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Sand liquefaction is one of the important causes of site damage during earthquakes. About half of the cases of ground damage in earthquakes are caused by sand liquefaction. After liquefaction loss of strength occurs in the soil around the foundation or pile foundation, large deformation occurs in the soil, which makes the structure sink or deform and even causes the destruction of the structure. If the liquefaction of sand occurs near the wharf, the permanent displacement of sheet pile or caisson wharf will occur.
In addition, liquefaction of sand can also cause instability and settlement of natural or artificial slopes. On another hand, buried cavity structures such as pipelines and tanks may float when the surrounding soil is liquefying, which often causes very serious consequences. In summary, sand liquefaction is a natural disaster phenomenon with wide range, great harm and difficulty to control, so it is necessary for its identity. Firstly, this paper summarizes the basic knowledge of sand liquefaction mechanism, influencing factors, discriminant methods and preventive measures.
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After that, I summarize the research direction of liquefaction briefly. Finally, combined with my recent research direction (CoupledEulerian-Lagrangian large deformation theory), the idea of applying CEL large deformation to sand liquefaction is put forward.
Sand liquefaction refers to the phenomenon that the pore water pressure increases and the effective stress decrease when the saturated loose powder and fine sand are vibrated. From a mechanical point of view, it can be said that the shear strength of a substance tends to disappear under certain conditions.
For sand, its shear strength mainly depends on the friction resistance between solid particles. When the normal pressure between sand particles equals or approaches to zero, the friction resistance equals or approaches zero, and the sand presents a liquid state.
Sandblast: Because of the tendency of compaction of loosely soil particles caused by earthquakes, excessive pore-water pressure occurs in the soil, and the elevated excessive pore-water pressure seeks passages to rush out of the ground. It brings out the soil particles and forms conical sediments around the eruption hole, similar to small volcanic craters, which is called the sand boiling phenomenon. Sand boiling can cause site damage and uneven ground settlement.
Flow slide is the most catastrophic site damage caused by liquefaction. It is consisted by completely liquefied soil and a complete soil layer covering the liquefied layer, which can reach tens of meters or even wider. It may slide along the slope at a speed of 10 kilometers per hour. In the 1964 Alaska earthquake, the coastal underwater slippage took away many port facilities and caused coastal surges, resulting in secondary disasters along the coast. When the surface slope above the liquefied soil layer is small enough to not cause flow slide, ground cracks may occur due to gravity and vibration inertia force, i.e. horizontal opening, inclination and differential settlement of the ground, or even wave pushing and shaking. This liquefaction phenomenon will cause extensive damage to underground facilities such as ground structures and pipelines.
Cyclic activity is a phenomenon of intermittent liquefaction and limited flow deformation during cyclic shear process, which is caused by the intermittent action of soil volume contraction and dilation, resulting in the rise and fall of pore-water pressure. It mainly occurs in saturated cohesiveless soils with medium density and relatively dense.
Not all kinds of soils can liquefy. It can be concluded that liquefaction will not occur in rocks and clay soils, because the cohesion of soil particles in these soils is relatively strong. The liquefaction occurs mainly in loose sandy soil which is slowly accumulated. In addition, liquefaction occurs only in saturated soils, the pore-water pressure will suddenly rise and lead to the destruction of soil skeleton only in saturated soils.
In addition to the above two necessary conditions, there are many factors affecting sand liquefaction, which can be summarized into four categories: soil conditions, drainage conditions, static conditions and dynamic conditions. For soil conditions, its type, density, uniformity, structural form and stress history will have a great impact. The difference of drainage conditions controls the pore-water pressure between soil particles. When the drainage conditions are good, the accumulated pore-water can be discharged in time, which dissipates the pore-water pressure and thus inhibits liquefaction. The weight of overlying soil, the form and magnitude of dynamic load belong to the category of mechanical conditions, and the change of mechanical conditions is the most direct inducing factor of sand liquefaction. It can be seen from that there are many influencing factors of foundation liquefaction, and the influence of each factor on foundation liquefaction is highly non-linear. It is difficult to accurately distinguish liquefaction of foundation and evaluate the degree of damage with statistics, simplified model and single mechanics theory.
This method determines the anti-liquefaction strength of soil based on the laboratory test with simulated site conditions. At the same time, the seismic stress index is calculated by the design seismic data, and the liquefaction is judged by comparing the magnitude of the two methods. The main indoor tests are: various types of cyclic triaxial compression test, resonance column test, cyclic shear, cyclic torsional shear, shaking table, centrifuge model test.
This method is mainly used to distinguish liquefaction of saturated sand in foundation of large buildings and geotechnical structures. It can be simulated in the laboratory according to the specific shape of the building, site boundary and drainage conditions, and the results can be corrected according to practical experience. There are some defects in this method, such as difficulty in sampling, stress release and great difference between stress state and soil foundation. Therefore, the determination of test parameters and how to simulate the field situation of soil better are keys to improve the reliability of laboratory test methods.
The basic principle of the discriminant method is as follows: in the macro seismic liquefaction and non-liquefaction area, based on the data of the discriminant index measured by field test, through analysis, statistics and summary, the relationship between the macro seismic disaster data and the empirical formula or liquefaction demarcation line are established to determine whether liquefaction will occur or not. The most common methods of analysis are data analysis and criterion discrimination.
