Page 63 - EQTips_Eng

IITK-BMTPC Earthquake Tip 31

Why do Buildings Sink into the Ground during Earthquakes? page 2
disintegration of the large soil mass behind it.
Therefore, structures may sink and destabilize, if
Refill
supported by such soil (Figure 4b). Soil movement Vibrator mate
due to lateral spreading can exert significant
stresses and damage in foundations and bridge
abutments causing damages. Original Compacted
Soil Soil

(a)

Seepage Flow
Excess
pore
Soil that can
liquefy water

Stone Columns pressure
release
(a)
Surcharge Surcharge

Soil that can liquefy (b)
Original GL
(b)
Lowering of the ground
Figure 4: Lateral spreading of slopes – (a) steep Soil that can liquefy
slopes slide, and (b) gentle slopes subside New GL

Parameters Influencing Liquefaction
Susceptibility of a site to liquefaction depends on
following factors:
(1) Soil properties: Uniformly-graded, fully-saturated
cohesionless (sandy) soils are more prone to
liquefaction than well-graded soils. Also, fine sands
Liquefied Soil
are more susceptible to liquefaction than coarse
sands, gravelly soils, silts or clays. Sands with some
clay content also can liquefy. Loose sands have (c)
much higher tendency to liquefy than dense sands.
(2) Earthquake shaking characteristics: As shaking Figure 5: Measures to overcome liquefaction –
three options
intensity increases, soil is subjected to higher stresses
and strains, and is more susceptible to liquefaction.

Also, strong shaking with longer duration causes a Related - Earthquake Tip
larger number of stress and strain cycles in soil; this
Tip 31: What is important in foundations of earthquake-resistant
increases chances for liquefaction. buildings?

(3) Geologic conditions: Some geologic processes sort Resource Material
soils into uniform grain size and deposit them
Kramer,S.L., (1996), Geotechnical Earthquake Engineering, Prentice
loosely, as in Aeolian, Fluvial and Colluvial deposits; Hall, Inc., New Jersey, USA
such soils are candidates for liquefaction. Towhata,I., (2008), Geotechnical Earthquake Engineering, Springer-
Measures to overcome Liquefaction Verlag, Berlin

To overcome liquefaction one may:
(a) Increase liquefaction-resistance of soil, e.g., densify Authored by:

loose deposits (by vibro-compaction, impact C.V.R.Murty
compaction and pressure grouting) or replace Indian Institute of Technology Jodhpur, India
susceptible soil (by vibro-replacement) (Figure 5a); Sponsored by:
(b) Reduce earthquake demand on soil, e.g., release of Building Materials and Technology Promotion
pore water pressure by allowing drainage of water Council, New Delhi, India
(by stone columns), or increase confinement by
adding a surcharge (Figure 5b); This release is a property of IIT Kanpur and BMTPC. It may
(c) Remove soil layers that can liquefy, or use be reproduced without changing its contents with due
acknowledgement. Suggestions or comments may be sent
anchored piles as foundations going through to: nicee@iitk.ac.in. To see all IITK-BMTPC Earthquake Tips, visit
liquefiable soil layers (Figure 5c). www.nicee.org or www.bmtpc.org.