Soil liquefaction is a phenomenon characterized by the reduction of the soil’s shear strength and stiffness because of the buildup of pore pressure in the soil’s skeleton. A cohesionless soil’s shear strength is determined by the amount of effective stress that the soil is subjected to and the angle at which the internal friction is taking place (Sassa 138).
The liquefaction is more prevalent in soils that range from sandy to silty, which are loose and saturated. However, the phenomenon has also been found to occur in gravels and non-plastic silts soil types. Under the saturation state, water usually completely displaces air between the individual soil particles. The water then exerts pressure on these soil particles, thereby influencing how closely such particles are actually compressed together. When subjected to earthquake shaking or any other rapid loading mechanism, the loose saturated soils often tend to settle and become more compact. While water pressure is quite low before an earthquake, the earthquake shaking itself usually causes the water pressure to increase to a level where it forces the soil particles to move altogether. However, the cyclic stress application period is usually too short as compared to that necessary for the discharge of water contained in the soils. The shrinking of the soil volume usually takes time, thus, the soil can be characterized by gradual build-up of pore pressure. Since effective stress and pore pressure are inversely related, the effective stress would be zero at the instance when the pore pressure equals the overall stress. This state will make sands entirely lose their stiffness/firmness and shear strength, often temporarily. Thus, this would make the structures built above or supported within the liquefied soil layer be prone to considerable damage or destruction.
Ground Failure Resulting From Soil Liquefaction
The ground failures resulting from liquefaction can be grouped into four major categories, which include, flow failure, lateral spread, ground oscillation, and the loss of bearing capacity (Naeim 365). Apart from the above-mentioned failures, other failures resulting from the liquefaction process include settlement, increased pore water pressure, sand boils, and several kinds of deformation.
Flow failures are more common in sloping areas having an inclination of five percent or more, and normally occur in the form of landslides on a larger scale. The damage caused by flow failures surpasses by far the damage that can be caused by any type of ground failure attributed to liquefaction. Flow failures usually take place when the strength of a saturated mass of granular soil is lost after initial liquefaction, making it to flow similar to a heavy liquid, thereby creating large land deformations (Sassa 54).
Unlike flow failures, lateral spreads usually occur on very gentle slopes, where the saturated mass of soil shifts towards a free-face, for example a cliff. Lateral displacements can vary from several feet to tens of feet in situations where conditions are more susceptible. Such displacements often go together with the cracking of the ground and differential vertical displacement. The occurrence of lateral displacements normally disrupts the foundations of building structures, and cause rupturing or disconnection of utility infrastructures, such as pipelines.
- Ground Oscillation
Liquefaction may not permit lateral movement in grounds that are too flat. However, liquefaction at depth can potentially cause separations at the surface by disengaging the overlying soil blocks, thereby making the blocks to mobilize backward and forward over the liquefied layer. Ground oscillation is usually goes along with the opening and closing of crevices, and damages of grid-like structures, for instance, pavements. The occurrence of the phenomenon in a build-up environment can cause a serious clean-up problem, and can result in damage and disruption if it comes along with ground settlement.
Loss of Bearing Capacity
This phenomenon occurs when liquefaction, which had initially taken place in a layer of sand, situated a few meters under a footing, advances upwards through the overlying layers of sand, resulting in the subsequent weakening of the soil supporting a structure. The weakening will make the soil lose its strength, thereby resulting in large deformations that can cause large settlements of structures.
Factors Affecting Liquefaction Susceptibility
The liquefaction of cohesionless soils is determined by a number of factors. These factors include soil types and grain size distribution, relative density, vertical effective stress, the degree of saturation, thickness of sand layer, earthquake loading characteristics, and the age and origin of the soils.
Soil Types and Grains-Size Distribution
Soils whose resistance to deformation is mobilized by friction between particles are usually highly susceptible to liquefaction. When factors such as grain shape, uniformity coefficient, and relative density are all equal, the cohesionless soil’s frictional resistance would be expected to decrease with a decrease in the soil’s grain size (Terzaghi, Peck and Mesri 2002).
Laboratory experimental results and numerous field case histories have showed relative density among the most critical factors influencing the liquefaction phenomenon. Liquefaction has been found to occur mostly in both clean and silty saturated sands whose relative density is below 50 percent. It has been observed that soils having a high relative density have a tendency of dilating when cyclic shearing occurs, thereby resulting in the generation of negative pore pressures and their increased resistance to shear stress. However, liquefaction cannot occur at a relative density of about 75 percent, as it falls within the lower limit of relative density.
