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Soil liquefaction, also called earthquake liquefaction, ground failure or loss of strength that causes solid soil to behave temporarily as a viscous liquid. The phenomenon occurs in water-saturated unconsolidated soils affected by seismic S waves (secondary waves), which cause ground vibrations during earthquakes. Although earthquake shock is the best known cause of liquefaction, certain construction practices, including blasting and soil compaction and vibroflotation (which uses a vibrating probe to change the grain structure of the surrounding soil), produce this phenomenon intentionally.
Poorly drained fine-grained soils such as sandy, silty, and gravelly soils are the most susceptible to liquefaction.
The destructive power of liquefaction was abruptly brought to the world's attention by two large earthquakes in 1964”the March 27 Great Alaskan (Mw = 9.2) and the June 16 Niigata, Japan (Mw -- 7.5). Liquefaction-induced lateral spread during the Alaskan earthquake distorted more than 250 highway and railway bridges, damaging most beyond repair and causing several to collapse. Flow failures in shoreline areas during this same event carried port facilities out to sea, while returning waves overran coastlines causing additional deaths and destruction.
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In total, more than 50% of the Alaskan earthquake damage was caused by liquefaction-induced ground failure.
During the Niigata earthquake, liquefaction reduced bearing strength beneath many buildings, causing settlement and tipping. At other localities, lateral spreads collapsed bridges, severed pipelines, and wreaked havoc on pile foundations and other underground structures. Research following those two events has clarified mechanisms controlling liquefaction, produced procedures for predicting its occurrence, and generated methods for mitigating damaging effects. Field and laboratory tests indicate that liquefaction and subsequent ground deformation comprise complex phenomena that are difficult to model either physically or analytically.
Thus, empirical procedures have become the standard of practice for evaluation of liquefaction resistance, prediction of ground deformation, and design of remedial measures.
The formal definition of soil liquefaction is "the transformation of a granular soil from a solid state to liquefied state as a consequence of increased pore water pressure and reduced effective stress". Liquefaction occurs as seismic waves propagate through saturated granular sediment layers, which induces cyclic shear deformation and collapse of loose particulate structures. As collapse occurs, contacts between grains are disrupted and loads previously carried through particle-to-particle contacts are transferred to the interstitial pore water. This load transfer generates increased pore water pressure and concomitant decrease of intergranular or effective stress. As pore water pressures increase, the sediment layer softens, allowing greater deformation and an accelerated rate of collapse of particulate structures. When the pore pressure reaches a certain critical level, the effective stress approaches zero and the granular sediment begins to behave as a viscous liquid rather than a solid, and liquefaction has occurred. With the soil in a liquefied and softened condition, ground deformations occur readily in response to static or dynamic loading. The amount of deformation is a function of loading conditions, amplitudes and frequencies of seismic waves, the thickness and extent of the liquefied layer, the relative density and permeability of the liquefied sediment, and the permeability of surrounding sediment layers.
Liquefaction may lead to any one of several forms of ground failure, depending on surface loads, site geometry, and the depth, thickness, and extent of the liquefied layer. Ground failures are divided into two general categories depending on whether induced ground movements are primarily lateral or vertical.Ground failures associated with lateral ground displacements are of three types: flow failure, lateral spread, and ground oscillation[4]. The bounds between these failure types are transitional, with the type of failure and amount of displacement dependent on local site conditions.Buildings constructed on loose soil pitch and tilt easily when liquefaction occurs, since the soil no longer supports the structures' foundations. In contrast, structures anchored to bedrock or stiff soils in earthquake-prone areas suffer less damage, because less vibration is transmitted through the foundation to the structure above. In addition, buildings anchored to bedrock have a reduced risk of pitching and tilting.One of the most severe episodes of liquefaction in modern times occurred in China during the Tangshan earthquake of 1976. Some scientists estimate that an area of more than 2,400 km2 was subjected to severe liquefaction, which contributed to the extensive damage that took place in the southern part of the city.