Total stress method is based on the non-linearvariation curve of secant shear modulus and equivalentdamping with strain amplitude obtained from laboratorytests, and the approximate solution obtained by iterationis represented by the shear stress comparison methodproposed by Seed et al. It regards the foundation asahomogeneous soil with a horizontal free surface, ignoringthe additional stress caused by the building on the surfaceof the foundation, which greatly simplifies the problem. The disadvantage of the total stress method is that it cannot determine the development of pore-water pressure andreveal the essence of sand liquefaction, which has been less applied.
The generation, development and dissipation of pore water pressure in soil under earthquake directly affect the liquefaction process of soil. Effective stress method is based on this point, aiming at the law of pore-water pressure increase and diffusion change in soil under vibration load. According to cyclic triaxial or shear test, the formula of pore water pressure increase is determined. In the formula, the pore-water pressure gauge is expressed as a function of shear stress, normal stress, number of stress cycles or related physical quantities. After that, it is applied to the effective stress analysis.
Liquefaction prevention methods can be divided into two categories: one is to prevent liquefaction of foundation (through foundation reinforcement); the other is to prevent liquefaction even if liquefaction occurs, it will not affect the use of structures (through structural design).
The first kind of foundation reinforcement method can be further divided into three kinds according to the corresponding liquefaction principle. (1) The method of preventing soil skeleton from destroying; (2) The method of destroying soil skeleton, but it is difficult for soil particles to suspend in water; (3) Method (1) and method (2) are used comprehensively. Among these principles, the methods based on principle method (1) include compaction, solidification, replacement and so on. On the contrary, there are some methods to reduce the shear stress caused by seismic vibration, such as increasing effective stress by lowering groundwater level, restraining deformation by underground wall (steel sheet pile, wall solidification, etc.). The method based on the method (1) can prevent the destruction of soil skeleton, of course, it can also avoid the rise and fall of excess pore water pressure. The methods based on the method (2) include setting up drainage layer, replacing (replacing with good drainage material) and so on, which can make the foundation unsaturated by lowering the groundwater level. These methods are based on the premise that the soil skeleton will be destroyed, so a certain degree of subsidence will occur.
For the second kind of method, even if the foundation liquefies, it will not affect the use of buildings. The damage and liquefaction mechanism caused by liquefaction must be considered in design. The basic prevention principle is to transfer the resistance of the foundation to the structure after liquefaction. In other words, structural reinforcement or reduction of building gravity is the most common method.
Through consulting the relevant information, several noteworthy problems in the research of sand liquefaction are summarized.
Great progress has been made in previous studies on sand liquefaction. However, due to its complexity, the study of sand liquefaction is still one of the most important topics in civil engineering and earthquake disaster research. There are still many problems worth studying from theory to practical application, which is an important research direction in the future.
Because the factors affecting sand liquefaction are very complex, it is difficult to take these factors into account completely. Common discriminant methods often consider only a few of them. In addition, the extent and mode of the influence of some factors on liquefaction are still unclear, which is still in the preliminary stage of research and often leads to mis-judgement. Therefore, a more reasonable comprehensive identification method of soil liquefaction and classification criteria of liquefaction damage grade can effectively reduce the probability of failure, which needs further development. In addition to theoretical discrimination, the field identification method of sand liquefaction is also an important part of protection. Therefore, it is necessary to develop practical analysis methods and techniques for site liquefaction risk analysis and liquefaction hazard assessment suitable for engineering applications.
The study of dynamic interaction between foundation soil and structure is the key to the prevention and control of liquefaction disasters. The biggest difficulty is the lack of measured data, which leads to many uncertainties in the analysis process and greatly limits the application of this method in the design. In order to solve this problem, it is the most direct and effective method to carry out laboratory test of small scale model, field test of large scale model or prototype test research. The experiment can not only analyze the mechanism of interaction, but also provide an important means to verify the theoretical analysis method.
Because of the randomness and uncertainty of seismic load and the spatial variability of soil, the application of probability method and reliability analysis is also the focus of future research.
There are still many works to be done in the study of liquefaction large deformation. In the aspect of damage analysis of various engineering structures under large liquefaction deformation, there are considerable problems in both mechanism understanding and seismic design method in the code. Inclination analysis of buildings on liquefied foundation, lateral bearing capacity evaluation of pile foundation in liquefied soil layer, dynamic response of Metro in liquefied soil layer are all urgent problems to be solved in engineering. It is necessary to develop new analytical methods on the basis of new understanding and research ideas, starting from the mechanism. It is very important to establish practical analytical methods which are easy to be accepted and mastered by engineers.
Regular cyclic load is the most important way of dynamic analysis and calculation at present, but there is little research on irregular dynamic load. The study of the latter should be strengthened.
Previous studies on liquefaction mainly involved bridges, railways, wharfs, water conservancy facilities, underground structures and lifeline projects in liquefied areas. In recent years, with the increasing scale of high-rise and super-high-rise buildings, the theoretical and practical problems of high-rise buildings have been carried out one after another. However, there are still few reports about the damage of superstructure in liquefied area at home and abroad. Therefore, the impact of liquefaction in seismic area on superstructure, especially high-rise buildings, should be given sufficient attention and
Review and Exploration of Sand Liquefaction. (2019, Dec 02). Retrieved from https://studymoose.com/review-and-exploration-of-sand-liquefaction-essay
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