Ground failure associated with liquefaction can be manifested in several forms.
These include sand boils, lateral spreads, flow failures, ground oscillation, bearing capacity failures, and post-liquefaction settlement.
Sand boils: Although not strictly a form of ground failure because alone they do not cause ground deformation, sand boils are diagnostic evidence of elevated pore water pressure at depth and an indication that liquefaction has occurred (NRC 1985). On level ground, high pore water pressure caused by liquefaction may cause water venting to the ground surface. If the flowing pore water rises quickly enough, it can carry sand particles through cracks up to the ground surface, where they will settle and form conically shaped sand deposit.
Lateral spreads involve lateral displacement of large superficial blocks of soil driven by gravitational and inertial force as a result of liquefaction in subsurface layers (NRC 1985) (Figure 2.6). Movement occurs in response to the combined gravitational and inertial forces generated by earthquakes. Lateral spreads develop normally on very gentle slopes (between 0.3 and 3 degrees) (Youd et al. 1978). Youd (1978) has reported displacements associated with lateral spreads ranging from one meter to tens of meters.
Such lateral displacement can pull buildings apart and destroy buried pipelines. During many earthquakes, lateral spreads may produce more damage than any other form of liquefaction (NRC 1985).
Figure 2.6 Schematic Diagram of a Lateral Spread (Youd 1992)
Flow failures are perhaps the most catastrophic ground failures caused by liquefaction (NRC 1985) (Figure 2.7). They generally occur in saturated loose sands with slopes ranging between 10 and 20 degrees (Youd et al. 1978). If flow failure happens, large amounts of material may flow many tens of meters at relatively high speeds of tens
of km/h. The movement will not stop until the driving forces are reduced to values less than the viscous shear resistance of the flowing material. The moving soil may be composed of completed liquefied mass, or of intact material block floating in liquefied material.
Figure 2.7 Diagram of a Flow Failure Caused by Liquefaction and Loss of Strength of Soils Lying on a Steep Slope (Youd 1992)
Bearing capacity failures: When the soil supporting a building or other structures liquefies and loses its strength, the ability of soil to support foundations is reduced. Buildings will tilt and settle down (Figure 2.8). During 1964 Niigate earthquake in Japan (Figure 2.1) and 1999 Kocaeli earthquake in Turkey (Figure 2.2), there were buildings suffered bearing capacity failures and tilted severely. This kind of failures generally occurs in deposits of saturated cohesionless soil that extend from near the ground surface to a depth of at least half of the building width. If the deposit where liquefies is shallower, then differential settlement, but not overturning of the structure, can be resulted (Youd et al. 1978).
Ground Oscillation: When the ground is flat or the slope is too gentle to allow lateral displacement, liquefaction at depth may decouple overlying soil layers from the underlying ground, allowing the upper soil to oscillate back and forth and up and down in the form of ground waves (NCR 1985). These oscillations are usually accompanied by opening and closing of fissures and fracture of rigid structures such as pavements and pipelines (Figure 2.9).
Figure 2.8 Diagram of Structure Tilted Due to Loss of Bearing Strength. (Youd 1992)
Figure 2.9 Diagram of Horizontal Ground Oscillation Caused by Liquefaction in the Cross-Hatched Zone Decoupling the Surface Layers From the Underlying Ground (Youd 1992)
Settlement: In many cases, the weight of a structure will not be great enough to cause the large settlements associated with soil bearing capacity failures described above.
However, smaller settlements may occur as soil pore-water pressure dissipate and the soil consolidates after the earthquake (EERI 1994). These settlements are ordinarily small, but they also may cause considerable damage. Densification and ground settlement is commonly associated with and enhanced by liquefaction.
The ground failures associated with liquefaction can cause great damage to structures sitting on it. Building foundation can slide or unevenly settle, cause the tilt and damage of building, or collapse of bridges; the buried pipes would rupture; buried structures with a bulk unit weight lower than that of liquefied soil would be lifted up.
Liquefied soil also exerts much higher pressure on retaining walls, which can make them to tilt and slide. The movement of retaining wall can cause settlement of retained soils and collapse of structures on the ground surface. Increased water pressure can also trigger landslide and cause damage to dam or road.