1. Liquefaction Kevin Carr Compression Dilation
Stockton University
Where Can Liquefaction Occur?
Soils most susceptible to liquefaction are well sorted, loosely compacted, highly saturated sands or silts. These conditions are common in low lying areas near bodies of water, around river deltas and beaches. Risk for liquefaction is determined by susceptibility, i.e., soil type and saturation, and potential for ground shaking.
This map (left) shows the susceptibility to liquefaction in the San Francisco Bay Area.
(Knudsen, 2006)
What Is Liquefaction?
Liquefaction Resistant Structures
Liquefaction is the process by which soils lose strength or stiffness as a result of seismic or other sudden loading, and take a liquid-like form. This usually occurs in sandy or silty, water-saturated soils, near rivers, lakes, and beaches (Johansson, 2000). Liquefaction can take two forms, static or flow liquefaction which occurs in loose soils and usually results in large movements, and seismic or cyclic mobility in moderate or dense soils, which is generally less dramatic (Burton & Scobie, n.d.)
Several techniques exist to reduce the likelihood and effects of liquefaction. One is to simply avoid building on liquefiable soils. If it is necessary to build on liquefiable soils, however, other options exist.
Resistant structures can
be built to mitigate damage
when liquefaction does occur.
Buildings can be placed on
stiff foundation mats, which
help to span small areas of
liquefiable soils. Pilings can
also be driven deep into the earth, through liquefiable soils, into stronger soil or bedrock below. These must be built not only to support the vertical load, but also to resist, or move with, lateral shifts. Gas and water lines should also be built with flexible joints, to resist breaking (Johansson, 2000)
Manifestations and Effects of Liquefaction
Bearing-Capacity Failure
Soil bearing buildings, pilings, or other loads, loses strength and sinking occurs.
Niigata, Japan, 1964
Magnitude 7.5 earthquake
Apartment buildings tilted
and partially sunk.
Can be seen in quicksand.
(Khaldoun, Fiser, Wegdam, Bonn, 2005)
Soil Improvement
Soils can be improved to prevent the occurrence of liquefaction. This can be ultimately achieved by compacting the soil, by improving the drainage, or both.
How Does it Work?
Vibroflotation
Vibrating probe inserted into ground causes soil to collapse and become densified.
Can be backfilled with gravel to provide drainage.
When a shear force is applied to loose, saturated soil, it is compressed, and so the water pressure between particles increases. If sufficient time is allowed, and if the surrounding soil is permeable enough to allow for drainage, the water is then driven off. However, if the pressure is applied rapidly enough, the water pressure pushes the particles apart. This breaks the friction between particles, known as effective stress, allowing them to move freely around each other.
If the soil is already compact, a shear force first wants to compress the soil. However, particles almost instantaneously need to shift as they are pushed past each other. This causes a dilation, increasing space between particles, which decreases the water pressure. This suction effect wants to pull the particles close together again, increasing the effective stress. The soil is alternately strengthening and weakening in rapid succession, and is therefore known as cyclic mobility (Burton & Scobie, n.d.).
Lateral Spreading
Soil slides on deeper
layer of liquefied soil
Occurs on gentle
slopes (Youd, 1973).
Dynamic Compaction
Heavy weight dropped repeatedly to compact soil.
Significant damage to surface occurs.
Can cause failure of levees and
embankment structures
Motagua River,
Guatemala, 1976.
(Johansson, 2000)
Compaction Piles
Concrete or timber piles installed in grid pattern.
Densifies and reinforces surrounding soil.
Compaction Grouting
Cement injected into liquefiable soil.
Pressure compacts surrounding soil.
Good option for existing structures.
Can be injected at an angle.
(Johansson, 2000)
Flow Failure
Large movement of soil
down steep slope
Mudslides, quick clays.
Lake Merced, San
Francisco, 1957
(Saundry, 2010)
Compression
Other methods include excavation and removal of liquefiable soils, densification by blasting, permeation grouting, drains, and edge containment structures to prevent lateral spreading (Knudsen, 2006).
Sand Boils
Liquefied sand pushed from buried layer to
surface.
Forms sand volcanoes
(Knudsen, 2006)
Conclusion
Understanding and evaluating the risks associated with liquefaction begins with understanding the processes which cause it. By understanding how liquefaction can manifest, and what areas are susceptible, we can take necessary precautionary measures to avoid both loss of life and economic disasters. It is important to know the risks, so that problem areas can be avoided when possible, and preventative measures can be taken where necessary.
References
Burton, S., & Scobie, G. (n.d.). Liquefaction Characteristics. Retrieved April 12, 2016, from
Johansson, J. (2000, January). Soil liquefaction. Retrieved April 12, 2016, from
Khaldoun, A., Eiser, E., Wegdam, G. H., & Bonn, D. (2005). Liquefaction of Quicksand Under Stress. Nature, 437(7059),
635th ser. Retrieved April 22, 2016.
Knudsen, K. (2006, August 18). Liquefaction. Retrieved April 12, 2016, from
Nance, D., & Murphy, B. E. (2015). Physical Geology Today. New York, NY: Oxford University Press.
Saundry, P. (2010, July 18). Landslide. Retrieved April 21, 2016, from
Youd, T. L. (1973). Liquefaction, Flow, and Associated Ground Failure. Geological Survey Circular, 688th ser., 1-12.
Retrieved April 21, 2016.
Dilation