Liquefaction

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Soil Liquefaction is the process by which saturated, unconsolidated soil or sand is converted into a suspension. It is commonly observed in quicksand, quick clay, turbidity currents, and as a result of earthquake shock in unconsolidated sediments. It can be caused when flowing water reduces the friction between sand particles (as from an underground spring), or when a sudden change in pressure or repeated shock acting on water saturated or supersaturated sediments (as in an earthquake). Although the effects of liquefaction had been observed and understood for years, it was dramatically brought to the attention of engineers and seismologists in 1964 during the Niigata, Japan and Alaska earthquakes. It was a major factor in the destruction in San Francisco's Marina District during the 1989 Loma Prieta earthquake.

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Liquefaction in Construction

A few construction sites (Irvine, California is a classic example) must address problems with the soil suddenly losing its sheer strength due to liquefaction. [1] (Sheer strength is the property of soil that allows a building to sit on top of the soil rather than sink down into it.) Excavation and even road construction must be mindful that running normal earth-moving equipment across a spot repeatedly will draw water up to the surface and cause liquefaction.

Note: You know you have created a problem with liquefaction when you walk across a spot and it feels as if you are walking across a mattress.

Changing the path of construction equipment is necessary to avoid the pumping action of the tires on the soil. While this may not seem like much of a problem, larger pieces of equipment such as a Caterpillar® tractor-scraper could bury itself up to the axles in a couple of passes if it rolls over the same spot too much. [2] Lesser problems could cause delays such as the wait time needed to allow such areas to dry out prior to paving parking lots and roads.

Construction for Taco Bell Headquarters in Irvine, California had to be modified to accommodate the local soils conditions. A relatively impervious layer of rock several feet below the surface of the soil traps the water above and lets it pool, creating a swampy condition just below the surface resembling the look and consistency of peanut butter. This condition covers several square miles of Irvine. An underground garage in that area is not practical.[3]

When flood conditions threaten an earthwork levee or embankment, locals may feel compelled to reinforce the levee with sandbags. Often they will load the heavy bags onto a big truck and drive the truck parallel to the levy and unload the bags. This constant movement of a heavy truck over the same ground will sometimes create a pumping action, and liquefaction of the soil near the levee. This will weaken the integrity of the levee. Locals should use lighter trucks and multiple paths to drop off the bags and thus lessen the likelihood of liquefaction.[4]

If you have already caused liquefaction, you can mitigate the problem by mixing in a cement-based product designed to strengthen the structural integrity of the soil. Also, special mats designed to span such soil conditions have been used with mixed results.[5]

Liquefaction in earthquakes

For a more detailed treatment, see Earthquake liquefaction.

The shock or repeated shock of earthquake waves can cause water-saturated soil to rearrange itself in such a way that it essentially becomes a suspension of solids in the liquid. Heavy structures on such areas can suddenly sink or shift. Buried objects can shift and relatively low density objects can float to the surface.

"Often during earthquakes, fine-grained water-saturated sediments may lose their former strength and form into a thick mobile mudlike material. The process is called liquefaction. The liquefied sediment not only moves about beneath the surface but may also rise through fissures and “erupt” as mud boils and mud 'volcanoes.'"[6]
"... the ground shaking reduces the strength of earth material on which heavy structures rest. Parts of many major cities, particularly port cities, have been built on naturally occurring bodies of soft, unconsolidated clay-rich sediment (such as the delta deposits of a river) or on filled areas in which large amounts of loose earth materials have been dumped to build up the land level. These water-saturated deposits often experience a change in property known as liquefaction when shaken by an earthquake. The material loses strength to the degree that it becomes a highly fluid mud, incapable of supporting buildings, which show severe tilting or collapse."[7]

This can be demonstrated on a small scale by saturating a bucket of sand with water; place a stone on the currently solid top of the sand, then repeatedly strike the side of the bucket with a hammer and watch the rock sink.

The initial packing density of the sand plays an important role in whether the soil deposit will liquefy, as shown in earthquake liquefaction.

Liquefaction in quicksand

For a more detailed treatment, see 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 can't escape, it creates liquefied soil that can no longer support weight. Quicksand can be formed by standing or 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.

Liquefaction in quick clay

For a more detailed treatment, see 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, 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. [1]

Turbidity currents

For a more detailed treatment, see Turbidity current.

Submarine landslides are turbidity currents and consist of a flow of water saturated sediments flowing downslope. An example occurred 18 November 1929, when an earthquake 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.[8]

External links

References

  1. Alex Shrugged was a soils technician many years ago in the Southern California area. Most building sites do not experience problems with liquefaction but if they do, it is a well-known problem.
  2. Alex Shrugged has personally observed a tractor-scraper buried up to its axles in Irvine California. It remained stuck for several days until a fellow with a small D6 tractor working on the site right next to ours offered to dig it out. The D6 is a relatively small bulldozer that could ride on top of the mushy soil without getting trapped.
  3. While Alex Shrugged did not work on the Taco Bell project personally, he did speak with the soils engineer who did and who related this story to him.
  4. Alex Shrugged watched a TV report on flooding and as part of the report was the efforts of the locals sandbagging the levee. The reporter pointed directly at the spot where liquefaction was already occurring near the levee. The camera zoomed in and in the midst of that liquefaction were the tire tracks of a truck, first going left and then right. It was the place where the truck turned to follow the path parallel to the levee to drop off the sandbags. The levee collapsed shortly thereafter.
  5. Alex Shrugged has tried all sorts of ways to mitigate the problem of liquefaction, but nothing he wishes to put in writing beyond this. Check with the local contractors or soils engineer for suggestions. Everything Alex Shrugged ever did to mitigate this problem was cleared with a soils engineer.
  6. Harold L. Levin, Contemporary Physical Geology, 2nd edition (New York: Saunders College Publishing, 1986).
  7. Arthur N. Strahler, Physical Geology (New York: Harper & Row, Publishers, 1981), p. 202.
  8. 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.
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