Soil liquefaction occurs when a cohesionless saturated or partially saturatedsoil substantially losesstrength andstiffness in response to an appliedstress such as shaking during anearthquake or other sudden change in stress condition, in which material that is ordinarily a solid behaves like a liquid. Insoil mechanics, the term "liquefied" was first used byAllen Hazen[1] in reference to the 1918 failure of theCalaveras Dam inCalifornia. He described the mechanism of flowliquefaction of theembankment dam as:
If the pressure of the water in the pores is great enough to carry all the load, it will have the effect of holding the particles apart and of producing a condition that is practically equivalent to that ofquicksand... the initial movement of some part of the material might result in accumulating pressure, first on one point, and then on another, successively, as the early points of concentration were liquefied.
The phenomenon is most often observed in saturated, loose (lowdensity or uncompacted), sandy soils. This is because a loosesand has a tendency tocompress when aload is applied. Dense sands, by contrast, tend to expand in volume or 'dilate'. If the soil is saturated by water, a condition that often exists when the soil is below thewater table orsea level, then water fills the gaps between soil grains ('pore spaces'). In response to soil compressing, thepore water pressure increases and the water attempts to flow out from the soil to zones of low pressure (usually upward towards the ground surface). However, if theloading is rapidly applied and large enough, or is repeated many times (e.g., earthquake shaking, storm wave loading) such that the water does not flow out before the next cycle of load is applied, the water pressures may build to the extent that it exceeds the force (contact stresses) between the grains of soil that keep them in contact. These contacts between grains are the means by which the weight from buildings and overlying soil layers is transferred from the ground surface to layers of soil or rock at greater depths. This loss of soil structure causes it to lose itsstrength (the ability to transfershear stress), and it may be observed to flow like a liquid (hence 'liquefaction').
Although the effects of soil liquefaction have been long understood,engineers took more notice after the1964 Alaska earthquake and1964 Niigata earthquake. It was a major cause of the destruction produced inSan Francisco'sMarina District during the1989 Loma Prieta earthquake, and in thePort of Kobe during the1995 Great Hanshin earthquake. More recently soil liquefaction was largely responsible for extensive damage to residential properties in the eastern suburbs and satellite townships ofChristchurch during the2010 Canterbury earthquake[2] and more extensively again following the Christchurch earthquakes that followed inearly andmid-2011.[3] On 28 September 2018, anearthquake of 7.5 magnitude hit the Central Sulawesi province of Indonesia. Resulting soil liquefaction buried the suburb of Balaroa and Petobo village 3 metres (9.8 ft) deep in mud. The government of Indonesia is considering designating the two neighborhoods of Balaroa and Petobo, that have been totally buried under mud, as mass graves.[4][needs update]
Thebuilding codes in many countries require engineers to consider the effects of soil liquefaction in the design of new buildings and infrastructure such as bridges, embankment dams and retaining structures.[5][6][7]
Soil liquefaction occurs when theeffective stress (shear strength) of soil is reduced to essentially zero. This may be initiated by either monotonicloading (i.e., a single, sudden occurrence of a change in stress – examples include an increase in load on an embankment or sudden loss of toe support) or cyclic loading (i.e., repeated changes in stress condition – examples includewave loading orearthquake shaking). In both cases a soil in a saturated loose state, and one which may generate significant pore water pressure on a change in load are the most likely to liquefy. This is because loose soil has the tendency to compress when sheared, generating large excessporewater pressure as load is transferred from the soil skeleton to adjacent pore water during undrained loading. As pore water pressure rises, a progressive loss of strength of the soil occurs as effective stress is reduced. Liquefaction is more likely to occur in sandy or non-plastic silty soils but may in rare cases occur in gravels and clays (seequick clay).
A 'flow failure' may start if the current strength of the soil falls below the strength required to maintain the equilibrium of a slope or footing of a structure. This can occur due to monotonic (single-event) loading or cyclic loading, and can be sudden and catastrophic. A historical example is theAberfan disaster. Casagrande[8] referred to this type of phenomena as 'flow liquefaction', although a state of zero effective stress is not required for this to occur.
'Cyclic liquefaction' is the state of soil when large shear strains have accumulated in response to cyclic loading. A typical reference strain for the approximate occurrence of zero effective stress is 5% double amplitude shear strain. This is a soil test-based definition, usually performed via cyclictriaxial, cyclic directsimple shear, or cyclictorsional shear type apparatus. These tests are performed to determine a soil's resistance to liquefaction by observing the number of cycles of loading at a particular shear stress amplitude required to induce 'fails'. Failure here is defined by the aforementioned shear strain criteria.
