WHAT IS LIQUIFACTION?
During heavy ground shaking by earthquakes, liquefaction occurs when the pressure exerted by the water present in saturated soil becomes so great that the soil particles become ‘suspended’ in the water. A soil deposit that is liquefied behaves like the better known phenomena: quicksand.
A few key terms:
· Saturated soils: soils in which the space (voids) between the soil particles is completely filled with water
· Pore water pressure: pressure exerted on particles of soil by the water in the voids. Most of the time this pressure is relatively low (hydrostatic) and results in an equilibrium condition of effective stress state. However, there are some circumstances in which rapidly increased stresses can cause the pore water pressure to increase.
In more technical terms, liquefaction is imminent when the pore water pressure (u) equals the total overburden stress (svo). This creates an effective stress state equal to zero
(svo’ = [svo – u] = 0).
Due to the forces exerted by gravity, soil particles naturally rest upon each other and, depending on the properties of the soil, form sort of grid that is relatively stable (or can be made so by compaction or other construction practices). During liquefaction the water pressures become high enough to counteract the gravitational pull on the soil particles and effectively ‘float’, or suspend, the particles. The soil particles can then move freely with respect to each other. Since the soil is no longer behaving as an inactive grid of particles, the strength and stiffness of a liquefied soil is significantly decreased, often resulting in a variety of structural failures. (The picture at right shows overturned apartment buildings in Niigata, Japan due to liquefaction in 1964. Picture below shows an example of lateral spread failure due an earthquake in Kobe, Japan in 1995.)
Typically when we discuss liquefaction due to a seismic event, we are addressing “cyclic liquefaction”, which occurs when repeated cycles of shearing generate an accumulation of porewater pressures. However, if the soil is very loose sand, “flow liquefaction” can occur from first time loading during site development. Also, “quasi liquefaction” describes a state of partial liquefaction of a soil deposit that does not propagate fully throughout the site; however the subsurface liquefaction response still negatively affects structures at the surface.
If liquefaction occurs beneath a surface that has hardened as a result of compaction, weathering, or some other process; ‘sand boiling’ can occur. The water pressures build below the surface to the point that the water breaks through the solid surface much like a bubble in boiling water. The pictures at left and below illustrate craters left behind as evidence of sand boiling.
On the US West Coast, these sand boils are normally about one to three feet in diameter (0.3 to 1 meter), see photos at left and below right. In the New Madrid Seismic Zone, the level of sand liquefaction was so extensive that the sand boils in this region are called “sand blows” since they generally are 10 to 100 feet diameter (3 to 30 meters).
Figures 1 and 2 show a typical view of soil grains in an unexcited, saturated deposit. The blue column on the right indicates the magnitude of porewater pressure present. The arrows in Figure 2 indicate the forces created by the interactions of the soil grains. Figure 3 shows elevated water pressure created by additional loading as from a seismic event. The increased water pressure acts to ‘float’ the grains and thereby decreases the interaction between grains, thus causing the characteristic properties of liquefaction.