02 - Underground

Soil properties under a building influence the response of the building and its contents to seismic shaking. The design of the building, its contents, and especially its foundations, must consider the soil properties and how these perform under earthquake loading. Certain soil conditions can amplify the shaking at the surface. The soil itself can sustain damage or even failure if it is not treated. Some examples of such soil failure modes include shear failure, lateral spread, slope instability, or liquefaction. All these lead to weakening of soil under the building foundations resulting in building tilting and settling with associated damage to the building, contents and connected infrastructure.

Hazards

The likelihood of damage due to either soil failure or increased (amplified) shaking increases under the following conditions. It is to be noted that the upper 30 m of soil layer are considered to affect the shaking at the surface.

  • Landfills: Sites built on landfills exhibit higher shaking levels, due to amplification effects of such soft soil conditions.
  • Sandy and silty soils with shallow ground water levels (say closer than 10 m to the surface). Saturated soils when subjected to shaking are highly susceptible to liquefaction, where the water is ejected to the surface weakening the soil under the foundations and resulting in building tilting, settlement or even potential collapse.
  • Sites gently sloping towards rivers or other water bodies. Such conditions are conducive to lateral spread in which large blocks of the site shift downslope creating large fissures in the soil and damaging the foundations.
  • Ground accelerations are relatively large (1 m/sec2 or higher) and of long duration. Local structural design codes give an indication of the design ground accelerations at a site but not the duration of shaking, which is difficult to predict. Liquefaction, where saturated sand or silty soil becomes almost liquid, occurs when such soils are subject to shaking at a certain acceleration and for a relatively long time.

Controls

For new structures, a geotechnical investigation is always conducted to determine the type of foundations required even before starting design of the structure. If the structure is in an earthquake zone additional geotechnical tests to determine liquefaction potential should be conducted. Most structural design codes, especially in seismic countries, define the soil parameters, which require special strengthening measures. Besides suitable foundations, eg. piles, soil strengthening measures may need to be applied as well. These include lowering the ground water table, replacing the soil, strengthening the soil by compaction or geotextiles or other measures.

Foundation strengthening measures for existing building include, eg. injection grouting of the soil, micropiling, dewatering and densification, geotextiles, etc..