The term 'cyclic mobility' refers to the mechanism of progressive reduction of effective stress due to cyclic loading. This may occur in all soil types including dense soils. However, on reaching a state of zero effective stress such soils immediately dilate and regain strength. Thus, shear strains are significantly less than a true state of soil liquefaction.
Liquefaction is more likely to occur in loose to moderately saturated granular soils with poordrainage, such as siltysands or sands andgravels containing impermeablesediments.[9][10] Duringwave loading, usually cyclic undrained loading, e.g.seismic loading, loose sands tend to decrease involume, which produces an increase in theirpore water pressures and consequently a decrease inshear strength, i.e. reduction ineffective stress.
Deposits most susceptible to liquefaction are young (Holocene-age, deposited within the last 10,000 years) sands andsilts of similar grain size (well-sorted), in beds at leastmetres thick, and saturated with water. Such deposits are often found alongstream beds,beaches,dunes, and areas where windblown silt (loess) and sand have accumulated. Examples of soil liquefaction includequicksand, quick clay,turbidity currents and earthquake-induced liquefaction.
Depending on the initialvoid ratio, the soil material can respond to loading either strain-softening or strain-hardening. Strain-softened soils, e.g., loose sands, can be triggered to collapse, either monotonically or cyclically, if the static shear stress is greater than the ultimate or steady-state shear strength of the soil. In this caseflow liquefaction occurs, where the soil deforms at a low constant residual shear stress. If the soil strain-hardens, e.g., moderately dense to dense sand, flow liquefaction will generally not occur. However, cyclic softening can occur due to cyclic undrained loading, e.g., earthquake loading. Deformation during cyclic loading depends on thedensity of the soil, the magnitude and duration of the cyclic loading, and amount of shear stress reversal. If stress reversal occurs, the effective shear stress could reach zero, allowing cyclic liquefaction to take place. If stress reversal does not occur, zero effective stress cannot occur, and cyclic mobility takes place.[11]
The resistance of the cohesionless soil to liquefaction will depend on the density of the soil, confining stresses, soil structure (fabric, age andcementation), the magnitude and duration of the cyclic loading, and the extent to which shear stress reversal occurs.[12]
Three parameters are needed to assess liquefaction potential using thesimplified empirical method:
The interaction between the solid skeleton and pore fluid flow has been considered by many researchers to model the material softening associated with the liquefaction phenomenon. The dynamic performance of saturatedporous media depends on the soil-pore fluid interaction. When the saturated porous media is subjected to strong ground shaking, pore fluid movement relative to the solid skeleton is induced. The transient movement of pore fluid can significantly affect the redistribution of pore water pressure, which is generally governed by the loading rate,soil permeability,pressure gradient, andboundary conditions. It is well known that for a sufficiently highseepage velocity, the governing flow law in porous media is nonlinear and does not followDarcy's law. This fact has been recently considered in the studies of soil-pore fluid interaction for liquefaction modeling. A fully explicit dynamicfinite element method has been developed forturbulent flow law. The governing equations have been expressed for saturated porous media based on the extension of the Biot formulation. The elastoplastic behavior of soil under earthquake loading has been simulated using a generalizedplasticity theory that is composed of ayield surface along with a non-associated flow rule.[18]
Pressures generated during large earthquakes can force underground water and liquefied sand to the surface. This can be observed at the surface as effects known alternatively as "sand boils", "sand blows" or "sand volcanoes". Such earthquake ground deformations can be categorized as primary deformation if located on or close to the ruptured fault, or distributed deformation if located at considerable distance from the ruptured fault.[19][20]
The other common observation is land instability – cracking and movement of the ground down slope or towards unsupported margins of rivers, streams, or the coast. The failure of ground in this manner is called 'lateral spreading' and may occur on very shallow slopes with angles only 1 or 2 degrees from the horizontal.
One positive aspect of soil liquefaction is the tendency for the effects of earthquake shaking to be significantlydamped (reduced) for the remainder of the earthquake. This is because liquids do not support ashear stress and so once the soil liquefies due to shaking, subsequent earthquake shaking (transferred through ground byshear waves) is not transferred to buildings at the ground surface.