Vertical Effective Stress
An increase in effective vertical stress has been found to result in increases in the soil’s bearing capacity and shear strength, which in turn increases the shear strength needed to initiate liquefaction and lowers the potential for liquefaction. It has been established from field observations that saturated soils found 15-18 meters cannot possibly liquefy. The liquefaction of sandy soils cannot occur if the effective overburden pressure surpasses 190kN/m2.
Degree of Saturation
Liquefaction can only take place in saturated soils. It is only settlement resulting from densification in the course of shaking that can be of considerable concern. There is little information regarding the possibility of liquefaction in sands that are partially saturated. However, laboratory test results have been showing that the soil’s liquefaction resistance tends to increase with a decrease in the degree of saturation. Furthermore, it has been evident that sand samples having low degree of saturation tend to become liquefied only when subjected to severe and lengthy period of earthquake shaking.
Thickness of Sand Layer
The liquefied soil layer has to be considerably thick for it to cause widespread damage at the ground surface level. The pressure resulting from the uplifting process and the amount of water discharged from the liquefied soil layer can potentially cause ground damage, for instance fissuring events. However, in a situation where the liquefied layer is relatively thin, the surface layer can possibly prevent the liquefaction’s effects from reaching the level ground surface where damage is witnessed.
Earthquake Loading Characteristics
The cohesionless soil’s vulnerability to liquefaction when an earthquake occurs is determined by the magnitude and the number stress/strain cycles it has been subjected to by the earthquake shaking. These factors or conditions are thereafter connected to the earthquake shaking’s intensity, major frequency, and even its duration.
Age and Origin of the Soils
The soil grains found in natural alluvial and fluvial deposits are normally loosely packed. Such deposits are often young, weak, and lack any form of additional strength resulting from the cementation and aging processes. It has been noted that alluvial deposits that are older than the late Pleistocene (10, 000 – 130, 000 years) cannot possibly liquefy, unless subjected to excessive enormous earthquake loading. Furthermore, alluvial deposits formed during the late Holocene era (1,000 years or less) can possibly liquefy; while those formed during the earlier Holocene (1,000-10,000 years) appear to be fairly liquefiable.
The soil’s ability to liquefy can be evaluated using several methods. The evaluation of liquefaction in soils has generally been undertaken using the simplified procedure, which was initially developed by Seed and Idriss, and is derived from the Standard Penetration Test (SPT). This evaluation method has been revised and updated several times since it was first put forward in 1971. The Seed-Idriss simplified procedure used to assess or evaluate liquefaction resistance is comprised of two basic parameters, which include:
- The level of cyclic loading that the soil is subjected to as a result of the earthquake. This level is expressed as a cyclic stress ratio (CSR).
- The soil’s resistance to liquefaction. It is expressed as a cyclic resistance ratio (CRR).
The soil’s CRR is mainly founded on the empirical correlations to factors such as the Standard Penetration Test (SPT), Cone Penetration Test (CPT), or shear wave velocity (Vs) (Davis et al. 608). Such empirical correlations have been derived from documented data regarding the liquefied and non-liquefied soils in a number of previous earthquakes. The empirical relationship that is widely used to determine soil’s resistance to liquefaction compares CRR with rectified SPT resistance (N1)60, from locations in which liquefaction occurred or failed to occur during the previous earthquakes. The ratio showing the soil’s resistance to liquefaction (CRR) to the stress triggered by the earthquake (CRR) is referred to as the factor of safety (F.S) against liquefaction triggering (Bozorgnia and Bertero 167). The soil can be said to be prone to triggering of liquefaction if the FS is less than 1.0, and not vulnerable to liquefaction if the FS is greater than 1.0.
Mitigation of Liquefaction Hazards
The chances of liquefaction occurring can be eliminated through carrying out densification, dewatering, and stabilization. Other methods essential in mitigating liquefaction hazards include the use of shallow or deep foundations that have specifically designed to absorb any occurrence of liquefaction and the resulting vertical and horizontal deformations that are capable of posing considerable risk. In most cases, a mitigation strategy may constitute the implementation of a set of techniques or concepts, for instance, densification, reinforcement, and mixing. In order to minimize cost while simultaneously achieving a reasonable risk level, one can create shallow or deep foundations that are specially designed to work along with partial ground improvement techniques.
The preferred mitigation method is determined by the scale of liquefaction and its related consequences. Furthermore, the mitigation costs have to be taken into consideration in relation to achieving an acceptable level of risk. The first option to avoiding liquefaction hazards is avoiding construction of structures on or within soils susceptible to liquefaction. However, if construction has to be undertaken on soils susceptible to liquefaction, the liquefaction hazards would mainly be mitigated through two basic methods, which include soil improvement and structural improvement.
