydrodynamic pressure direction downward; Cut off the flow of groundwater. The specific measures are:

Dry period construction method. The groundwater level is low in the dry season, the water level difference between the inside and outside of the foundation pit is small, and the hydrodynamic pressure is small, so it is not easy to produce quicksand.

Digging and throwing large stones. Dig the earthwork in sections, so that the excavation speed exceeds the sand speed, immediately after digging to the elevation, lay bamboo, reed mats, and throw large stones, in order to balance the hydrodynamic pressure, the quicksand is pressed. This method is suitable for the treatment of local or slight quicksand.

The water-stopping curtain method will be continuously water-stopping support structures (such as continuous sheet piles, deep stirring piles, dense drainage cast-in-place piles, etc.) to drive a certain depth below the bottom surface of the foundation pit to form a closed water-stopping curtain, so that the groundwater can only seep from the lower end of the supporting structure to the foundation pit, increase the seepage path of groundwater flowing from the pit to the foundation pit, reduce the hydraulic slope of the brother, thereby reducing the hydrodynamic pressure and preventing the generation of quicksand.

Artificial lowering of the groundwater level. That is, the well point precipitation method (such as light well point, tube well point, jet well point, etc.) is used to reduce the groundwater level below the bottom surface of the foundation pit, and the seepage of groundwater is downward, and the direction of the hydrodynamic pressure is also downward, so that the water can not seep into the foundation pit, which can effectively prevent the occurrence of quicksand. Therefore, this method is widely used and reliable.

In addition, the underground diaphragm wall, compacted grouting method, soil freezing method, etc., are used to prevent the groundwater from flowing into the foundation pit to prevent the occurrence of quicksand.

Principle of quicksand control:

Reduce or eliminate the head difference of groundwater inside and outside the foundation pit, such as first using a well point outside the foundation pit to lower the groundwater and then excavation, or using tools such as grab buckets to carry out underwater excavation in the undrained foundation pit.

Increasing seepage paths, such as sheet piles driven along the pit wall to a depth beyond the pit floor, are long enough to make the head gradient in the protected soil smaller than the near danger gradient.

At the upward seepage outlet, the surface is covered with permeable material to balance the dynamic hydraulics (this method is mostly used downstream of the dam).

Less cutting down trees, less pollution. More greening, more environmental protection. This is also a measure to prevent and control quicksand.

1. The quicksand phenomenon is mainly caused by the high pressure of the moving water of the groundwater, and the direction of the liquid of the moving water pressure is opposite to the direction of gravity of the soil, and the soil is not only affected by the buoyancy of the water, but also by the action of the moving water pressure, and has a tendency to lift upward. When the positive force of dynamic water pressure is equal to or greater than the water immersion density of the soil, the soil particles are in a suspended state and flow into the foundation pit together with the groundwater, that is, the quicksand phenomenon occurs. 2. When the excavation of the foundation pit reaches below the groundwater level, and the soil is fine sand or silt, and the precipitation of the sump is used, under the action of a certain hydrodynamic pressure, the soil under the pit will form a flow state, and flow into the pit with the groundwater, and this phenomenon is called quicksand phenomenon, which is very common in the desert.