Studies of liquefaction features left by prehistoric earthquakes, calledpaleoliquefaction orpaleoseismology, can reveal information about earthquakes that occurred before records were kept or accurate measurements could be taken.[21]
Soil liquefaction induced by earthquake shaking is a major contributor tourban seismic risk.
The effects of soil liquefaction on the built environment can be extremely damaging. Buildings whose foundations bear directly on sand which liquefies will experience a sudden loss of support, which will result in drastic and irregular settlement of the building causing structural damage, including cracking of foundations and damage to the building structure, or leaving the structure unserviceable, even without structural damage. Where a thin crust of non-liquefied soil exists between building foundation and liquefied soil, a 'punching shear' type foundation failure may occur. Irregular settlement may break underground utility lines. The upward pressure applied by the movement of liquefied soil through the crust layer can crack weak foundation slabs and enter buildings through service ducts and may allow water to damage building contents and electrical services.
Bridges and large buildings constructed onpile foundations may lose support from the adjacent soil andbuckle or come to rest at a tilt.
Sloping ground and ground next to rivers and lakes may slide on a liquefied soil layer (termed 'lateral spreading'),[22] opening largeground fissures, and can cause significant damage to buildings, bridges, roads and services such as water, natural gas, sewerage, power and telecommunications installed in the affected ground. Buried tanks and manholes may float in the liquefied soil due tobuoyancy.[22] Earth embankments such as floodlevees andearth dams may lose stability or collapse if the material comprising the embankment or its foundation liquefies.
Over geological time, liquefaction of soil material due to earthquakes could provide a dense parent material in which thefragipan may develop through pedogenesis.[23]
Mitigation methods have been devised byearthquake engineers and include varioussoil compaction techniques such as vibro compaction (compaction of the soil by depth vibrators),dynamic compaction, andvibro stone columns.[24] These methods densify soil and enable buildings to avoid soil liquefaction.[25]
Existing buildings can be mitigated by injecting grout into the soil to stabilize the layer of soil that is subject to liquefaction. Another method called IPS (Induced Partial Saturation) is now practicable to apply on larger scale. In this method, the saturation degree of the soil is decreased.
Quicksand forms when water saturates an area of loose sand, and the sand is agitated. When the water trapped in the batch of sand cannot escape, it creates liquefied soil that can no longer resist force. Quicksand can be formed by standing or (upwards) flowing underground water (as from an underground spring), or by earthquakes. In the case of flowing underground water, the force of the water flow opposes the force of gravity, causing the granules of sand to be more buoyant. In the case of earthquakes, the shaking force can increase the pressure of shallow groundwater, liquefying sand and silt deposits. In both cases, the liquefied surface loses strength, causing buildings or other objects on that surface to sink or fall over.
The saturated sediment may appear quite solid until a change in pressure, or a shock initiates the liquefaction, causing the sand to form a suspension with each grain surrounded by a thin film of water. This cushioning gives quicksand, and other liquefied sediments, a spongy, fluidlike texture. Objects in the liquefied sand sink to the level at which the weight of the object is equal to the weight of the displaced sand/water mix and the objectfloats due to itsbuoyancy.
Quick clay, known asLeda Clay inCanada, is a water-saturatedgel, which in its solid form resembles highly sensitiveclay. This clay has a tendency to change from a relatively stiff condition to a liquid mass when it is disturbed. This gradual change in appearance from solid to liquid is a process known as spontaneous liquefaction. The clay retains a solid structure despite its high-water content (up to 80% by volume), becausesurface tension holds water-coated flakes of clay together. When the structure is broken by a shock or sufficient shear, it enters a fluid state.
Quick clay is found only in northern countries such asRussia,Canada,Alaska in the U.S.,Norway,Sweden andFinland, which were glaciated during thePleistocene epoch.
Quick clay has been the underlying cause of many deadlylandslides. In Canada alone, it has been associated with more than 250 mapped landslides. Some of these are ancient, and may have been triggered by earthquakes.[26]
Submarine landslides areturbidity currents and consist of water-saturated sediments flowing downslope. An example occurred during the1929 Grand Banks earthquake that struck thecontinental slope off the coast ofNewfoundland. Minutes later,transatlantic telephone cables began breaking sequentially, further and further downslope, away from theepicenter. Twelve cables were snapped in a total of 28 places. The exact times and locations were recorded for each break. Investigators suggested that a 60-mile-per-hour (100 km/h) submarine landslide or turbidity current of water-saturated sediments swept 400 miles (600 km) down thecontinental slope from the earthquake's epicenter, snapping the cables as it passed.[27]
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