Soil liquefaction


Soil liquefaction

Soil liquefaction describes the behavior of loose saturated unconsolidated soils, i.e. loose sands, which go from a solid state to have the consistency of a heavy liquid, or reach a liquefied state as a consequence of increasing porewater pressures, and thus decreasing effective stress, induced by their tendency to decrease in volume when subjected to cyclic undrained loading (e.g. earthquake loading). Liquefaction is more likely to occur in loose to moderate granular soils with poor drainage, such as silty sands or sands and gravels capped or containing seams of impermeable sediments [Youd, T.L., and Idriss, I.M. (2001). "Liquefaction Resistance of Soils: Summary report from the 1996 NCEER and 1998 NCEER/NSF Workshops on Evaluation of Liquefaction Resistance of Soils", "Journal of Geotechnical and Geoenvironmental Engineering", ASCE, 127(4), 297-313] . Deposits most susceptible to liquefaction are young (Holocene-age, deposited within the last 10,000 years) sands and silts of similar grain size (well-sorted), in beds at least several feet thick, and saturated with water. Such deposits are often found along riverbeds, beaches, dunes, and areas where windblown silt (loess) and sand have accumulated. Some examples of liquefaction include quicksand, quick clay, turbidity currents, and earthquake liquefaction.

Depending on the initial void 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 case "flow 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 will depend on the density 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, then "cyclic liquefaction" can take place. If stress reversal does not occur, zero effective stress is not possible to occur, then "cyclic mobility" takes place [Robertson, P.K., and Fear, C.E. (1995). "Liquefaction of sands and its evaluation.", "Proceedings of the 1st International Conference on Earthquake Geotechnical Engineering", Tokyo] .

The resistance of the cohesionless soil to liquefaction will depend on the density of the soil, confining stresses, soil structure (fabric, age and cementation), the magnitude and duration of the cyclic loading, and the extent to which shear stress reversal occurs [Robertson, P.K., and Wride, C.E. (1998). "Evaluating Cyclic Liquefaction Potential using the cone penetration test." "Canadian Geotechnical Journal", Ottawa, 35(5), 442-459.] .

Although the effects of liquefaction have been long understood, it was more thoroughly brought to the attention of engineers and seismologists in the 1964 Niigata, Japan and Alaska earthquakes. It was also a major factor in the destruction in San Francisco's Marina District during the 1989 Loma Prieta earthquake.

Earthquake liquefaction

Earthquake liquefaction is a major contributor to urban seismic risk. The shaking causes increased pore water pressure which reduces the effective stress, and therefore reduces the shear strength of the sand. If there is a dry soil crust or impermeable cap, the excess water will sometimes come to the surface through cracks in the confining layer, bringing liquefied sand with it, creating sand boils, colloquially called "sand volcanoes".

Studies of liquefaction features left by prehistoric earthquakes, called paleoliquefaction or paleoseismology, can reveal a great deal of information about earthquakes that occurred before records were kept or accurate measurements could be taken. [http://earthquake.usgs.gov/research/hazmaps/whats_new/workshops/CEUS-WORKSHP/Tuesday/NE-Tuttle.2.pdf]

Quicksand

Quicksand forms when water saturates an area of loose sand and the ordinary sand is agitated. When the water trapped in the batch of sand cannot escape, it creates liquefied soil that can no longer support weight. 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 shock initiates the liquifaction 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 object "floats" due to its buoyancy.

Quick clay

Quick clay, also known as "Leda Clay" in Canada, is a unique form of highly sensitive clay, with the tendency to change from a relatively stiff condition to a liquid mass when it is disturbed. Undisturbed quick clay resembles a water-saturated gel. When a block of clay is held in the hand and struck, however, it instantly turns into a flowing ooze, a process known as spontaneous liquefaction. Quick clay behaves this way because, although it is solid, it has a very high water content, up to 80%. The clay retains a solid structure despite the high water content, because surface tension holds water-coated flakes of clay together in a delicate structure. When the structure is broken by a shock, it reverts to a fluid state.

Quick clay is only found in the northern countries such as Russia, Canada, Alaska in the U.S., Norway, Sweden, and Finland, which were glaciated during the Pleistocene epoch.

Quick clay has been the underlying cause of many deadly landslides. 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. [http://geoscape.nrcan.gc.ca/ottawa/landslides_e.php]

Turbidity currents

Submarine landslides are turbidity currents and consist of water saturated sediments flowing downslope. An example occurred during the 1929 Grand Banks earthquake that struck the continental slope off the coast of Newfoundland. Minutes later, transatlantic telephone cables began breaking sequentially, farther and farther downslope, away from the epicenter. Twelve cables were snapped in a total of 28 places. 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 the continental slope from the earthquake’s epicenter, snapping the cables as it passed. [Bruce C. Heezen and Maurice Ewing, “Turbidity Currents and Submarine Slumps, and the 1929 Grand Banks Earthquake,” American Journal of Science, Vol. 250, December 1952, pp. 849–873.]

Effects

Liquefaction can cause damage to structures in several ways. [ [http://www.es.ucsc.edu/~es10/fieldtripEarthQ/Damage1.html Damage Caused by EarthQuakes ] ] 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. Liquefaction causes irregular settlements in the area liquefied, which can damage buildings and break underground utility lines where the differential settlements are large. Pipelines and ducts may float up through the liquefied sand. Sand boils can erupt into buildings through utility openings, and may allow water to damage the structure or electrical systems. Soil liquefaction can also cause slope failures. Areas of land reclamation are often prone to liquefaction because many are reclaimed with hydraulic fill, and are often underlain by soft soils which can amplify earthquake shaking. Soil liquefaction was a major factor in the destruction in San Francisco's Marina District during the 1989 Loma Prieta earthquake. Mitigating potential damage from liquefaction is part of the field of geotechnical engineering.

See also

* Paleoseismology
* Dry quicksand
* Atterberg limits
* Mud volcano
* Sand volcano or sand blow
* Thixotropy

References

External links

* [http://www.ce.washington.edu/~liquefaction/html/main.html Soil Liquefaction]
* [http://www.pnsn.org/SEIS/EQ_Special/WEBDIR_01022818543p/quakestory.html Shaking, Liquefaction on Harbor Island] , one of the few known live observations of an earthquake liquefaction event by a seismologist


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