Soil Improvement Options
The techniques for soil improvement include densification, stabilization and replacement, or drainage techniques. These techniques can be implemented in ways that would enable them eliminate the liquefaction potential either fully or partially, as determined by the loads and deformations that the structure has been designed to tolerate.
Vibro-compaction, vibro-replacement, and deep dynamic compaction are the most common techniques used for in-situ densification of soils prone to liquefaction. While similar equipments can be used for vibro-compaction and vibro-replacement techniques, the difference is that these two techniques achieve densification of soils at depth using different backfill material. For example, vibro-compaction uses a sand backfill, whereas stone is used as a backfill material in the vibro-replacement technique. Vibro-compaction has proved to be generally effective when the soils to be densified are sands having less than about 10 percent fine-grained material capable of passing through the No. 200 sieve. Conversely, vibro-replacement tends to be more effective in soils with less than 15-20 percent fine-grained material.
The deep dynamic compaction entails subjecting the ground surface to enormous impact energy in order to densify and firmly compact the sub-surface soils. It is achieved through lifting weights of about 10 to 30 tons using standard, modified, or specially designed machines, and dropping them from approximately 15 to 35 meters heights. The impact energy resulting from the free-fall is controlled through selection of weight, drop height, the number of drops to be made per point, and the grid’s spacing. The limitations of the deep dynamic compaction technique include the resulting vibrations, flying matter/materials, and noise.
Grouting and Soil Mixing Techniques
These techniques often seek to bond the soil particles together or minimize the void space within the liquefiable soil by introducing grout materials into them through permeation, mechanical mixing, or even jetting.
These techniques usually attempt to lessen the build-up of pore pressure through provision of highly conductive drainage path which facilitates fast dissipation of pore pressure to minimize the soil’s susceptibility to liquefaction. The drainage techniques are implemented through installation of drains comprised of sand, gravel, and or synthetic materials. While gravel and sand drains are often placed vertically, the installation of synthetic wick drains is normally done at several angles. The efficacy of measures for reducing liquefaction hazard can be enhanced through using drainage techniques together with other kinds of techniques meant for soil improvement.
Structural Improvement Options
Structural mitigation for liquefaction hazards can at some instances prove more economical as compared to using soil improvement mitigation techniques. However, it should be noted that structural mitigation can have little or no effect at all on the soil itself, thus may fail to minimize the liquefaction potential. This implies that liquefaction and its associated ground deformations can still occur after implementing structural mitigation measures. The appropriate ways of undertaking structural mitigation can be determined by the expected earthquake magnitude and the resulting type of deformation. For instance, structural mitigation measures cannot be practical in most cases where liquefaction-triggered flow slides or considerable lateral spreading is expected. However, structural mitigation can be economical and technically feasible in situations where soil deformation is expected to occur mainly through vertical settlement.
In cases where the structure is relatively small and light in weight, for instance, that housing a single family, establishing a slab foundation system can prove useful. Such a structure can also be supported by continuous spread footings comprised of isolated footings that have been connected together using grade beams. In contrast, a mat foundation is more appropriate for heavier buildings. It is important to design such mats in a way that would enable them bridge over the local areas of settlement. Furthermore, piles that extend to unliquifiable soil or bedrock situated below potentially liquefiable soils can be used.
It is evident that liquefaction is a serious seismic hazard that deserves more attention in the designing of new constructions and evaluation of existing ones. Its occurrence is largely determined by the nature of the soil and the area’s geology. Furthermore, the effects of liquefaction cannot only be catastrophic, but costly as well. The evaluation of the liquefaction phenomena can be undertaken using several methods. However, geotechnical often uses the simplified procedure that was firstly developed by Seed and Idriss. There exist several methods that can be effective in the mitigation of liquefaction hazards. These methods entail improving the conditions of soils vulnerable to liquefaction by making them more compact and dense, and also through improving the structures that are constructed on soils that are potentially liquefiable.
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Davis, Craig, Xiuli Du, Masakatsu Miyajima, and Liping Yan. International Efforts in Lifeline Earthquake Engineering: Proceedings of the Sixth China-Japan-Us Trilateral Symposium on Lifeline Earthquake Engineering. , 2014. Internet resource.
Naeim, Farzad. The Seismic Design Handbook. New York: Chapman & Hall, 1989. Print.
Sassa, Kyoji. Progress in Landslide Science. Berlin: Springer, 2007. Internet resource.
Terzaghi, Karl, Ralph B. Peck, and Gholamreza Mesri. Soil Mechanics in Engineering Practice. New York [etc.: Wiley, 1996. Print.