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Learning

Earthquake Design
31 and
Earthquake Tip
Construction

Why do Buildings Sink into the Ground during Earthquakes?

What is Liquefaction of Soils? 0
A special situation arises during earthquake Stress in Soil
shaking in sandy (cohesionless) soils that are loose and
saturated with water. Horizontal shaking of the earth Liquefied
soil
at the bedrock level is transmitted upwards to Depth
overlying layer(s) of soil. Saturated loose cohesionless
soils have voids between soil particles filled with
water. During strong ground shaking, loose sand
tends to densify; this tends to compress water, but

because water is incompressible, it tends to escape out. Shear Stress Demand Shear Strength Capacity
Water cannot drain out quickly from the soil (Figure imposed by Earthquake of Soil

1a), and therefore pore water pressure increases in soil; Figure 2: Liquefaction of soil layer – liquefied soil
this reduces the effective stress between soil particles. At layer may be embedded at a depth beneath the
some stage the effective stress may become almost zero. ground surface
In that situation, since soil strength depends on this
effective stress, the soil may loose its shear strength Physical Consequences of Liquefaction
completely and behave like a liquid; this phenomenon During liquefaction, cohesionless soil-water mixture
is called liquefaction. Buildings and structures rested on tends to behave like a liquid, and hence the ground
such soils can topple and sink into the ground (Figure tends to flatten out. For instance, embankments may
1b). Depending on soil properties and ground motion collapse while the depth of ponds may reduce. This
characteristics, the earthquake may impose shear stress can have serious detrimental effects on structures.
demand in soil at some depth that exceeds shear strength (1) Sinking and uplifting of structures
capacity of soil; soil liquefies over this depth (Figure 2). As the cohesionless soil-water mixture liquefies,
structures tend to settle or sink into the ground
(Figure 3). In many cases, some parts of the
building may sink more than the others, leading to
tilting of the building. Similarly, buried structures
tend to uplift and float up to the surface, because

Water their overall density is lower than the liquefied soil.
Table
Cohesionless
Water-soil
soil Photo: EERI Annotated Slide Set, 1999
mixture
moves
upwards
Rock
(a)

(a) (b)
Figure 3: Sinking and uplift of structures –

(a) Sinking of a building, and (b) uplift of sewage

tank, during 1964 Niigata Earthquake, Japan

(2) Slope failures and lateral spreading

When soil at a lower level looses its strength to

hold any load, the overlying soil layer may slide

laterally, especially when slope is steep (>~5%) and
the original soil is loose. This can cause landslides
(b)
Figure 1: Soil liquefaction during earthquake extending over hundreds of meters of motion of

shaking – (a) Process of liquefaction, and (b) soil mass (Figure 4a). In both loose and dense soils,

Collapse of buildings during 1964 Niigata when the slope is gentle (<~3%), forward
Earthquake, Japan
movement of a large soil mass can cause