The liquefaction of the soft lake sediment upon which central Mexico City was built amplified the effects of the 1985 earthquake, the epicentre of which was located hundreds of miles away. In addition, the liquefaction of the ground beneath the Mission and Market districts in San Francisco during the 1906 earthquake caused several structures to pitch and collapse. These districts were built on poorly filled reclaimed wetlands and shallow-water areas.Liquefaction may also contribute to sand blows, which are also known as sand boils or sand volcanoes. Sand blows often accompany the liquefaction of sandy or silty soil. With the collapse of the soil's granular structure, the density of the soil increases. This increased pressure squeezes the water out of the pore spaces between the soil grains and expels wet sand from the ground. Sand blows have been observed in the aftermath of several earthquakes, including the New Madrid earthquakes of 1811"12, the Tangshan earthquake of 1976, the San Francisco"Oakland earthquake of 1989, and the Christchurch earthquakes of 2010"
Critical porosity ratio method The critical porosity ratio method is a method to distinguish liquefaction by determining the empirical value of the critical porosity ratio of saturated sand and the actual porosity ratio of soil. In 1936, Casagrande first put forward the critical porosity ratio method according to the shear shrinkage of low density sand and the expand of high density sand, and changed the state of shear failure. The pore ratio corresponding to the constant lower volume is regarded as the critical pore ratio. The critical pore ratio method is also the earliest method used in the world to evaluate the liquefaction potential of sandy soil. But because the critical porosity ratio is greatly affected by load conditions and lateral boundary conditions, the critical porosity ratio is used as a single evaluation index to judge the sandy soil liquid.At present, this method has been rarely used. Castro and Poulos et al.[5-7] proposed a method based on the work of Casagrande Steady-state strength method for retaining the original stable state of soil when flow slip failure occurs. It is considered that only loose sand may occur flow slip failure, and determined the steady-state line of the relationship between pore ratio and consolidation pressure was established. The possibility of liquefaction is determined by comparing the measured porosity ratio with the steady-state line. Zhao chenggang et al.[8] investigated the liquefaction discrimination process of steady-state intensity method and steady-state line.
It is pointed out that the steady-state strength and steady-state deformation do not exist under any conditions, and the uniqueness of the steady-state line, and the line shape is analyzed and discussed.Energy Assessment MethodIn 1982, Davis et al.[9] proposed a method based on the concept of energy through the survey data of seismic liquefaction. It is assumed that the increase or decrease of pore water pressure is related to the energy dissipation of soil during earthquake. For a given earthquake magnitude, based on Gutenberg-Richter(1956) earthquake total radiation energy calculation method to determine its energy for a given site. In this theory, the pore water pressure increased with the energy dissipation in the soil, and Davis assumed that the increase of pore water pressure and energy attenuation had a certain linear relationship. Through a large number of earthquake site investigation, the ratio of the pore pressure increase with the effective overburden pressure is determined by summarizing the data and analyzing the test results. Law et al.[10] and Trifunac[11] constructed the liquefaction possibility respectively on the basis of the seismic energy calculation method proposed by Gutenberg Richter. Arias (1970) proposed the calculation method for measuring the intensity of earthquakes, and Kayen et al.[12] used Arias intensity method to calculate the ground total energy of the earthquake and established the energy discrimination method for evaluating the liquefaction possibility of the site. Based on a large number of field survey data, the energy discriminant method adopts the parameters such as magnitude, epicentral distance, effective overburden pressure and standard penetration number.
The relationship between the increase of pore pressure and the effective overburden pressure is established. However, when the epicenter distance is very small, the above hypothesized energy attenuation relation may not be reasonable, and it is not suitable for liquefaction identification of near-field sites. In addition, in the survey data selected by Divis, the number of standard penetration strikes is all less than 20, and when the number of measured penetration strikes is more than 20, the empirical formula needs further revision. Since the effective overburden pressure is proportional to the buried depth of sand layer, and the initial effective overburden pressure in the selected site are less than 135kPa, it should be carefully used for liquefaction identification of deep saturated sand.Shear wave velocity methodShear wave velocity is an important index of soil dynamic characteristics and reflects the degree of soil compaction and consolidation. In fact, the larger the initial shear modulus is, the smaller the deformation of soil. Shear wave velocity method has been widely used in the field of geotechnical engineering because of its advantages of small size, fast test speed and high efficiency. According to the basic theory of cyclic strain method, Dobry et al.[13] proposed the method of determining the liquefaction potential of saturated sand soil by using shear wave velocity.
