CONDITIONS FOR LIQUEFACTION

Liquefaction

The term liquefaction refers to the physical change that occurs when certain soils are shaken and transformed from solid ground capable of supporting a structure to a quicksand-like liquid with a greatly reduced ability to bear the weight of a building.

Liquefaction hazard

Liquefaction poses a real, identifiable hazard to structures built on the ground or buried beneath the surface. Damage to buildings caused by liquefaction can result in structural collapse and loss of life or injuries. Loss of the soil capacity to support the weight of a structure can have disastrous effects.

The damaging effects of liquefaction were dramatically displayed during the October 17, 1989 Loma Prieta earthquake. This magnitude 7.1 event triggered soil liquefaction over a wide area, but particularly in the Marina district of San Francisco about 50 miles from the epicenter. Many buildings were damaged or destroyed due to foundation failure and thousands of homes were left without gas or water when buried utility lines ruptured due to liquefaction-induced lateral spreading. Natural gas from ruptured gas lines ignited and consumed a block-wide area. Firefighting was hampered because of the disabled water system.

Factors to occur liquefaction

Three critical factors must be present for sediments to be prone to liquefaction. The sediment must be

1) saturated with ground water,

2)composed of sand or silt-sized particles, and

3) compacted fairly loose.

For liquefaction to occur, all three factors must be present at the same time; for example, neither a loosely compacted, dry sand, or a saturated, densely compacted sand would be prone to liquefaction because one of the three critical liquefaction elements is missing. The Liquefaction Potential Map for Salt Lake County shows that the most liquefaction-prone areas (the High and Moderate areas) are located along the valley floor, tributary stream channels, and near the Great Salt Lake. Soils in foothill areas are generally less susceptible to liquefaction because they are coarser, and not saturated by shallow groundwater

Ground Water

Sediments must be saturated with ground water in order to liquefy during an earthquake. A shallow “perched” water table will contribute to liquefaction conditions and should not be disregarded or confused with deeper water levels recorded in culinary water well logs. Fluctuations in the shallow ground water level will also affect liquefaction conditions. Seasonal or cyclical “wet “periods often cause ground water levels to rise, saturating shallower sediments, and perhaps increasing the liquefaction potential in an area.

Grain Size

The size of the sediment particles controls the size of the pore spaces. This is critical in clay and fine silt grains (those less than 1/32 of an inch in diameter) because, although water can fill the small pore spaces, the flow of water between pores becomes so restricted that liquefaction becomes difficult. Gravel particles (larger than 1/5 of an inch in diameter) pose a different situation. Due to the much larger mass of the grains and generally higher porosity, the great intensity and duration of ground shaking that is required to induce liquefaction rarely occurs, except in the largest earthquakes. Generally, only sands and coarse silts combine the optimum grain mass and pore-space geometry to liquefy, given the intensity of shaking expected in a moderate to large Wasatch Front earthquake. The sands and silts must also be relatively “clean” for liquefaction to occur. This means that liquefaction is most likely to occur in sands and coarse silts with a uniform grain size. A clayey-sand, for example, would have a reduced liquefaction potential because the clay-sized particles fit between the sand grains, “tightening” up the framework and increasing soil cohesion.

Soil Density

Loose compaction of the soil also contributes to the liquefaction potential. The more densely the grains are compacted in the framework, the greater the earthquake-shaking intensity, or acceleration, needed to raise pore pressures enough to shift the grains. It is unlikely that a typical Wasatch Front earthquake could provide sufficient shaking to induce liquefaction in very densely compacted soils.

Soil density generally increases with the age and depth of deposits. Sediments tend to compact over time and with burial, increasing their density. Historically, liquefaction has been observed mainly in sediments less than 45 feet below the ground surface.