Under certain influence, the properties of sand below the groundwater level change, showing a special phenomenon similar to liquid, which is the phenomenon of sand liquefaction disaster. Like the collapsibility of loess, expansion and contraction of expansive soil, freezing and thawing of frozen soil and thixotropy of silt, it is a special geotechnical engineering disaster.
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What is said here is under the action of strong earthquakes, but in fact, not only earthquakes will cause sand liquefaction, but also earthquakes, explosions and mechanical vibrations will cause sand liquefaction.
Discussion on the disaster phenomenon of sand liquefaction
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Under the action of strong earthquakes, the properties of sandy soil below the groundwater level may change obviously, making it appear as liquid. This phenomenon is called sand liquefaction disaster. Sand liquefaction disaster directly affects the rapid development of urban construction in China, and it is an important link in seismic safety evaluation, seismic fortification and earthquake damage prediction in China. From the Tangshan earthquake, Osaka earthquake, Hualien earthquake in Taiwan, Turkey earthquake and so on in recent decades.
First, the macro phenomenon of sand liquefaction:
1. Sand blasting and water gushing. This is the most obvious macroscopic sign of sand liquefaction. Liquefied sand, like pressurized liquid, will emerge from the ground under the pressure of the upper soil layer, where sand blasting and water gushing are serious, large areas of farmland and crops are buried, and channels and wells are blocked.
2. The bank collapsed. The riverbeds and ditches of highways and railways are covered weakly, and the sand layers here are more likely to liquefy. Due to the existence of free surface, river banks, embankments and subgrade often sink, crack and collapse, and bridges or other facilities are seriously damaged.
3. The ground splits and sinks. Liquefied sand is often sprayed onto the ground from cracks in the ground. On the other hand, the liquefaction of sand will often aggravate the ground cracks, and the liquefied sand layer will aggravate the destruction of the superstructure when it is redeposited.
Second, the geological background of sand liquefaction
Sand liquefaction occurs underground, which is closely related to certain geological conditions. Under certain earthquake action and geological background, whether liquefaction can occur, the scale and severity of earthquake damage are closely related to the type and state of soil. A large number of examples have proved that the groundwater level in areas with serious sand blasting leakage is generally shallow, rarely exceeding 3 meters. According to relevant data, when the groundwater level is 3-4 meters deep, sand blasting leaks rarely.
Of course, under the same conditions, saturated, loose, clean and uniform silt is the easiest to liquefy. When considering the liquefaction problem, we should first make clear the strength, and then make clear whether there are liquefiable soil types (from coarse sand to light loam) in the depth range where liquefaction may occur according to the survey data of the site. If there is this kind of soil, it is necessary to check whether the classification index of this kind of soil is within the above limit according to the test data, and finally study the soil and groundwater.
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Sand liquefaction
Sand liquefaction
Saturated loose powder and fine sand suddenly decompose under the action of vibration, showing liquid phenomenon. What is its mechanism? Saturated loose powder and fine sand tend to move and become denser under vibration, and the stress is transferred from sand skeleton to water. Due to the poor permeability of fine sand, pore water pressure will increase sharply. When the pore water pressure reaches the total stress value, the effective stress will drop to zero, particles will be suspended in water, and sand will be liquefied. After sand liquefaction, pore water moves from bottom to top under the action of excess pore water pressure. If there is no covering layer with worse permeability on the upper part of the sand layer, groundwater will overflow to the surface in a large area; If there is a cohesive soil layer with weak permeability above the sand layer, when the excess pore water pressure exceeds the strength of the overburden, the groundwater will carry sand particles to break through the overburden or spray out of the surface along the cracks in the overburden, resulting in water spraying and sand blasting. Earthquake, explosion and mechanical vibration can all cause sand liquefaction, especially earthquake, which has a wide range and great harm. The prevention and control of sand liquefaction mainly starts from two aspects: preventing sand liquefaction and preventing or reducing uneven settlement of buildings. Including reasonable selection of venues; Take measures such as vibroflotation, tamping, explosion and pile compaction to improve the compactness of sand; Drainage reduces the pore water pressure of sand; Raft foundation and deep pile foundation with soil replacement, sheet pile enclosure and good integrity are adopted.