On the basis of potential test, Dobry suggested that the critical shear strain value for liquefaction of saturated sand should be about 4 ~ 10, and derived the liquefaction discriminant formula. The semi-empirical and semi-theoretical formula has been adopted by the national bureau of standards (NBS). Tokimatsu et al.[14] determined the relationship between the modified critical shear wave velocity and the liquefaction stress ratio through a large number of experiments. In 1984, A new method of seismic liquefaction identification based on shear wave velocity is proposed in China. In the same year, Shi zhaoji[15] proposed that the suggested value of critical shear strain given by Dobry, which was too conservative.
Based on the measured data of Haicheng earthquake and Tangshan earthquake, the formula for determining the liquefaction of silt with the critical shear wave velocity as the index is established and extended to different seismic intensity. Zhou yanguo et al. [33] combined undrained water Based on the test data of more than 70 liquefaction points, a critical shear wave method is proposed By comparison, the calculated results of this method are in good agreement with the field investigation results, which also verifies the anti-liquefaction strength and Empirical conclusion of linear correlation of elastic shear modulus. Based on the liquefaction phenomenon of sand and gravel soil in Wenchuan earthquake, Cao zhenzhong et al.[16] considered that the modified shear wave velocity discriminant is no longer applicable to sandy gravel soil. The shear wave liquefaction model of sandy gravel soil was established by selecting intensity, groundwater depth, soil depth, shear wave velocity reference value and gravel content as evaluation factors.
The shear wave velocity method is based on a large number of in-situ test data and can directly reflect the properties of undisturbed soil. As the technology of geotechnical testing becomes more and more mature, many scholars have established various forms of liquefaction discrimination models by using this index. But due to different sand samples and shear wave velocity, it is not easy to determine an exact critical value.Cone Penetration Test MethodCone Penetration Test (CPT) is an in-situ test method for geotechnical engineering. It was firstly used in railway construction in Sweden in 1917. Due to the advantages of small impact of soil sample disturbance, fast identification speed and simple operation, this method has become a common in-situ test of geotechnical engineering. In 1984, Olsen [17] proposed the method of determining the cyclic resistance ratio (CRR) by means of cone penetration resistance and lateral wall friction resistance. In order to simplify the calculation, the stress index was used to normalize the penetration resistance of the cone. Robertson et al. [18] directly calculated the cyclic resistance using CPT test results. Moss et al.[19] proposed the deterministic and nondeterministic liquefaction evaluation method based on CPT based on the statistics of global liquefaction sites.
The results show that when the normalized CPT test result is used as the variable, the weight of the effect of the overburden effective pressure on liquefaction is reduced, and it is suggested to adopt the iterative method to modify the secondary stress on liquefaction. Zhou shengen[20] conducted a large number of static sounding tests in different intensity areas of Tangshan earthquake, and established the liquefaction model using the epicentral distance, groundwater level, buried depth of sand layer, and coverage area. Li zhaoyan et al.[21] conducted a study on the applicability of static penetration method in determining liquefaction in Bachu earthquake. The results showed that the success rate of the standard CPT method in identifying the liquefaction site is 55%. The soil properties of the soil layer in this area was quite different from the data sources of static sounding in the standard, which is not suitable for this area. After nearly 100 years of engineering application, the static sounding method has accumulated a large number of data and practical experience, and has broad application. With the improvement of the precision of the test instrument, the disturbance to the undisturbed soil can be avoided to the greatest extent, the influence of human factors can be reduced, and the test results can more precisely reflect the character of the soil.
Where a liquefaction hazard has been identified, mitigative measures are required to eliminate or reduce the hazard to an acceptable level. These measures may include any one or a combination of the following actions: (1) avoidance of the hazard through zoning restrictions or relocation of facilities to safer sites; (2) strengthening of the structure to withstand the effects of liquefaction; (3) strengthening of the ground to prevent liquefaction and damaging ground deformations; and (4) evaluation and acceptance of the risk where hazard to life and limb is minimal. All of these measures have been used effectively to reduce damage. For example, well-reinforced shallow foundations force differential lateral displacements into shear of soil layers beneath the foundation rather than fracture of the foundation and superstructure. Similarly, pile or other deep foundations that transfer structural loads to competent layers beneath the liquefiable sediments have proven effective in preventing damage to structures at sites where little or no lateral ground movement was generated. Insurance has been used as a protection against large financial loss by many individuals and industries at localities where the threat of injury or loss of life is small.
An Overview of Soil Liquefaction Process. (2019, Aug 20). Retrieved from https://studymoose.com/an-overview-of-soil-liquefaction-process-essay
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